OLDUKÇA FAYDALI!!! - Determination of Power Losses in HVDC Converter Stations

IEEE Std 1158-1991

IEEE Recommended Practice for Determination of Power Losses in High-Voltage Direct-Current (HVDC) Converter Stations

SponsorSubstations Committeeof theIEEE Power Engineering Society

Approved September 26, 1991

IEEE Standards Board

Abstract: A set of standard procedures for determining and verifying the total losses of a high-voltagedirect-current (HVDC) converter station is recommended. The procedures are applicable to all parts of theconverter station and cover standby, partial-load, and full-load losses and methods of calculation andmeasurement. All line-commutated converter stations used for power exchange in utility systems arecovered. Loss determination procedures for synchronous compensators or static var compensators are notincluded.Keywords: converter stations, power losses, HVDC converter stations

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Copyright © 1992 by the Institute of Electrical and Electronics Engineers, Inc.

All rights reserved. Published 1992. Printed in the United States of America

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ii

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iii

Foreword

(This foreword is not a part of IEEE Std 1158-1991 , IEEE Recommended Practice for Determination of Power Lossesin High-Voltage Direct-Current (HVDC) Converter Stations.)

This recommended practice was prepared by Working Group I6 of the DC Converter Stations Subcommittee of theSubstations Committee of the IEEE Power Engineering Society.

Preparation of this recommended practice was begun during 1986 for the purpose of establishing guidelines andcriteria for the determination of power losses in high-voltage direct-current (HVDC) converter stations. It presents anup-to-date summary of procedures for determining and verifying the total losses of an HVDC converter station.

The members of the working group that developed this recommended practice were:

H. P. Lips, Chair

G. AsplundV. BurtnykB. L. DamskyT. F. GarrityG. Juette

E. KolodziejW. O. KramerA. J. MolnarE. A. OlavarriaI. K. Tasinga

R. ThallamD. R. TorgersonJ. VithayathilM. L. Woodhouse

The following persons were on the balloting committee that approved this document for submission to the IEEEStandards Board:

W. AckermanB. AfsharC. AntonopulosS. ArnotA. BakerN. BarbeitoG. BartokL. BergstromJ. BetzK. BlackC. BlattnerW. BlockS. BoggsP. BolinG. BreuerS. BrownJ. BurkeJ. CannonR. CarberryR. ChopraD. ChristieE. CounselM. CukB. L. DamskyP. DanforsF. DenbrockW. DickC. DiemondW. DietzmanT. DoernC. DurandG. Flaig

L. FergusonD. GarrettH. GildenJ. GrzanA. HabanD. HarrisR. HarrisonM. HickJ. HolladayM. HolmD. C. JohnsonZ. KapelinaG. KaradyR. KeilF. KlugeD. KoenigT. KolendaA. KollarE. KolodziejW. O. KramerT. KrummreyL. KurtzD. LairdL. LaskowskiA. LeiboldJ. LemayC. LindebergH. P. LipsW. LongJ. McDonaldT. McLenahanR. MatulicS. Meliopoulos

A. J. MolnarD. MukhedkarP. NanneryE. A. OlavarriaJ. OrrellJ. OswaldS. PatelR. PerinaK. PetterssonT. PinkhamJ. QuinataJ. ReeveD. RishworthD. RussellJ. SabathD. SchaferH. SchneiderF. ShainauskasJ. SlappB. SojkaR. St. ClairW. SwitzerE. TaylorR. ThallamD. R. TorgersonI. VancersL. VolfR. WehlingW. WernerM. L. WoodhouseB. WrayR. Youngs


iv

When the IEEE Standards Board approved this standard on September 26, 1991, it had the following membership:

Marco W. Migliaro, Chair Donald C. Loughry, Vice Chair

Andrew G. Salem, Secretary

Dennis BodsonPaul L. BorrillClyde CampJames M. DalyDonald C. FleckensteinJay Forster*David F. FranklinIngrid FrommThomas L. Hannan

Donald N. HeirmanKenneth D. HendrixJohn W. HorchBen C. JohnsonIvor N. KnightJoseph L. Koepfinger*Irving KolodnyMichael A. LawlerJohn E. May, Jr.

Lawrence V. McCallDonald T. Michael*Stig L. NilssonJohn L. RankineRonald H. ReimerGary S. RobinsonTerrance R. Whittemore

*Member Emeritus

Mary Lynne NielsenIEEE Standards Project Editor


v

CLAUSE PAGE

1. Introduction.........................................................................................................................................................7

1.1 Purpose and Scope ..................................................................................................................................... 71.2 References.................................................................................................................................................. 91.3 Definitions and Abbreviations ................................................................................................................... 9

2. Value of Losses .................................................................................................................................................10

2.1 Introduction.............................................................................................................................................. 102.2 Losses....................................................................................................................................................... 102.3 Concept of Establishing Loss Evaluation Factors.................................................................................... 10

3. Determination of Total HVDC Converter Station Loses..................................................................................11

3.1 Methodology ............................................................................................................................................ 113.2 Summation of Equipment Losses ............................................................................................................ 123.3 Ambient Conditions ................................................................................................................................. 123.4 Operating Parameters ............................................................................................................................... 13

4. Determination of Equipment Losses.................................................................................................................14

4.1 Thyristor Valve Losses ............................................................................................................................ 144.2 Converter Transformer Losses................................................................................................................. 204.3 AC Filter Losses....................................................................................................................................... 234.4 Shunt Capacitor Bank Losses .................................................................................................................. 244.5 Shunt Reactor Losses ............................................................................................................................... 244.6 DC Smoothing Reactor Losses ................................................................................................................ 254.7 DC Filter Losses....................................................................................................................................... 264.8 Auxiliaries and Station Service Energy Consumption............................................................................. 274.9 Other Equipment Losses .......................................................................................................................... 28

5. Bibliography......................................................................................................................................................30

Annex A (informative) Alternative Methods for Establishing Loss Evaluation Factors .............................................32

Annex B (informative) Losses in HVDC Thyristor Valves .........................................................................................36

Annex C (informative) Losses in HVDC Converter Transformers ..............................................................................45

Annex D (informative) Auxiliaries and Station Service Energy Consumption ...........................................................50


Copyright 1992 IEEE All Rights Reserved 7

IEEE Recommended Practice for Determination of Power Losses in High-Voltage Direct-Current (HVDC) Converter Stations

1. Introduction

1.1 Purpose and Scope

It is the purpose of this document to recommend a set of standard procedures for determining and verifying the totallosses of a high-voltage direct-current (HVDC) converter station. The procedures are applicable to all parts of theconverter station and cover standby, partial-load, and full-load losses and methods of calculation and measurement.

HVDC converter stations consist of a number of different pieces of equipment, as shown in Fig 1. The lossmechanisms vary among various pieces of equipment, and it is the consensus of the industry that the total losses cannotreadily be determined by either factory or field testing alone. When in operation at rated conditions, these stationsdevelop losses typically in the range of 0.005 to 0.01 per unit of the converter station rating for each conversion.

Therefore, it has become an accepted practice to establish the losses of the total HVDC converter station by adding thelosses from each piece of equipment as derived from component tests and calculations, as applicable. Due to thecomplexity of the overall converter station and to the nonlinear voltage and current waveforms encountered in itsoperation, different methods have been used for establishing total station losses, sometimes giving differing results.

The recommendations contained in this standard apply to all line-commutated converter stations used for powerexchange in utility systems. In some applications, synchronous compensators or static var compensators (SVC) maybe connected to the ac bus of the HVDC converter station. However, loss determination procedures for such equipmentare not included in this document.


8 Copyright 1992 IEEE All Rights Reserved

IEEE Std 1158-1991 IEEE RECOMMENDED PRACTICE FOR DETERMINATION OF

Figure 1— Typical HVDC Equipment for One Pole



POWER LOSSES IN HVDC CONVERTER STATIONS IEEE Std 1158-1991

1.2 References

This recommended practice shall be used in conjunction with the following publications:

[1] IEC Pub 76, Power transformers.1

[2] IEC Pub 146-1-3 (1991), Semiconductor converters—General requirements and line commutated convertors—Part1-3: Transformers and reactors.

[3] IEC Pub 289 (1988), Reactors.

[4] IEC Pub 747-6 (1983), Semiconductor devices—Discrete devices—Part 6: Thyristors.

[5] IEEE C57.12.90-1987, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and RegulatingTransformers; and Guide for Short-Circuit Testing of Distribution and Power Transformers (ANSI).2

[6] IEEE C57.21-1990, IEEE Standard Requirements, Terminology, and Test Code for Shunt Reactors Rated Over500 kVA.

[7] IEEE Std 3-1982, IEEE Recommended Practice for the Selection of Reference Ambient Conditions for TestMeasurements of Electrical Apparatus (ANSI).

[8] IEEE Std 18-1980, IEEE Standard for Shunt Power Capacitors (ANSI).

[9] IEEE Std 100-1988, IEEE Standard Dictionary of Electrical and Electronic Terms (ANSI).

[10] IEEE Std 1030-1987, IEEE Guide for Specification of High-Voltage Direct-Current Systems Part I—Steady-StatePerformance (ANSI).

1.3 Definitions and Abbreviations

critical service loads: Station auxiliary loads that are sensitive to power supply disturbances and that have animmediate effect upon power transmission or whose outages could cause damage to the equipment.

essential loads: Those station auxiliary loads necessary to maintain full output of the station.

nonessential loads: Those station auxiliary loads not immediately necessary to maintain full HVDC station output.

standby losses: The losses produced with the HVDC converter station energized, but with the valves blocked.

station auxiliary losses: The electric power required to feed the HVDC station auxiliary loads.

total operating losses: The total station losses produced with the converter station energized and the valves operating.

SVC: Static var compensator.

i/V: Current versus voltage.

1IEC publications are available from IEC Sales Department, Case Postale 131, 3 rue de Varembré, CH 1211, Genève 20, Switzerland/Suisse. IECpublications are also available in the United States from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13thFloor, New York, NY 10036, USA.2IEEE publications are available from the Institute of Electrical and Electronics Engineers, Service Center, 445 Hoes Lane, P.O. Box 1331,Piscataway, NJ 08855-1331, USA.




2. Value of Losses

2.1 Introduction

When evaluating the economic costs of losses in a conventional ac substation, a distinction is made between no-loadand load losses. This approach is due to the fact that transformer losses are the dominating factor in ac substations andthat transformer losses can be broken down into two portions. One part, which is caused by magnetization, is presentwhenever the transformer is energized and is practically constant over the entire load range. It is called the no-loadlosses. The other portion is proportional to the load current squared and is called load losses.

On the other hand, there is no single piece of equipment that produces dominating losses in an HVDC converterstation. (See Table A.1 in Appendix A for a typical breakdown of station losses.) The losses in an HVDC converterstation are composed of the losses of various pieces of equipment, each of which has its own loss versus loadrelationship. For example, valve losses are not proportional to the load current squared (see Appendix B).Furthermore, when the station is energized, but in the standby mode described below, the tbyristor valves will beblocked, and different loss mechanisms will apply from those that are found in normal operation. A further variationin the losses comes about because some equipment may be connected only at certain load levels. (For example, somefilters and cooling towers may be required only at higher loadings.) Therefore, realistic values of the station losses canonly be obtained if the losses are calculated at several points between standby (no-load) and full load.

2.2 Losses

When evaluating the losses of HVDC converter stations, it is recommended that two types of losses be distinguished:standby losses and total operating losses.

2.2.1 Standby Losses

These are the losses produced with the HVDC converter station energized but with the valves blocked. Under thiscondition, the ac filters and shunt compensating equipment are not required, and they are not connected to the ac bus.Smoothing reactors and dc filters, if any, are also not energized. However, the station service and cooling equipmentare connected as required for immediate pickup of load. (Refer to Section 7.4.5 of IEEE Std 1030-1987 [10] forfurther explanation of the standby condition.) Standby losses can be considered as being equivalent to “no-load” lossesin conventional ac substation practice.

2.2.2 Total Operating Losses

These are the total station losses produced with the converter station energized and the valves operating. When theconverter valves are operating, the dc current is not permitted to drop below a defined minimum current, which isdesign-dependent and varies from project to project. Operating losses may be required to be evaluated at any loadcurrent between the minimum and maximum values. It is important to keep in mind that some of the equipment in theHVDC station is not used at all load levels; the ac filters are an example. Therefore, the calculation of the losses for aparticular station loading must specify which apparatus is online for that load condition. In general, all equipmentshould be assumed to be connected, which is required to meet the specified performance at the particular load level.Total operating losses minus the standby losses can be considered as being equivalent to “load losses” in conventionalac substation practice.

2.3 Concept of Establishing Loss Evaluation Factors

Various methods are in use in the industry for making an economic evaluation of the cost of losses in electricalequipment; in particular, several methods have been suggested for use with HVDC converter stations (see




Appendix A). To date, however, no consensus has emerged that would allow the recommendation of a single techniquefor this evaluation.

2.3.1 General Principles

Most evaluation techniques recognize that the cost of equipment losses involves two factors:

1) A capacity cost, representing the capital cost required to increase the capacity of the electrical supply systemso that it can supply the power losses in the equipment.

2) An energy cost, representing the cost of supplying the actual energy consumed in the equipment. Althoughthe energy cost actually occurs over the life of the equipment, for comparison purposes, it is reduced to asingle lump-sum cost through a present value calculation.

In principle, then, the cost of the losses in the equipment is the sum of the capacity cost plus the present value of theenergy cost over the expected life of the equipment.

2.3.2 Concepts as Applied to HVDC Converter Stations

However, when the principles of 2.3.1 are applied to an HVDC converter station, there are several complicatingfactors. For example

1) As indicated in 2.1 and 2.2, the losses in the converter station vary in a complex way as a function of the loadlevel.

2) In general, the HVDC converter station does not operate at a single load level, but the dc loading is dispatchedto meet the needs of the ac systems. Therefore, the losses in the converter station will vary over time.

3) As indicated in 3.3, the losses in the converter station are dependent on the ambient conditions. (For example,for a given dc loading, the losses may be higher in the summer than in the winter.)

4) The economic value of the losses is dependent on the conditions in the system. (For example, losses thatoccur when the ac system is base loaded are usually less costly than losses occurring at peak load conditions.)

These factors are unique to each HVDC application, as will be the accounting practice of each utility. It is not withinthe scope of this document to recommend how the evaluation of losses should be carried out, but some examples aregiven in Appendix A. The underlying principle is that the owner of the station should establish a set of “loss evaluationfactors” that are project-specific. These loss evaluation factors should be expressed in dollars per kilowatt of stationloss ($/kw), should be defined for each of the load levels of interest from standby to full load, and must take intoaccount the percentage of time that the station will operate at each load level. The economic evaluation of stationlosses would then be arrived at as the sum of the losses at each load level times the evaluation factor for that load level.

3. Determination of Total HVDC Converter Station Losses

3.1 Methodology

As a general principle, it would be desirable to determine the efficiency of an HVDC converter station by the directmeasurement of its energy losses. However, there are practical difficulties that prevent such a measurement, includingthe following:

1) Attempts to determine the station losses by subtracting the measured output power from the measured inputpower must recognize that such measurements have inherent inaccuracy, especially if performed at highvoltage dc. Moreover, the losses of an HVDC converter station at full load are generally less than 1% of thetransmitted power. Therefore, the difference between the measured input and output power is a smalldifference between two large quantities and, as such, is not likely to be a sufficiently accurate indication of theactual station losses.




2) In some special circumstances, it may be possible to arrange a temporary test connection in which the twoconverters are operated from the same ac source and are also connected together via their dc terminals. In thisconnection, the ac source only provides enough power to make up the losses in the circuit. However, it mustalso provide var support and commutating voltage for the two converters. Here again, there are practicalmeasurement difficulties.

Because of the problems described above, it is recommended that the total station losses be calculated from the lossesof the individual equipment. The equipment losses should be determined according to Section 4.

3.2 Summation of Equipment Losses

The total losses of the HVDC converter station should be arrived at as the sum of the losses of each piece of equipment.It is important to note that the actual losses in each piece of equipment will depend on the ambient conditions underwhich it operates, as well as on the operating conditions or duty cycles to which it is applied. Therefore, in order forthe summation of the individual losses to be a sufficiently accurate representation of the actual total HVDC converterstation losses, the ambient and operating conditions for each piece of equipment must be defined, based on the ambientand operating conditions of the entire HVDC converter station.

For some equipment or components, losses or electrical characteristics are measured at the factory under standardizedambient and operating conditions. In these cases, the results should be related to the actual conditions in the HVDCconverter station through well-recognized calculation procedures.

The fundamental principle of this recommended practice is that the determination of the equipment losses is, in fact,based on physical tests on the actual equipment or components. Therefore, the sum of the equipment losses does givea dependable measure of the losses for the entire HVDC converter station.

3.3 Ambient Conditions

3.3.1 Introduction

A set of standard reference ambient conditions for determining power losses in HVDC converter stations isrecommended below. In the event that equipment standards have established a reference ambient condition for lossdetermination, the applicable method referenced in the equipment standard shall be used and the losses so obtained becorrected to reflect the standard reference ambient conditions indicated below for determining the total converterstation losses.

3.3.2 Standard Reference Temperature

3.3.2.1 Outdoor Standard Reference Temperature

In view of its history of acceptance and use in international comparisons of mechanical and electrical standards, anoutdoor ambient temperature of 20 °C should be used as the standard reference temperature for determining the totalconverter station losses [7].

3.3.2.2 Cooling-Medium Standard Reference Temperature

Forced air or cooling fluids such as water are used to conduct heat from a piece of equipment and can influence thetemperature rise and associated losses of certain pieces of equipment. Therefore, the cooling-medium temperaturesand flow rates need to be established as a basis for determining the total converter station losses. Since the cooling-medium parameters are highly design related, the temperatures and flow rates should be established between the userand the supplier and utilized as a basis for determining the total converter station losses.




3.3.3 Standard Reference Altitude

The reference altitude to be used for the evaluation of total converter station power losses should be the actual altitudeof the installation in question.

3.4 Operating Parameters

The losses of an HVDC converter station depend on its operating parameters according to a complex relationship (seeAppendix A). In addition to ac system voltage and converter station loading, the converter firing angle introduces anadditional variable. Also, the concept of reactive power compensation should be considered, as it would have animpact on the requirement for connecting shunt capacitors or shunt reactors. Whereas for rated conditions, theoperating parameters of an HVDC converter station would be well defined by the performance specification and theresultant design criteria, at partial load, the control strategy adopted may have a major impact on total station losses.

3.4.1 General

In general, HVDC converter station losses should be determined for nominal ac system voltage and frequency. Thetransformer tap-changer should be assumed to be in the position corresponding to nominal ac system voltage.

3.4.2 Rated Load

Where a converter station is expected to be operated at rated load for most of the time, station losses should bedetermined for this condition only, assuming nominal firing angle, that shunt compensation and filtering equipmentare connected to meet the performance requirements, and that auxiliary equipment (e.g., cooling-water pumps) is inoperation to cope with the standard reference temperature (see 3.3).

3.4.3 Varying Load

Where the converter station is expected to be operated at widely varying loads or for reactive power compensation andac voltage control, a prospective load duration curve should be used to determine a number of representative loadingconditions; e.g., typically three to five.

Total converter station losses should be determined for these loading conditions, assuming that dc current, converterfiring angle, shunt compensation, and filtering equipment are consistent with the respective loading condition and theperformance requirements specified. Station service loads and auxiliary equipment should be connected to support therespective loading condition, assuming the standard reference temperature.

The losses at the individual loading conditions should then be weighted with factors representative of the system of theparticular utility; e.g., as described in [B3], and added to obtain the equivalent losses for evaluation.

3.4.4 Standby

For the standby mode, converter transformers should be assumed to be energized and the thyristor valves blocked. Allfilters and reactive power compensation equipment should be assumed to be disconnected. Station service loads andauxiliary equipment (e.g., cooling-water pumps) should be assumed to be connected as required for immediate pickupof load for the converter station.




4. Determination of Equipment Losses

4.1 Thyristor Valve Losses

The procedures set out below will provide a total loss figure for valves based on the summation of individual losscomponents. The individual components of loss are determined by applying standardized calculation methods. Datashould be obtained, where possible, from verifiable factory measurements.

All losses are determined on a per-valve basis, in which a “valve” is taken to be one arm of a three-phase, line-commutated converter bridge.

Standby losses are defined for the valves energized but blocked.

The auxiliary power losses of the thyristor valve cooling equipment are included in the auxiliary power energyconsumption of the total converter station (see 4.8).

Valve losses and the methods adopted in the following subsections for their prediction are discussed more fully inAppendix B.

4.1.1 Thyristor Conduction Loss per Valve: W1

This is the principal loss arising from the passage of load current through the thyristors of the valve (see B.3.1.3 andB.5.1).

(1)

where

n = The number of series levels in the valve

= The current independent component of the ON-state voltage of the average thyristor (see NOTE 1), in volts

= The slope resistance of the ON-state characteristic of the average thyristor (see NOTE 1), in ohmsu = The commutation overlap calculated for the operating conditions for which losses are being determined, in

radiansI = The mean dc bridge current for which losses are being determined, in amperes

NOTES:

1 — and are determined from the fully spread ON-state voltage measured at the appropriate current and junctiontemperature (see 4.1.10). The average value of ON-state voltage should be readily available from production records. Ifparallel connection of thyristors is employed, the appropriate current is the bridge current divided by the number of parallelpaths.

2 — When the dc-side harmonic currents (root sum of squares) are more than 10% of the dc component, the modified formulagiven in B.5.1 should be used.

4.1.2 Thyristor Spreading Loss per Valve: W2

This is an additional conduction loss of the thyristors arising from the delay in establishing full conduction of thesilicon (see B.3.1.2 and B.5.2).

(2)

W1nI3----- Vo RoI

2π u–2π

--------------- +=

Vo

Ro

Vo Ro

W2 nf v1 t( ) v2 t( )–( ) i⋅ t( ) ωdω t α=

ω t α 120 u+ +=

∫ t=




where

n = The number of series levels in the valvef = The system frequency, in hertzV1(t) = The instantaneous ON-state voltage of the average thyristor at the appropriate junction temperature

measured with a trapezoidal current pulse exhibiting the correct amplitude and commutation overlapperiods (see 4.1.10), in volts

V2(t) = The predicted instantaneous ON-state voltage of the average thyristor at the same junction temperature forthe same current pulse but with the conducting area fully established, in volts

i(t) = The instantaneous current in the thyristor, in amperesα = The firing delay angle, in degreesu = The commutation overlap angle, in degreesω = 2πf, in radians/second

Typically, instantaneous ON-state voltage data, including the effects of spreading, are not available from productionrecords. Measurements of typical thyristor ON-state voltage, including spreading, should therefore be obtained duringthe valve periodic firing and extinction type test or, alternatively, from a separate type test in the laboratory.

4.1.3 Other Valve Conduction Losses per Valve: W3

These are the conduction losses in the main circuit of the valve due to components other than the thyristors (seeB.3.3.1.1 and B.5.3).

(3)

where

I = The mean de bridge current for which losses are being determined, in amperesR = The dc resistance of the valve terminal-to-terminal circuit excluding the thyristors, in ohmsu = The commutation overlap angle calculated for the operating conditions for which losses are being

determined, in radians

The value of R should be determined by direct measurement as a room-temperature type test on a representative valvesection that includes all elements of the main circuit of a valve in the correct proportions, but in which the thyristorshave been replaced by copper blocks of the appropriate dimensions. Alternatively, the dc resistance may be calculated,in which case the calculating methods should be documented.

4.1.4 “DC” Voltage-Dependent Loss per Valve: W4

This is the loss in the shunt resistive impedance of the valve arising from the voltage appearing between valveterminals during the nonconducting interval. It includes losses in dc grading resistors, thyristor off-state and reverseleakage resistance, resistance of coolant in coolant pipes, resistivity effects of the structure, other grading networks,fiber optics, etc. (see B.3.3.2 and B5.4.) [B9].

(4)

where

RDC = The effective off-state dc resistance of a complete valve determined by measuring the current drawn duringthe valve terminal-to-terminal dc voltage type test, in ohms

u = The commutation overlap angle calculated for the operating conditions for which losses are beingdetermined (see Note 1), in radians

W3I2R3

--------2π u–

2π---------------

=

W4

VL2

2πRDC----------------- 4π

3------

34

------- 2cos α 2α 2u+( )cos+( ) 78---

3m 2 m–( )4

--------------------------+ 2u 2sin α 2α 2u+( )sin–+( )–+=




α = The firing delay angle, in radiansVL = The rms line-to-line commutating voltage referred to the valve winding (see Note 2), in voltsm = L1/(L2 + L1), where L1 is the inductance, referred to the valve winding, between the commutating voltage

source and the point of common coupling of the star and delta bridge valve winding connections. L2 is theinductance referred to the valve winding between the point of common coupling and the valves (seeNote 3).

NOTES:

1 — Eq 4 is only valid for .

2 — The commutating voltage source can normally be taken as the ac filter bus.

3 — When separate transformers supply the star and delta bridges, L1 = 0; hence, m = 0. When three-winding construction isemployed, a common line winding impedance and mutual coupling effects of the two secondary windings give real values forL1, which may be positive or negative. For more complicated transformer arrangements, e.g., filters connected to a low-voltage tertiary winding, the effective values of L1 and L2 must be determined with care.

With the valves in standby mode, that is, energized but blocked, the contribution to standby losses is

(5)

4.1.5 Damping Loss per Valve (Damping-Resistor-Dependent Term): W5

This is the component of loss that is dependent on the value of the resistive elements of those circuits that are accoupled via series capacitors arising from the voltage appearing between valve terminals during the nonconductinginterval. The principal circuits involved are the damping circuits (see B.3.2.1 and B5.5) [B9].

(6)

where

VL, α, u, and m are as defined in Eq 4f = The system frequency, in hertzCac = The effective value of valve terminal-to-terminal capacitance (see Notes 3, 4, and 5), in faradsRac = The effective value of the associated series-connected resistance (see Notes 3, 4, and 5), in ohms

NOTES:

1 — Eq 6 is only valid for .

2 — The above equation is derived from [B10]. A different equation can be derived from [B4], giving results that agree to within± 0.5% of the total thyristor valve losses. The difference in equations is due to differing, but equally valid, simplificationsmade in the two references.

3 — Unlike the case of dc OFF-state resistance, it is not possible to determine the values of Cac and Rac readily from, for example,the ac power frequency test. Since “stray” capacitance will have negligible influence on the current flow in the associatedresistors, the design values for the grading network components should be used to derive Cac, and Rac.

4 — If the valve employs more than one grading network branch that incorporates series-connected R and C, then each branchmust be evaluated separately and the results summed.

uπ6--- 30°( )<

W4(standby)

VL2

3RDC--------------=

W5 2πf2VL2Cac

2 Rac

4π3

------3

2-------–

3 3m2

8----------------- 6m2 12m– 7–( )u

4---

78---

9m4

-------39m2

32-------------–+

2αsin + + + +

78---

3m4

-------3m2

32----------+ +

2α 2u+( )sin3m16

-----------3 3m2

8-----------------+

2αcos–3m16

----------- 2α 2u+( )cos+

=

uπ6--- 30°( )<




5 — If energy is extracted from one of the grading network branches for energizing the valve firing electronics, then it should bedemonstrated that the additional losses are either negligible or else within the allowance made in the loss declaration.

With the valves in standby mode, that is, energized but blocked, the contribution to standby losses is given by:

(7)

where

(8)

Notes 4 and 5 of Eq 6 also apply.

4.1.6 Damping Loss per Valve (Change of Capacitor Energy Term): W6

This is the component of loss arising from the change in stored energy in the circuit capacitances as a result of the“step” changes in the voltage waveform that appears between the valve terminals during the nonconducting interval.The principal circuits involved are the damping circuits (see B.3.2.2 and B.5.5) [B9].

(9)

where

VL, f, α, u, and m are as defined in Eqs 4 and 6Chf = The sum of the effective terminal-to-terminal capacitance of all capacitive grading network branches

within the valve (whether incorporating series resistors or not), plus the total effective stray capacitancebetween valve terminals arising from externally connected equipment (see Notes 2 and 3)

NOTES:

1 — Equation 9 is only valid for .

2 — The capacitances within the valve may be derived from the design values of the grading network components. The externalstray capacitance arises predominantly from the windings and bushings of the converter transformer (plus separate wallbushings if fitted), all of which can be measured at manufacture. Surge arresters, busbars, and the valve structure itselfcontribute to the stray capacitance, but these components are small and calculated values will suffice.

3 — While the capacitances internal to each valve are likely to be virtually identical, the external stray capacitance variesaccording to the position of the valve within the bridge. For determination of station loss, the average value of straycapacitance should be used.

4.1.7 Turn-Off Losses per Valve: W7

Additional losses are generated in the thyristors and damping resistors due to reverse current flow in the thyristors atturn-off (see B.3.1.4, B.3.2.3, and B5.6). The reverse current arises from the stored-charge property of the thyristors.The additional losses are given by

(10)

W5(standby)

RacVL2

3Zac2

---------------=

Zac Rac2 1

2πfCac------------------

2+ ohms=

W6

VL2 fChf 7 6m2+( )

4------------------------------------------ αsin2 α u+( )sin2+[ ]=

uπ6--- 30°( )=

W7 Qrrf 2VL α u ωto+ +( )sin=




where:

Qrr = The average value of thyristor stored charge measured at the junction temperature, di/dt, and recoveryvoltage appropriate to the service conditions for which losses are being determined (see 5.1.10), incoulombs

f = The system frequency, in hertzVL = The rms line-to-line commutating voltage referred to the valve winding, in voltsα = The firing delay angle, in radiansu = The commutation overlap angle, in radiansω = 2πf, in radians/secondto = A time determined from the relationship

where

= The commutating di/dt measured at current zero

NOTE — Qrr is a parameter normally recorded in production. It is important that the value of Qrr used is the full integral of reversecurrent, not an approximate triangulation such as that proposed in IEC Pub 747-6 (1983) [4].

4.1.8 Reactor Losses per Valve: W8

Reactor winding loss and the overwhelming majority of reactor core eddy current loss (and/or reactor damping resistorloss) have already been accounted for (see B.3.3.1.2, B.5.8, and Note 2 of this subsection). Depending on the design,reactor core hysteresis loss may not be negligible. A dc magnetization curve for the core material(s) should bedetermined for the minor loop of excitation that an HVDC valve reactor normally experiences. The dc B-H curveshould be established from a magnetizing force no greater than that arising from the peak of the reverse current at turn-off in one polarity to well into the saturated region in the other, and back again. From the area enclosed by the loop, acharacteristic hysteresis loss in joules per kilogram can be determined and applied to the design of the reactor inquestion; i.e.,

(11)

where:

nL = The number of reactor cores in a valveM = The mass of each core, in kilogramsk = The characteristic loss, in joules/kilogramf = The system frequency, in hertz

NOTES:

1 — Eq 11 assumes a single major excursion of the B-H loop per cycle. Oscillatory valve current at turn-on (if present) and valvegrading network displacement current flowing through the reactor during the blocking interval will result in additional minorexcursions of the B-H loop, but these normally contribute negligible additional losses.

2 — If the saturation current level for the reactor is high in relation to rated bridge current, and normal commutation di/dt is alsohigh (corresponding to small overlap angles at rated conditions), then additional reactor core eddy current losses will begenerated during the commutation periods. If this is the case, it should be demonstrated that these additional losses are eithernegligible or else within the allowance made in the loss declaration (see B.5.8).

to

Qrr

didt-----

i 0=

-------------------- seconds( )=

didt-----

i 0=

W8 nL M k f⋅ ⋅ ⋅=




4.1.9 Total Valve Losses

Total operating losses per valve (WT) are given by:

(12)

Standby losses per valve are given by:

(13)

Losses per 12-pulse converter are simply 12 times the total losses per valve (WT).

4.1.10 Temperature Effects

All components have electrical characteristics that are temperature sensitive. However, it is normal experience that theonly component with temperature-sensitive characteristics that can materially affect valve losses is the thyristor itself.

In 4.1.1 and 4.1.2, the ON-state voltage is the temperature-sensitive parameter, while in 4.1.7 it is the stored charge.

Junction temperature, which cannot be directly measured, is normally determined by adding to the coolant temperature(Tc) a value that is the product of the junction power dissipation (Wj) and the thermal resistance between the junctionand the coolant (RθJC), viz:

(14)

Tc is directly available, while Wj must be determined by a summation of thyristor conduction and switching losses andRθJC by laboratory measurement or calculation. RθJC comprises not only the thermal resistance of the thyristor but alsothe (flow dependent) thermal resistance of the heat sink to which it is mounted and the interface between the two.

If thyristor production test data is obtained at a different current or temperature than those appropriate to the specificapplication, then correction factors may need to be applied. If this is the case, then the ON-state voltage (Vf) should beconsidered to be described by an expression of the following form:

(15)

where

I = The on-state current, in amperesTj = The junction temperature, in °CVo, Ro, α and β are constants characteristic of the thyristor being used

Alternatively, the correction factor may be determined graphically from test data obtained on sample thyristors.

In the case of stored charge, production measurements should ideally be made at the desired conditions or at conditionsthat are more severe than the service duty. In this context, “more severe” means at higher values of temperature,commutating di/dt, and recovery voltage. If correction factors are required, these may be determined graphically fromtest data obtained on sample thyristors.

WT Wn

n 1=

n 8=

∑=

Wstandby W4(standby) W5(standby)+=

Tj Tc Wj+ RθJC⋅=

Vf Vo 1 αTj+( ) IRo 1 βTj+( )+=




4.2 Converter Transformer Losses

4.2.1 Introduction

The currents flowing through the windings of converter transformers contain harmonics whose magnitudes depend onthe operating parameters of the converter station. The current-dependent load losses in the transformer due to thenonsinusoidal current waveshapes are greater than those that would occur with a sinusoidal current of the same rmsvalue at fundamental frequency. No consensus has yet been reached in the industry on how load losses in HVDCconverter transformers should be calculated or measured. Various methods have been proposed or are in use (seeSections C3 through C7). Of these, only the two methods in Sections C6 and C7 reflect the change in losses that occurswith a change in harmonic content of the transformer current.

Until an appropriate equipment standard is developed, either one of the two methods described in 4.2.3 should be usedto determine the load losses of HVDC converter transformers.


In the standby mode, with the transformers energized and the valves blocked, the transformer standby losses are theno-load losses. The no-load losses (core losses) may be determined in the same manner as for ac powertransformers [5].

4.2.3 Operating Losses

In the operating mode, with the valves conducting, the transformer operating losses can be taken as the sum of themagnetizing losses (core losses) and the current dependent (load) losses.

Under load-carrying conditions, harmonic voltages are imposed on the transformer. However, the effect of theharmonic voltages on the exciting current of the transformer, relative to the effect of the fundamental frequencycomponent of voltage, is negligible (see Section C1). Hence, the core losses under load may be considered to be equalto the no-load losses at the tap position corresponding to the load level considered, and with nominal voltage applied.

Two methods of determining load losses are described below.

4.2.3.1 Method 1

This method consists of measuring the transformer effective resistance at various harmonic frequencies andcalculating the total load losses as the sum of the losses at each harmonic (see Section C6, [B2], [B6], and [B7]).

The procedure consists of the following three steps:

1) Measure the effective resistance of the transformer at various harmonic frequencies in a manner similar tothat used for measuring the load losses of a transformer at fundamental frequency; i.e., by applying a sourcevoltage of the appropriate frequency to the low-voltage windings of the transformer under test with the high-voltage winding short-circuited. Measurements at about 10 frequencies between the fundamental and the50th harmonic are considered sufficient, preferably at the 1st, 2nd, 3rd, 5th, 7th, 11th, 13th, 20th, 30th, and50th harmonics. A best-fit smooth curve showing the variation of effective resistance with frequency shouldthen be developed.The measurements on three-phase transformers require a three-phase voltage source. For three-windingtransformers, the measurements should be made on each valve-side winding with the other valve-sidewinding open-circuited.

2) Calculate the magnitude of the six-pulse characteristic harmonic currents in each winding of the convertertransformer for the particular load level and corresponding operating parameters (see 3.4).The characteristic harmonic currents should be calculated from the equations given in Section C6.Noncharacteristic harmonic currents may be neglected unless special situations warrant this consideration.




Three-winding transformers used for 12-pulse operation often have two primary winding halves connected inparallel to achieve equal stray impedances for the delta and wye paths. Hence, the primary windings, as wellas the valve windings, carry 6-pulse currents.

3) Calculate the total load loss by summing the losses due to each of the characteristic harmonic currents (In),including the fundamental frequency current, by using Eq 16, which gives the load loss on a per-phase basis.

(16)

where

Rn = The effective resistance, in ohms, at harmonic nIn = The current, in amperes, at harmonic nn = The harmonic number

The magnitudes of the harmonic currents become quite small at the high harmonic numbers, and the losses due tothose currents are also small. It is considered that losses beyond the 49th harmonic can be neglected.

4.2.3.2 Method 2

Method 2 is an approximate method of determining load losses that does not require measurements at harmonicfrequencies. It is based on a typical relationship between the effective resistance of converter transformers (in per unitof the fundamental frequency resistance) and frequency. The relationship was developed from measurements made inaccordance with Method 1 on several converter transformers supplied on recent HVDC projects (see Section C7).

The procedure consists of the following steps:

(1) Measure the load loss of the converter transformer at power frequency in accordance with IEEE C57.12.90-1987 [5].

Calculate the effective resistance at fundamental frequency (R1) from the measurements:

(17)

where

PL = The measured load loss in watts for one phase at current I in amperes

(2) Determine the effective resistance at other harmonic frequencies

(18)

where kn is taken from Table 1.

Load loss In2

n 1=

n 49=

∑ Rn=

R1

PL

I2------=

Rn R1 kn( )=




Table 1— Variation of Transformer Resistance With Frequency

(3) Proceed as in steps (2) and (3) of 4.2.3.1.

4.2.3.3 Comments on Method 1 and Method 2

Method 1 is the more accurate method but requires instrumentation and measurement techniques that may not bereadily available to all manufacturers. The accuracy of the method is directly dependent on the accuracy of themeasurements.

Method 2 is considered to provide a reasonable estimate of load losses for converter transformers of typical design.However, the accuracy of the method may decrease for transformers whose designs depart significantly from those onwhich the tests were made.

Calculation of harmonic currents flowing through the transformer are required for Method 1 and Method 2.

4.2.4 Auxiliary Power Losses

The auxiliary power losses of the converter transformer should be included in the auxiliary power energy consumptionof the total converter station (see 4.8). They may be measured separately during factory test or during measurement ofthe converter station energy consumption.

4.2.5 Total Transformer Losses

In the standby mode, transformer losses are given by 4.2.2. With the converter station in operation, the totaltransformer losses are given by 4.2.3.

Harmonic Number (n) Relative Resistance (kn)*

*In per-unit of resistance at n = 1.

1 1.00

3 2.29

5 4.24

7 5.65

11 13.00

13 16.50

17 26.60

19 33.80

23 46.40

25 52.90

29 69.00

31 77.10

35 92.40

37 101.00

41 121.00

43 133.00

47 159.00

49 174.00




4.3 AC Filter Losses

4.3.1 Introduction

The principal function of the ac filters in an HVDC converter station is to provide a low-impedance shunt for theharmonic currents generated by the converter. Typically, single-tuned, multiple-tuned, and high-pass filters are used inHVDC converter stations.

For purposes of loss determination, the ac system should be assumed to be open-circuited so that all harmonic currentsare considered to flow into the ac filters.

In the standby mode, ac filters are not connected to the ac system and, therefore, generate no losses.

When the converter is operating, the determination of ac filter losses should be based on characteristic harmoniccurrents of the converter, which should be calculated for each load level and with consistent operating parameters(see 3.4). For calculating the converter harmonic currents, the formulas given in Section C6 should be used [B4]. Theharmonic current flowing in each filter branch should then be calculated from the total converter harmonic current andshould be used as a basis for determining the losses in each filter component.

4.3.2 Filter Capacitor Losses

The fundamental frequency losses in the filter capacitors should be determined in the same way as for shunt capacitorbanks (see 4.4). The three-phase Mvar rating of the capacitor bank should be calculated from the capacitance value andthe fundamental frequency voltage across the capacitor bank. Because of the low power factor of capacitors, the lossesdue to harmonic currents can be considered to be very small and should be neglected.

4.3.3 Filter Reactor Losses

The fundamental and harmonic currents in the filter reactors should be calculated. The impedance of the reactor atfundamental frequency and the quality factors at the fundamental and harmonic frequencies should be measured at thefactory. The reactor losses should then be determined by the equation

(19)

where

n = The harmonic numberILn = The calculated current through reactor at nth harmonic, in amperesXLn = The reactor reactance at nth harmonic, XLn = nXL1, in ohmsQn = The average quality factor for all reactors of the same item measured at the nth harmonicPR = The filter reactor loss

4.3.4 Filter Resistor Losses

The losses in the filter resistors should be calculated for the fundamental and harmonic currents together. Theresistance value of the resistor should be determined by factory measurements. The rms current through the filterresistor should be calculated. The losses in each resistor are

(20)

PR

ILn( )2XLn

Qn------------------------

n 1=

n 49=

∑=

Pr RIR2=




where

R = The resistor value, in ohmsIR = The rms current through the resistor, in amperesPr = The filter resistor losses, in watts

4.3.5 Total Filter Losses

In the standby mode, the ac filter losses are zero. With the converter station in operation the total filter losses shouldbe obtained by summing the losses of all capacitors, reactors, and resistors that make up a filter.

4.4 Shunt Capacitor Bank Losses

Shunt capacitors are sometimes used in addition to harmonic filters to provide reactive support to the ac system. Shuntcapacitor banks are made from a matrix of series- and parallel-connected capacitor cans.

In the standby mode, shunt capacitor banks are not connected to the ac system and therefore generate no losses.

When the converter station is operating, power losses in shunt capacitor banks should be determined for those loadlevels of the converter station at which such banks will be connected to the ac bus. Fundamental frequency losses of allcapacitor units manufactured under the same contract should be measured during production tests according to IEEEStd 18-1980 [8] and expressed in watts per kilovar. The power losses of the entire bank, P, should then be calculatedby:

(21)

where

P1 = The mean value of the losses of one capacitor unit averaged over all units manufactured under the samecontract and expressed in watts per kilovar

S = The three-phase kvar rating of the capacitor bank at nominal system voltage and frequency

The losses so calculated should be used without further correction to in-service ambient temperatures.

4.5 Shunt Reactor Losses

In the standby mode, shunt reactors are not connected to the ac system and therefore generate no losses.

When the converter station is operating, shunt reactors may be connected to the ac bus of an HVDC converter stationto compensate for capacitive currents from ac harmonic filters, particularly at light load. Their duty does not differfrom conventional applications in ac transmission systems.

Losses of shunt reactors in HVDC converter stations should be measured during factory tests in accordance withIEEE C57.21-1990 [6] and corrected to the maximum winding temperature excluding hot spots, calculated for thestandard ambient conditions (see 3.3).

Shunt reactor losses should only be included in the total converter station losses for those load levels at which it isintended that shunt reactors will be connected to the ac bus.

P P1S=




4.6 DC Smoothing Reactor Losses

4.6.1 Introduction

DC smoothing reactors are used in HVDC converter stations to filter the direct current and voltage, to limit converterdc side overcurrents during transient conditions, and to protect the converter from steep front overvoltages originatingfrom the dc overhead line (in transmission systems) or from the other converter (in back-to-back systems).

DC smoothing reactors may be of the following types:

1) Air core, air insulated, naturally cooled, and mounted on a separate insulating support structure2) Oil or gas cooled, oil/gas insulated, and mounted in a tank

The latter type of reactor may also have an iron shell core with an air gap, to limit the external stray magnetic field.

The current through the smoothing reactor is direct current with superimposed harmonies, mainly characteristicharmonics. Small amounts of noncharacteristic harmonics may also occur, particularly when the ac system isunbalanced.


DC smoothing reactors are connected in series to the dc terminal of the converter. Under standby conditions, thesmoothing reactor current and voltage are both zero; therefore, losses do not occur.

4.6.3 Load Losses

The dc loss of the smoothing reactor should be established during factory tests according to IEC Pub 289 (1988) [3],and IEC Pub 76 [1] using the direct current magnitude corresponding to the desired load level. Correction for theambient temperature at the test should be made to relate the loss to the standard reference temperature (see 3.3).

The winding loss due to harmonic currents should be determined by calculation. The calculation should use theharmonic current amplitudes applicable to the appropriate load level and the corresponding harmonic resistance. Theharmonic resistance should be determined by bridge measurement.

In smoothing reactors of a tanked construction, with an iron shell core, magnetization losses caused by harmoniccurrents may be a few percent of the total smoothing reactor losses. For a 12-pulse system, the magnetization lossesshould be calculated with the following empirical procedure:

(22)

(23)

(24)

(25)

where

Pm = The magnetization loss, in wattsPo = The direct current losses, in watts

Pm 0.125Ph 0.125Pe+( )Po=

Phn

In

Io----

n p.u.( )=

Pen

In

Io----

2n0.5 p.u.( )= for n 10>( )

Pe2

In

Io----

2n2 p.u.( )= for n 2=( )




Ph = Σ (Phn) = The hysteresis loss component, in per-unitPe = Σ (Pen) = The eddy-current loss component, in per-unitIn = The harmonic current of order n, in amperesIo = The dc current, in amperesn = The harmonic number

4.6.4 Auxiliary Losses

Air-cored, air-insulated smoothing reactors do not usually require auxiliary supplies.

Tanked smoothing reactors generally have forced cooling. The auxiliary power losses of the smoothing reactor shouldbe included in the auxiliary power energy consumption of the total converter station (see 4.8). They may be measuredseparately during factory test or during measurement of the converter station energy consumption.

4.6.5 Total Smoothing Reactor Losses:

In the standby mode, the smoothing reactor losses are given by 4.6.2. With the converter station in operation, thesmoothing reactor losses are given by 4.6.3.

4.7 DC Filter Losses

4.7.1 Introduction

The principal function of the dc filters, in conjunction with the dc smoothing reactor, is to provide a low-impedanceshunt for the harmonic currents generated by the converter, thus reducing the level of harmonic currents on the dc lineand preventing noise generation in adjacent wire communication systems. No dc filters are used in back-to-backsystems. The dc filter configuration may vary from one to several filter branches, depending on system requirements.Components that contribute to dc filter losses are the capacitor banks, dc filter reactors, and dc filter resistors.

DC filters are connected between the high- and low-voltage terminals of the converter. Under standby conditions, thedc filter current and voltage are both zero; therefore, losses do not occur.

When the converter is operating, the dc filter losses should be determined for normal operating parameters, at theappropriate load level, using factory loss measurements and calculated harmonic currents. Losses should be calculatedby determining the losses in each filter component and totalling them.

4.7.2 Filter Capacitor Losses

Losses in the dc filter capacitors are made up from losses in the dc grading resistors and harmonic losses.

The losses in the grading resistors are calculated by using the total resistance of the capacitor bank as determined fromthe mean value of all grading resistors per capacitor unit obtained from production tests, and the capacitor bankconfiguration, using

(26)

where

ER = The rated bank voltage, in voltsRc = The total bank resistance, in ohmsPc = The filter capacitor losses, in watts

Pc

ER( )2

Rc--------------=




Losses due to the harmonic current in the capacitor bank are very small because of the low power factor and should beneglected.

4.7.3 Filter Reactor Losses

The reactor losses should be determined by calculating the harmonic currents in the reactor for the appropriate loadlevel and corresponding operating parameters (see 3.4) and by measuring the reactor reactance and quality factor at theharmonic frequencies during factory tests. The reactor losses are given by the equation

(27)

where

n = The harmonic numberILn = The calculated current through the reactor at nth harmonic, in amperesXLn = The reactor reactance at nth harmonic, in ohmsQn = The quality factor measured at the nth harmonic

4.7.4 Filter Resistor Losses

The resistor losses should be calculated considering all harmonic currents together. The resistance value of the resistor,R, should be determined by factory measurements.

The rms value of the current through the resistor should be calculated for the appropriate load level of the converterstation and corresponding operating parameters (see 3.4). The losses in each resistor are

(28)

where

R = The resistance value, in ohmsIR = The rms current through the resistor, in amperes

4.7.5 Total Filter Losses

In the standby mode, the dc filter losses are zero. With the converter station in operation, the total filter losses areobtained by summing the losses of all capacitors, reactors, and resistors that make up the dc filters.

4.8 Auxiliaries and Station Service Energy Consumption

4.8.1 General Considerations

The auxiliary power supply system should be designed to meet the availability and reliability performance, safety, andmaintenance requirements and applicable standards specified for the HVDC converter station. The auxiliary powersystem and subsystems should allow operation at full rated dc power, with either one of the primary sources out ofservice. The loads supplied by the auxiliary power system can be grouped into three categories.

PR

ILn( )2XLn

Qn------------------------

n 1=

n 49=

∑=

PR RIR2=




4.8.1.1 Critical Service Loads

These are loads that are sensitive to power supply disturbances and that have an immediate effect on powertransmission or whose outages could cause damage to the equipment. In general, critical loads are supplied by aredundant, uninterruptible dc supply system. Critical loads include, but are not limited to, those listed in Table D.1.

4.8.1.2 Essential Loads

Essential loads are those necessary to maintain full output of the station. The loss of supply will result in reduction orcomplete loss of station output without any damage to equipment. Loads in this category can tolerate power loss forshort periods of time to permit automatic changeover to alternative sources. Examples of essential loads are includedin Table D.2.

4.8.1.3 Nonessential Loads

These loads are those auxiliaries not immediately necessary to maintain full HVDC station output. Loads in thiscategory will tolerate longer outages and selective load shedding could be implemented. A list of typical nonessentialloads is included in Table D.3.

4.8.2 Determination of Auxiliary System Energy Consumption

The electrical power required to feed the HVDC station loads is referred to as station auxiliary losses. This power willvary depending on several factors, such as station service facilities, operating requirements, and ambient conditionsprevailing. In general, because of the diversity factor, the maximum power supplied to the station auxiliaries iscommonly only about 60% of the connected load.

The total HVDC station auxiliary losses should be determined for the appropriate load levels of the HVDC converterstation, using consistent operating parameters (see 3.4) under steady-state and normal operating conditions, by directmeasurements on the main feeder at either source. Station service auxiliaries during special circumstances, such asrapid cooling or heating of valve halls for maintenance purposes, short-time overload, or under transient conditions,should not be considered in the evaluation of auxiliary losses.

A similar procedure is required to determine the station auxiliary power losses for standby operation. In this case, onlythose auxiliary loads essential for the immediate pickup of converter load shall be connected.

To account for load variations with time, a series of measurements should be taken over a defined time period, and theresults should be averaged.

It should be recognized that in many installations there will be equipment that is not within the responsibility of themain supplier. The loads of this equipment should be measured separately at the respective bus and subtracted from theoverall loss measurement for contractual purposes.

4.9 Other Equipment Losses

4.9.1 General Considerations

In addition to the main components discussed in 4.1 through 4.8, other loss-producing equipment exists in an HVDCconverter station, such as surge arresters, temporary overvoltage (TOV) suppressors, instrument transformers, radiointerference (RI) and power-line carrier (PLC) filters, high-voltage switchgear, etc. All of these are connected bybuswork, including bushings, and cables.




Note that, in some applications, synchronous compensators or SVCs may be connected to the ac bus of the HVDCconverter station. However, loss determination procedures for such equipment are not considered within the scope ofthis document. (See Section 1.)

It is general utility practice to disregard losses in buswork, bushings, and cables of ac substations because of theirminor significance. The same approach should be taken for the determination of losses in HVDC converter stations.Likewise, losses of conventional surge arresters, instrument transformers, RI and PLC filters, and high-voltageswitchgear should be neglected.

4.9.2 TOV Suppressor Losses

At present, there is no standard covering the application, design, or loss determination of TOV suppressors. TOVsuppressors represent a special application of metal oxide surge arresters to limit power-frequency temporaryovervoltages to little above nominal; e.g., 1.25 p.u., In doing so, TOV suppressors have to absorb relatively largeamounts of energy. Therefore, their voltage rating (MCOV) must be low as compared to conventional surge arresters,and a sufficient number of units must be connected in parallel.

Due to the low voltage rating of TOV suppressors and the resulting thermal stability constraints, TOV suppressors mayeither be connected to the system only temporarily, or they may be connected for long periods and equipped withspecial cooling to maintain thermal stability in the presence of the high level of continuous losses associated with thismethod of use.

Losses in switched TOV suppressors should be disregarded for the purpose of determining HVDC converter stationlosses, as these devices are only temporarily connected to the system for very short periods.

Losses in TOV suppressors that can be connected for long periods should be determined according to the followingprocedure:

1) Obtain a sufficiently detailed i/v (current-versus-voltage) characteristic per arrester from zero to peak steady-state operating voltage (see Fig 2) for the applicable continuous operating temperature as related to thespecified ambient conditions (see 3.3).

2) Assume stiff ac voltage of a level consistent with specified operating parameters (see 3.4).3) Multiply the instantaneous voltage with the corresponding arrester current as per the i/v characteristic for

sufficient data points between zero and peak steady-state system voltage to obtain a power curve extendingover a quarter-cycle of power frequency. Integrate over the quarter-cycle to obtain energy dissipation in watt-seconds. Multiply by 4f, where f is the power system frequency in hertz.

4) Add losses of all parallel arresters per phase and all three phases, as applicable.

Energy consumed by special cooling systems of permanently connected TOV suppressors should be included in thestation auxiliary losses (see 4.8) by having such cooling systems connected during station auxiliaries lossmeasurements.




Figure 2— TOV Arrester Loss Determination

5. Bibliography

[B1] Egide, E., Felber, A., Schmied, A., and Titscher, G. Converter transformers and smoothing reactor of the HVDCtie Duernrohr, presented at the CIGRE SC 14 Meeting in Vienna, Austria, 1983.

[B2] Forrest, J. A. C. “Harmonic Load Losses in HVDC Converter Transformers.” Paper 90WM224-6 PWRD,presented at the IEEE/PES Winter Meeting in Atlanta, Georgia, February 5–9, 1990.

[B3] Harrison, R. E. and Burtnyk, V. “Evaluation of Losses in HVDC Transmission Systems.” Fourth InternationalConference on AC and DC Power Transmission, IEE Conference Publication No. 255. pp. 31–36.

[B4] Kimbark, E. W. Direct Current Transmission, volume I. New York: John Wiley & Sons, Inc., 1971.

[B5] Laughton & Say, eds. Electrical Engineer’s Reference Book. Butterworth’s, 14th Edition, 1985, Chapter 34.




[B6] Ram, B. S. and Forrest, J. A. C. Study of HVDC converter transformer temperature and hot spots due to effectsof harmonic currents. Canadian Electrical Association, CEA Report 211 T 423, September 1985.

[B7] Ram, B. S., Forrest, J. A. C., and Swift, G. W. “Effect of Harmonics on Converter Transformer Load Losses.”IEEE Transactions on Industry Applications, vol. 11, 1975, pp. 165–171.

[B8] Sacharoff, G. Influence des harmoniques de courant sur la valour des pertes parasites des transformateursalimentent des groupes de conversion. CIGRE 12–73 (WG 02) 09, 1973.

[B9] Tennakoon, S. B. and Woodhouse, M. L. “Calculation of Valve Damping Circuit Losses in 12-Pulse HVDCConverters,” presented at IEEE Twelfth Conference and Exposition on Overhead and Underground Transmission andDistribution, Dallas, Texas, September 22–27, 1991.

[B10] Uhlmann, E. Power Transmission by Direct Current. New York: Springer-Verlag, 1975.




Annex A Alternative Methods for Establishing Loss Evaluation Factors

(Informative)

(These appendixes are not a part of IEEE Std 1158-1991, IEEE Recommended Practice for Determination of Power Losses in High-Voltage Direct-Current (HVDC) Converter Stations, but are included for information only.)

A.1 Introduction

When evaluating the economic costs of losses in a conventional ac substation, a distinction is made between no-loadand load losses. This approach is due to the fact that transformer losses are the dominating factor in ac substations andthat transformer losses can be broken down into two portions. One part, which is caused by magnetization, is presentwhenever the transformer is energized and is practically constant over the entire load range. It is called the no-loadlosses. The other portion is proportional to the load current squared and is called load losses.

In an HVDC converter station, there is no single piece of equipment producing dominating losses. From the literature([B3], [B5]), a typical breakdown of HVDC converter station losses is as follows:

Table A.1—Typical Breakdown of HVDC Converter Station Losses

The issue is further complicated because for thyristor valves in the energized but blocked state, different lossmechanisms apply than in normal operation (see Appendix B). In addition, valve losses are not proportional to the loadcurrent squared. Also, various equipment may be connected depending on load level (e.g., filters, cooling towers). Asa result, total HVDC converter station losses normally are higher at part load then they would generally be for acequipment.

In normal utility practice, the cost of losses is based on the cost of generating capacity required to supply the peaklosses in the system and the cost of energy to supply the energy losses. In most cases, load duration curve, load factor,and loss factor are taken into account. Details of calculation procedures vary between utilities, for good reasons. Anyprocedure used for evaluating energy losses of HVDC converter stations should recognize that the loss factor issubstantially greater than for ac substations at the same load factor. Hence, the relationship between load factor andloss factor generally used for ac equipment may not be appropriate.

Since the loss characteristic of an HVDC converter station differs from that of a normal ac substation, variousalternative methods have been used or suggested for determining loss evaluation factors for HVDC converters. It isbeyond the scope of this document to recommend any particular method. However, some examples are given insubsequent sections for illustration.

Item Total Losses (%)

Converter Transformers No-Load “fixed” losses Load “variable” losses

12–1427–39

Thyristor valves 32–35

DC smoothing reactors 4–6

AC filters 7–11

Other losses 4–9




A.2 Method A

In this method, the values used in evaluation of an HVDC project differed from the normal system values because theproject justification was based principally on sale of surplus energy to a neighboring utility. It was assumed that theresources at the sending end would operate the same regardless of the losses. Therefore, the impact of losses would beon the receiving end rather than on the sending end. This differed from normal utility practice, where load is fixed andresources are added to supply power for losses. Based on the HVDC system loading curve and experience with actualusage, an annual load factor of 39% and a loss factor of 45% were determined. The factor for constant losses, such ascore losses, was 100%.

The value of displaced power in the receiving system was approximated at 55 mills/kWh and the discount rate at 9.3%.This was a composite value based on differing rates used by various participating utilities. Since resources would notbe added for the purpose of energy exchange, there was no peaking component in the value of losses. The compositeservice life of the project was determined to be 28 years.

The resulting loss evaluation factors were as follows:

Factor fc = (1.0)(0.055)(8760)(9.861) = $4,700/kWFactor fv = (0.45)(0.055)(8760)(9.861) = $2,100/kW

where

9.861 = PW at 9.3% for 28 yearsfc = The evaluation factor for losses arising from having the complete terminal energized at nominal voltage

and ready for immediate operation. This loss includes converter transformers, valves (blocked), ac and dcfilters, and first-stage cooling equipment, as applicable.

fv = The evaluation factor for losses arising from operating the HVDC converter station as a rectifier at ratedpower, including converter transformers (including additional cooling and pumps), valves (includingcooling and all associated equipment), smoothing reactor, station service, etc.

Loss evaluation was performed for standby and rated load.

A.3 Method B

This method uses mainly conventional practices for determining the cost of losses; however, an empirical relationshipbetween variable loss and load is assumed. This empirical relationship takes into account that losses of an HVDCstation, at part load, are greater than the value obtained if a load squared relationship were used.

The cost of losses is divided into:

1) A demand portion, which is the cost in dollars per kilowatt of installing system capacity, and2) An energy portion, which is the present worth of the energy that will be used by each kilowatt of loss during

the book life of the converter station, converted to dollars per kilowatt.

The values are leveled, that is converted to yearly values, and then the sum is divided by the fixed-charge rate for theconverter station and any other appropriate factors; e.g., transmission efficiency, tax rate, transformer loading factor,etc.

For load losses, the following formula is suggested in Method B for the Load Loss Cost Rate (LLCR):

( A-1)LLCR

LIC( ) DRF( ) LECL( ) LF( )+ET( ) FCRC( ) IF( )

-----------------------------------------------------------------------=




where

LIC = The leveled total system investment cost, in dollars per kilowatt-year, of the additional generation andtransmission system capacity needed to supply the power used by the losses, including the cost of financingthat investment

DRF = The demand responsibility factor. It represents the portion of the system’s capability allocated to meet thelosses of the converter station.

LECL = The leveled energy and operating costLF = The loss factor which is empirical for the converter as a whole. It can be described as:

( A-2)where

k1 = Between 0.8 and 0.9k2 = Between 0.1 and 0.2k1 + k2 =1.0 and the values for k1 and k2 have to be determined for each converter station, based on loss

calculations at two or three load values.SLF = The converter station loading factor

ET = The efficiency of transmissionFCRC = The fixed charge rate for the converter stationIF = The increase factor

For no-load losses, the following formula is suggested in Method B for the No-Load Loss Cost Rate (NLLCR):

( A-3)

where

LECN = The leveled energy and operating cost (no-load)FCRT = The fixed charge rate for the ac transmission system

Note that the above factors are partly derived from other system economic data, discussion of which would be beyondthe scope of this document.

The use of LLCR and NLLCR as discussed above would be the same as with other methods: they are multiplied withthe respective converter station losses to obtain the evaluated cost of losses.

A.4 Method C

This method suggests that separate evaluation factors be used for the different types of equipment in an HVDCconverter station. The evaluation factor consists of capacity charge plus energy charge and is calculated according tothe formula:

( A-4)

( A-5)

Fuel charge represents the system average fuel costs in dollars/kilowatt hour leveled over the book life of the HVDCfacility or other selected evaluation period. The fixed charge rate along with the capacity charge and the fuel charge

LF2 k1 SLF( )2 k2 SLF( )+=

NLLCRLIC LECN+

ET( ) FCRT( ) IF( )--------------------------------------------=

Evaluation Factor $ kW⁄( ) Capacity Ch e $ kW⁄( )arg Energy Ch e $ kW⁄( )arg+=

Energy Ch e $ kW⁄( )argFuel Ch e $ kW⁄( )arg Equivalent Hours×

Fixed Ch e arg Rate------------------------------------------------------------------------------------------------------------=




will depend on the internal conditions of the utility and requirements and assumptions made, and are found to varysubstantially between utilities.

Equivalent hours in a year are calculated based on an annual load duration curve of the HVDC facility, operatingconditions coincident with this curve, and knowledge of how the latter affect losses in the equipment being evaluated.

As compared to general practice, where evaluation factors for load losses and no-load losses for the total converterstation are established, in Method C, separate evaluation factors are derived for each major piece of equipment, basedon the above equations. For this purpose, separate loss duration curves are developed, based on the converter stationload duration curve, and assuming a loss/load relationship (e.g., losses proportional to load squared for a transformerwith fundamental losses constant at nominal system voltage for ac filters).

This procedure will result in different equivalent hours in the above equation, all other factors remaining unchanged,and thus different evaluation factors for each category of equipment will be obtained. These factors are then applied tothe losses of the respective equipment and the resultant cost of equipment losses are added up to obtain the evaluatedcost of the total converter station losses.

A.5 Method D

This method is based on utilizing values for no-load losses and load losses (in dollars per kilowatt), which are the sameas those used for ac systems where the loss factor is based on losses at part load being proportional to the load squared.In this method, the magnitude of the load losses are “adjusted” to reflect the higher load losses at part load for anHVDC station. The adjustment is made by calculating an equivalent load loss at rated load that, if the loss did followa squared relationship, would result in the same energy losses over the load duration curve as the actual energy losses.This method is described in [B3].




Annex B Losses in HVDC Thyristor Valves

(Informative)

B.1 Introduction

Valve losses are a complex and controversial issue. This is because the magnitude and distribution of the individualloss components are not easy to predict and are difficult to verify by measurement.

Manufacturers need to know in detail how and where losses are generated, since this affects component and equipmentratings. Each manufacturer has established his or her own techniques for quantifying the diverse loss producingmechanisms, which may involve the use of advanced computer modeling techniques that are proprietary in nature.

Purchasers are interested in a verifiable loss figure, which allows equitable bid comparison and which can form thebasis of a performance guarantee. At present, there is no internationally agreed basis by which different bids should becompared or by which the design of a successful bidder should be assessed.

This appendix discusses the principal source of valve losses and comments on the recommended procedures set out in5.1 of this recommended practice for arriving at a total loss figure for valves.

B.2 Generation of Losses

Valve losses are generated whenever the integral over a full cycle of the V × I product, measured at the valve terminals,is not equal to zero. In practical valves, losses are generated during four distinct periods in each power frequency cycle:at turn-on, during conduction of load current, at turn-off, and during the blocking interval between conduction periods.

B.2.1 Turn-On

At turn-on, the thyristors are triggered into conduction by the application of a gate pulse. Initially exhibiting highimpedance, each thyristor switches to a low but finite impedance state in a time-scale characteristic of the thyristorused. Saturating inductance is usually incorporated into the valve to limit the rate of discharge of circuit straycapacitance, which is initially charged to high voltage. The stored energy in the stray capacitance is dissipated aslosses. The energy stored in the valve-damping capacitors, which are also initially precharged, is dissipated bydischarging through the associated damping resistors of the capacitors. At turn-on, energy is absorbed from the circuitand stored in the magnetic fields associated with the valve reactors. Some energy is dissipated in the reactor coresduring this process.

B.2.2 Conduction

During the conduction phase, the valve is characteristically of low (but not zero) impedance; and it is during this timethat load current flows, leading to conduction losses in the valve.

B.2.3 Turn-Off

Firing of the next valve in sequence causes the valve load current to commutate to zero and the thyristors (and theirassociated grading networks) to experience negative recovery. Negative recovery excites an oscillation between theinductances and capacitances of the circuit, which is damped by the valve-damping resistors, giving a characteristicloss. Practical thyristors do not cease conduction at current zero but a short time after. The current flow that isestablished during this brief period of reverse conduction flows through the source electromotive force (EMF),resulting in additional energy being fed into the circuit. This energy is dissipated as extra turn-off losses in both thethyristors and the damping resistors (see B.3.1.4 and B.3.2.3). Energy, previously stored in the magnetic fields of thereactors, is released back into the circuit. As is the case during turn-on, some energy will be dissipated in the reactorcores during this process.




B.2.4 Blocking

During the nonconducting interval, the valve is characteristically of high (but not infinite) impedance. The valveexperiences a complex voltage waveshape (Fig B.1) containing portions of power frequency sine waves, highfrequency “jump” voltages, and a dc off-set. Application of this voltage to the impedance of the nonconducting valveleads to a characteristic power loss in the resistive component of that impedance.

Figure B.1 —Typical Valve Voltage and Current Waveforms in a Three-Phase Bridge Converter

B.3 Sources of Losses

A valve is made from many components; however, most full-load losses (typically 85–95%) are generated in just twocomponents: the thyristors and the damping resistors (see Fig B.2). The source of the losses in these components isdescribed below and, for completeness, mention is made of the other small but not negligible sources of losses.




Figure B.2 —Simplified Equivalent Circuit of a Typical Thyristor Valve

B.3.1 Thyristors

Five principal loss mechanisms are present.

B.3.1.1 Thyristor Turn-On Switching Loss

When gated, the thyristor impedance falls rapidly, but not instantaneously, to a low value. As the current builds, theinstantaneous V × I product is very large, sometimes exceeding 100 kW in each thyristor. Fortunately, this period lastsonly a few microseconds in each cycle; hence, the average turn-on switching loss is very much lower than the peak.

B.3.1.2 Spreading Loss

Within each thyristor, conduction is initially established only in the immediate vicinity of the gate electrode. Theconducting area then expands to occupy the whole of the silicon slice. For large area devices of the type used for




HVDC, this process may take several milliseconds. During this time, the forward volt drop is higher than the “fullyspread” value normally quoted in thyristor data sheets.

B.3.1.3 ON-State Loss

This is the classical loss component derived from the multiplication of load current by the fully spread ON-statevoltage of the thyristors. In a real circuit, valve current is not an ideal rectangular pulse of 120° electrical conductionlength, and since thyristor forward volt drop is a nonlinear function of current, dc current ripple and commutationoverlap both affect the actual losses.

B.3.1.4 Turn-Off Switching Loss

As mentioned in B.2.3, turn-off does not occur at current zero, but a short time later. As a result, reverse current isflowing in the thyristor when it begins to develop reverse blocking voltage. As the thyristor impedance increases, thereverse current decays to a low value. During this process, energy is dissipated at the thyristor junction that is equal tothe integral of the V × I product with respect to time over the period of interest.

B.3.1.5 OFF-State and Reverse Leakage Loss

When subjected to the blocking voltage between conduction intervals, the thyristors conduct a finite leakage currentthat, although small, is generally conducted at high voltage. The resulting loss forms part of the blocking loss of thevalve.

B.3.2 Damping Resistor Loss

Four principal components of loss are present.

B.3.2.1 Damping-Resistor-Dependent Term

During the blocking interval, a current with complex waveshape flows in the damping circuit. One component of thiscurrent produces a loss that is dependent on the value of the damping resistance. Other components, as describedbelow, produce components of loss that are independent of the value of resistance.

B.3.2.2 Change of Capacitor Energy Term

During firing and recovery of the valves in a bridge, the voltage across nonconducting valves is subjected to fastvoltage jumps. Under these conditions,the damping capacitors exhibit low impedance, and the resistors control thecurrent flow. Classically, the loss due to these voltage jumps (Vjump) is taken as

( B-1)

where

C = The effective valve terminal-to-terminal capacitance (including strays), in microfaradsf = The system frequency, in hertz

This calculation technique is imprecise, since the real jumps are not step changes; hence, the damping resistor loss isoverestimated. Additionally, all other components in the circuit are assumed to be without losses; as a result, allcapacitor energy is predicted as being dissipated in the damping resistors. In practice, the thyristor junctions, the valvereactor cores, the valve surge arresters, other valve grading networks, and the converter transformers all absorb afraction of this energy, thereby further reducing actual damping resistor loss.

12---C f Σ ∆Vjump( )2⋅ ⋅




B.3.2.3 Circuit Inductance Stored Energy Term

As discussed in B.2.3 and B.3.1.4, reverse current is established in the valve at turn-off. This current flows in thecircuit inductances and hence represents a source of magnetic energy equal to 1/2 LIo

2, where Io is the peak reversecurrent at turn-off. This magnetic energy, which is predominantly dissipated in the damping resistors, is not accountedfor by the classical equations, since these assume ideal extinction at current zero.

B.3.2.4 Damping Resistor Loss (Dispersion Effect)

At turn-off, not all tbyristors in the valve turn off simultaneously. Those that recover first do so at a higher voltage thanthe per-level average, while those that recover last recover at a lower voltage than the per-level average. Since dampingresistor losses due to change in capacitor energy are proportional to the jump voltage squared, the dispersion in turn-off instants leads not only to nonuniform distribution of damping loss within a valve but also to an increase in the totalloss. The magnitude of this effect is directly related to the spread of thyristor turn-off characteristics.

It should be noted that a similar dispersion in thyristor turn-on times, while also leading to nonuniform distribution ofturn-on loss, does not normally lead to any significant increase in total loss.

B.3.3 Other Losses

These may conveniently be divided into current-related losses and voltage-related losses.

B.3.3.1 Current-Related Losses

B.3.3.1.1 Conductor Losses

During the ON state, I2R losses are generated in the resistances of the reactor windings, busbars, connections, heatsinks, pressure interfaces, etc. In some designs, high-frequency effects may be important.

B.3.3.1.2 Reactor Core Losses

Two types are present: hysteresis loss and eddy-current loss. These are a function of the magnetic material used and thegeometry and make-up of the core. Hysteresis loss is related to the frequency and the amplitude of flux swing in thecore, while the eddy-current loss is related to the frequency and the rate of change of flux squared in the core. In somereactor designs, the apparent eddy-current loss is increased by inclusion of a parallel-connected reactor dampingresistor. Valve firing is the dominant event that produces reactor core eddy-current loss.

B.3.3.1.3 Miscellaneous Losses

Extremely small but nonzero losses are also produced as a result of electromechanical displacement and strayelectromagnetic induction effects.

B.3.3.2 Voltage-Related Losses

B.3.3.2.1 DC Grading Circuits

High ohmic value resistors are connected across each thyristor to ensure uniform distribution of direct voltage. Theseincur a loss during the blocking period of the valve.

B.3.3.2.2 Water Circuit Losses

Water-cooled valves employ high-purity water that nevertheless exhibits finite conductivity. The water columnscontained in the cooling pipes, therefore, conduct electricity that in turn leads to power loss.




B.3.3.2.3 Electronics Losses

Most valves employ electrically triggered thyristors. The associated electronics usually extract energy from a localgrading network. The energy consumed by the electronics and a proportion of the losses generated in that part of thepower supply that regulates the energy in-feed contribute small additional losses.

This component is highly design-dependent and can only be considered on a case-by-case basis. The actual energyrequired by the electronics is likely to be small (1–2 W per thyristor level). The circuit used to extract this energy fromthe valve circuit is likely to be the biggest source of losses, the magnitude of which will depend largely on what is donewith energy that is surplus to the requirements when the converter is operating at conditions other than the minimumcase for which the electronics are required to be fully active. If the valve damping network is the source of energy forthe electronics (the normal arrangement), then only that component of the damping network displacement current thatflows through the power supply, which produces losses dependent on the value of series resistance, will genuinely addto valve losses. The component of displacement current arising from the change in capacitor-stored energy, whilepossibly leading to large actual dissipation in the electronics, will be off-set by reduced losses elsewhere.

B.4 Losses During Standby

Standby losses are usually defined with the valves energized but blocked. Under these conditions, current-related andswitching-related losses can be ignored. As a result, only B.3.1.5, B.3.2.1, B.3.3.2.1, B.3.3.2.2, and B.3.3.2.3contribute to the standby losses.

B.5 Discussion of Procedures for Loss Determination

The recommended procedures set out in 4.1 of this recommended practice will provide a total loss figure for valvesbased on the summation of individual loss components. The individual components of loss are determined by applyingstandardized calculation methods using data obtained, where possible, from verifiable factory measurements.

The answers will not be precise, but it is believed that all major loss mechanisms are covered and that the result willbe more accurate than could be achieved by direct measurement at site.

Three broad categories of loss are considered: current-dependent losses, voltage-dependent losses, and switchinglosses. Fig B.1 shows the classical valve voltage and current waveshapes for a three-phase bridge. In the treatment ofvoltage-dependent loss, the voltage waveshape appearing during the nonconducting interval is idealized, in thatcommutation jumps are step changes without overshoot. The formulas presented for determining the voltage-dependent losses are a generalized form of the classical equations that allow for interaction effects between the twobridges of a 12-pulse group when there is some common impedance in the commutation path. When commonimpedance is present, commutation jumps, additional to those shown in Fig B.1, are present.

Depending on the application and the valve technology used, different loss components will assume differentsignificances. Therefore, the order in which the individual loss mechanisms are listed in 4.1 is not intended to reflectany particular ranking.

The components of loss, W1 to W8, are believed adequate to cover most valve designs. Designs employing novelcomponents or circuit configurations, or equipped with unusual auxiliary circuits that could materially affect valvelosses, should be assessed on their own merits.

All losses are determined on a per-valve basis, in which a “valve” is taken to be one arm of a three-phase line-commutated converter bridge.

B.5.1 Thyristor Conduction Loss per Valve: W1 (Eq 1)

This is the principal loss arising from the passage of load current through the thyristors of the valve.




The calculation method proposed is idealized in that dc side harmonic currents are ignored and the commutation ofcurrent during the periods of overlap approximated to a straight line. The former is only justified provided the dc sideharmonic current (root sum of squares) remains well below 10% of the dc component. If this is not the case, then Eq 1should be modified to be:

( B-2)

where

In2 = The sum of the squares of the rms values of the individual dc-side harmonic currents

B.5.2 Thyristor Spreading Loss per Valve: W2 (Eq 2)

The thyristor spreading loss is thyristor-type and application dependent and can only be quantified by directmeasurement. Illustrative wave-shapes are shown in Fig B.3.

Figure B.3 —Typical Thyristor Current and ON-State Voltage Waveshapes Illustrating the Effect of Spreading

W1n VoI⋅

3----------------

n Ro⋅3

------------- I2 In2+( ) 2π u–

2π---------------

+=




B.5.3 Other Valve Conduction Losses per Valve: W3 (Eq 3)

Comments as in B.5.1 apply. Room temperature dc resistance measurement/calculation is proposed that willunderestimate actual losses. If calculated conduction losses exceed 1% of the total valve losses, then correction fortemperature and/or frequency effects should be applied.

B.5.4 “DC” Voltage-Dependent Loss per Valve: W4 (Eq 4)

This is calculated as V2/R, the formula shown being arrived at by integrating the valve-voltage-squared waveform overthe full cycle [B9]. As stated in the introduction to Section B5, the commutation jumps are taken as step changeswithout overshoot. This results in negligible error in the prediction of dc voltage-dependent loss, whatever the value ofα or u (up to the maximum of u = 30°). If u exceeds 30°, the converter is commutating in double (or triple) overlapmode, and different formulas will apply. If this situation occurs, an expert should be consulted.

B.5.5 Damping Loss per Valve: W5 (Eq 6) and W6 (Eq 9)

This is calculated as I2R for those grading circuits in the valve containing series-connected resistance and capacitance[B9]. The formulas are arrived at by summing the integrals of the displacement current squared over each segment ofthe valve voltage waveform. The resultant form of the equation has one term (Eq 6) that produces a loss componentdependent on the value of R and a second term (Eq 9) that produces a loss component independent of the value of R.The formulas, which are a derivative of that published by Uhlmann [B10], are valid for u < 30°, where u is the overlapangle. As for the dc voltage-dependent loss (B.5.4), if u exceeds 30°, then different formulas apply, and an expertshould be consulted.

An underlying assumption in the derivation of the formulas is that step changes in the valve voltage waveform areseparated by sufficient time for the voltage on the capacitors to settle to the sine wave rate. Certain notches in the valvevoltage waveform have a width equal, to u, so that for small angles of u (u < 3 Rac · Cac), this may not be the case, anderrors will increase. The direction of error will, however, always be towards overestimation.

As stated in B.3.2.2, the second term (Eq 9) gives a loss component corresponding to the change in stored energy in thecapacitances of the system. While, for convenience, this loss is assigned to the resistors of the valve-dampingnetworks, the actual distribution of these losses within the valve is more complex.

It should be noted that Eq 9 does not allow for the turn-off dispersion effect referred to in B.3.2.4. Modern techniquesnormally permit such close matching of thyristor characteristics that the resulting influence on total valve losses issmall and may be neglected.

B.5.6 Turn-Off Losses per Valve: W7 (Eq 10)

As mentioned in B.3.1.4 and B.3.2.3, additional losses are generated in the thyristors and damping resistors due toreverse current flow in the thyristors at turn-off. The reverse current arises from the stored charge property of thethyristors. The real processes are very complex. However, since the reverse current flows through the source EMF, theadditional energy fed into the circuit, which is dissipated as losses, is given by

( B-3)

where

VL, α, u, and f are as defined in 4.1irr(t) = The instantaneous reverse current of the average thyristor at turn-off, in amperest = The time, in secondst1 = A time, sufficiently after current reversal, for irr(t) to have decayed to a value comparable with the reverse

leakage current level, in radians

W7 2f irr t( ) VL ωtsin td⋅ω t α u+=

ω t α u t1+ +=∫=




Since the integral of the reverse current is the reverse recovered charge, and since the average line-to-line voltage overthe period of interest is approximately equal to the instantaneous sine wave voltage, at a time after current zero whenhalf the stored charge has been extracted, Eq B-3 can be simplified to that shown in Eq 10.

B.5.7 Valve Turn-On Losses

As mentioned in B.3.1.1, switching losses are generated at the thyristor junctions at turn-on. In addition, the valvesaturating reactors experience eddy-current core loss and/or reactor-damping resistor loss if fitted. Some high-frequency busbar losses may be generated. All these components of loss are real; however, unlike turn-off losses,which arise from current flow through the source EMF, these losses represent part of the energy stored in thecapacitances of the circuit immediately prior to turn-on, whose discharge currents do not flow through the source EMF.Eq 9 accounts fully for these losses, although its formulation ascribes all the losses to the damping resistors. Thereality is that actual damping resistor dissipation at turn-on is less than predicted by the classical formula, thedifference being attributable to dissipation elsewhere in the circuit. Thus, turn-on losses have already been accountedfor, and no further loss component need be added.

B.5.8 Reactor Losses per Valve: W8 (Eq 11)

This equation allows for the component of reactor loss attributable to hysteresis in the reactor core. Winding loss isincluded in Eq 3, and the overwhelming component of eddy-current core loss in Eq 10. As stated in B.3.3.1.2, eddy-current core loss is dependent on the rate of change of flux squared. High rates of change of flux are associated withhigh di/dt in the reactor, coincident with the reactor cores being unsaturated. This combination normally only occursat turn-on during the discharge of circuit capacitances (see B.5.7). Note 2 of 4.1.8 recognizes that in somecircumstances, additional eddy-current losses may be present. Whether these are important or not will depend on thereactor design and the specific application. If eddy-current losses outside the valve firing interval (defined as beingfrom thyristor triggering to collapse of valve anode voltage to near zero) exceed 0.5% of total valve losses, then thisadditional loss should be declared and a suitable type test performed.




Annex C Losses in HVDC Converter Transformers

(Informative)

C.1 Introduction

The load currents in HVDC converter transformers differ from those in ac power transformers in that the currentsflowing in the windings of a converter transformer contain significant amounts of harmonics. The load losses in theconverter transformer that occur due to those harmonic currents are greater than those arising from a fundamentalfrequency current of the same rms magnitude.

No agreement has been reached in the industry on how load losses of an HVDC converter transformer should bedetermined with adequate accuracy for loss evaluation purposes. The major issue is how to account for the load lossescaused by the harmonic currents, which vary with load, transformer leakage reactance, and converter firing angle.Various methods have been proposed in the past, some of which are summarized in subsequent subsections.3

With respect to no-load (excitation or core) losses and auxiliary power losses, there is no difference in principlebetween conventional power transformers and HVDC converter transformers.

During standby operation (transformers energized with the valves blocked), the applied voltage is the normal acsystem voltage, which usually contains no appreciable harmonics. Hence, the core losses (no-load losses) may bedetermined in the same manner as for ac power transformers.

During operation with the valves unblocked, harmonic voltages appear on the valve windings due to the flow ofharmonic currents through the transformer. The magnitudes of the harmonic voltages vary with the operatingparameters of the converter station.

The harmonic voltages on the line windings are much lower in magnitude than those on the valve windings due to thelow harmonic impedance of the ac filters. The magnitudes also vary with converter operation.

In converter transformers of typical design, where the line winding is located adjacent to the core, the effect of theharmonic voltages on the exciting current of the transformer, relative to the effect of the fundamental frequencycomponent of voltage, is negligible. The core losses of the transformer, therefore, are not significantly affected byconverter operation. Hence, the core losses during load conditions may be considered to be equal to those duringstandby conditions at the same rms ac voltage.

C.2 Components of Load Loss

The converter transformer load losses consist of three main components:

1) Winding dc resistance loss (I2R loss)2) Winding stray loss3) Stray loss in the structural and external parts, such as the core, clamps, and tank

The I2R loss can be determined from the rms magnitude of the actual current and the de resistance of the winding. Thewinding stray loss varies with harmonic content of the current waveform. The loss can be considerably reduced inmagnitude by using transposed cable with individual strands of small dimensions. The other stray losses also vary withharmonic content of the current. The magnitude of the other stray losses depends on the amount of stray flux in thecore, clamps, and tank; the positioning of magnetic shielding; and other design factors.

3A pertinent equipment standard is under consideration by the Transformers Committee of the IEEE.




C.3 Method A

Method A is given in IEC Pub 146 (1991) [2] and was developed primarily for industrial applications in which thetransformer ratings are generally smaller and the per-unit losses greater than those for HVDC converter transformers.

In this method, the load losses of the converter transformer are measured in the factory in a short-circuit test carried outin the same manner as for ac power transformers, except that the short-circuit current is the IEC rated current, whichis somewhat greater than the rms value of the actual current waveform.

The IEC rated current is a sinusoidal fundamental frequency current with an rms magnitude equal to the rms value ofthe (theoretical) rectangular current waveform, assuming zero commutating reactance (zero transformer reactance andno overlap angle during commutation). The magnitude of the IEC rated current is typically about 3% larger than therms value of the actual current, as indicated in Fig C.1.

Figure C.1—IEC Rated Current Compared to Actual Current

The higher value of the loss so obtained, due to the larger-than-actual rms current, is intended to approximate theactual loss that would occur with the actual current that contains the harmonies. This method, when applied toconverter transformers for large HVDC systems, gives losses that are lower than the actual losses. In addition, it doesnot take into account the effect of increased harmonic current content during operation at high converter firing angle.




C.4 Method B

This method for estimating load loss in converter transformers was presented by Sacharoff to CIGRE Working Group12-02 in 1973 [B8]. Two mathematical expressions were presented:

(1) If x is less than or equal to 40%, then

( C-1)

(2) If x is greater than 40%, then

( C-2)

where

x = The total fundamental frequency stray loss expressed as a percentage of the I2R lossI = The IEC rated current, in amperesR = The dc resistance, in ohms

The second and third terms in the square brackets represent the winding stray loss and the other stray losses,respectively. This method does not explicitly take into account the actual operating parameters, such as the firing andoverlap angles, which have an impact on the magnitude of the harmonic currents.

C.5 Method C

Method C was presented in a paper [B1] to CIGRE Study Committee 14 in 1983. It is based on short-circuit testscarried out on the converter transformers for the Duernrohr back-to-back HVDC station at several frequencies. Theload loss calculated from the measurements was found to be about 26% higher than those measured by Method A.Based on these measurements, the following empirical expression was developed for the load loss:

( C-3)

where

P The load loss per Method A, in wattsI The IEC rated current, in amperesR The dc resistance, in ohms

The factor 0.75 is applied to rated load conditions only. Measurements were made on three-phase two-windingtransformers with a reactance of about 20%, and a firing angle delay of about 18° was considered.

Although the expression may be considered preferable to Method A, it may not be applicable to transformers ofdifferent design, particularly if the relative magnitudes of the winding stray loss and the other stray losses are differentfrom those in the transformer tested. In addition, the method does not reflect the effect of changes in transformerreactance and converter firing angle, and thus in magnitude of harmonic currents.

Load loss 100 6( ) 2.5( ) x 10–( ) 1.2( )+ +[ ]I2R 100⁄=

Load loss 100 x 34–( ) 2.5( ) 30( ) 1.2( )+ +[ ]I2R 100⁄=

Load loss P 0.75 P I2R–( )+=




C.6 Method D

Method D is based on measurements of the effective resistance of the converter transformer at harmonic frequencies,and on calculation of losses from the effective resistance and calculated magnitudes of harmonic currents ([B2], [B6],[B7]). It can be summarized as follows:

(1) The effective resistance of the transformer is measured at various harmonic frequencies in a manner similar to thatused for measuring the load losses of a transformer at fundamental frequency; i.e., by applying a source voltage of theappropriate frequency to the low-voltage windings of the transformer under test with the high-voltage winding short-circuited. Measurements at about 10 frequencies between the fundamental and the 50th harmonic are consideredsufficient, preferably at the 1st, 2nd, 3rd, 5th, 7th, 11th, 13th, 20th, 30th, and 50th harmonics. The measured resistancecan be conveniently expressed in per-unit of the resistance at fundamental frequency. A best-fit smooth curve showingthe variation of effective resistance with frequency can then be developed.

The measurements on three-phase transformers should be made with a three-phase voltage source. For three-windingtransformers, the measurements should be made on each valve-side winding with the other valve-side winding open-circuited.

(2) The magnitude of the six-pulse characteristic harmonic currents in each winding of the converter transformer iscalculated. Three winding transformers used for 12-pulse operation often have two primary winding halves connectedin parallel to achieve equal stray impedances for the delta and wye paths. Hence, the primary windings, as well as thevalve windings, carry six-pulse currents.

The characteristic harmonic currents are calculated from the following:

( C-4)

where

In = The current, in amperes, at harmonic nn = The harmonic numberXt = The converter transformer reactance at fundamental frequency, in ohmsF = [k1

2 + k22 − 2k1 k2 cos (2α + u)]1/2

En = The equivalent line-to-neutral voltage on the valve side of the converter transformercalculated bymultiplying the line-to-neutral voltage on the line side by theturns ratio (including the effect of tapposition), in volts rms

α = The firing angle, in degreesu = The overlap angle, in degrees

(3) The total load loss is then calculated by summing the losses due to each of the characteristic harmonic currents,including the fundamental frequency current, using Eq C-5, which gives the load loss on a per-phase basis.

In

3 F En⋅ ⋅π n Xt⋅ ⋅---------------------=

k1

n 1–( )u2---sin

n 1–---------------------------------=

k2

n 1+( )u2---sin

n 1+----------------------------------=




( C-5)

where

Rn = The effective resistance, in ohms, at harmonic n Other terms are as aboveOther terms are as above

The magnitudes of the harmonic currents become quite small at the high harmonic numbers, and the losses due tothose currents are also small. It is considered that losses beyond the 49th harmonic can be neglected.

C.7 Method E

Measurements made in accordance with Section C6 on several converter transformers supplied on recent HVDCprojects by two manufacturers indicate that the effective resistances at various frequencies, when expressed in per-unitof the resistance at fundamental frequency, are similar for all transformers. The differences in effective resistance fromthe average value, for the same values of firing angle and overlap angle, resulted in differences in total load losses ofless than ± 2%. Hence, it is considered that use of the “typical” relationship between effective resistances (in per-unit)and harmonic number will provide a reasonable estimate of load losses while avoiding the need for measurements atharmonic frequencies.

The “typical” relationship between effective resistance (in per-unit) and harmonic number is shown in Table 1. The“typical” relationship may be modified periodically as additional data becomes available from measurements on othertransformers.

The procedure for determining load losses is similar to that in Section C6, except that the effective resistance of thetransformer at harmonic frequencies is determined from the measured effective resistance at fundamental frequencyand the typical relationship with frequency in Table 1.

It should be recognized that the load losses of converter transformers whose design departs significantly from those onwhich measurements have been made, particularly in terms of other stray losses, may differ from the losses calculatedby this method.

Load loss In2Rn

n 1=

n 49=

∑=




Annex D Auxiliaries and Station Service Energy Consumption

(Informative)

D.1 General Considerations

The auxiliary power system interfaces with the following systems:

1) Valve cooling system2) Transformer cooling system3) Switchyard equipment cooling systems4) HVAC system5) Converter control and protection system6) Switchyard equipment control and protection system7) Fire protection and communication system8) Building auxiliaries9) Water supply system10) Uninterruptible power supply system

The equipment and devices that are part of these systems are grouped according to their critical nature. The followingare sample loads typical of a 1200 MW, 500 kV ac, ± 500 kV dc HVDC converter station, where the total stationauxiliary peak losses would be approximately 0.09% of the converter station rating.

D.2 Critical Loads

The critical loads are usually supplied from a 24 V dc distribution system, and the estimated loads given in Table D.1are provided on a bipole basis.

Table D.1 —Critical LoadsDevice Approximate Load (watts)

Mimic control 300

Sequence of events recorder 120

Thyristor monitor panel 120

Valve base electronics 300

Converter control and protection 500

Cooling system control 150

Bipole controller 500

Annunciator controller 120

Miscellaneous station control panels 250

Miscellaneous station protection panels 300

Fire protection systems 150

Fault location cabinet 250

HVDC metering panels 500




D.3 Essential Loads

The essential loads could be ac and dc loads, usually supplied from 480 V ac motor control centers and 125 V dcdistribution systems, and the estimation given in Table D.2 is per converter:

Table D.2 —Essential Loads

Equipment Approximate Load

Converter transformers (three-, one-phase units), pumps, fans, heaters

300 kVA

Smoothing reactors (two units), pumps, fans, heaters 150 kVA

24 V dc battery chargers 25 kVA

125 V dc battery chargers 60 kVA

Wet surface air coolers 85 kW

Valve cooling-water pump 95 kW

Air handling unit 75 kVA

Condensing units (two per converter) 150 kVA

Recirculating fan 20 kW

Return fan 12 kW

Water treatment plant 150 kVA

AC switchyard, 480 V, three-phase, equipment loads panel 300 kVA

AC filter yard, 480 V, three-phase, equipment loads panel 150 kVA

DC switchyard, 480 V, three-phase, equipment loads panel 300 kVA

Auxiliary control unit 2 kW

Cooling control unit 2 kW

Valve hall-ground switches 8 kW

Transformer control unit 2 kW

Revenue metering equipment 2 kW

Converter process control unit 2 kW

Main switchgear power circuit breaker (PCB) control 4 kW

Protection systems (transformer, line, busbar, filter, etc.) 15 kW

AC switchyard panel equipment control 120 kW

AC filter yard panel equipment control 120 kW

DC switchyard panel equipment control 120 kW

Balance of plant, emergency lights (miscellaneous panels) 100 kW




D.4 Nonessential Loads

An estimate of the nonessential loads is given in Table D.3.

Table D.3 —Nonessential Loads

Equipment Approximate Load

Lifting equipment (hoists, cranes, roll-up doors) 75 kVA

Lighting system 20 kW

Secondary heating and cooling systems 50 kW

Spare converter transformer (pumps, fans, heaters) 75 kVA

Spare smoothing reactor 50 kVA

Yard and building receptacles 15 kVA

Secondary distribution panels 150 kVA



IEEE Std 1158-1991

Figure D.1—Typical Arrangement of Station Auxiliaries


OLDUKÇA FAYDALI!!! - Determination of Power Losses in HVDC Converter Stations

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