Analysis of the Requirements in Selected Grid Codes

8/6/2019 Analysis of the Requirements in Selected Grid Codes

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Analysis of requirements in selected Grid

Codes

Willi Christiansen & David T. Johnsen


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Analysis of requirements in selected Grid Codes Willi Christiansen & David T. Johnsen

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AbstractThis Report contains studies about the connection requirements of wind power generating

units. Six Grid Codes from Canada, Denmark, Ireland, Germany, Scotland and UK have

been compared and analyzed. The subject is relevant due to the lack of information about

generic connection requirements for wind power generating units. The purpose with this

Report is to outline and analyze the most restringing wind power connection requirements.

The studies are divided into an analysis of the connection requirements regarding

continuous operation modus and a second part regarding operation during fault sequences.

The first part (static analysis) concentrates on the continuous load flow conditions. The

second part is an analysis of the dynamic requirements in the Grid Codes. This analysis

concentrates mainly on the requirement of the fault ride through capability of each wind

turbine generator.

In the first part, the most restringingpower factor requirements are described. Due to the

worst case requirement, a wind turbine should be able to run continuously at full

production with apower factor of 0.90 lagging to 0.95 leading. Furthermore the maximum

required voltage and frequency range are outlined. The voltage range is more than 10% of

the nominal voltage. The frequency requirement is within the range from 47.5Hz to 52Hz.

This requirement seems disproportionate high.The result of the second part is a curve showing the requiredfault ride through capability. It

is required that a wind turbine under no circumstances is allowed to trip within the first

150ms. A voltage duration curve describes the voltage limits which define the voltage level,

in which a wind turbine has to continue operating.

The Report is completed with a theoretical review of different limiting load scenarios,

wherein the outlined restrictions can lead to problematic operation situations for the wind

power generating unit. The summarizing conclusion is that it is not sufficient only to obey

the requirements of the Grid Code individually. The requirements in the Grid Code should

rather be seen in its entirety and in relation to the given network. The studies have shownthat even if generic Grid Code requirements can be defined, detailed and challenging

network analyses have to be made for each wind power connection query, whereininformation about the type of wind turbine and of the given network is taken into

consideration.


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Preface .................................................................................................................................................2Abstract ...............................................................................................................................................31. Introduction ....................................................................................................................................52. Selection of Grid Codes..................................................................................................................63. Static regulations during continuous operation ..........................................................................7

3.1. Power factor regulations ...........................................................................................................73. 1. 1. The most restringing power factor regulation .................................................................11

3.2. Power Curtailment...................................................................................................................133.3. Voltage range and control .......................................................................................................143.4. Remote voltage control ............................................................................................................163.5. Frequency ................................................................................................................................173.6. Flicker......................................................................................................................................183.7. Harmonics................................................................................................................................19

4. Dynamic regulation during fault sequences...............................................................................204.1. Fault ride trough requirements................................................................................................20

4. 1. 1. Exceptions from the fault ride through requirements......................................................234.2. Repeating fault sequences........................................................................................................23

5. Examples of limiting worst case scenarios .................................................................................245.1. Scenario 1, fast voltage changes in the transmission system...................................................245.2. Scenario 2, high voltage and pf requirements .........................................................................245.3. Scenario 3, weakening of the network .....................................................................................255.4. Scenario 4, active power recovering after a voltage dip .........................................................25

6. Discussion......................................................................................................................................267. Conclusion.....................................................................................................................................288. References .....................................................................................................................................30Appendix ...........................................................................................................................................31


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1. Introduction

The aim of this Report is to describe the most essential conditions for connecting wind turbines to

the grid. The Grid Codes of selected countries with interesting technical and economical conditionswithin the field of wind power are taken into consideration in the analysis.

The technical specifications of the chosen Grid Codes are divided into static and dynamicrequirements.The static part of the Grid Code examination consists of the subjects regarding the continuousoperation of the wind farm. Following themes will be included in the static part: voltage control,quality of voltage, pf requirements, power curtailment, frequency and flicker.The dynamic part of the Grid Code examination consists of subjects regarding the operation of windturbines during fault sequences and disturbances in the Grid. Following themes will be included inthe dynamic part: fault ride through capabilities and fault recovery capabilities.

The most restringing conditions, seen from the perspective of the wind turbine, are outlined anddescribed. The most restringing conditions are outlined in the formulation of generic connectionrequirements. The focus of this examination is on the dynamic requirements in the Grid Codes.

Worst case scenarios are presented to highlight sections wherein the generic Grid Code might not besufficient to ensure the stability and the quality of the electrical systems. The scenarios will bediscussed and suggestions as to how the connection queries of wind farms could be solved, arebriefly mentioned.

The purpose of outlining the requirements of a generic Grid Code is to give an idea of the technical

qualifications, which should be satisfied by a generic wind turbine model. This Report willtherefore be a summation of the requirements in selected Grid Codes. It has to be pointed out thatthis Report does not consist of a complete generic Grid Code in itself.


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2. Selection of Grid CodesThe selection of Grid Codes is based upon a number of preset considerations. It is important toarchive a broad variation of Grid Codes in order to obtain a realistic generic Grid Code. The

countries are selected in the view of interesting wind power aspects like technical possibilities andgeographical limitations. Furthermore following conditions must be fulfilled for the choices of GridCodes:

Wind power potential Detailed section regarding wind power (or non-synchronous generators) within the

Grid Code Interesting network characteristics (island, weak/strong network, high penetrations of

wind power)

All together, six Grid Codes are selected for the analysis of a generic Grid Code.

Among the chosen Grid Codes is Denmark [2] due to the high penetration of wind power. Ireland[4] is selected because it is an island-system and therefore of great interest. The Grid Code of EON[3] (a German transmission system Operator (TSO)) is chosen due to the important wind powermarket in Germany and due to detailed technical descriptions in the Grid Code of EON.Furthermore the Grid Code from Scotland [5] and the UK [6] are analyzed because of the detailedconnection of non-synchronous generating unit section and because of the high wind potential inUK and Scotland. Finally the Grid Code from the Canadian TSO [1], AESO, is taken intoconsideration in order to archive a contribution from an oversee Grid Code. The Canadian GridCode is not included in the frequency analysis due to the fact that the Canadian system operates at60Hz.Parts of the Spanish and of the Australian Grid Codes have also been taken into account, but these

Grid Codes have not been analysed in detail.


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3. Static regulations during continuous operation

The first part of the Grid Code analysis concentrates on the static requirements. Some of the

requirements contains time limits or operates with time ranges. The values are however stillconstant and the semi-static (time ranges) regulations are therefore included in this static analysis.The second part (chapter 4. ) concerns the dynamic requirements in the Grid Codes. Theserequirements do primarily concern the desired behaviour of the Wind Turbine Generator (WTG)during faults and disturbances.

Unless described otherwise, the static requirements refer to the behaviour and to the power flow atthe connection point of the Wind Farm Power Station (WFPS) to the transmission grid. Theoperation of a single WTG is therefore not of interest in this static requirements chapter. Contrary isit in chapter 4. (dynamic requirements), where the operation and measurements of each WTGapplies.

3.1. Power factor regulations

The power factor regulation concerns the reactive power consumption of the Wind Farm PowerStation (WFPS). A simple induction generator, with no additional capacitors attached, will duringnormal operation consume reactive power. This reactive power has to be produced somewhere inthe grid. It is preferred that the WFPS is reactive power neutral, since the distribution of reactivepower is relatively cost intensive. The requirements to the reactive power and to the power factorare relatively similar in the different Grid Codes.

The static phase angel requirements are listed in the following Table 3.1. The Wind Farm Power

Stations (WFPS) shall be capable of operating at any point within the power factor ranges.

For the avoidance of doubt, a generating unit operating at lagging power factor delivers reactivepower into the transmission system.

Canada Denmark Germany Eon

Ireland Scotland UK

Static, continuespower factor

0.90laggingto 0.95leading

Q/Prated= 0 toQ/Prated= 0.1 at

fullproductionandthrough astraightline to

0.95 lagging to0.95 leadingfor a ratedactive power

capacity 100

0.95 laggingto 0.95leading at fullproduction.

32.6MVArper 100MWinstalled,activecapacity from100%

0.95laggingforproduction

between100% -20%. 0.95leadingforproduction

0.95lagging to0.95leading


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Q/Prated= -0.1 toQ/Prated= 0 at zeroproduction

MW thepower factor isvoltagedependent.***

production to50%production.0.95 laggingto 0.95leading from50%production toidle.*

between100% to50%**

Table 3.1 power factor requirements of the selected Grid Codes.

*The requirements to the reactive power production in the Irish Grid Code are complex compared tothe other Grid Codes. The idea of varying the power factor dependent on the active production isillustrated much clearer on Figure 3-1. It can be observed that the relation between reactive powerand active power must remain constant within the area from 100% to 50% active power production.


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Figure 3-1 The Irish power factor requirements

The black triangle below the 10 % production line on Figure 3-1 indicates that the reactive poweroutput during operations below 10% must be altered, if the voltage limit is reached.


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**The Scottish reactive power requirements diverse from the requirements in the UK when theactive power production gets below 20% of the rated active power. The Scottish case is illustratedin the following Figure 3-2:

Figure 3-2. The Scottish power factor requirements

The inductive reactive power limits are reduced linearly below 50% Active Power output as shownin Figure 3-2. The reactive power limits for active power output below 20 % shall be adjustablewithin the area of Q = - (5% of rated MW output) to Q = 5% of rated MW output.

*** EON: With active power output, each generating unit with a rated power of 100 MW must

meet the range of reactive power provision shown in Figure 3-3 as a basic requirement at thenetwork connection point. Additional requirements may have to be met in individual cases.In addition to any of the other Grid Codes, the German Grid Code [3] contains information on thereactive power requirements during different voltage situations. The reason for including the actualvoltage at the bus is the fact that a high reactive power production induce higher voltages, which isundesired if the initial voltage is at a high level already. Thereby the power factor control becomes apart of the voltage regulations.The case for the remaining Grid Codes is that it must be possible to operate between 0.95-0.9

lagging at full production, even if the voltage is at 1.1p.u. This could cause unnecessary stress to thecomponents due to the increasing voltage.


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Figure 3-3. The German power factor requirements for Prated above 100MW

When changing the reactive power output, step changes corresponding to a reactive power of morethan 2.5 % of the network connection capacity in the high voltage network and 5 % in the extra highvoltage network are not permissible. No step changes smaller than 500 kVAr will be required.

3. 1. 1. The most restringing power factor regulation

The most restringing power factor requirement (black line on Figure 3-4 and Figure 3-5) consists of

a combination of the power factor requirements listed in Table 3.1. The most restringing conditionat full output is the Canadian [1] 0.90 lagging to 0.95 leading. At lower production levels thevariable power factor of the Irish power factor requirements will surpass the restrictive Canadianregulations. As seen in Figure 3-1, the reactive power production requirement of the Irish Grid Code[4] surpasses the Canadian when the power factor gets below 0.95 leading and 0.90 lagging.Following Figure 3-4 illustrates the Irish power factor requirements including the Canadianintensifications. The main contribution of the Canadian Grid Code is the power factor of 0.90lagging, which is applicable from 100% to 70% active production.


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Figure 3-4, most restringing power factor settings

Figure 3-5, most restringing reactive power settings


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The actual reactive production can be seen in the Figure 3-5. The required limits are defined by thecontinuous black line.The Wind Farm Power Stations shall be capable of operating at any point within the power factorranges. Furthermore it shall be possible to order reactive regulation requirements via remote controland locally. Depending on the operational situation, the system operator changes the desired MVAror voltage reference. In general it must be possible, independent of the rated power, to run throughthe agreed design range for the power factor at rated active power output within a few minutes. Theentire process must be possible as often as requested.

The issue regarding the generic power factor requirements is that the requirements are based on atvoltage level around 1.0p.u. In the transmission system a low voltage must be supported by anincreased reactive power production and vice versa. The opposite reaction will only cause furthervoltage deviation.Only the German Grid Code [3] includes the actual voltage level as a part of the power factorrequirements (see Figure 3-3).

3.2. Power Curtailment

The need for power curtailment occurs when it is not possible to compensate for the loss of windpower by up rating conventional plants. To avoid this situation the wind power is curtailed. Thissituation normally occurs during the daily load increase when the conventional plants have tocompensate for both the increasing load demand and for the declining wind production. Anotherreason for curtailing wind is if the wind production increases beyond the load consumption. Thiswill increase the inertia of the transmission system which causes a frequency increase.

The curtailment amount is dependent on installed capacity, location, wind forecasting reliability andeconomics and can not be outlined precisely. But it must in any case be possible to reduce the

power output in every operating condition and from any operating point to a maximum power valuewhich corresponds to a percentage value based on the network connection capacity. The reductionof the power output to the signalled value must take place with at least 10% of the networkconnection capacity per minute without the system being disconnected from the network.1 The mainindication of a production surplus is a frequency increase. Therefore the power output must bereduced from a frequency of 50.5 Hz according to Figure 3. A gradient of 5 % per second applies.When the frequency deviation decreases, the power output must be increased accordingly. The re-establishment of the active power supplied to the network must not exceed a maximum gradient of10% of the network connection capacity per minute.

1 [3] E.ON Netz GmbH, Grid Code for high and extra high voltage, 1. August 2003 Page 23


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Voltage Range

-25

-20

-15

-10

-5

0

5

10

15

20

25

400 275 220 150 132 110

Nominal Voltage

V/Vnomi

npercent

1 Hour

15 min.

Continious Time

Figure 3-7, specified worst case voltage range

- note that the operating interval for the 132V nominal voltage is 1 hour for voltages between 10%above nominal voltage to 18% above nominal voltage.

3.4. Remote voltage control

Wind Farm Power Stations (WFPS) shall have a continuously-variable, continuously acting, closedloop voltage regulation system with similar response characteristics to a conventional automatic

voltage regulator.

The voltage regulation system shall be adjustable by the TSO by signalling a voltage set point forthe voltage at the connection point. The voltage regulation system shall act to regulate the voltage atthis point by continuous modulation of the reactive power output within its reactive power range,and without violating the voltage step emissions. The set-point shall be adjustable within 95% to105% of rated voltage.

The response speed of the voltage regulation system, following a step change in voltage at theconnection point, shall be such that the wind farm power station achieves 95 % of its steady-statereactive power response within 1 second.

It is only in the Irish [4] and the Canadian [1] Grid Code that these remote voltage control optionsare specified in detail.


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3.5. Frequency

The Following Table 3.4 shows the frequency requirements of the European Grid Codes:

Frequency range requirement

Minimum Time DelayFrequency(Hz) Denmark[2] Germany[3] Ireland[4] Scotland[5] UK [6]

52 Hz t to 53 Hz 3 min % % % %

51.5 Hz to 52 Hz 30 min % 60 minContinuousOperation

ContinuousOperation

51.0 Hz to 51.5 Hz 30 minContinuousOperation

60 minContinuousOperation

ContinuousOperation

50.5 Hz to 51.0 Hz 30 minContinuousOperation


ContinuousOperation

49.5 Hz to 50.5 HzContinuousOperation

ContinuousOperation

ContinuousOperation

ContinuousOperation

ContinuousOperation

49.5 Hz to 47.5Hz



ContinuousOperation

47.5 Hz to 47.0 Hz 3 min % 20 sec 20 sec 20 sec

>47.0 Hz % % 20 sec 20 sec 20 sec

Table 3.4, Frequency range requirement

The design of generators plant and apparatus must enable operation in accordance with thefrequency range in Table 3.4. Due to the frequency requirements in the table, wind farms arerequired to be capable of operating continuously between 47.5Hz and 52Hz and time limitedbetween 47 and 47.5Hz likewise as between 52 and 53. This is a relative wide range in relation torealistic events. Nevertheless a model of a wind turbine should be able to operate within this range.

Generic frequency range requirement

Frequency (Hz) Code of operation

52 Hz to 53 Hz 3 min

51.5 Hz to 52 Hz Continuous Operation

51.0 Hz to 51.5 Hz Continuous Operation




47.5 Hz to 47.0 Hz 20 sec

>47.0 Hz 20 sec

Table 3.5, Generic frequency range requirement

The case is different for Canada due to a system frequency of 60Hz. The Canadian frequencyrequirements are not included in the analysis since there is no basis for comparison.


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In addition the active power output may be reduced in the outside values within the frequencyranges (see chapter 3.2. ). The WFPS should remain connected to the transmission system duringrate of change of transmission system frequency values of up to 0.5 Hz per second. Automaticisolation from the network due to the frequency is only permissible at frequencies below 47Hz andabove 53Hz. Upon reaching 47Hz or 53Hz the unit must be automatically isolated from the networkwithout delay.

No additional WTG shall be started while the transmission system frequency is above 50.2Hz.

3.6. Flicker

Flicker is defined as a single, rapid change of the RMS voltage. Transmission system step changescan occur due to switching in and out of capacitors, lines, cables, transformers and other plant.Voltage fluctuations at a point of common coupling with a fluctuating load directly connected to thetransmission system shall not exceed the lines on following figure:

Figure 3-8, voltage flicker limits

Flicker shall not exceed 3% at any time. The maximum permissible values for rapid voltage

changes from wind farms in the connection point are shown on Figure 3-8. The red line indicatesthe limits in the Danish Grid Code [2]. The black lines indicate the Canadian Grid Code [1].

The flicker contribution from the wind farm in the connection point shall be limited so that the shortterm flicker (Pst), determined as a weighted average of the flicker contribution over ten minutes is


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below 0.30. The long term flicker value (Plt), determined as a weighted average of the flickercontribution over two hours shall be limited so that Plt is below 0.20.

The flicker contributions Pst and Plt are defined in IEC 61000-3-7 ( Electromagnetic compatibility).

It is primarily the Danish Grid Code [2], which contains restrictive rules about flicker. The Irish andthe German Grid Codes do not mention flicker.

3.7. Harmonics

Wind Farm Power Stations connected to the transmission system shall be capable of withstandingthe levels of harmonic distortion liable to be present on the transmission system. Theseelectromagnetic compatibility levels are not directly specified in any of the Grid Codes but theDanish. The Grid Codes from UK [6] and Scotland [5] refer to the standard ER G5/4 Planning Levels for Harmonic Voltage Distortion and the Connection of Non-Linear Loads to the

transmission systems and Public Electricity Supply Systems in the United Kingdom. The Canadian

Grid Code [1] refers to the IEEE standard 519-1992 Recommended Practices and Requirementsfor Harmonic Control in Electrical Power Systems. The requirements of the mentioned standardsare not included in the report.

The requirements in the Danish Grid Code [2] regarding harmonics are outlined as follows:

The harmonic disturbance Dn for each individual harmonic shall be defined as:

The total harmonic effective distortion THD shall be defined as:

Dn shall be lower than 1 per cent for 1 < n < 51 in the connection point.THD shall be smaller than 1.5%.

In general, the Danish Grid Code [2] limits the individual harmonics as a function of the nominalvoltage. Furthermore a limit of the THD is presented.


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4. Dynamic regulation during fault sequences

4.1. Fault ride through requirements

Phase swinging or power oscillations must not result in triggering of the generating unit protection.The turbine-generator unit control must not excite any phase swinging or power oscillations.The Wind Turbine Generator (WTG) shall be equipped with voltage and frequency relays fordisconnection of the wind farm at abnormal voltages and frequencies. The relays shall be setaccording to agreements with the regional grid company and the system operator. The protectivefunctions of the wind turbine shall include settings and time delays meeting the requirementsdescribed in this section.

All countries have afault ride through capability figure. These requirements do only concern shortcircuits in the transmission system, and not short circuits within the Wind Farm Power Station

(WFPS).Following fault ride through figures are partly the most restricting ones. In the following theScottish [5], the Irish [4] and the German [3] fault ride through figures are presented:

Figure 4-1, the Scottish fault ride through requirement


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the operating range of the generating unit, these types of faults must not result in instability orisolation from the network.Furthermore, Figure 4-4 shows that wind turbine generator units shall be capable of continuousoperation down to 90% of rated voltage at the connection point.

In addition to the requirement that the WTG must remain connected to the transmission system, theWTG shall have the technical capability to provide the following functions:The wind WTG shall provide active power in proportion to retained voltage and maximize thereactive current to the transmission system without exceeding WTG limits during the voltage dip inthe transmission system. The maximization of reactive current is described in detail in the belowFigure 4-5;The wind farm power station shall provide its maximum available active power as quickly as thetechnology allows with a gradient of at least 20% of the rated power per second. Within the greyarea in Figure 4-4 the active power increase can take place at 5% of the rated power per second.This power increase should in any event occur within one second of the transmission systemvoltage recovering to 0.90pu of the normal operating range.

The generating units must support the voltage within a disturbed network. If a voltage drop of morethan 10% of the root mean square (RMS) of the generator terminal voltage occurs, the generationunit must be switched to voltage support, according to following figure:

Figure 4-5, Amount of reactive current feed for voltage support with a fault in the network


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After the fault identification, the network voltage support must be provided within 20ms byproviding reactive power at the generator terminals with a factor of 2% of the rated current perpercent of the voltage drop. The maximization of reactive current shall continue for at least 600ms,or until the transmission system voltage recovers to the normal operational range of thetransmission system; whichever is the sooner.

The transient phenomenon, with regard to the reactive power consumption after the voltage returnsto the normal operation range, must be completed after 400ms. After this time the reactive powerexchange must take place as it is specified on the basis of the normal operational schedule.

4. 1. 1. Exceptions from the fault ride through requirements

Wind Turbine Generator units are not required to ride through transmission system faults that causea forced outage of a radial line to the wind farm (isolation of the WFPS). Nor shall wind turbinegenerator units ride through faults that occur on the lower voltage networks of the wind turbine.

The requirements described in Figure 4-4 and Figure 4-5 do also not apply when the wind turbine

power station is operating at:- less than 5% of its rated power;- during very high wind speed conditions, when more than 50% of the wind turbine

generator units in the power station have been shut down or disconnected under anemergency shutdown sequence to protect the plant and apparatus.

4.2. Repeating fault sequences

A wind turbine generation unit shall have sufficient capacity to meet the above mentionedrequirements. Besides it shall be able to withstand the impacts from faults in the grid whereunsuccessful automatic reclosure takes place without necessitating disconnecting the generation

unit. The unit shall have capacity to meet following three independent sequences:

at least two single-phase earth faults within two minutes

at least two two-phase short circuits within two minutes

at least two three-phase short circuits within two minutes

Additionally, there shall be sufficient energy reserves (emergency power, hydraulics andpneumatics) for the following three independent sequences:

at least six single-phase earth faults with five-minute intervals

at least six two-phase short circuits with five-minute intervals

at least six three-phase short circuits with five-minute intervals


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5. Examples of limiting worst case scenariosThe aim of this chapter is to analyze the consequences of the requirements in the Grid Codes duringdifferent load scenarios. Suggestions on to how the wind turbines should operate and respond

during stressing load situations, without violating the requirements in the Grid Code, are given.

5.1. Scenario 1, fast voltage changes in the transmission system

A Wind Farm Power Station (WFPS) is connected to the transmission system through an automatictap changing transformer. During steady state load conditions, the transformer keeps the voltage onthe low voltage side of the transformer close to constant.The transformer settings ensures a voltage of approx. 1pu at the connection point of the WFPS,even if the pre-fault voltage on the transmission system is relatively low (0.9pu).

It is then assumed that a fault occurs on the transmission system. The fault is cleared by the

protection system within 200ms. The post-fault voltage on the transmission system is now relativelyhigh (1.1 pu) due to switching within the transmission system. The automatic tap-changingtransformer cannot tap fast enough to compensate for the sudden increase of the voltage. Thereforethe post-fault tap position is equally to the pre-fault tap position. This means that the post-faultvoltage on the low voltage side of the transformer is higher (in pu) than the voltage in thetransmission system. If the voltage gets above 1.2pu, the wind turbines within the WindDistribution System (WDS) may trip due to the settings of their protection system.

This example shows that relative simple occurrences can result in tripping of the wind turbines,even though the Grid Code is obeyed. It can be summarized that the individual requirements in theGrid Code are adapted to the local conditions of the transmission system. It is therefore necessary to

simulate extreme load conditions including the WFPS, analyzing how the requirements in the GridCode should be handled.

5.2. Scenario 2, high voltage and pf requirements

A requirement in the Grid Codes is that a WFPS has to be able to run continuously during highvoltage situations (above +10%). The reactive power requirements of a Wind Turbine Generator(WTG) are simultaneously within the pf range of 0.9 lagging to 0.95 leading at full production. Thecombination of a high voltage situation and a pf of 0.9 lagging (the wind turbine produces reactivepower) do not seem realistic. A production of reactive power will furthermore increase the voltage.During a high voltage situation, the WTG should therefore prevent it self from producing at fulllagging.

This example shows again that it might not be sufficient to fulfil the individual Grid Coderequirements independently. The Grid Code requirements should be seen as a combination withrealistic network conditions.This consideration is what can be observed on Figure 3-3. A pf of 0.95 lagging has only to beobtained at low voltages and visa versa.


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5.3. Scenario 3, weakening of the network

The third scenario consists of a WFPS with the installed capacity of several hundreds of megawatts,which is installed in a remote end of the transmission system. A nearby transmission fault results in

the tripping of one of two parallel lines to that section of the system. The wind generation ridesthrough the fault and following the fault clearing, the wind generation increases back to or near theoriginal pre-disturbance megawatt value. However, with the weakened link to that part of thesystem and the significant level of power interchange between that area and the rest of thetransmission network, there may not be enough dynamic reactive power reserve in the vicinity ofthe wind farms to maintain voltage stability. As an example, such dynamic requirements might beprovided by the application of a static VAr compensator (SVC), to regulate transmission systemvoltage immediately after a severe disturbance, and thereby ensuring a fast and stable voltagerecovery.

5.4. Scenario 4, active power recovering after a voltage dipIn most Grid Codes it is required that the active power returns after a fault occurrences within onesecond, as soon as the voltage is above 0.90pu. During a fault ride through event the active power ofa wind turbine decreases. This is a consequence of the low voltage and of the decreasing mechanicalpower input.The voltage return after a fault sequence is dependent on the level of the short circuit power of thetransmission system. The voltage will return to the pre-fault value immediately in a robust system.Areas with high penetration of wind power are typically not strong networks. This can be expectedespecially if the fault clearance mechanism trips an OHL line (Scenario 3).Due to the Grid Code requirement the wind turbine will increase its active power production assoon as the voltage is above 0.9pu. The voltage can as a consequence of this get unstable and

significant voltage backswings can occur.A suggested solution to this event is that the active power return should occur less rapid. If thevoltage is kept beneath 0.9pu at the terminals of the wind turbine, the active power recovery can bespread out over a longer period of time. The oscillation, overshot and instability of the recoveringvoltage can thereby be kept at a minimum. A consequence of this is a temporary loss of activepower.

With this strategy, the pre-fault conditions can be achieved within 10 seconds. This could be ahelpful strategy to satisfy the requirements in the Grid Code in weak networks, but it requires anumber a simulations.


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This is an example of how some of the complicated requirements in the Grid Code can be fulfilledconcurrently while still taking the technical limits of the network into account.

The above example shows that detailed network analyses and problem solutions has to be includedin the connection queries of wind farms. If the Grid Code has to be satisfied in weak networks, asecure and continuously operation of the wind farm could become a challenging issue.

This conclusion is one of the most important results from the analysis of the most restricting GridCode requirements. It is not sufficient to obey the regulations of the Grid Code individually; theGrid Code should rather be seen in its entirety and in relation to the given network.Therefore network analyses have to be made for each wind farm connection query and the technicallimits of the specific transmission system should be taken into account. The solutions will showhow a specific generating unit can comply with the Grid Code requirements.


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7. ConclusionThis Report contains studies of wind power connection requirements, which are described in GridCodes from different countries. The countries are chosen in the view of interesting wind power

aspects like technical possibilities and geographical limitations. Denmark is chosen due to the highpenetration of wind power. Ireland is an island-system, and it is therefore of great interest. The GridCode of EON (a German transmission system Operator (TSO)) is chosen due to the important windpower market in Germany and due to the detailed Grid Code of EON. Furthermore the Scottish andthe Grid Code from UK are analyzed. Finally the Grid Code from the Canadian TSO, AESO, istaken into consideration.

The wind turbine connection requirements from the mentioned countries have been compared. Themost restringing regulations have been outlined with the purpose of demonstrating which technicalchallenges generic wind turbine models should be able to handle during continuous operation andduring faults. The analysis has been divided into a static and a dynamic part. The static section

concentrates on the continuously operation while the dynamic part describes the behaviour duringfault sequences and disturbances in the grid.

Many of the technical aspects are only mentioned in some Grid Codes. The explanation for this isthat each Grid Code reflects the technical conditions of the respective transmission system. Everycountry has its own special technical issues to deal with, and this speciality is reflected in the GridCode.

The static part has shown two important aspects. Firstly the significant requirement of the pf rangeis conspicuously. The most restringing conditions is that a wind farm at its connection point to thetransmission system running at full active power production shall be able to vary its power factor

(pf) between 0.9 lagging to 0.95 leading. Lagging means feeding reactive power into the grid. It hasto be pointed out that the pf range is only between 0.95 leading to lagging, if just the Europeanrequirements are taking into consideration.The second significant requirement is the huge frequency range, wherein the wind farm has to beable to operate continuously. A wind farm power station is required to run continuously between47.5Hz and 52Hz. The lower limit reflects the Irish transmission system, whereas the upper limit of52Hz reflects the German system. Beside this frequency range, continuous operation in a voltagerange of more than 10% of the nominal voltage is required.

The analysis of the dynamic requirements has shown that a wind turbine under no circumstances isallowed to trip during the first 150ms of a voltage drop sequence. This requirement reflects the

demand for keeping the generating wind turbine units in operation. This can avoid the loss of activepower production. In case of a longer duration of a fault sequence, the operating limit is describedwith a voltage duration curve. The Curve describes in detail, when a wind turbine may trip.There are given detailed descriptions on how the reactive current and the active power productionshould act during and after a fault sequence. Especially the active power recovery following a faultis important, since an immediate active power recovery could cause voltage fluctuations in a weak


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network. Especially the German Grid Code has included detailed requirements on the active powerrecovery.

Finally some worst case scenarios are presented. They demonstrate how complicated it is to avoidviolating some of the Grid Code requirement under different load conditions. All Grid Coderequirements should be satisfied independently of each other. Simultaneously most technicalrequirements have a mutual influence on each other.

Summarized it can be concluded that it is not sufficient to obey the requirements of the Grid Codeindividually. The Grid Code should rather be seen in its entirety and in relation to the givennetwork, in which the wind farm is going to be connected. It requires challenging planning andconnection studies if the general Grid Code requirements, which have been derived by comparingthe Grid Codes from different countries, have to be satisfied.


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8. References[1]Wind Power Facility, Technical Requirements, Grid Code from Alberta Electric System

operator, Canada

[2]Wind turbines connected to grids with voltages above 100 kV, Technical regulations for theproperties and the regulation of wind turbines, Grid Code from the Danish TSO,energinet.dk

[3]Grid Code, High and extra high voltage, EON Netz, German TSO

[4]WFPS1, Wind Farm Power Station Grid Code Provisions, ESB National Grid, Irish TSO

[5]Scottish Grid Code, Scottish Hydro-Electric Transmission Ltd, Scottish TSO

[6]The Grid Code, Revision 12, National Grid Electricity Transmission plc, TSO in UK

[7]Integration of Wind Energy into the Alberta Electric System, Electric System ConsultingABB Inc

[8]Large Scale Integration of Wind Energy in the European Power Supply, analysis, issues andrecommendations, A report by EWEA, January 2006-01-18


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Appendix

Appendix 1. Power curtailment scheme

Type of

regulation PurposePrimary

regulation aim

Absolute productionconstraint

Limit the wind farm's current power production in the connection pointto a maximum, specifically indicated MW value. Constraintsmay be necessary to avoid overloading of the power grid.

Limit production to optionalMWmax

Delta productionconstraint

It must be possible to reduce the power production of the wind farm bya desired power value compared to what is possible at present, therebysetting aside regulating reserves for the handling of critical powerrequirements.

Limit production byMWdelta

Balanceregulation

The power production of the wind farm must be adjusted to the currentpower requirement with a view to maintaining the power balance of thebalance responsible market player and/or the system operator.downward/upward regulation of production must be possible.

Change current productionby -MW/+MW with the setgradient and maintain theproduction at this level

Stop regulation The wind farm must maintain the power production at the current level(if the wind makes it possible). The function results in stopfor upward regulation and production constraints if the wind increases.

Maintain current production

Power gradientconstrainer

For system operational reasons it may be necessary for wind turbines tolimit the maximum speed at which the power output changes in relationto changes in wind speed. The power gradient constrainer is to ensurethis.

Power gradients do notexceed the maximumsettings

System protection System protection is a protective function which must be able toautomatically downward regulate the power production of the wind

farm to a level which is acceptable to the power system. In the case ofunforeseen incidents in the power system (for instance forced outage oflines), the power grid may be overloaded at the risk of power systemcollapse. The system protection regulation must be able to rapidlycontribute to avoiding system collapse.

Downward regulate powerproduction automatically on

the basis of an externalsystem protection signal


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Appendix 2. Nomenclature

Grid Code Description of the connection

conditionsLagging The phase of the current is

behind the phase of the voltageLeading The phase of the current is

ahead the phase of the voltageOHL Over Head LineP Active power [MW]pf Power factorpu Per UnitQ Reactive Power [MVAR]

RMS Root mean square

SVC Static Var CompensatorTSO Transmission System OperatorUCTE Union for the Co-ordination of

Transmission of ElectricityWDS Wind Distribution SystemWFPS Wind Farm Power StationWTG Wind Turbine GeneratorTHD The total harmonic effective

distortion

Analysis of the Requirements in Selected Grid Codes

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