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

Transcription

1 Analysis of requirements in selected Grid Codes

2 This page was intentionally left blank 1

3 Preface This report has been submitted to Ørsted DTU, Section of Electric power Engineering, Technical University of Denmark (DTU). It has been carried out within the areas of electrical power engineering and implementation of wind power. The Report is a preprojekt, which will be used as a guideline for a final Master thesis in spring We, David T. Johnsen and Willi Christiansen, have carried out this project in a cooperation with Siemens Wind Power, Energi E2 and Ørsted DTU, section of electric power engineering (DTU). The project was developed in January We are thankful to our supervisors Arne Hejde Nielsen from Ørsted-DTU, Kim Høj-Jensen and Jørgen Nygaard Nielsen from Siemens Wind Power. A special thank also to Troels Sørensen from Energi E2, who has followed the project with interest. Lyngby den Willi Christiansen, wil-li@arcor.de David T. Johnsen, david.t.johnsen@gmail.com 2

4 Abstract This 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 restringing power factor requirements are described. Due to the worst case requirement, a wind turbine should be able to run continuously at full production with a power 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 required fault 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 shown that even if generic Grid Code requirements can be defined, detailed and challenging network analyses have to be made for each wind power connection query, wherein information about the type of wind turbine and of the given network is taken into consideration. 3

5 Preface...2 Abstract Introduction Selection of Grid Codes Static regulations during continuous operation Power factor regulations The most restringing power factor regulation Power Curtailment Voltage range and control Remote voltage control Frequency Flicker Harmonics Dynamic regulation during fault sequences Fault ride trough requirements Exceptions from the fault ride through requirements Repeating fault sequences Examples of limiting worst case scenarios Scenario 1, fast voltage changes in the transmission system Scenario 2, high voltage and pf requirements Scenario 3, weakening of the network Scenario 4, active power recovering after a voltage dip Discussion Conclusion References...30 Appendix...31

6 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 conditions within 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 dynamic requirements. The static part of the Grid Code examination consists of the subjects regarding the continuous operation 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 wind turbines during fault sequences and disturbances in the Grid. Following themes will be included in the dynamic part: fault ride through capabilities and fault recovery capabilities. The most restringing conditions, seen from the perspective of the wind turbine, are outlined and described. The most restringing conditions are outlined in the formulation of generic connection requirements. 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 be sufficient to ensure the stability and the quality of the electrical systems. The scenarios will be discussed and suggestions as to how the connection queries of wind farms could be solved, are briefly 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 will therefore be a summation of the requirements in selected Grid Codes. It has to be pointed out that this Report does not consist of a complete generic Grid Code in itself. 5

7 2. Selection of Grid Codes The selection of Grid Codes is based upon a number of preset considerations. It is important to archive 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 and geographical limitations. Furthermore following conditions must be fulfilled for the choices of Grid Codes: 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 power market 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 detailed connection of non-synchronous generating unit section and because of the high wind potential in UK and Scotland. Finally the Grid Code from the Canadian TSO [1], AESO, is taken into consideration in order to archive a contribution from an oversee Grid Code. The Canadian Grid Code is not included in the frequency analysis due to the fact that the Canadian system operates at 60Hz. 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. 6

8 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 still constant 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. These requirements 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 at the connection point of the Wind Farm Power Station (WFPS) to the transmission grid. The operation of a single WTG is therefore not of interest in this static requirements chapter. Contrary is it in chapter 4. (dynamic requirements), where the operation and measurements of each WTG applies Power factor regulations The power factor regulation concerns the reactive power consumption of the Wind Farm Power Station (WFPS). A simple induction generator, with no additional capacitors attached, will during normal operation consume reactive power. This reactive power has to be produced somewhere in the grid. It is preferred that the WFPS is reactive power neutral, since the distribution of reactive power is relatively cost intensive. The requirements to the reactive power and to the power factor are 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 reactive power into the transmission system. Static, continues power factor Canada Denmark Germany Eon 0.90 lagging to 0.95 leading Q/Prated = 0 to Q/Prated = 0.1 at full production and through a straight line to 0.95 lagging to 0.95 leading for a rated active power capacity < 100MW For a rated active power capacity > Ireland Scotland UK 0.95 lagging to 0.95 leading at full production. 32.6MVAr per 100MW installed, active capacity from 100% 0.95 lagging for production between 100% - 20% leading for production 0.95 lagging to 0.95 leading

9 Q/Prated = -0.1 to Q/Prated = 0 at zero production MW the power factor is voltage dependent. *** production to 50% production lagging to 0.95 leading from 50% production to idle.* between 100% to 50%** 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 to the other Grid Codes. The idea of varying the power factor dependent on the active production is illustrated much clearer on Figure 3-1. It can be observed that the relation between reactive power and active power must remain constant within the area from 100% to 50% active power production. 8

10 Figure 3-1 The Irish power factor requirements The black triangle below the 10 % production line on Figure 3-1 indicates that the reactive power output during operations below 10% must be altered, if the voltage limit is reached. 9

11 **The Scottish reactive power requirements diverse from the requirements in the UK when the active power production gets below 20% of the rated active power. The Scottish case is illustrated in 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 shown in Figure 3-2. The reactive power limits for active power output below 20 % shall be adjustable within 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 the network 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 the reactive power requirements during different voltage situations. The reason for including the actual voltage at the bus is the fact that a high reactive power production induce higher voltages, which is undesired if the initial voltage is at a high level already. Thereby the power factor control becomes a part of the voltage regulations. The case for the remaining Grid Codes is that it must be possible to operate between lagging at full production, even if the voltage is at 1.1p.u. This could cause unnecessary stress to the components due to the increasing voltage. 10

12 Figure 3-3. The German power factor requirements for P rated above 100MW When changing the reactive power output, step changes corresponding to a reactive power of more than 2.5 % of the network connection capacity in the high voltage network and 5 % in the extra high voltage network are not permissible. No step changes smaller than 500 kvar will be required 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 condition at full output is the Canadian [1] 0.90 lagging to 0.95 leading. At lower production levels the variable power factor of the Irish power factor requirements will surpass the restrictive Canadian regulations. 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 Canadian intensifications. The main contribution of the Canadian Grid Code is the power factor of 0.90 lagging, which is applicable from 100% to 70% active production. 11

13 Figure 3-4, most restringing power factor settings Figure 3-5, most restringing reactive power settings 12

14 The actual reactive production can be seen in the Figure 3-5. The required limits are defined by the continuous black line. The Wind Farm Power Stations shall be capable of operating at any point within the power factor ranges. Furthermore it shall be possible to order reactive regulation requirements via remote control and locally. Depending on the operational situation, the system operator changes the desired MVAr or voltage reference. In general it must be possible, independent of the rated power, to run through the agreed design range for the power factor at rated active power output within a few minutes. The entire process must be possible as often as requested. The issue regarding the generic power factor requirements is that the requirements are based on at voltage level around 1.0p.u. In the transmission system a low voltage must be supported by an increased reactive power production and vice versa. The opposite reaction will only cause further voltage deviation. Only the German Grid Code [3] includes the actual voltage level as a part of the power factor requirements (see Figure 3-3) Power Curtailment The need for power curtailment occurs when it is not possible to compensate for the loss of wind power by up rating conventional plants. To avoid this situation the wind power is curtailed. This situation normally occurs during the daily load increase when the conventional plants have to compensate for both the increasing load demand and for the declining wind production. Another reason for curtailing wind is if the wind production increases beyond the load consumption. This will increase the inertia of the transmission system which causes a frequency increase. The curtailment amount is dependent on installed capacity, location, wind forecasting reliability and economics 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 value which corresponds to a percentage value based on the network connection capacity. The reduction of the power output to the signalled value must take place with at least 10% of the network connection capacity per minute without the system being disconnected from the network. 1 The main indication of a production surplus is a frequency increase. Therefore the power output must be reduced 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 reestablishment of the active power supplied to the network must not exceed a maximum gradient of 10% 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 13

15 Figure 3-6, power curtailment requirements The power curtailment concerns the right end of the curve in Figure 3-6 when the frequency increases above 50.5Hz. Furthermore it must be possible to constrain the active power gradient such that extreme increases in active power during a short period of time are avoided. Again the parameters and range of the active power gradients are of varying sizes and depends on the installed capacity, location etc. The settings must apply with the TSO requirements. A table containing the main issues regarding the active power curtailment is listed in Appendix 1. The table is originally listed in the Danish Grid Code [2] Voltage range and control The Wind Farm Power Station (WFPS) must be able to run at rated voltage plus a specified voltage range. The voltage range depends on the level of the voltage on the transmission system, which varies from country to country. Voltage range Denmark [2] Germany [3] Ireland [4] Limited time periods* -10% 5% -3% 13% -5% 10% -20% 10% -10% 20% -10% 18% 400 kv -8% 10% 150kV -13% 12% 132kV -13% 12% 400 kv 150kV 132kV X 400 kv -13% 5% 220 kv -9% 12% 110 kv -10% 12% X 400 kv 220 kv 110 kv 14

16 Voltage range Scotland [5] UK [6] Canada [1] Limited time periods* ±5% 400 kv -10% 5% 400 kv ±10% 275 kv ±10% 275kV ±10% 132 kv ±10% 132kV ±10% 400 kv ±10% 400 kv ±15% 275 kv ±10% 275kV ±20% 132 kv ±10% 132kV Table 3.2, permitted voltage ranges * Denmark (1 hour), Scotland (15 min), UK (15 min) The TSO will provide the voltage operating range It can be observed in Table 3.2 that the specified voltage range varies between the different countries. Additionally some countries allow higher voltage ranges over a limited time period. Following specified voltage ranges are the most restringing requirements: Specified voltage range Nominal Limited time period Voltages voltage range 15 min 1 hour 400 kv -13% +10% -13% -20% -13% -20% 275 kv ±10% -10% -15% & 10% 15% X 220 kv -13% +12% % X 150 kv -3% +13% -3% -10% & -3% -10% & 10% 20% 10% 20% 132 kv ±10% -10% -20% & 10% 20% & 10% 18% 110 kv -13% +12% X X Table 3.3, specified worst case voltage range Table 3.3 shows the continuous operating voltage range in regard to the nominal network voltage. Normal operation of the WFPS should be possible within the specified range and time limit. It has to be pointed out that exceptions are mentioned in most of the Grid Codes. Greater or lesser variations in voltage to those set out above can bee agreed in relation to a particular connection site. The following Figure 3-7 is a graphic display of the voltage range in the generic Grid Code. It can be seen how the general requirement for all nominal voltages is continues operation at voltages around ±10% of the nominal voltage. 15

17 Voltage Range V/Vnom in per cent Hour 15 min. Continious Time Nominal Voltage 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 Remote voltage control Wind Farm Power Stations (WFPS) shall have a continuously-variable, continuously acting, closed loop 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 for the voltage at the connection point. The voltage regulation system shall act to regulate the voltage at this 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% to 105% of rated voltage. The response speed of the voltage regulation system, following a step change in voltage at the connection point, shall be such that the wind farm power station achieves 95 % of its steady-state reactive power response within 1 second. It is only in the Irish [4] and the Canadian [1] Grid Code that these remote voltage control options are specified in detail. 16

18 3.5. Frequency The Following Table 3.4 shows the frequency requirements of the European Grid Codes: Frequency range requirement Frequency Minimum Time Delay (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 min 51.0 Hz to 51.5 Hz 30 min 50.5 Hz to 51.0 Hz 30 min 49.5 Hz to 50.5 Hz 49.5 Hz to 47.5 Hz Operation 30 min Operation Operation Operation Operation 60 min 60 min Operation 60 min Operation Operation Operation Operation Operation Operation Operation Operation Operation Operation 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 generator s plant and apparatus must enable operation in accordance with the frequency range in Table 3.4. Due to the frequency requirements in the table, wind farms are required to be capable of operating continuously between 47.5Hz and 52Hz and time limited between 47 and 47.5Hz likewise as between 52 and 53. This is a relative wide range in relation to realistic 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 Operation 51.0 Hz to 51.5 Hz Operation 50.5 Hz to 51.0 Hz Operation 49.5 Hz to 50.5 Hz Operation 49.5 Hz to 47.5 Hz 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 frequency requirements are not included in the analysis since there is no basis for comparison. 17

19 In addition the active power output may be reduced in the outside values within the frequency ranges (see chapter 3.2. ). The WFPS should remain connected to the transmission system during rate of change of transmission system frequency values of up to 0.5 Hz per second. Automatic isolation from the network due to the frequency is only permissible at frequencies below 47Hz and above 53Hz. Upon reaching 47Hz or 53Hz the unit must be automatically isolated from the network without delay. No additional WTG shall be started while the transmission system frequency is above 50.2Hz Flicker Flicker is defined as a single, rapid change of the RMS voltage. Transmission system step changes can 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 the transmission 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 indicates the 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 short term flicker (Pst), determined as a weighted average of the flicker contribution over ten minutes is 18

20 below The long term flicker value (Plt), determined as a weighted average of the flicker contribution over two hours shall be limited so that Plt is below The flicker contributions Pst and Plt are defined in IEC (Electromagnetic compatibility). It is primarily the Danish Grid Code [2], which contains restrictive rules about flicker. The Irish and the German Grid Codes do not mention flicker Harmonics Wind Farm Power Stations connected to the transmission system shall be capable of withstanding the levels of harmonic distortion liable to be present on the transmission system. These electromagnetic compatibility levels are not directly specified in any of the Grid Codes but the Danish. 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 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. The requirements of the mentioned standards are 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 nominal voltage. Furthermore a limit of the THD is presented. 19

21 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 for disconnection of the wind farm at abnormal voltages and frequencies. The relays shall be set according to agreements with the regional grid company and the system operator. The protective functions of the wind turbine shall include settings and time delays meeting the requirements described in this section. All countries have a fault ride through capability figure. These requirements do only concern short circuits 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 the Scottish [5], the Irish [4] and the German [3] fault ride through figures are presented: Figure 4-1, the Scottish fault ride through requirement 20

22 Figure 4-2, The Irish fault ride through requirement Figure 4-3, The German (EON) fault ride through requirement for a near by generator fault Figure 4-1 to Figure 4-3 show the fault ride through requirement from the Scottish, the Irish and the German Grid Code. Altogether the three figures represent the most restringing requirements. Following Figure 4-4 demonstrates the worst case Grid Code requirements. The fault ride through requirement is defined as following: A Wind Turbine Generator (WTG) shall remain connected to the transmission system for transmission system voltage dips on any or all phases, where the transmission system voltage, measured at the HV terminals of the grid connected transformer, remains above the heavy black line in Figure 4-4: U/U n 90% 80% 70% 45% Fault ride through capability of wind farm power stations 15% 0% (ms) 1500ms 2683ms 3 minutes Figure 4-4, Fault ride through capability of wind farm power stations Figure 4-4 shows that each generating unit shall remain transiently stable and connected to the system without tripping for a close-up solid three-phase short circuit fault or any unbalanced short circuit fault on the transmission system with a total fault clearance time of up to 150ms. Throughout 21

23 the operating range of the generating unit, these types of faults must not result in instability or isolation from the network. Furthermore, Figure 4-4 shows that wind turbine generator units shall be capable of continuous operation 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, the WTG shall have the technical capability to provide the following functions: The wind WTG shall provide active power in proportion to retained voltage and maximize the reactive current to the transmission system without exceeding WTG limits during the voltage dip in the transmission system. The maximization of reactive current is described in detail in the below Figure 4-5; The wind farm power station shall provide its maximum available active power as quickly as the technology allows with a gradient of at least 20% of the rated power per second. Within the grey area 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 system voltage 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 more than 10% of the root mean square (RMS) of the generator terminal voltage occurs, the generation unit 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 22

24 After the fault identification, the network voltage support must be provided within 20ms by providing reactive power at the generator terminals with a factor of 2% of the rated current per percent 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 the transmission system; whichever is the sooner. The transient phenomenon, with regard to the reactive power consumption after the voltage returns to the normal operation range, must be completed after 400ms. After this time the reactive power exchange must take place as it is specified on the basis of the normal operational schedule Exceptions from the fault ride through requirements Wind Turbine Generator units are not required to ride through transmission system faults that cause a forced outage of a radial line to the wind farm (isolation of the WFPS). Nor shall wind turbine generator 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 an emergency shutdown sequence to protect the plant and apparatus Repeating fault sequences A wind turbine generation unit shall have sufficient capacity to meet the above mentioned requirements. Besides it shall be able to withstand the impacts from faults in the grid where unsuccessful 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 and pneumatics) 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 23

25 5. Examples of limiting worst case scenarios The aim of this chapter is to analyze the consequences of the requirements in the Grid Codes during different 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 Scenario 1, fast voltage changes in the transmission system A Wind Farm Power Station (WFPS) is connected to the transmission system through an automatic tap changing transformer. During steady state load conditions, the transformer keeps the voltage on the 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 relatively high (1.1 pu) due to switching within the transmission system. The automatic tap-changing transformer cannot tap fast enough to compensate for the sudden increase of the voltage. Therefore the post-fault tap position is equally to the pre-fault tap position. This means that the post-fault voltage on the low voltage side of the transformer is higher (in pu) than the voltage in the transmission system. If the voltage gets above 1.2pu, the wind turbines within the Wind Distribution 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 the Grid 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 Grid Code should be handled 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 high voltage 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. The combination of a high voltage situation and a pf of 0.9 lagging (the wind turbine produces reactive power) 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 full lagging. This example shows again that it might not be sufficient to fulfil the individual Grid Code requirements independently. The Grid Code requirements should be seen as a combination with realistic network conditions. This consideration is what can be observed on Figure 3-3. A pf of 0.95 lagging has only to be obtained at low voltages and visa versa. 24

26 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 rides through the fault and following the fault clearing, the wind generation increases back to or near the original pre-disturbance megawatt value. However, with the weakened link to that part of the system and the significant level of power interchange between that area and the rest of the transmission network, there may not be enough dynamic reactive power reserve in the vicinity of the wind farms to maintain voltage stability. As an example, such dynamic requirements might be provided by the application of a static VAr compensator (SVC), to regulate transmission system voltage immediately after a severe disturbance, and thereby ensuring a fast and stable voltage recovery Scenario 4, active power recovering after a voltage dip In most Grid Codes it is required that the active power returns after a fault occurrences within one second, as soon as the voltage is above 0.90pu. During a fault ride through event the active power of a wind turbine decreases. This is a consequence of the low voltage and of the decreasing mechanical power input. The voltage return after a fault sequence is dependent on the level of the short circuit power of the transmission 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 expected especially 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 as soon 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 the voltage is kept beneath 0.9pu at the terminals of the wind turbine, the active power recovery can be spread out over a longer period of time. The oscillation, overshot and instability of the recovering voltage can thereby be kept at a minimum. A consequence of this is a temporary loss of active power. With this strategy, the pre-fault conditions can be achieved within 10 seconds. This could be a helpful strategy to satisfy the requirements in the Grid Code in weak networks, but it requires a number a simulations. 25

27 6. Discussion The investigations of the requirements in the Grid Codes from selected countries have shown several technical aspects, which have to be taken into consideration in wind turbine connection queries. The technical aspects of the requirements vary in the different Grid Codes. Some aspects reflect regional differences between the transmission grids. As an example, the frequency range can be mentioned. The worst case Grid Code requires a continuously operation between 47.5 Hz to 52 Hz. This is a wide range, and it can hardly be imagined that any country in Europe operates continuously within this range. The explanation for the lower limit of the range is that Ireland has a relatively weak network, in which the frequency can drop to 47.5Hz. The transmission systems of the "Union for the Coordination of Transmission of Electricity" (UCTE), which is the association of transmission system operators in continental Europe, have a frequency which is relatively fixed to 50Hz. It does certainly not drop down to 47.5Hz under normal operation. Therefore the frequency range of 47.5Hz to 52Hz reflects the technical differences between the transmission grids of Ireland and of continental Europe. Furthermore it was shown that the worst case requirement to the voltage range is above ±10%. During limited time operation the voltage range is as wide as ±20% in some cases. This requirement does not seem justifiable seen from the perspective of other electrical components within the transmission system, which do not withstand such a wide voltage range. It should therefore be called into question, why wind turbines should withstand voltage ranges that are far above the ranges of other technical equipments within the transmission system. The Grid Codes guidance for installation recommends the installation of an automatic tap-changing transformer at the connection point of the Wind Farm Power Station (WFPS) to adjust for the deviation in voltage. However the problem will only be solved in steady state. A tap-changing transformer can not adjust for immediate voltage deviation, which may occur during disturbances (see scenario 1 in chapter 5. ). In the previous chapter a load scenario was presented wherein the issues regarding momentarily active power recovery after a fault clearance were described. It was suggested to raise the post-fault active power production slowly. This could prevent voltage oscillations and overshoots. The requirements of the German Grid Code might be the solution to this problem. The German Grid Code describes in detail an active power recovery procedure, which should initiate no later than 1 second after the voltage has returned to the pre-fault value. The procedure allows the Wind Turbine Generator to delay the active power recovery. This regulation is a notable advance compared to the requirements in other Grid Codes, who demand the active power to return immediately after the voltage has reached 90% of its rated value. 26

28 This is an example of how some of the complicated requirements in the Grid Code can be fulfilled concurrently 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 included in the connection queries of wind farms. If the Grid Code has to be satisfied in weak networks, a secure 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 Grid Code requirements. It is not sufficient to obey the regulations of the Grid Code individually; the Grid 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 technical limits of the specific transmission system should be taken into account. The solutions will show how a specific generating unit can comply with the Grid Code requirements. 27

29 7. Conclusion This Report contains studies of wind power connection requirements, which are described in Grid Codes 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 high penetration of wind power. Ireland is an island-system, and it is therefore of great interest. The Grid Code of EON (a German transmission system Operator (TSO)) is chosen due to the important wind power market in Germany and due to the detailed Grid Code of EON. Furthermore the Scottish and the Grid Code from UK are analyzed. Finally the Grid Code from the Canadian TSO, AESO, is taken into consideration. The wind turbine connection requirements from the mentioned countries have been compared. The most restringing regulations have been outlined with the purpose of demonstrating which technical challenges generic wind turbine models should be able to handle during continuous operation and during 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 during fault sequences and disturbances in the grid. Many of the technical aspects are only mentioned in some Grid Codes. The explanation for this is that each Grid Code reflects the technical conditions of the respective transmission system. Every country has its own special technical issues to deal with, and this speciality is reflected in the Grid Code. The static part has shown two important aspects. Firstly the significant requirement of the pf range is conspicuously. The most restringing conditions is that a wind farm at its connection point to the transmission 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 has to be pointed out that the pf range is only between 0.95 leading to lagging, if just the European requirements are taking into consideration. The second significant requirement is the huge frequency range, wherein the wind farm has to be able to operate continuously. A wind farm power station is required to run continuously between 47.5Hz and 52Hz. The lower limit reflects the Irish transmission system, whereas the upper limit of 52Hz reflects the German system. Beside this frequency range, continuous operation in a voltage range 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 is allowed 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 active power production. In case of a longer duration of a fault sequence, the operating limit is described with 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 production should act during and after a fault sequence. Especially the active power recovery following a fault is important, since an immediate active power recovery could cause voltage fluctuations in a weak 28

30 network. Especially the German Grid Code has included detailed requirements on the active power recovery. Finally some worst case scenarios are presented. They demonstrate how complicated it is to avoid violating some of the Grid Code requirement under different load conditions. All Grid Code requirements should be satisfied independently of each other. Simultaneously most technical requirements have a mutual influence on each other. Summarized it can be concluded that it is not sufficient to obey the requirements of the Grid Code individually. The Grid Code should rather be seen in its entirety and in relation to the given network, in which the wind farm is going to be connected. It requires challenging planning and connection studies if the general Grid Code requirements, which have been derived by comparing the Grid Codes from different countries, have to be satisfied. 29

31 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 the properties 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 Consulting ABB Inc [8] Large Scale Integration of Wind Energy in the European Power Supply, analysis, issues and recommendations, A report by EWEA, January

32 Appendix Appendix 1. Power curtailment scheme Type of regulation Absolute production constraint Purpose Limit the wind farm's current power production in the connection point to a maximum, specifically indicated MW value. Constraints may be necessary to avoid overloading of the power grid. Primary regulation aim Limit production to optional MWmax Delta production constraint Balance regulation Stop regulation Power gradient constrainer System protection It must be possible to reduce the power production of the wind farm by a desired power value compared to what is possible at present, thereby setting aside regulating reserves for the handling of critical power requirements. The power production of the wind farm must be adjusted to the current power requirement with a view to maintaining the power balance of the balance responsible market player and/or the system operator. downward/upward regulation of production must be possible. The wind farm must maintain the power production at the current level (if the wind makes it possible). The function results in stop for upward regulation and production constraints if the wind increases. For system operational reasons it may be necessary for wind turbines to limit the maximum speed at which the power output changes in relation to changes in wind speed. The power gradient constrainer is to ensure this. System protection is a protective function which must be able to automatically downward regulate the power production of the wind farm to a level which is acceptable to the power system. In the case of unforeseen incidents in the power system (for instance forced outage of lines), the power grid may be overloaded at the risk of power system collapse. The system protection regulation must be able to rapidly contribute to avoiding system collapse. Limit production by MWdelta Change current production by -MW/+MW with the set gradient and maintain the production at this level Maintain current production Power gradients do not exceed the maximum settings Downward regulate power production automatically on the basis of an external system protection signal 31

33 Appendix 2. Nomenclature Grid Code Lagging Leading OHL P pf pu Q Description of the connection conditions The phase of the current is behind the phase of the voltage The phase of the current is ahead the phase of the voltage Over Head Line Active power [MW] Power factor Per Unit Reactive Power [MVAR] RMS SVC TSO UCTE WDS WFPS WTG THD Root mean square Static Var Compensator Transmission System Operator Union for the Co-ordination of Transmission of Electricity Wind Distribution System Wind Farm Power Station Wind Turbine Generator The total harmonic effective distortion 32