Grid Connection - Wind Energy - Revision history

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Ranisha: /* Equivalent Synchronous Machine Model */

2014-08-29T14:13:11Z

Equivalent Synchronous Machine Model

Ranisha at 14:12, 29 August 2014

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Ranisha at 14:11, 29 August 2014

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AnkeMaria at 07:41, 27 July 2011

2011-07-27T07:41:45Z

@@ Line 36: / Line 36: @@
 For this reason, only a probabilistic assessment considering the actual wind speed conditions at the site can provide an answer to the question, which mitigation option should finally be applied.<br/>
-<br/>[[File:Outage wind park example.JPG|thumb|center|180px|Figure 1: Outage of one of the parallel 132kV lines between Laingsburg and Boskloof]] <ref>id. pp.8</ref><br/>
+<br/>[[File:Outage wind park example.JPG|thumb|center|180px|Figure 1: Outage of one of the parallel 132kV lines between Laingsburg and Boskloof]] [[Grid Connection - Wind Energy#toc|►Go to Top]]<br/>
 <br/>
@@ Line 50: / Line 48: @@
 In the case that no more detailed information is available, which is usually the case in an early planning stage; a typical power curve of a wind generator can be used as equivalent wind farm power curve. Effects, like wind-farm internal losses etc. can be considered in the equivalent wind farm power curve.<br/>Based on average wind speed and shape factor, the distribution of wind-speeds can be calculated, based on a Weibull distribution. Together with the equivalent wind farm power curve, a wind generation duration curve for one wind farm can be calculated showing the probability for the case that the generated wind power exceeds a given level (see Figure 3).<br/>The energy that cannot be delivered to the system can then be evaluated by calculating the area between that part of the red curve that is above the blue line indicating the wind farm limit and the blue line itself. For a monetary analysis, this area has to be multiplied by the assumed tariff.<br/>
-<u><br/></u>
+<br/>
-<u>'''The example of Laingsburg (Western Cape, [[South_Africa_Energy_Situation|South Africa]]):'''</u><br/>
+<u>'''The example of Laingsburg (Western Cape, [[South Africa Energy Situation|South Africa]]):'''</u><br/>
 In case of the wind farm at Laingsburg, there were no wind speed data available. For this reason, the corresponding assessment has been carried out for an assumed average wind speed of 7 m/s and 8 m/s. this assessment will provide a good idea about the order of magnitude of the monetary losses associated with a wind farm limitation and at the same time show the sensitivity of this value against average wind speed.<br/>Capping a single wind farm with 7m/s average wind speed at 80% results in a not delivered energy of approximately 5% of the totally available energy. This would be equivalent to around 19 000 MWh/year, which could not be delivered. Assuming a tariff of 1,25 Rand/kWh, the resulting economic losses to the wind farm operators would be in the order of magnitude of 23 750 000 R/year.<br/>If instead 8m/s average wind speed is assumed an considering also an 80% power output limit, the results indicate a not delivered energy of approximately 7,5% of the totally available energy. This would be equivalent to around 37 000 MWh/year, which could not be delivered, resulting in 46 250 000 R/year of losses.<br/>
@@ Line 60: / Line 58: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Limit the Wind Farm Output in the Case of an Actual Line Failure<br/> ==
@@ Line 68: / Line 66: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Dynamic Line Rating Systems<br/> ==
@@ Line 91: / Line 89: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 = Impact on Voltage Variations at the Connection Point and the Surrounding Subtransmission Network<br/> =
@@ Line 102: / Line 100: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Reactive Power Control Modes<br/> ==
@@ Line 114: / Line 112: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Impact Studies<br/> ==
@@ Line 138: / Line 136: @@
 [[File:PV curves for wind-Farm coshi.JPG|thumb|center|180px|Figure 4: PV curves considering a cosphi(P)Characteristic]] [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Impact on Short Circuit Currents<br/> ==
@@ Line 144: / Line 142: @@
 In the case of a grid impact study, the main purpose of short circuit studies is to verify that the short circuit rating of existing equipment such as bus bars, circuit breakers, lines or cables remains within the required limits.<br/>Additionally, short circuit studies are required for verifying the settings of line or transformer protection relays in the network.<br/>
-<u></u>
 '''<u>Special Modeling Considerations:</u>'''<br/>
@@ Line 162: / Line 160: @@
 <br/>This means that doubly-fed induction machine wind generators have a considerable contribution to peak short circuit currents.<br/>In the longer term however, when the controllers have taken over again, fault currents will be in a range of normal operating currents, depending on the reactive current control concept of the turbines. Because there are many Grid Codes around the world that require the injection of reactive currents during situations of low voltages and because voltage support usually helps the turbines to ride through low voltage conditions, many wind generators inject reactive currents during situations of low voltages and thereby contribute to AC-components of short circuit currents.<br/>
+<br/>
 '''<u>Equivalent Synchronous Machine Model:</u>'''
@@ Line 203: / Line 201: @@
 Harmonic current injection studies are typically carried out based on wind turbine measurement report according to IEC 61400-21, which lists the worst-case harmonic current injection at each harmonic frequency at which a current injection >0,1% of rated current can be observed.<br/>
-== Superposition<br/> ==
+Because it is very much unlikely, the all wind generators in a wind farm inject worst case harmonic currents with the same phase angle at the same time, assumptions for the diversity with which harmonic currents are injected into the grid by the different wind generators can be made.<br/>The generally applied international standard that defines rules for these assumptions is IEC 61000-3-6.
 <u>The basic consideration of these rules is that:</u><br/>
@@ Line 242: / Line 240: @@
 It is common practice to install a power quality meter in the grid connection point that monitors harmonic voltages at the PCC and harmonic currents injected by the wind farm during wind farm operation.
+<br/>
 = Impact on Flicker<br/> =
@@ Line 273: / Line 271: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 == Technical Requirements<br/> ==
@@ Line 290: / Line 288: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
+<br/>
 = Further Information =
@@ Line 301: / Line 299: @@
 = References =
 *GTZ 2009: Grid Integration of Wind Energy in the Western Cape. p. 7
 <references /><br/>
 [[Category:Wind]]

@@ Line 1: / Line 1: @@
-[[Portal:Wind|► Back to Wind Portal]]<br/>
+► Back to [[Portal:Wind|Wind Portal]] / [[Portal:Grid|Grid Portal]]
-[[Portal:Grid|► Back to Grid Portal]]
+= Overview <br/> =
 <u>For analyzing the grid impact of a wind farm connection at (exemplary)<ref>Content of this page from: GTZ 2009: Grid Integration of Wind Energy in the Western Cape. p. 7 http://www.gtz.de/de/dokumente/gtz2009-en-grid-study-western-cape.pdf</ref> 120kV, the following main aspects have to be studied:</u>
@@ Line 20: / Line 18: @@
 In a first step, it is required to verify that the existing network capacity is able to take the additionally generated power. For this purpose, credible contingencies, such as n-1 branch outages, have to be analyzed.<br/>An n-1 analysis based on load flow calculations of all surrounding lines leads to the conclusion that there is a thermal overload problem in the case that one circuit between (in the example that follows) Laingsburg and Boskloof is on outage and the wind farm production is above 120MW. For all other contingencies, no thermal problems can be identified.<br/>
-<br/>The worst case situation with a wind farm production of 150MW and the outage of one of the parallel circuits is depicted in Figure 1. As shown in this diagram, the circuit that will remain in service will carry a load of 127% of its thermal rating.<br/><u>The general options for mitigating such problems are:</u><br/>
+<br/>The worst case situation with a wind farm production of 150MW and the outage of one of the parallel circuits is depicted in Figure 1. As shown in this diagram, the circuit that will remain in service will carry a load of 127% of its thermal rating.
 *Reinforce the existing line or build a new line.
 *[[Grid Connection - Wind Energy#Probabilistic Assessment of the Non Delivered Energy in the Case of Wind Farm Limitation|Limit the wind farm]] during all times.
@@ Line 36: / Line 36: @@
 For this reason, only a probabilistic assessment considering the actual wind speed conditions at the site can provide an answer to the question, which mitigation option should finally be applied.<br/>
-<br/>[[File:Outage wind park example.JPG|frame|center|180px|Figure 1: Outage of one of the parallel 132kV lines between Laingsburg and Boskloof]] <ref>id. pp.8</ref><br/>
+<br/>[[File:Outage wind park example.JPG|thumb|center|180px|Figure 1: Outage of one of the parallel 132kV lines between Laingsburg and Boskloof]] <ref>id. pp.8</ref><br/>
 [[Grid Connection - Wind Energy#toc|►Go to Top]]<br/>
 == Probabilistic Assessment of the Non Delivered Energy in the Case of Wind Farm Limitation<br/> ==
@@ Line 187: / Line 189: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
 = Impact on Harmonics<br/> =
@@ Line 281: / Line 284: @@
 <references /><br/>
 [[Category:Wind]]

@@ Line 152: / Line 152: @@
 <br/>This means that doubly-fed induction machine wind generators have a considerable contribution to peak short circuit currents.<br/>In the longer term however, when the controllers have taken over again, fault currents will be in a range of normal operating currents, depending on the reactive current control concept of the turbines. Because there are many Grid Codes around the world that require the injection of reactive currents during situations of low voltages and because voltage support usually helps the turbines to ride through low voltage conditions, many wind generators inject reactive currents during situations of low voltages and thereby contribute to AC-components of short circuit currents.<br/>
-==== <br/>'''Equivalent Synchronous Machine Model'''<br/> ====
+==== Equivalent Synchronous Machine Model ====
 Because of the mentioned difficulties with the modeling of variable speed wind generators for steady state short circuit analysis, simplified modeling approaches are required for the inclusion of variable speed wind generators in IEC 60909 or ANSI C37-type short circuit calculations.<br/>
@@ Line 186: / Line 186: @@
 [[Grid Connection - Wind Energy#toc|►Go to Top]]
 = Impact on Harmonics<br/> =

@@ Line 246: / Line 246: @@
 = Further Information =
-*[https://energypedia.info/Portal:Wind Portal:Wind]<br/>
+*[[Portal:Wind|Portal:Wind]]<br/>
 <br/>
@@ Line 254: / Line 255: @@
 <references /><br/>
 [[Category:South_Africa]]

@@ Line 183: / Line 183: @@
 *High order harmonics are typically resulting from self commutated power electronics converters. Their phase angel is therefore highly uncorrelated.
+<br/>
 For considering these effects, IEC 61000-3-6 defines formulas allowing calculating the superposition of harmonic voltage distortions resulting from harmonic current injection of different harmonic sources.<br/>For wind generation, Summation Law 2 according to IEC 61000-3-6 is typically applied.<br/>Summation Law 2 is based on the following formula for each harmonic order h and each harmonic source i:<br/>[[File:Formula harmonic order h harmonic source i.JPG|frame|center|180px|Formula: harmonic order h harmonic source i]] <ref>id. p. 19</ref>
@@ Line 210: / Line 210: @@
 *Switching actions within in the wind farm.
+<br/>
 Flicker assessment for wind farms is typically carried out according to IEC 61000-3-7 and IEC 61400-21. The method requires the calculation of the minimum short circuit level and impedance angle at the PCC.<br/><u>Based on the impedance angle, the maximum values for:</u><br/>
@@ Line 219: / Line 219: @@
 *Max. number of switching operations N120<br/>
+<br/>
 for the relevant impedance angle at the grid connection point must be taken from a wind turbine measurement sheet according to IEC 61400-21.<br/><u>These values together with the minimum fault level at the grid connection point allow calculating:</u><br/>
@@ Line 227: / Line 227: @@
 for one wind generator.<br/>For assessing the Flicker of an entire wind farm, IEC 61400-21 states that it can be assumed that the flicker relevant components of wind turbulence and other flicker-relevant aspects are uncorrelated within the wind farm.<br/>For this reason, the following formula can be applied for calculating the flicker of an entire wind farm with n turbines based on the flicker of one individual WTG:<br/>[[File:Formula flicker.JPG|frame|center|153px|Formula: Flicker of individual WTG]]<ref>id. p. 22</ref><br/>With Psti being the short-term flicker of one individual WTG. This means in other words, that the total flicker of a wind farm with n WTGs is lower by a factor of 1/ n than the flicker that would be caused by an individual WTG with n times the size.<br/>
+<br/>
 == Technical Requirements<br/> ==
@@ Line 238: / Line 238: @@
 *Because of the overall amount of planned wind generation in the Cape, all wind generators should be equipped with '''fault ride-through (FRT)''' capability for avoiding WTG trips in case of line faults.
+<br/>
 For standardizing the technical characteristics of wind generators in a country, it is recommended to extend the national grid code by Connection Conditions for wind generation. These Connection Conditions should consider existing standards with respect to the connection of thermal and hydro power plants and make provision of the particular technical characteristics of wind generators.<br/>
+<br/>
 = Further Information =
@@ Line 248: / Line 248: @@
 *[https://energypedia.info/Portal:Wind Portal:Wind]<br/>
 = References =
 <references /><br/>
 [[Category:Wind]]

@@ Line 38: / Line 38: @@
 '''The example of Laingsburg (Western Cape, South Africa):'''<br>
-In case of the wind farm at Laingsburg, there were no wind speed data available. For this reason, the corresponding assessment has been carried out for an assumed average wind speed of 7 m/s and 8 m/s. this assessment will provide a good idea about the order of magnitude of the monetary losses associated with a wind farm limitation and at the same time show the sensitivity of this value against average wind speed.<br>Capping a single wind farm with 7m/s average wind speed at 80% results in a not delivered energy of approximately 5% of the totally available energy. This would be equivalent to around 19 000 MWh/year, which could not be delivered. Assuming a tariff of 1,25 Rand/kWh, the resulting economic losses to the wind farm operators would be in the order of magnitude of 23 750 000 R/year.<br>If instead 8m/s average wind speed is assumed an considering also an 80% power output limit, the results indicate a not delivered energy of approximately 7,5% of the totally available energy. This would be equivalent to around 37 000 MWh/year, which could not be delivered, resulting in 46 250 000 R/year of losses.<ref>id. pp.9</ref><br>
+In case of the wind farm at Laingsburg, there were no wind speed data available. For this reason, the corresponding assessment has been carried out for an assumed average wind speed of 7 m/s and 8 m/s. this assessment will provide a good idea about the order of magnitude of the monetary losses associated with a wind farm limitation and at the same time show the sensitivity of this value against average wind speed.<br>Capping a single wind farm with 7m/s average wind speed at 80% results in a not delivered energy of approximately 5% of the totally available energy. This would be equivalent to around 19 000 MWh/year, which could not be delivered. Assuming a tariff of 1,25 Rand/kWh, the resulting economic losses to the wind farm operators would be in the order of magnitude of 23 750 000 R/year.<br>If instead 8m/s average wind speed is assumed an considering also an 80% power output limit, the results indicate a not delivered energy of approximately 7,5% of the totally available energy. This would be equivalent to around 37 000 MWh/year, which could not be delivered, resulting in 46 250 000 R/year of losses.<br>
 [[Image:Wind speed example.JPG|frame|center|Figure 2: Wind speed distribution and power duration curve for an average wind speed of 7m/s]]
@@ Line 44: / Line 44: @@
 <ref>id. p. 11</ref><br>
 <br>
 === Limit the Wind Farm Output in the Case of an Actual Line Failure <br>  ===
-Alternatively, if network operation guidelines allow so, it would be much more economical to limit the wind farm output only under situations, in which one of the circuits between Laingsburg and Boskloof is actually out of service, e.g. because of a planned or an unplanned outage.<br>In cases, in which there is only a minor overload on the remaining circuit, like in the situation here, it would be possible to limit the wind farm output by a manual post-fault action executed by the network operator.<br>Alternatively, or in situations in which there is a major overload (above the emergency rating of the line) it is advisable to install an automatic intertrip-schemes that trips some of the wind turbines in the wind farm in the case that the circuit breakers of one of the two parallel circuits are opened.<ref>id.</ref><br>
+Alternatively, if network operation guidelines allow so, it would be much more economical to limit the wind farm output only under situations, in which one of the circuits between Laingsburg and Boskloof is actually out of service, e.g. because of a planned or an unplanned outage.<br>In cases, in which there is only a minor overload on the remaining circuit, like in the situation here, it would be possible to limit the wind farm output by a manual post-fault action executed by the network operator.<br>Alternatively, or in situations in which there is a major overload (above the emergency rating of the line) it is advisable to install an automatic intertrip-schemes that trips some of the wind turbines in the wind farm in the case that the circuit breakers of one of the two parallel circuits are opened.<br>
 === Dynamic Line Rating Systems <br>  ===
@@ Line 63: / Line 63: @@
 *Measurement of ambient conditions
-or combinations of some of the above listed measurements. Together with a thermal model of the line, it is possible to predict the actual rating of the line at any moment in time.<ref>id. pp. 11</ref><br>
+or combinations of some of the above listed measurements. Together with a thermal model of the line, it is possible to predict the actual rating of the line at any moment in time.<br>
 == Impact on Voltage Variations at the Connection Point and the Surrounding Subtransmission Network<br>  ==
@@ Line 71: / Line 71: @@
 *Identifying the required var-control mode (voltage/droop/const. power factor)
 *Identifying the required reactive power range
-*Identifying the need of additional reactive compensation devices in the system.<ref>id. p. 12</ref>
+*Identifying the need of additional reactive compensation devices in the system.
 === Reactive Power Control Modes<br>  ===
@@ Line 77: / Line 77: @@
 The least cost effective solution for a wind farm is the operation at constant power factor close to unity of each individual WTG, for which no park-controller is required. This is the classical approach for small wind farms (only a few turbines) connected to distribution grids (&lt;=33kV).<br>
-In case of wind farms connected to subtransmssion or transmission levels, the system operators usually asks the wind farm operator to maintain a constant power factor, a constant reactive power setting or even the voltage at the grid connection point.<br>Hardware costs of the WTGs depend on the required power factor range (or reactive power range) of each individual WTG. In case of wind farms connected to a transmission or subtransmission level, the power factor range required at the grid connection point is not equal to the power factor range of each WTG because the reactive power need of the step-up transformers and the main grid-transformer has to be covered by the WTGs too. For this reason, there is usually additional shunt capacitors connected near to the grid connection point for complying with reactive power requirements of the system operator.<br>When requiring the operation with unity power factory at the grid connection point, the power factor range of typical WTGs is usually sufficient for covering the reactive power demand of wind farm internal components.<br>When asking for a wider power factor range at the grid connection point, e.g. 0,95, WTGs with wider reactive power range and/or additional shunt capacitors are usually required.<br>Voltage control requires a feed-back loop of the voltage and continuously varying reactive adjustment of the WTGs. Depending on the dynamic requirements (speed of operation) and the required reactive power range of the WTGs, it will be possible to realize voltage control by WTGs and additional shunt capacitors or even additional dynamic reactive power compensation devices, such as SVC or STATCOM might be required, which can have a very high cost impact.<br>Droop control refers to a combined voltage-reactive power control defined by a v-Q characteristic, according to which reactive power is adjusted. Droop control is typically used for fast acting reactive power compensation devices, such as STATCOMs or SVCs.<ref>id. pp.12</ref><br>
+In case of wind farms connected to subtransmssion or transmission levels, the system operators usually asks the wind farm operator to maintain a constant power factor, a constant reactive power setting or even the voltage at the grid connection point.<br>Hardware costs of the WTGs depend on the required power factor range (or reactive power range) of each individual WTG. In case of wind farms connected to a transmission or subtransmission level, the power factor range required at the grid connection point is not equal to the power factor range of each WTG because the reactive power need of the step-up transformers and the main grid-transformer has to be covered by the WTGs too. For this reason, there is usually additional shunt capacitors connected near to the grid connection point for complying with reactive power requirements of the system operator.<br>When requiring the operation with unity power factory at the grid connection point, the power factor range of typical WTGs is usually sufficient for covering the reactive power demand of wind farm internal components.<br>When asking for a wider power factor range at the grid connection point, e.g. 0,95, WTGs with wider reactive power range and/or additional shunt capacitors are usually required.<br>Voltage control requires a feed-back loop of the voltage and continuously varying reactive adjustment of the WTGs. Depending on the dynamic requirements (speed of operation) and the required reactive power range of the WTGs, it will be possible to realize voltage control by WTGs and additional shunt capacitors or even additional dynamic reactive power compensation devices, such as SVC or STATCOM might be required, which can have a very high cost impact.<br>Droop control refers to a combined voltage-reactive power control defined by a v-Q characteristic, according to which reactive power is adjusted. Droop control is typically used for fast acting reactive power compensation devices, such as STATCOMs or SVCs.<br>
 <br>
@@ Line 92: / Line 92: @@
 <br>
-According to the results shown in Figure 4, the maximum voltage at the wind farm connection point would exceed the usual +5% limit and the voltage variation against active power variations would be in the range of approximately 5%. Hence operation with unity power factory would lead to unacceptable voltage variations.<br>It can further be observed from Figure 4 that voltage step changes because of line outages in the neighborhood of the wind farm are within very reasonable limits (around 1%) and don’t cause any problem.<br>The typical approach for mitigating voltage problems, which are due to varying active power levels, is to operate the wind farm with an active power dependent power factor (absorbing vars). This is a very cost effective solution because such a reactive power or power factor characteristic can easily be implemented inside the controller of each individual WTG and doesn’t require any voltage feed-back or global wind farm controller.<br>The resulting PV-curve is depicted in Figure 5. These curves are based on a wind farm with a minimum power factor of 0,95 (absorbing) at the wind farm connection point. The voltage now stays within the required voltage band and voltage variations against active power variations stay within acceptable limits of less than 2%. <ref>id. pp. 16</ref><br>
+According to the results shown in Figure 4, the maximum voltage at the wind farm connection point would exceed the usual +5% limit and the voltage variation against active power variations would be in the range of approximately 5%. Hence operation with unity power factory would lead to unacceptable voltage variations.<br>It can further be observed from Figure 4 that voltage step changes because of line outages in the neighborhood of the wind farm are within very reasonable limits (around 1%) and don’t cause any problem.<br>The typical approach for mitigating voltage problems, which are due to varying active power levels, is to operate the wind farm with an active power dependent power factor (absorbing vars). This is a very cost effective solution because such a reactive power or power factor characteristic can easily be implemented inside the controller of each individual WTG and doesn’t require any voltage feed-back or global wind farm controller.<br>The resulting PV-curve is depicted in Figure 5. These curves are based on a wind farm with a minimum power factor of 0,95 (absorbing) at the wind farm connection point. The voltage now stays within the required voltage band and voltage variations against active power variations stay within acceptable limits of less than 2%. <br>
 <br>
@@ Line 102: / Line 102: @@
 [[Image:PV curves for wind-Farm coshi.JPG|frame|center|Figure 4: PV curves considering a cosphi(P)Characteristic]] <ref>id. p. 18</ref><br>
 <br>
 == Impact on Short Circuit Currents<br>  ==
@@ Line 128: / Line 128: @@
 *The equivalent transient reactance typically defines the machine’s behavior during situations, in which the controllers have taken over again.
-Unfortunately, this approach is not able to provide very accurate results for the short circuit contribution of wind farms. It should only be used for planning studies, when the short circuit rating of existing substations, lines and transformers has to be verified.<br>In the case that accurate short circuit currents have to be calculated, e.g. for analyzing malfunctioning of protection relays, time domain simulations based on accurate dynamic WTG models are required.<ref>id. p.14</ref><br>
+Unfortunately, this approach is not able to provide very accurate results for the short circuit contribution of wind farms. It should only be used for planning studies, when the short circuit rating of existing substations, lines and transformers has to be verified.<br>In the case that accurate short circuit currents have to be calculated, e.g. for analyzing malfunctioning of protection relays, time domain simulations based on accurate dynamic WTG models are required.<br>
 <br> [[Image:Model aggregation.JPG|frame|center|Figure 6: Aggregated Wind Farm Model for System Impact Studies]] <br>
@@ Line 152: / Line 152: @@
 *Contribution to transient shc-current (Iks): 0,67 kA
-The short circuit contribution of the wind farm is not critical in this particular example because the fault level at the wind farm connection point is very low and the substation is operated far below its short circuit rating.<ref>id. pp. 18</ref><br>
+The short circuit contribution of the wind farm is not critical in this particular example because the fault level at the wind farm connection point is very low and the substation is operated far below its short circuit rating.<br>
 <br>
 == Impact on Harmonics<br>  ==
@@ Line 181: / Line 181: @@
 <br>'''In-the-Field Measurements'''<br>It is common practice to install a power quality meter in the grid connection point that monitors harmonic voltages at the PCC and harmonic currents injected by the wind farm during wind farm operation.<br>
 <br>
 == Impact on Flicker<br>  ==
@@ Line 205: / Line 205: @@
 *• Short-term and long term flicker value (Pst/Plt) due to switching actions
-for one wind generator.<br>For assessing the Flicker of an entire wind farm, IEC 61400-21 states that it can be assumed that the flicker relevant components of wind turbulence and other flicker-relevant aspects are uncorrelated within the wind farm. <br>For this reason, the following formula can be applied for calculating the flicker of an entire wind farm with n turbines based on the flicker of one individual WTG:<br>[[Image:Formula flicker.JPG|frame|center|Formula: Flicker of individual WTG]]<ref>id. p. 22</ref><br>With Psti being the short-term flicker of one individual WTG. This means in other words, that the total flicker of a wind farm with n WTGs is lower by a factor of 1/ n than the flicker that would be caused by an individual WTG with n times the size.<ref>id. pp.21</ref><br>
+for one wind generator.<br>For assessing the Flicker of an entire wind farm, IEC 61400-21 states that it can be assumed that the flicker relevant components of wind turbulence and other flicker-relevant aspects are uncorrelated within the wind farm. <br>For this reason, the following formula can be applied for calculating the flicker of an entire wind farm with n turbines based on the flicker of one individual WTG:<br>[[Image:Formula flicker.JPG|frame|center|Formula: Flicker of individual WTG]]<ref>id. p. 22</ref><br>With Psti being the short-term flicker of one individual WTG. This means in other words, that the total flicker of a wind farm with n WTGs is lower by a factor of 1/ n than the flicker that would be caused by an individual WTG with n times the size.<br>
 == Technical requirements<br>  ==
@@ Line 217: / Line 217: @@
 *Because of the overall amount of planned wind generation in the Cape, all wind generators should be equipped with fault ride-through (FRT) capability for avoiding WTG trips in case of line faults.
-For standardizing the technical characteristics of wind generators in a country, it is recommended to extend the national grid code by Connection Conditions for wind generation. These Connection Conditions should consider existing standards with respect to the connection of thermal and hydro power plants and make provision of the particular technical characteristics of wind generators.<ref>id. pp. 40</ref><br>
+For standardizing the technical characteristics of wind generators in a country, it is recommended to extend the national grid code by Connection Conditions for wind generation. These Connection Conditions should consider existing standards with respect to the connection of thermal and hydro power plants and make provision of the particular technical characteristics of wind generators.<br>
 == References  ==