Review of European Grid Codes for Wind Farms and Their Implications for Wind Power Curtailments

17th Int'l Wind Integration Workshop | Stockholm, Sweden | 17 – 19 October 2018 Review of European Grid Codes for Wind Farms and Their Implications for Wind Power Curtailments

Elis Nycander and Lennart Söder Electric Power and Energy Systems, KTH Royal Institute of Technology, Stockholm, Sweden [email protected]

Abstract—In order to accommodate the increasing wind Chinese grid codes, and [9] compares US and Chinese grid power penetration in power systems grid codes for wind power codes. An important aspect is the need for harmonization of plants are being continuously updated by TSOs. In this paper grid codes in different countries [5] [6]. we review several European grid codes for wind power released from 2015-2018. Specifically, we focus on the comparison of Table 1: Reviewed grid codes the new ENTSO-E grid code Requirements for Generators, released in 2016, with national grid codes to see to what Country TSO Year EU area Ref extent these are in agreement and how this contributes towards Denmark (DK) Energinet 2016 Nordic region [10] harmonization of grid codes within Europe. Also, we discuss Ireland (IE) Eirgrid 2015 Ireland and [11] the implications of the grid codes for performing curtailments Northern Ireland of wind power plants. UK National Grid 2018 UK [12] Germany (DE) TenneT 2017 Continental Europe [13]* Index Terms—Wind power, grid codes, RfG EU ENTSO-E 2016 - [14] * Preliminary version, for offshore wind I.INTRODUCTION In order to ensure a reliable power system TSOs or other In this paper recent grid codes of several European coun- regulating authorities specify grid codes with requirements tries are compared, as detailed in Table 1. Also the recent that must be fulfilled by generators that want to connect to ENTSO-E grid code Requirements for Generators (RfG) is the power system. Grid codes are important for operators included. This came into force on May 17 in 2016 and is an and developers of power plants since they impose minimum attempt by the EU to increase harmonization of European technical requirements that power plants must satisfy. grid codes. Individual member states have three years to The grid codes for generating units have traditionally been comply with RfG. Thus a comparison of RfG with national specified with synchronous generation in mind. As the same European grid codes can shed light on recent developments requirements could not be met by wind power generators of grid codes and harmonization efforts within Europe. Al- these were not required to comply with standard grid codes though RfG has been dicussed elsewhere, for example in [15] for generators. Instead the rules for wind farms were aimed in relation to fault ride through requirements, to the authors’ at ensuring that these did not degrade system reliability [1]. best knowledge no concise and comprehensive comparison As the amount of installed wind power capacity increased of RfG with the national grid codes it is replacing has been it was recognized that grid codes must be adopted for made. wind farms to ensure that the increasing share of wind The technical specifications in grid codes can be divided power does not endanger the reliability of the power system. into several categories as listed below. The remainder of As an example, fixed speed wind turbines (Type 1) have this paper compares the grid codes in Table 1 regarding limited low-voltage ride through capabilities and were often requirements corresponding to the different categories. The designed to automatically disconnect from the grid during sections are voltage dips [2] [3]. However, if many wind farms disconnect II Tolerance for voltage and frequency deviations under during a disturbance this can cause the voltage to drop normal operation further, leading to a cascading failure and voltage collapse. III Active power control and frequency regulation Thus current grid codes for wind farms include require- IV Reactive power control and voltage regulation ments for low-voltage ride through capabilities. Also, im- V Behaviour under grid disturbance provements in wind turbine technology allows wind farms It should be noted that the reviewed grid codes specify to contribute more actively towards power system stability. different requirements depending on the size and connection Modern wind turbines using full scale converters (Type 4) voltage of the wind farm. Larger wind farms connected and also doubly fed induction generators (Type 3) have at higher voltages have stricter requirements. In this pa- the possibility to control production of reactive power, thus per the focus is on the requirements for the largest wind helping to maintain voltage stability [4] [2]. farms connected at high voltages. Also, RfG has different Many articles comparing the grid codes for wind farms in specifications for different synchronous areas. These areas different countries exist. Both [5] and [6] compare several are the Nordic region, Ireland and Northern Ireland, Great European and North American grid codes, as well as a Britain, Continental Europe, as well as the Baltic region. The Chinese grid code in the former case. [7] compares Aus- national grid codes may then be compared with RfG for the tralian and international grid codes, [8] compare Danish and corresponding synchronous area.

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DK DK 30 s 30 s 30 min 30 min 110-300kV 60 min RfG NR max 60 min 300-400kV (set by TSO) RfG NR 30 min 30 min IE > 30 min (set by TSO) 400 kV 220 kV 110 kV 110 kV all voltages

IE 110-300kV 20 s RfG IE 60 min 60 min 300-400kV

RfG IE UK 275 kV 132 kV 275 kV 132 kV 90 min 90 min all voltages > 90 min (set by TSO) 110-300kV RfG UK UK 300-400kV 15 min 20 s 90 min 90 min 15 min

DE 60 min 20 min (only AC connected) RfG UK 20-60 min 20 s 60 min 110-300kV 90 min 90 min 15 min (set by TSO) RfG CE > 90 min (set by TSO) 20-60 min 60 min 300-400kV (set by TSO) 1 0.9 1.1 1.15 DE 0.85 0.95 1.05 1.118

90 min 90 min Voltage (pu) 20 s (only DC-connected) 15 min (only DC-connected)

RfG CE Figure 2: Voltage operation bands. The dark areas indicate 30 min

> 30 min (set by TSO) > previous (set by TSO) continuous operation. For RfG the ranges for 110-300 kV 47 47.5 48 48.5 49 49.5 50 50.5 51 51.5 52 and 300-400 kV are shown separately. Frequency (Hz)

Table 2: Minimum ROCOF Figure 1: Frequency operation bands. The dark areas indicate TSO ROCOF (Hz/s) continuous operation. The dashed lines show the normal Energinet 2.5 operating range for the frequency. Eirgrid 0.5 National Grid 2.5*, 1** TenneT 2.5 ENTSO-E set by TSO*** II.VOLTAGE AND FREQUENCY TOLERANCE * For DC connected wind farms **For all power generating modules Figure 1 shows the frequency tolerance bands specified ***2 Hz/s for DC-connected wind farms according to in the different grid codes. All codes except the Irish one ENTSO-E HVDC Network Code [18] require continuous operation in the range 49-51 Hz. From 47.5-49 Hz and 51-51.5 Hz operation is required for a prolonged time period. This period is longer for the isolated Hz/s, which will require generators to change their protection systems (Ireland and UK) compared to the Nordic and system settings [17]. Continental systems. Outside of this range disconnection from the system may be allowed in less than a minute. For example, the UK grid code states that outside of the range III.ACTIVE POWERAND FREQUENCY CONTROL 47.5-51.5 Hz disconnection by frequency-based relays may be allowed without specific agreements with the TSO. Figure 2 shows the voltage operating range specified in Active power regulation can generally be divided into the different grid codes. Generally operation between 0.9 automatic frequency response, where the active power is and 1.1 pu is required, with some differences depending on automatically adjusted by frequency-based governors, and the voltage level. manual control where the active power is changed according Additionally to the specified frequency ranges there are to issued set point values. minimum values specified for the rate of change of frequency RfG defines three different frequency response types, (ROCOF) that wind farms should be able tolerate without shown in Table 3. Limited Frequency Sensitive Mode for disconnecting from the grid, as shown in Table 2. With Over-frequencies (LFSM-O) is required by all wind farms the exception of the Irish grid codes the minimum ROCOF and requires them to reduce the active power for over- tolerance is 2.5 Hz/s. In Ireland the minimum ROCOF frequencies with a constant droop setting. Additionally large tolerance is 0.5 Hz/s (Grid Code Version 6.0 of 2015 [11]). wind farms should have the ability to operate in Limited To keep the ROCOF below 0.5 Hz/s Eirgrid has a limit Frequency Sensitivity Mode for Under-frequencies (LFSM- for the instantaneous renewable penetration at 65% as of U) or Frequency Sensitivity Mode (FSM) which both involve 2018 [16]. In order to further increase this limit towards an active power increase for under-frequencies, and thus 75% Eigrid intends to raise the ROCOF requirement to 1 require wind farms to be operated below available capacity.

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other generators) to be able to change their active power production according to set points issued by the TSO. Among the reviewed grid codes, those of National Grid and TenneT have already adopted the RfG specifications. The settings specified for the different modes are also shown in Table 3. Figure 3 shows the FSM from National Grid. With the settings specified by National Grid the frequency response of wind farms (and other generators) with limited frequency sensitivity mode will be activated only once the primary frequency response of generators in FSM is exhausted and the frequency is outside of the normal operating range. However, wind farms operated in FSM with the specified zero deadband will act inside the normal frequency range and thus provide primary frequency reserves. In contrast, with the FSM deadband specified by TenneT the Figure 3: Frequency Sensitivity Mode in UK wind farms with FSM will react only outside of the normal frequency range, which for the European synchronous area is 49.8-50.2 Hz. Thus offshore wind farms, to which [13] Table 3: Frequency sensitivity modes specified in ENTSO-E applies, are not expected to participate in the normal primary grid code frequency regulation according to TenneT. TSO/Mode Description ENTSO-E LFSM-O Limited Frequency Sensitivite Mode for Over-frequencies. Down-regulation for high frequencies. Threshold value 50.2 Hz - 50.5 Hz. Droop 2% - 12%. Response within 30 seconds. LFSM-U Limited Frequency Sensitivitey Mode for Under-frequencies. Up-regulation for low frequencies. Threshold value 49.5 Hz - 49.8 Hz. Droop 2%-12%. Response within 30 seconds. FSM Frequency Sensitivity Mode. Provide both up and down regulation with deadband of 0 Hz - 0.5 Hz and droop 2% - 12%. Response within 30 seconds. Reserves 1.5% - 10% of available capacity. National Grid LFSM-O 50.4 Hz threshold, 10% droop LFSM-U 49.5 Hz threshold, 10% droop FSM 0 Hz deadband, droop 3-5%, 10% of available capacity TenneT LFSM-O 50.2 Hz threshold, 5% droop LFSM-U 49.8 Hz threshold, 2% droop FSM 0.2 Hz deadband, 6% droop, 2% of available capacity Figure 4: Frequency response from Energinet with multiple droop values Table 4: Active power control modes specified by Energinet and Eirgrid

TSO Description Mode Energinet Absolute Active power set point. Set point issued continuously and changed power limits within 10s of received signal. System pro- Quick adjustment of production to predefined values (0%, 25%, 40%, tection 50%, 70% of nominal capacity). Adjustment completed within 10 seconds of signal. Individual wind generators may be stopped. Delta power Rolling reserves. Production at constant fraction of available capacity. limits Response to set point within 10 seconds. Frequency Down-regulation for high frequencies. Droop 2%-12%. Response to response frequency deviation within 15 seconds. Frequency Up and down regulation with deadband and droop specified by TSO. regulation Possibility for several droop value intervals for high frequencies. Response within 15 seconds, new parameters set in 10 seconds. Eirgrid Wind follow- Production following available capacity. ing Active power Production at set nominal value. Set point may be issued continuously set point and response should begin within 10s of received signal. Curve 1 Frequency response for over-frequencies with deadband and 2%-10% droop (4% default). 60% of frequency response within 5s and 100% within 15s. 120 days notice to change settings. Deadband ±0.2Hz or ±0.15Hz. Curve 2 Frequency response for over-frequencies and under-frequencies with Figure 5: Active power frequency response curves from deadband and 2%-10% droop (4% default). Reserve 5% of available Eirgrid capacity. 60% of frequency response within 5s and 100% within 15s. 120 days notice to change settings. Deadband ±0.15Hz. At the time of writing this paper Energinet and Eirgrid Apart from the requirements for frequency sensitivity, had not yet adapted the RfG specifications for frequency RfG also has a general requirement for wind farms (and sensitivity. The control modes for active power control

3 17th Int'l Wind Integration Workshop | Stockholm, Sweden | 17 – 19 October 2018 specified by these operators are summarized in Table 4, and Table 5: Reactive power limits specified in RfG and national example curves for the frequency response are shown in grid codes Figure 4 and 5. Power factor RfG Voltage RfG voltage TSO Q/Pmax lead lag Q/Pmax range (pu) range (pu) Both Energinet and Eirgrid require wind farms to have the Energinet 0.95 0.95 0.66 0.95 0.16 0.15 Eirgrid 0.95 0.95 0.66 0.66 0.218 0.218 possibility to have frequency response with different droop National 0.95 0.95 0.66 0.66 0.1 0.225 values for different frequency ranges. This is in contrast to Grid TenneT 0.925* 0.925* 0.82 0.75 0.2 0.225 RfG which only requires a single droop value in frequency * Outer limit in Figure 6 sensitivity mode. They may also require wind farms to operate with reserves to enable frequency response for under- frequencies, which applies to wind farms operating at curve 2 for Eirgrid (5% reserves) and to wind farms in delta power mode for Energinet. A notable difference is that 1 Energinet requires the settings for the frequency response to Energinet Eirgrid be changed within 10 seconds, while Eirgrid gives a notice 0.8 National grid TenneT time of 120 days as some operators may have to change RfG envelope the settings on site. The time frame to change frequency 0.6 sensitivity settings is not specified by RfG. max P/P

In addition to operating with reserves to allow wind 0.4 farms to participate in frequency regulation, the output may also be capped by absolute power limits in cases when 0.2 grid congestion or over- voltages put limits on production. Energinet also requires wind farms to be able to switch to 0 system protection mode, during which active power must -0.6 -0.4 -0.2 0 0.2 0.4 0.6 Q/P be restricted to any value from a predefined list within 10 max seconds. Figure 6: Reactive power profiles at nominal voltage. The A more recent development in terms of active power shaded area of National Grid may be included subject to control is for wind farms to have synthetic inertia, which agreement. For TenneT, the inner profile is the minimum is an active power control fast enough to emulate the effect basic requirement which may be extended by the TSO. of inertia in conventional generators. Such synthetic inertia may for example be used to limit the ROCOF during a large imbalance [19]. Although RfG gives the TSOs the right to Energinet 1.2 specify requirements for synthetic inertia such specifications Eirgrid are not included in any of the national grid codes. National Grid 1.15 TenneT RfG envelope

1.1

1.05 IV. REACTIVE POWERAND VOLTAGE CONTROL

1 U (pu) Reactive power requirements are usually speciﬁed using 0.95

PQ- and UQ- profiles. The PQ profile specifies the reactive 0.9 power requirement as a function of active power and the UQ profile specifies the reactive power requirement as a 0.85 function of the connection voltage. For low and high values 0.8 of active power and connection voltage the reactive power -0.6 -0.4 -0.2 0 0.2 0.4 0.6 Q/P requirements may be reduced. max Figure 6 shows the PQ profiles in the different grid Figure 7: Reactive power profiles at rated capacity codes and Table 5 lists the maximum Q/Pmax range and corresponding power factors. Note that Pmax refers to the RfG only specifies an outer envelope, shown in Figure rated capacity of the wind farm. The basic requirement in 6 within which the profile of the individual TSO must be all national grid codes except TenneT is operation between contained. RfG also specifies a region-specific maximum 0.95 power factor leading and lagging, corresponding to value for the Q/Pmax range. These values are shown in Table ±0.33 Q/Pmax. TenneT has a minimal requirement of ±0.14 5, together with the actual ranges of the profiles specified Q/Pmax which may be increased by the TSO. Also, unlike by the TSO. As can be seen the outer region of TenneT is the other TSOs, TenneT require the specified reactive power beyond what is allowed for that region of the EU, but the capability in the full range of active power operation, i.e. no minimum requirement of TenneT is well inside this range. decrease for operation at low capacity. However, it should Figure 7 shows the UQ profiles specified by the different be noted again that the TenneT grid code applies specifically TSOs. RfG specifies an outer envelope within which the to offshore wind farms. profiles need to be contained, as well as maximum values

4 17th Int'l Wind Integration Workshop | Stockholm, Sweden | 17 – 19 October 2018 for the voltage range within which full reactive power production may be required. The voltage ranges are shown 1 in Table 5. For Eirgrid, which requires full active power 0.9 production inside the whole voltage operation range up to 0.8 1.118 pu, the UQ profile lies outside of the RfG envelope. 0.7 In all the reviewed grid codes it is required that wind 0.6 farms should be able to operate within the given PQ-profiles Energinet Eirgrid 0.5 in three modes, namely reactive power control mode, power National grid TenneT factor control mode, and voltage control mode. The choice Voltage (pu) 0.4 RfG, 110 kV of control mode is done in coordination between the TSO 0.3 and the wind farm operator. In reactive power control mode the reactive power is independent of the active power and 0.2 voltage. Power factor control mode corresponds to operation 0.1 at a constant power factor. In voltage control mode the 0 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 reactive power is varied proportionally to the voltage within Time (s) the capability limits to keep the voltage stable. Figure 8: Low voltage ride through profiles Table 6: Specifications for Voltage Control

TSO Set-point Droop Deadband Regulation range (pu) speed ENTSO-E 0.95-1.05 2%-7% ±5% 90% in 1-5 s Energinet - - no 60 s Eirgrid - 1%-10% no 90% in 1 s National Grid 0.95-1.05, 2-7%, no 5 s 0.25 resolution 0.5% resolution TenneT 0.95-1.05, 2-5%, no 90% in 5 s 1 resolution 0.5% resolution

The requirements for voltage control are summarized in Table 6. RfG specifies a voltage set point in the range 0.95-1.05 pu, with a droop setting1 in the range 2-7% and a deadband of ±0.05 pu. 90% of the voltage adjustment should be completed within a 1-5s of the voltage change, as specified by the TSO. It can be seen that the grid codes Figure 9: Generic fault-ride through profile in RfG of National Grid and TenneT have adopted these voltage regulation requirements. However, they do not require any non-zero deadband settings. Eirgrid requires a droop in the strict version of the RfG profile for wind farms connected range 1-10% and 90% of reactive power adjustment in 1s. at or above 110 kV. Energinet do not specify a range for the droop and have a It can be seen in Figure 8 that both National Grid and lower requirement for the speed of the voltage regulation. TenneT have adopted the RfG low voltage ride through requirement, with a clearing time of 0.14 s and 0.15 s, V. REQUIREMENTSUNDER DISTURBANCES respectively. However, National Grid do not include a recovery time in the profile. Instead, it has a fault ride through The main requirement under grid disturbances is the requirement for faults that exceed 0.14 s, shown in Figure requirement for low voltage ride through, which means that 10. This curve specifies the required ride through time for a wind farm should not disconnect from the grid under a given voltage, so that e.g. a voltage of 0.5 pu must be a temporary voltage drop before a fault is cleared and tolerated for 0.71 s, and a voltage of 0.8 pu for 2.5 s. the voltage restored. These requirements are often specified In contrast to the RfG-compliant grid codes, Eirgrid and using voltage-time profiles showing the minimum transient Energinet do not require a zero minimum voltage but also voltage drop that a wind generator should withstand without have longer clearing times of 0.5 and 0.6 s, respectively. disconnection. Figure 8 shows the low voltage ride through However, since the voltage requirement is with regards to profiles specified in the reviewed grid codes. the voltage at the connection point of the wind farm to the RfG specifies a generic profile shown in Figure 9, with grid, the individual wind turbine generator will not see a some freedom for the individual TSO to set requirements. zero voltage even for the most severe faults. The clearing time, during which time the voltage is at the During a fault, priority may be given to providing reactive minimum level, may be between 0.14-0.15 s. The voltage power to support the voltage over maintaining active power recovery time may be between 1.5-3 s. The minimum voltage production. For example, Energinet require that reactive level is 0 for wind farms connected at or above 110 kV, and current is increased linearly to reach its maximum value as in the range 0.05-0.15 pu for wind farms connected below the voltage drops from 0.9 to 0.5 pu. Eirgrid, National Grid, 110 kV. The curve shown in Figure 8 corresponds to the least and TenneT require that wind turbines generate maximum reactive current during a fault. To this end a reduction of 1The percentage change in voltage that will cause the generator to go from maximal reactive power consumption to maximal reactive power active power during a voltage drop may be allowed. production, or vice versa After voltage recovery upon clearing of a fault wind farms

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comprehensive list of information to be exchanged will be agreed upon between the wind farm operator and the TSO. It should be stressed that the reviewed grid codes all come from countries which have a relatively high level of wind penetration and thus have come far in setting up the technical infrastructure required to accommodate large amounts of wind power. In other systems the information available to the TSO may be more limited. For example, the Swedish TSO SvK does not have access to online active power measurements from major generators, presumably because this has not been necessary for operation [20].

VII.CONCLUSION Figure 10: National Grid fault tolerance requirement for We have reviewed several European grid codes, with focus faults longer than 140 ms on the ENTSO-E RfG [14] which was released in 2016 and is now being implemented by European TSOs. Of the reviewed codes that of Danish Energinet [10] and Irish Eirgrid [11] are required to restore active power to pre-fault values. This have not yet been updated (as of August 2018) to agree with must be done fast enough to prevent frequency deviations RfG, while the more recent codes of British National Grid that may otherwise occur as a result of the active power [12] and German TenneT [13] are in agreement with RfG. imbalance. The specifications for post-fault active power To accommodate different practices by national TSOs and recovery are shown in Table 7. RfG does not specify the time different synchronous areas RfG leaves considerable flexi- for active power recovery. Comparing the other grid codes it bility for implementation by individual TSOs. Specification is seen that both Eirgrid and National Grid have quite strict of tolerance for the rate of change of frequency is left requirements for active power recovery within 0.5s, whereas to the TSOs where significant differences exist. For active Energinet and TenneT allow up to 5s. This reflects the fact power regulation individual TSOs are given the possibility it is more important to restore active power balance quickly to specify set points that should be followed. With regards to in a smaller system with less inertia, as frequency deviations reactive power requirements there are significant differences for a given imbalance will be more severe. between the minimum reactive power range in different synchronous areas. Table 7: Specifications for Post-Fault Active Power Recovery On the other hand, common operation modes for fre- TSO Requirement quency response and automatic voltage control are estab- ENTSO-E TSO must specify time and magnitude of recovery lished in RfG with parameters to be set by the individual Energinet Recovery within 5s TSOs. In summary, although significant harmonization of Eirgrid Tclear ≤ 140ms: 90% within 0.5s Tclear ≥ 140ms: 90% within 1s national grid codes has been made, different practices in National Grid 90% recovery within 0.5s different systems makes full harmonization a challenge. TenneT Recovery with gradient 10-20% of Pmax/s Regarding the concrete requirements, all reviewed codes include specifications for low voltage ride through capability. VI.DATA AND COMMUNICATION Further, there are requirements for all wind farms to provide In order to facilitate operation of the transmission system frequency response for over-frequencies by reducing output. wind farms are also required to make certain data and control Large wind farms are also required to be able to operate signals available to the TSO. However the requirements for in frequency control mode with reserves, providing both communication differ between the different countries. RfG upward and downward frequency response. In Ireland the specifies that the wind farm operator should be capable of reserve requirement is 5%, but is not applied to all wind exchanging information in real time or periodically with time farms. Large wind farms are also expected to be able to sampling, and that the TSO should specify a list of signals operate in voltage control mode, changing the reactive power to be made available by the wind farm operator. output to keep the voltage at a stable level. All the reviewed grid codes require basic SCADA infor- An important conclusion from this review is that with re- mation to be exchanged with the TSO but with some vari- gards to grid codes the necessary requirements for perform- ations. Energinet require power and voltage measurements ing active regulation of wind power such as curtailment are as well as the status and settings of different control modes already in place. Wind farms may be required to operate with to be communicated. Eirgrid also require all wind farms in reserves to be able to participate in frequency regulation for excess of 10 MW to provide meteorological data including both over and under frequencies. In case of local congestion wind speed, wind direction, temperature, and pressure. Na- or voltage limit violations, wind farms can be required to tional Grid requires exchange of SCADA data for those wind reduce their output to a set point issued by the TSO. How and farms which have a significant impact on the transmission to what extent farm operators are economically compensated system. It also requires wind farms to send available active for this is a question of market design which is not addressed power to the TSO. Finally, TenneT requires status of breakers in technical grid codes. and switches, measurement values, regulation settings, and Finally, it should also be noted that while new wind farms set point values to be made available. In most cases a must obey all grid code requirements, there can be older

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