ABB_Reactor Protection Explanation

7/22/2019 ABB_Reactor Protection Explanation

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HV SHUNT REACTOR SECRETSFOR PROTECTION ENGINEERS

By

Zoran GajiABB Sweden

Vsters, Sweden

Birger Hillstrm

ABB SwedenVsters, Sweden

Fahrudin MekiABB Inc.

Allentown, PA 18106

Presented to:30

thWestern Protective Relaying Conference

Spokane, WashingtonOctober 21-23, 2003


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HV SHUNT REACTOR SECRETSFOR PROTECTION ENGINEERS

Z. Gaji, B. Hillstrm F. Meki

ABB Sweden ABB Inc.Vsters, Sweden Allentown, PA 18106

Abstract: Viewed in the substation yard, an HV, oil immersed, shunt reactor does not differmuch from a power transformer, but in reality it is not that simple. There are distinct differencesbetween construction and operating characteristics of these two devices.

In order to explain the properties of shunt reactors numerous current and voltage waveformseither captured as disturbance recordings in the field or simulated by ATP [7] will be presented.On all these figures the nomenclature for current and voltage signals, as shown in Figure 1below, will always be used.

A

B

C

UA UB UC

IaN IbN IcN

IN

IC

IB

IA

3Io

Figure 1: Shunt Reactor Current & Voltage Signals

All presented current and voltage signals will be expressed in per unit system with shunt reactorrated data as a base.


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I. BASIS ABOUT HV SHUNT REACTORS

1.1 INTRODUCTION

Shunt reactors are designed for connection to the ends of high voltage transmission lines or tohigh-voltage cables for the purpose of controlling the line voltage by absorbing reactive power.Let us look at the equivalent circuit of the transmission line and see shunt reactor effect on theline parameters.

Figure 2:Transmission line equivalent circuit (uncompensated line)

In Figure 2, Vs and Is are the sending-end voltage and current, and Vr and Ir are the receiving-end voltage and current.Writing Kirchoff Voltage Law equation for the circuit in Figure2,

)2

*

(

YVr

IrZVrVs ++= (1)

IrZVrZY

Vs **)2

*1( ++= (2)

Also writing a Kirchoff Current Law equation at the sending end,

2

*

2

* YVsYVrIrIs ++= (3)

IrZY

VrZY

YIs *)2

*1(*)

4

*1( +++= (4)

=

Ir

Vr

CD

AB

Is

Vs (5)

where:

Vr

Z

Y/2 Y/2

Vs

Is Ir


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3

)(2

*1 perunit

ZYDA +== (6)

( )=ZB (7)

))(4

*1( S

ZYYC += (8)

Example 1:

A three-phase line, completely transposed 345kV, 124 miles has the following positive sequenceconstants:

)/(10*76.6

)/(563.00515.06 mileSjy

milejz

=

+=

From equations (6), (7) and (8)

04

0

0

08.9010*277.8

78.8429.70

159.09706.0

=

=

==

C

B

DA

From (5), the no-load receiving-end voltage is

LLrNL kVA

VsV 3.356

9706.0

8.345===

Figure 3 summarizes these results, showing a high receiving-end voltage at no-load and a lowreceiving-end voltage at full load. This voltage regulation problem becomes more severe as theline length increase.


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Figure 3:Voltage profiles of an uncompensated line

Assume that identical shunt reactors are connected from each phase to neutral at both ends of thesame line during light load conditions, providing 75% compensation (the reactors are removedduring heavy load conditions).

In this case the line constants are:

)(9010*1.2)75.01(10*4.8

)(78.8429.70044 SY

lzZ

==

==

From equations (6), (7) and (8)

004.0993.0 ==DA

From (5), the no-load receiving-end voltage is

LLrNL kVA

VsV 2.348993.0

8.345 ===

It may be concluded from the previous example that reactors reduce overvoltages during lightload conditions. However, shunt reactors can reduce line loadability if they are not removedunder full-load conditions.There are two general types of shunt reactors. One is dry-type reactor of an air core or core-lessdesign. These reactors are limited to voltages up to 34.5kV and are often installed on the tertiaryof a transformer.

V(x)

Vs

Receiving endSending end

No-load

Full-load

Short-circuit

RNLV

SIL

SRSIL VV

RFLV

0RSCV


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HV, oil immersed, shunt reactors are the most compact and cost-efficient means to compensatereactive power generation of long-distance, high-voltage power transmission lines, or extendedcable systems during light load conditions.

Two main application of the reactor can be identified, referring to Figure 4.: Shunt reactors that are continuously in service, generally used for EHV and long HV

lines/cables Switched shunt reactors are applied in the underlying system and near load centers

It is common for shunt reactors to be installed at both ends of EHV lines, and sized to prevent theline voltage from exceeding design value when energized from one end. Since there is usuallysome uncertainty as to which end of a line may be energized (or de-energized) first, shuntreactors are usually installed at both ends of line.

Figure 4:One-line diagram of line-connected switched shunt reactors

The shunt capacitance depends on type of transmission line, length of line and line voltage. Along distance 345kV transmission line will have a shunt capacitance around 3.14 F /mile

(1.12Mvar/mile). The corresponding shunt capacitance for the 345kV cable is almost 20 times aslarge as or about 22.4Mvar/mile. The shunt capacitance will be increased by increasing thetransmission voltage (proportional to the square of the transmission voltage).

Equivalent Pi of the

Long Line

G G52 52

52 52

Z

Y/2 Y/2


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1.2 SHUNT REACTOR GENERAL DESIGN CONCEPTS

Two different ways are used in building reactors, commonly referred to as gapped core andcoreless [1] & [2]. The gapped core reactor has a subdivided limb of core steel with air gapsinside the winding and no limb at all for the coreless concept.

It is easy to verify that the gapped core concept becomes more advantageous as the lossevaluation rate increases and particularly at higher system voltages. This is due to the higherenergy density that can be achieved in a gapped core reactor compared to a coreless reactor.

SHUNT REACTOR OPERATING CHARACTERISTICS

Linearity

For normal operating voltages there is a linear relationship between applied voltage and reactorcurrent (i.e. a small increase in voltage will result in a proportional increase in current). Magnetic

fluxes and flux densities are also proportional to the time integral of the applied voltage. With avoltage of sinusoidal shape the fluxes and flux densities are also proportional to the voltage. Thedeviation from a true sinusoidal shape in line voltage is in general negligible for normaloperating voltages.As the magnetic flux to a great extent has its path in magnetic core steel the core steel will getsaturated for flux densities above a certain level, the saturation point. Below and up to thesaturation point only a small current is needed to magnetize the core steel and the extra currentneeded to reach a marginal increase flux density is small. Once above the saturation point theextra current needed to further increase the flux density will be large.

Harmonic content

Steady state harmonics in reactor current arise from partial saturation in the magnetic circuit.These effects are in fact very small, and without practical importance for relaying andcommunication interference. Of all harmonics the third harmonic will be dominant. In the reactorneutral the third harmonics in the three phases add together and act like a zero sequence current.

Asymmetry between phases

The tolerances on asymmetry between phases of a three-phase reactor or between single-phaseunits forming a three-phase bank can be judged by the amount of residual harmonics. The resultis a zero sequence current in the neutral connection. Standards are realistic, but better tolerancesare possible to achieve. A usual figure is 0.5 %.


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II. HV SHUNT REACTOR SWITCHING

2.1 SWITCHING IN OF REACTORS, INRUSH CURRENT

The switching in of a reactor gives rise to inrush current a transient phenomenon related tosaturation in the shunt reactor magnetic circuit. In principle, it is the same story as inrush currentof a transformer, but there are differences. A reactor core keeps no remanence, because of the airgaps, which makes the whole thing easier. However, the damping of the asymmetric condition the dc component is slow, due to the inherent low losses in a shunt reactor. It is thereforenecessary to keep this phenomenon in mind when designing the relay protection system for HVshunt reactors.The instantaneous current values during shunt reactor switching in can be visualized from theFigure 5. Input data for all figures in this chapter are obtained from actual disturbance recordingsin the field. Depending on the switching instant the currents might have a dc component. Theworst condition is when the reactor phase is closed in at zero voltage. The flux will increase withthe voltage-time-area during the first half-cycle to a value twice the maximum flux in normaloperation. The current is proportional to the flux density, until reactor core saturation occurs.Above the point of saturation the current will increase faster than the flux.

17 18 19 20 21 22 23 24 25 26 27 28 29 30

2

0

2

4

IAIBIC

99,2MVA, 440kV, 60Hz Reactor

Cycles

Current[pu]

Figure 5: ShuntReactor Phase Currents during Asynchronized Switching

Without saturation, the first peak of the current with full dc offset would be 2 2 2.82 = timesrated current. The actual current peak might rise to a value in between 3 and 5.5 times dependingon the particular shunt reactor design details. One of the time intervals when reactor core goesinto saturation is clearly marked if Figure 5.For a three-phase reactor the different phases will experience different degrees of dc offset. Thecombination of the individual phase current offsets will give a neutral current rich in harmonicsand also with possibly dc offset from the zero line as shown in Figure 6 or Figure 14.


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17 18 19 20 21 22 23 24 25 26 27 28 29 300.75

0.0833

0.92

1.75

IN3Io

99,2MVA, 440kV, 60Hz Reactor

Cycles

Current[pu]

Figure 6: ShuntReactor Neutral Currents during Asynchronized Switching

The time to more or less fully balanced operation around zero flux in the core may be fairly longoften in order of seconds, but such condition is of no harm for the shunt reactor itself. In recentyears, so-called point on wave closing relays are available from switchgear manufacturers. Byusing these relays switching of different power system devices, including shunt reactors, can beperformed without a disturbance to the rest of the power system. Typical current waveformsduring the synchronized shunt reactor switching is shown in Figures 7, 8 & 9.

Figure 7: ShuntReactor Phase Currents during Synchronized Switching

4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 102

1

0

1

2

IAIBIC

150MVAr, 220kV, 50Hz Reactor

Cycles

Current[pu]


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4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 100.25

0.25

0.75

1.25

IN3Io


Cycles

Current[pu]

Figure 8: ShuntReactor Neutral Currents during Synchronized Switching

In order to obtain such disturbance free shunt reactor switching, circuit breaker poles must beprecisely closed in three consecutive phase voltage peaks as shown in the following figure.

5 5.22 5.44 5.67 5.89 6.11 6.33 6.56 6.78 72

1

0

1

2

IAIB

ICUAUBUC


Cycles

Current&V

oltage[pu]

Figure 9: ShuntReactor Phase Currents & Voltages during Synchronized Switching


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2.2 SHUNT REACTOR DISCONNECTION

Disconnection of small reactive current was at one time regarded as a dangerous operationbecause of the risk of current chopping and resulting switching overvoltage. Modern surgearresters are fully capable of handling this condition, and besides, the tendency of the circuit

breaker to chop reactor current is not so pronounced for typical HV shunt reactor rated currentvalues [1], [3].

However, the primary current chopping causes another, and maybe less known, transientphenomenon, which appears in the CT secondary circuit. This phenomenon is manifested as anexponentially decaying dc current component in the CT secondary circuit (see Figure 16, fortypical example). This secondary dc current has no corresponding primary current in the powersystem. The phenomenon can be simply explained as a discharge of the magnetic energy storedin the magnetic core of the current transformer. However these discharge secondary currents aretypically very small for shunt reactors and pose no effect on the reactor protection schemes withnumerical relays.


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From Figure 11 it is obvious that the overcurrent relays which use DFT filtering technique can beset more sensitive than the relays which use TRMS filter for its operation. Similar results can beobtained if similar analysis is performed for the neutral point current as well. All settingrecommendation in this document will be given for relays, which utilize DFT filtering technique(i.e. relays which effectively suppress the dc component and higher harmonic components in the

input quantity).

20 30 40 50 60 70 800

0.56

1.13

1.69

2.25

TRMSDFT

IC RMS value

Cycles

[pu]

Figure 11: Digital Filter Output for input signal shown in Figure 10


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IV. POSSIBLE PROTECTION PROBLEMS

DURING SHUNT REACTOR SWITCHING

It is well known fact that one of the principal difficulties with shunt reactor protection scheme isfalse operation during reactor energizing and de-energizing [4]. As explained previously, during

this period relatively high and long lasting dc current component typically causes most problemsfor protective relays. If the protection relays maloperate this typically happen some hundreds ofmillisecond or even 1 to 2 seconds after circuit breaker closing. What is most difficult tounderstand is why this problem often happens randomly and not with every reactor switchingattempt. Most problems are typically encountered with restricted ground fault protection,differential protection and ground fault protection during switching. Therefore performance ofthese three relays during switching in of the shunt reactor will be explained here in more details.

4.1 CURRENT TRANSFORMER PERFORMANCE DURING SWITCHING IN OF SHUNT REACTOR

It should be noted that HV shunt reactors are typically switched in and out at least once per day

or even more often depending on the power system loading patterns. As shown in Chapter IIduring switching in of shunt reactor relatively high and long lasting dc current component mightappear in one or more phases. This current waveform moves the operating point of CT magneticcore on the hysteresis curve in one direction and when the dc component diminish it leaves themain CT with certain level of residual (i.e. remanent) flux. During normal operation reactorcurrent is always around 1pu and therefore of a relatively low magnitude, which is never bigenough to move the operating point towards the origin. Therefore when next switching attemptcomes, depending on the moment of switching, residual flux in the CT core can increase ordecrease. Thus this mechanism will sooner or later cause CT saturation during reactor switch inoperation. This CT saturation then causes problems for protective relays, which lose the correctinformation about the primary current and therefore can maloperate. Such CT saturation event is

captured by numerical relay disturbance recorder and it is shown in Figure 12.

0 5 10 15 20 25 30 351

0

1

2

3

4

IC


Cycles

Current[pu]

Figure 12: Phase CT saturation during ShuntReactor Switching in

CT Saturation

Instant


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0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

DFT IC


Cycles

Current[pu]

Figure 13: Influence of Phase CT saturation on current DFT value calculation

This type of CT saturation is reflected in the CT secondary side as: loss of information about primary dc component reduced current magnitude

Figure 13 represents the DFT filter output value for the input current waveform as shown inFigure 12.

4.2 RESTRICTED GROUND FAULT RELAY PERFORMANCE DURING SWITCHING IN OF REACTOR

Modern numerical relays typically offer restricted ground fault protection of a low impedancetype. This gives the following benefits to the end user:

this relay can be applied with different type of CTs at the reactor bushing and at reactorneutral point (i.e. CTs doesnt need to be identical)

main CTs can be shared with other relays no galvanic connection is necessary between CTs at the reactor bushing and at reactor

neutral point in case of an internal fault no high voltages will appear in the CT secondary wiring

Typically these restricted ground fault relays of a low impedance type calculate the differential

current as a difference between zero-sequence currents at the reactor bushings and the reactorneutral point. As additional operating criteria they often use directional principle (i.e. producttype relays). However for shunt reactor protection these sometimes might not be enough toprevent maloperations. Lets have a look into the disturbance-recording file captured bynumerical relay, which is shown in Figure 14.


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0 5 10 15 20 25 30 351

0

1

2

IN3Io


Cycles

Current[pu]

Figure 14: Zero-sequence currents during ShuntReactor Switching in

The problem is that when one or more phase CTs saturate false 3Io current appears at the reactorbushings. Unfortunately this very often manifests as the current of opposite polarity incomparison with the neutral point current, which then causes the directional restricted groundfault relay (i.e. product type relay) to maloperate during reactor switching in.Calculated phase angle difference between neutral point current and zero-sequence current at thereactor bushing for the above event is shown in Figure 15:

5 10 15 20 25 30 3520

0

20

40

60

80

100

3Io-IN

Phase Angle Between IN & 3Io

Cycles

Angle[deg]

Figure 15: Calculated phase angle difference between IN and 3Io currents

Obviously it is necessary to have some additional means to restrain low impedance, restrictedground fault relay from maloperations during shunt reactor switching in. One very effectivemethod is to check the amount of second harmonic component in the shunt reactor neutral pointcurrent and adaptively prevent relay operation if the preset limit is exceeded.


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4.3 DIFFERENTIAL RELAY PERFORMANCE DURING SWITCHING IN OF REACTOR

Modern numerical relays typically offer differential protection of a low impedance type. Thisgives the following benefits to the end user:

this relay can be applied with different type of CTs at the reactor bushing and at reactor

star point (i.e. CTs doesnt need to be identical) main CTs can be shared with other relays no galvanic connection is necessary between CTs at the reactor bushing and at reactor

star point in case of an internal fault no high voltages will appear in the CT secondary wiring

Here the situation is little bit easier because the relay measures essentially the same current onboth ends of the protected winding. However again the long lasting dc component can causeuneven saturation of the two CTs and cause the relay maloperations. Lets have a look into thedisturbance recording file captured by numerical relay, which is shown in Figure 16.

0 2.08 4.17 6.25 8.33 10.42 12.5 14.58 16.67 18.75 20.83 22.92 252

0

2

4

ICIcN

99.2MVA, 440kV, 60Hz Reactor

Cycles

[pu]

Figure 16: Phase C winding currents during shunt reactor switching in

As it can be seen in Figure 16 due to uneven CT saturation on the two winding ends differentialprotection had unwanted operation and it has disconnected the shunt reactor from the powersystem. Thus if sensitive setting is required for the differential protection (i.e. 10-15% of thereactor rated current) it might be necessary to have some additional means to restrain lowimpedance differential protection relay from maloperations during shunt reactor switching in.One effective method is to enable second harmonic blocking feature commonly readily availablein numerical transformer differential relay. Second possibility is to delay the restraint differentialprotection operation only during reactor switching. In the same time in order to have secureoperation for heavier internal fault, the unrestrained differential level can be typically set downto 200% and without any time delay.


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4.4 GROUND OVERCURRENT RELAY PERFORMANCE DURING SWITCHING IN OF REACTOR

Numerical ground overcurrent relay might maloperate during reactor switching if it is set toosensitive. Typically in such cases either pickup current value or time delay are increased.However, another very effective method for such type of problem is to enable second harmonic

blocking feature for ground overcurrent relay, which is readily available in certain numericalprotections. Then the relay will check the second harmonic component level in the measuredinput current and prevent relay operation if the preset limit is exceeded.

V. SHUNT REACTOR BEHAVIOUR DURING

EXTERNAL AND INTERNAL FAULTS

Shunt reactors are connected in parallel with the rest of the power network. As shown inAppendix II shunt reactor can be treated as a device with the fixed impedance value. Thereforethe individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z).

Thus during external fault condition, when the faulty phase voltage is lower than the ratedvoltage , the current in the faulty phase will actually reduce its value from the rated value.Depending on the point on the voltage wave when external fault happens the reduce currentmight have superimposed dc component. Such behavior is verified by an ATP simulation and itis shown in Figure 17.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 161.5

0.98

0.47

0

0.57

1.08

1.6

IAIBIC

150MVA, 220kV, 50Hz Reactor

Cycles

Current[pu]

Figure 17: External Phase A to Ground Fault, Reactor Phase Currents

As a result, shunt reactor unbalance current will appear in the neutral point as shown in Figure18. However, this neutral point current will typically be less than 1 pu irrespective of the locationand fault resistance of the external fault.


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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 161.5

0.17

1.17

2.5

IN3Io


Cycles

Current[pu]

Figure 18: External Phase A to Ground Fault, Reactor Zero-sequence Currents

Similarly during an internal fault the value of the individual phase currents and neutral pointcurrent will depend very much on the position of the internal fault. Assuming that due to theconstruction details, internal shunt reactor phase-to-phase faults are not very likely, only twoextreme cases of internal phase to ground fault scenarios will be presented here.

In the first case the Phase A winding to ground fault, 1% from the neutral point has beensimulated in ATP. As a result the phase currents on the HV side (i.e. in reactor bushings) will bepractically the same as before the fault as shown in Figure 19.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 161.5

0.98

0.47

0

0.57

1.08

1.6

IAIBIC


Cycles

Current[pu]

Figure 19: Internal Phase A Winding to Ground Fault, Phase Currents


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However phase A current at the shunt reactor star point and common neutral point current willhave very big value due to so-called transformer effect. These currents can be so high to evencause CT saturation as shown in Figure 20 for the common neutral point current.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 165

1.67

8.33

15

IN3Io


Cycles

Current[pu]

Figure 20: Internal Phase A Winding to Ground Fault, Zero-sequence Currents

This type of the internal fault shall be easily detected and cleared by the differential, restrictedground fault or neutral point ground overcurrent protection, but not by reactor HV sideovercurrent or HV residual ground fault protections.

In the second case the Phase A to ground fault, just between the HV CTs and shunt reactorwinding (i.e. shunt reactor bushing failure) has been investigated. In this case the currents haveopposite properties. The phase A current on the HV side is very big (limited only by the powersystem source impedance and fault resistance), while the phase A current in reactor star pointwill have very small value due to a fact that phase A winding is practically short-circuited.As a result, shunt reactor unbalance current will appear in the neutral point. However, thisneutral point current will typically have a value around 1 pu (i.e. similar value as during externalground fault).

That type of the internal fault (i.e. shunt reactor bushing failure) shall be easily detected andcleared by the differential, restricted ground fault or HV side overcurrent or residual ground faultprotections. Neutral point ground overcurrent protection can operate with the time delay.

For internal ground fault in some other location in-between these two positions the shunt reactorcurrents will have values somewhere in the range limited by this two extreme cases.


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VI. SHUNT REACTOR TURN-TO-TURN PROTECTION SCHEMES

Turn-to-turn faults in shunt reactor present a formidable challenge to the protection engineer.The current and the voltage changes encountered during such fault are very small and thereforesensitive and reliable protection against turn-to-turn faults is difficult to achieve. At the same

time the longitudinal differential protection offers no protection at all for such faults. Hencespecial protection schemes shall be employed.

One such scheme, often used in certain countries, utilizes a fact that the HV shunt reactorwinding is often made of two half-windings connected in parallel (i.e. the HV lead is brought outat the mid point of the winding, and the two neutral leads at the bottom and the top of thewinding). This gives the opportunity to install two CTs in the winding star point (i.e. one in eachwinding part). Then so-called split phase differential protection can be utilized to detect turn-to-turn faults. However this protection scheme have the following drawbacks:

this special CT arrangement typically causes reactor manufacturing problems typically low CT ratio is required, which can cause longitudinal differential protection

problems during reactor switching in, if the same CTs are used for both differentialprotections

this scheme can be only used if the shunt reactor is specifically ordered with these CTs

Second turn-to-turn protection scheme for shunt reactors, successfully used in some othercounties, utilize the following facts:

HV power system voltages are well balanced during normal load conditions Modern HV, oil immersed shunt reactors have very small manufacturing asymmetry

between individual phases Shunt reactor winding impedance is approximately proportional to the square of the

number of active turns Short circuit between some number of turns will cause the decrease of the winding

impedance only in the faulty phase and corresponding small raise of the shunt reactorneutral point current

Currents during turn-to-turn fault are of the small magnitude and they will not produceany sufficient unbalance voltage

Any external cause of neutral point current (i.e. external phase to ground fault) will causeappearance of unbalance voltage which can be used to block the operation of turn-to-turnprotection scheme

In case of a bigger winding turn-to-turn fault which might cause the sufficient voltageunbalance, sensitive directional zero sequence relay connected on the shunt reactor HVside and set to look into the reactor shall be capable to detect such fault

This protection scheme was developed even before multifunctional numerical relays wereavailable. To implement such shunt reactor turn-to-turn protection scheme within multifunctionalnumerical relay utilizing its graphical configuration facilities, and readily available logical gates,timers etc. shall not be a big problem for a protection engineer.


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In order to verify above statements, shunt reactor behavior, for phase A winding 1% turn-to-turnfaults, is verified by an ATP simulation and it is shown in Figures 21 & 22. From these figures isobvious that the above-described scheme can be successfully implemented if the power systemitself is well balanced.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 161.5

0.98

0.47

0

0.57

1.08

1.6

IAIBIC


Cycles

Current[pu]

Figure 21: Internal Phase A Winding turn-to-turn fault, Phase Currents

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.08

0.033

0.013

0.06

IN3Io3Uo


Cycles

Current&V

oltage[pu]

Figure 22: Internal Phase A Winding turn-to-turn fault, Zero-sequence Quantities


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VII. SHUNT REACTOR MECHANICAL FAULT DETECTION

Similarly to the power transformers, HV oil immersed shunt reactors typically have build-inmechanical devices for internal fault or abnormal operating condition detection. Typically thefollowing built-in mechanical fault detection devices can be encountered within shunt reactor:

gas detection relay (i.e. Buchholz relay) with alarm and trip stage sudden pressure relay winding temperature contact thermometer with alarm and trip stage oil temperature contact thermometer with alarm and trip stage low oil level relay

These mechanical relays are excellent compliment to the electrical measuring relays previouslyexplained. Typically it is recommended to arrange that these mechanical relays trip reactorcircuit breaker independently from electrical relays. However signals from mechanical devicesshall be connected to binary inputs of numerical relays in order to get time tagging information,disturbance recording and event reporting in case of their operation.

VIII. TYPICAL SHUNT REACTOR CONTROL SCHEMES

The shunt reactors are generally designed for natural cooling with the radiators mounted directlyon the tank. However sometimes it is required to have some control action in the cooling circuitdepending on the status of the shunt reactor circuit breaker. The control action can be initiated bythe circuit breaker auxiliary contact or by operation of an overcurrent relay set to 50% of thereactor rated current. By using overcurrent relay secure control action is obtained when reactor isenergized regardless the circuit breaker auxiliary contact status.

In order to improve power system performance, lately it is often required by the electricalutilities to perform automatic shunt reactor in and out switching, by monitoring the busbarvoltage level. This functionality is quite easy to integrate into multifunctional, numerical relay.However user must carefully check relay performance regarding the following points:

over/under voltage relay with reset ratio or 1% or better is required for such application typically more than one over/under voltage level with independently settable time delays

are required within the relay over/under voltage relay shall be capable to operate only when all three voltages are

above/below set operate level or relay must be capable to measure and operate on thevalue of the positive sequence voltage


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IX. TRADITIONAL SHUNT REACTOR PROTECTION

AND CONTROL SCHEMES

Usually multifunctional numerical protection relays are used for both power transformer andshunt reactor protection. However, typically old protection schemes for shunt reactor protection,with just a few protection functions are still specified and applied today.

Two such traditional protection arrangements are shown in the following two figures.

IA

IBIC

IN

50/51

50G/51G

87N

Figure 23: Typical ShuntReactor Protection Scheme No1

The first protection scheme utilizes restricted ground fault protection (i.e. 87N) as reactor unitprotection. This protection shall trip instantaneously for all internal phase to ground faults. Forinternal phase-to-phase fault detection, overcurrent protection (i.e. 50/51) is utilized. Groundovercurrent protection (i.e. 50G/51G) is used as backup protection for ground faults and as mainprotection for circuit breaker pole disagreement condition.


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IcNIbN

IaN

IAIB

IC

50N/51N50/51

87

Figure 24: Typical ShuntReactor Protection Scheme No2

The second protection scheme utilizes differential protection (i.e. 87) as reactor unit protection.This protection shall trip instantaneously for all internal phase to phase and phase to groundfaults. Overcurrent protection (i.e. 50/51) is used as backup protection for internal phase-to-phase faults. Residual overcurrent protection (i.e. 50N/51N) is used as backup protection forground faults and as main protection for circuit breaker pole disagreement condition.

Actually it shall be noted that the numerical multifunctional relays can offer much morefunctionality than shown on the above two figures. Please refer to the following chapter to seeproposed shunt reactor protection scheme with multifunctional numerical protection relay.


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X. CONCLUSIONS

The paper has described a number of details regarding HV shunt reactors and their protectionand control schemes. In order to help the end user to properly select and apply multifunctionalnumerical relays for HV shunt reactor protection and control, an example of possible applicationof such relay, which utilizes DFT filtering technique, is presented in Figure 25.

IcN

IN

IbN

IaN

IAIB

IC

50N/51N50/51

50G/51G

87

4950/51 50/51

67N

87N

# 1

# 3# 2

UA

UB

UC

59 27 59N

Figure 25: Example of complete HV shunt reactor protection and control schemewith multifunctional, numerical relay

All proposed protection or control functions in Figure 25 are typically readily available inmultifunctional numerical transformer protection relays. However suitability of a particular relayto be used for shunt reactor application shall be carefully evaluated.

Table 1 gives the summary about each function from Figure 25 as well as some typical settingvalues [5]. The proposed settings shall be considered only as guidelines.

It is hoped that this paper will provide some guidance to those seeking assistance in HV shuntreactor protection and control issues.


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26

Function CommentTypical setting shown in percents

of the shunt reactor rating87=low impedancedifferential

protection

Check suitability for shunt reactorapplication with relay manufacturer.

Set restraint differential level to 10-15% with 2ndharmonic restrain set at 10%. Set unrestraintdifferential level 200%.

87N=lowimpedance,restricted groundfault protection

Check suitability for shunt reactorapplication with relay manufacturer.

Set differential level to 10%. Set operate angle fordirectional criteria to 65 deg. Relay shall includeadaptive 2ndharmonic restrain feature.

#1-50/51=HVovercurrent

protection

Backup protection, sensitive for internalfaults close to the reactor bushings.

Set low set to 130% with time delay in between0.6s and 1s. Set high set to 250% with time delay

of 0.1s. *#2-50/51=HVovercurrent

protection

Backup protection, sensitive for internalfault close to the reactor star point.

Set low set to 130% with time delay in between0.6s and 1s. Set high set to 200% with time delay

of 0.1s. *#3-50/51=HVovercurrent

protection

Used as circuit breaker failureprotection and indication that reactor isenergized for the cooling control logic.

Set low set to 30% with appropriate time delay asCBF protection. Set high set to 50% in order to

indicate that shunt reactor is energized. *

49=thermaloverload protection

Shall be used with great care. Shuntreactor overload can only be caused byovervoltage in a power system. That isthe exact time when reactors arerequired to be energized. Thus it mightcome in conflict with shunt reactorvoltage/reactive power controlfunctionality in the power system.

Specific manufacturing data are required in orderto properly set this function. Possible to usewinding/oil contact thermometer instead.

50G/51G=groundfault protection inreactor neutral point

Backup protection, sensitive for internalfault close to the reactor star point. Usedfor turn-to-turn fault detection logic.

Specific system data are required in order toproperly set this function.

50N/51N=groundfault overcurrent

protection in reactorHV side

Backup protection, sensitive for internalfaults close to the reactor bushings.

Set low set to 20% with time delay in between0.6s and 1s or even longer. Use 2ndharmonic

blocking. Set high set to 175% with time delay of0.1s. *

59N=unbalanceovervoltage

Used for turn-to-turn fault detectionlogic.


67=directionalground fault

protection

Used for turn-to-turn fault detectionlogic.


27&59=under/overvoltage

Used for automatic shunt reactorcontrol. Often more than one stagerequired.

Specific system data are required in order toproperly set these functions.

Table 1: List of functions for complete HV shunt reactor protection and control scheme

* These settings are proposed for HV shunt reactors with own circuit breaker. In case that theHV shunt reactor is directly connected to the HV line without its own circuit breaker, thesesettings have to be revised in order to prevent unwanted operation, when:

HV line is de-energized, due to low-frequency, long time constant transients which aredetermined by the combination of line capacitance and shunt reactor inductance [4]&[6]

One HV line phase is open during dead time of single pole autoreclosing cycle


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27

APPENDIX I

Basic relationships between shunt reactor most important quantities are shown here:

X

U

IXIUQ

22

***3===

(App_I.1)QPTotal *002.0 (App_I.2)

QPP TotalCu *0015.0*75.0 (App_I.3)

QPP TotalFe *0005.0*25.0 (App_I.4)

where: Q is shunt reactor rated reactive power in VAr U is shunt reactor rated phase-to-phase voltage in V I is shunt reactor rated phase current in A X is shunt reactor rated reactance in Ptotalis total shunt reactor losses in W (typically total shunt reactor losses are in order of

0,2% in accordance with references [1]&[2]) PCuis total shunt reactor copper losses in W (typically shunt reactor copper losses are in

order of 75% of the total losses in accordance with reference [1]) PFeis other shunt reactor losses (most of them are iron losses) in W

Above data are important in order to properly simulate HV shunt reactors.


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APPENDIX II

Typical HV shunt reactor magnetizing characteristic is shown in Figure 26.

0 1 2 3 40

0.5

1

1.5

2

2.5Shunt Reactor Characteristic

Current [pu]

Voltage[pu]

Figure 26: Typical magnetizing characteristic of a gapped core shunt reactor

The relation between voltage and current peak value in a HV shunt reactor can be described bytwo lines, one below saturation and the other above saturation. The point where the two linesintersect is called knee point. The relation between voltage and current peak value is shown inFigure 26. The knee point usually corresponds from 125% to 135% of the rated voltage. Theslope of the saturated part is 20% to 40% of the slope in the unsaturated region.

Below the knee point the reactor current is sinusoidal (i.e. this means that the relation betweenthe current peak value and the root-mean-square value of the current is fix and equal to 2 ) andits magnitude is directly proportional to the applied sinusoidal voltage. Thus for normaloperating voltages there is a linear relationship between applied voltage and reactor current (i.e. asmall increase in voltage will result in a proportional increase in current). Magnetic fluxes andflux densities are also proportional to the time integral of the applied voltages. With a voltage ofsinusoidal shape the fluxes and flux densities are also proportional to the voltage. The deviationfrom a true sinusoidal shape in phase voltage is generally negligible for normal operatingconditions of the power system.

For operation above the knee point the current peak value increases faster than the root-mean-square value, and the quotient between the peak value and the root-mean-square value exceedsvalue 2 .


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BIBLIOGRAPHY1. Carlson, ., Shunt Reactor Manual, ABB Power Technology Products/Transformers,

Ludvika-Sweden, 2002-08-202. ABB Transformatori, Pamphlet Shunt Reactors, Roma-Italy3. Switchgear Committee of the IEEE Power Engineering Society, IEEE Application Guide

for Shunt Reactor Switching, IEEE Std C37.015-19934. IEEE Power Systems Relaying Committee, IEEE Guide for the Protection of ShuntReactors, ANSI/IEEE C37.109-1988

5. Nylen, R., Shunt Reactor and SVC Protection Application Guide, ABB Relays, Vsters-Sweden, 1988-11-17

6. Elmore, W. A., editor, Protective Relaying: Theory and Applications, Marcel Dekker, Inc.,1994.

7. ATP is the royalty-free version of the Electromagnetic Transients Program (EMTP). Formore info please visit one of the following web sites: http://www.eeug.de/ orhttp://www.ee.mtu.edu/atp/

BIOGRAPHICAL SKETCHES

Zoran Gajiwas born in former Yugoslavia in 1965. He received his BSEE with honors from Universityof Belgrade, Yugoslavia in 1990 and GDE in Computer Engineering from Witwatersrand University,Johannesburg-RSA in 1995. Since 1993 he has been working in the area of power system protection andcontrol within ABB Group of companies, where he had various engineering positions. Currently he has aposition of Protection Application Specialist with ABB Automation Technologies, SubstationAutomation, Sweden. He is a member of IEEE and PES. Zoran has published several technical papers inthe relay protection area. His main working areas are computer applications for protection and control ofelectrical power systems, protection and control algorithms for microprocessor based relays and powersystem simulation. Zoran is co-holder of two patents.

Birger Hillstrm was born in Sweden 1944. He received his M.Sc.E.E degree from Chalmers TechnicalUniversity in Goteborg Sweden, 1968. He was employed as development engineer at ASEA, Sweden,1970 and is currently working as development project manager at ABB Automation Technologies,Substation Automation, Sweden. During ten years he was relay laboratory manager and was thenresponsible for development of a digitally controlled, analogue power system simulator and was after thatpart of a specification group of a real time digital power system simulator. His special areas of interest aretransient network analysis, development and testing of line and transformer static and numerical relays.

Fahrudin Meki was born in former Yugoslavia in 1967. He received his BSEE with honors fromSarajevo University, Bosnia and Herzegovina in 1991 where he also worked as research assistant. Hereceived his MSEE degree from Istanbul Technical University, Turkey in 1996. Since 1996 he has been

working in the area of power system protection and control within ABB, where he had variousengineering positions. Currently he is Senior Application Engineer with the Substation Automation andProtection Division, ABB Inc, in Allentown, PA. Fahrudin has published several technical papers in thearea of protection and reliability. He is currently responsible for the application and technical issuesassociated with ABB relays. He is a member of IEEE.

ABB_Reactor Protection Explanation

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