7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms Comparison of high technical demands on grid connected wind turbines defined in international Grid Codes Tobias Bublat, Phone: +49 (0)40 36149 7267, Fax: +49 (0)40 36149 1720, tobias.bublat@gl-group.com Tobias Gehlhaar, Phone: +49 (0)40 36149 7577, Fax: +49 (0)40 36149 1720, tobias.gehlhaar@gl-group.com Germanischer Lloyd Industrial Services GmbH, Business Segment Wind Energy, Steinhöft 9, 20459 Hamburg, Germany Abstract -- Since grid operators changed technical standards for grid connected wind turbines by publishing relevant Grid Codes, it is well known that some of them are extra demanding and also have influence on turbine development process. The article shall give an overview on the highest demands given in a purposive selection of regulations. A comparison will show deviations in height and in the point of reference of requirements. The necessity of straight forward verification procedures will be shown. Such procedures, based on this comparison, solve problems and discrepancies partly coming from imprecise definition or inadequately made explanations of some requirements. Index Terms -- Certification, FACTS, FRT, Grid Code, Grid Code Compliance, Grid Connection Compatibility, Grid Support, IGCC, LVRT, Wind Turbine, Type Certificate (GCC) I. INTRODUCTION The number of grid connection regulations mostly called Grid Codes has risen to an almost unmanageable amount since the first TSOs (Transmission System Operators) have defined and published changed requirements for the connection of wind turbines and wind farms to the high-voltage TS (Transmission System) in the late 90’s. System Operators and authorities of countries, where high penetration of wind energy in the electricity system is expected in the upcoming years, have to take the challenge to ensure grid integration of wind turbines without any difficulties. Wind turbines shall be capable of showing a behavior during normal operation but also during grid failure similar as required for conventional generating units such as coal fired power plants and other synchronous generating units. Wind farms shall ride through faults in the TS and they moreover shall make a contribution to voltage control for stability purposes. Unfortunately connection conditions given in international Grid Codes differ from one country to another. System Operators define special requirements depending on the respective TS infrastructure and control area. This often causes confusion for all parties concerned, as they are possibly not aware of the different requirements or unscheduled changes in some Grid Codes. Manufacturers are constantly challenged to adapt the design of their turbines with the latest requirements of the network operators. Wind turbines shall be configured in a way that they are in compliance with the technical regulations. Straight forward verification procedures have to be improved and harmonized proving the wind turbines grid connection compatibility and showing compliance with the requirements given in the respective Grid Codes. A uniform procedure helps manufacturers, developers as well as System Operators and should include the following steps: 1. 2. 3. 4. 5. 6. 7. Evaluation of Grid Codes and extraction of requirements Comparison of Grid Code requirements Creation of a test plan based on recognized testing procedures Measurement by accredited testing laboratories (ISO 17025) and issuing of a final measurement report. Observation of steps 1 to 4 and assessment of measurements by an accredited certification body (DIN EN 45011) issuing a Type Certificate (GCC) for Grid Connection Compatibility afterwards. Validation of a wind turbine model and site assessment regarding Grid Code Compliance of the wind farm. Simplified grid connection negotiations on basis of project certified wind farms based on type certified wind turbines. A uniform basis will lead to easier contract negotiations with System Operators. The verification and certification procedure for a turbine type (steps 1 to 5) needn’t be repeated for every site. Such a Project Certificate (GCC) will be required for wind farms connected to the medium voltage Distribution System in Germany after 2011 the latest [8]. In Spain, Project Certificates (GCC) are required since January 01, 2008 [9]. 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms Details for a straight forward verification process are given in section III of this paper. II. COMPARISON OF GRID CODE REQUIREMENTS relevant target markets is indispensable. The following comparison shall give an overview on the latest connection conditions given in a purposive selection of Grid Codes from TSOs from all over the world (table no.1). International Grid Code comparison (IGCC): steps 1 and 2 For the verification and certification of the wind turbines Grid Connection Compatibility (GCC) a determination of connection requirements applicable to wind turbines in Country / Region TSO Author Title Grid Code Date of issue GB NGET NGET Germany E.ON E.ON Scandinavia All Nordel China All CEPRI Poland PSE PSE INSTRUKCJA RUCHU I EKSPLOATACJI SIECI PRZESYLOWEJ 2006 Australia NEMMCO AEMC National Electricity Rules 2007 Grid Code – High and extra high voltage 2008 April 2006 Nordic Grid Code Technical Rule for Connecting Wind Farm to Power System (Recommendations) 2005 Fig. 1. Grid Codes from different TSOs [10] Only the most important and technically sophisticated requirements shall be discussed. The comparison of requirements is divided into the following topics: A. B. C. D. E. Operation during frequency and voltage deviations Frequency bound power limitation Active power control Power factor correction and voltage control Grid support including LVRT (Low Voltage Ride Through) The grid connection requirements for wind turbines given in above listed Grid Codes are valid at the PCC (Point of Common Coupling) with the TS. The PCC is mostly defined on the high-voltage side of the wind farms step-up transformer, but exceptions prove the rule. Wind farms are divided into embedded and non-embedded systems in Great Britain [1]. Non-embedded generating units are directly connected to the TS whereas embedded units only have an indirect connection to the TS by another user (e.g. a wind farm or Distribution System). Embedded and non-embedded units need to fulfill different requirements. Furthermore different requirements have been defined for the two TS areas Scotland and England/Wales, which are also depending on the rated power output of the wind farm. The PCC of a wind farm might also be defined on a remote TS bus-bar in Great Britain [1]. Germany is divided into 4 control areas managed by the TSOs EnBW, E.ON, Vattenfall Europe Transmission and RWE. Smaller wind farms or single wind turbines being connected to the Distribution System are currently not covered by the connection conditions of the German TSOs. But new approaches in the German energy sector are going in that direction and a new Grid Code [8] for the connection of wind turbines and other renewable energy technologies to the medium voltage distribution system will be finalized by the federal association of German power industry bdew soon. Similar requirements will then apply to the connection of wind turbines at lower voltage levels, too. The Australian Grid Code defines requirements for the connection to both systems, the Distribution System and the TS [6]. Prior to the year 2007 wind farms and wind turbines had been classified as unscheduled units due to their intermitted power source and were exempted from most requirements. Since March 2007 new amendments for wind turbines have been published by the Australian Energy Market Commission AEMC and had been assimilated in the Grid Code. These connection conditions are also applicable to wind turbines and are similar to those from other Grid Operators. The only difference is that different levels of difficulty have been specified. Requirements have been subdivided to an automatic access standard, a minimum access standard and a negotiated access standard. For wind farms complying with the automatic access standard grid access won’t be denied [6]. A. Operation during frequency and voltage deviations Wind turbines are required to continuously maintain active power output, even if voltage and frequency levels 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms exceed values defined for the TS under normal operating conditions. In Diagram 2 it can be seen that those operation areas differ from one country to another. They can general be divided in three main areas. Fig. 2. Required wind farm operation depending on voltage, frequency and time The first area is the frequency range around the nominal value of grid frequency, where normal operation of the wind farm and all single wind turbines is required without limitations. At lower frequency values (second area) wind farms shall at least operate for specified minimum times which in most cases are linked to system voltage, a prohibited reduction of the active power output level or a combination of both. Automatic reduction in active power output during low frequency events should not be bigger than 5% of the available power in Great Britain for instance [1]. Exceptions have to be made for a change in wind speed. B. Frequency bound power limitation If the system frequency rises above the upper limit of the normal operation area (third area) wind turbines are generally required to reduce their active power output with a given rate of change in most countries. In this power reduction mode the rate of change (Gradient G) is either bound to the respective change in frequency or shall be accomplished within a specified time range. G= ∆P kW ∆t s or G= ∆P ∆f kW f The following gradients or times can be extracted from Grid Codes: 0,4 PAvailable (at each wind turbine) Hz E.ON G= CEPRI Power reduction permitted, but not specified PSE Constant power reduction rate down to P= 0% Pr at 50,5Hz and a rise up to 51,5Hz or by disconnection of single wind turbines NGET G≥ 0,2 PAvailable Hz (Limited Frequency Sensitive Mode; reduction must be achieved within t= 10s) Nordel Required as proportional power reduction as a function of system frequency. Detailed settings will be defined by the TSO. AEMC Minimum access standard for wind farms at Transmission level and P = 30MW: Loading level shall be reduced by an amount of P= 50% Pr • within t= 3s, and holding the output level until frequency recovers to normal operation frequency band 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms G≥ • AEMC Q= ±0,395 Pr, 0,167 PAvailable s for 0 < P = Pr, U = Ur ±10% Nordel Capability to operate with zero reactive power exchange required at PCC or by disconnecting the wind farm within t= 1s E. Grid support including Low Voltage Ride Through (LVRT) C. Active power control Wind farms with a nominal capacity equal to 50 MW or higher located in Great Britain or wind farms in Australia for instance might be instructed to make a contribution to frequency control if agreed upon [1] [6]. They will then be required to provide Primary, Secondary and High Frequency response. Wind farms are generally exempted from this requirement in other countries, but might provide operating reserve as an ancillary service. For implementing such control area wind farms shall be capable of varying their active power output accordingly managed by signals provided by the System Operator. Defined set points shall be adjusted with gradients similar to those described under B., but the rate of change differs from one country to another. not less than G = e.g. E.ON ∆S N kVA ∆t min = In the past wind farms and wind turbines have been disconnected during grid failure as fast as possible. This might lead to a collapse of the TS in circumstances due to the high penetration of wind energy in some regions and a resulting loss of thousands MW wind power generation. For avoiding such scenarios grid operators decided that wind farms shall run through grid faults without disconnection as it is required for conventional power plants. Wind turbines shall stay connected for specified voltage and time ranges as shown in Diagram 4. Disconnection is generally not permitted for voltage values above the lines shown in Diagram 4 and above limit line 1 in Diagram 5 for instance. In addition, some grid operators require contribution to voltage stabilization and system recovery during and after grid failure. 0,1 ⋅ S N min D. Power factor correction and voltage control Wind farms are required to balance voltage deviations at the PCC or the respective connection point by adjusting their reactive power exchange and moreover by settling up predetermined power factors. Voltage deviations due to load flow changes shall be compensated. Examples are shown below. E.ON Phase to phase voltages [kV] 440 253 127 420 245 123 Fig. 4. LVRT requirements of TSOs 380 220 110 350 193 96 Power factor 1 0,95 0,95 inductive capacitive Fig. 3. Power factor control range E.ON [2] Provided that 49,5 Hz < f < 50,5 Hz P = Pr CEPRI Depends on the wind turbines generator system and grid configuration. Voltage regulation is required at PCC PSE 0,975ind - 0,975cap, for 0 < P = Pr 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms Dip duration above 140ms: Restoration of active power output to 90% of PPre-disturbance within 1.0s after the voltage returns to U= 90% of rated system voltage. AEMC Restoration of active power output to 95% of PPre-disturbance within 100ms after fault clearance (Automatic access standard). III. STRAIGHT FORWARD VERIFICATION PROCEDURE The 7 steps of this approach, named in the introduction will be explained in the following section in more detail. Fig. 5. LVRT requirement E.ON With the exception of the Polish System Operator LVRT capability is required for voltage dips down to zero voltage at the PCC [5]. With respect to the older Grid Code E.ON has dramatically changed the LVRT requirements in the Code of 2006 [2]. The voltage-time-diagram (Diagram 5) is now divided in more areas defining the turbines behaviour during grid failure. Below 1500ms and depending on the respective area brief disconnection by the wind turbines might be allowed, if defined re-synchronisation times are kept. This will be agreed with the System Operator. If system voltage does not recover to 80% of the highest phase-to-phase value measured at the low voltage side of each turbine transformer a quarter of all turbines in the wind farm shall be disconnected by automatic protection devices after 1.5s, 1.8s, 2.1s and 2.4s [2]. Wind farms shall stay connected during 3-phase faults in the TS until the primary protection system clears the fault in Australia. For all other types of fault the wind farm shall operate for the longest time expected for the respective breaker fail protection to clear the fault or the highest of 430ms and the time needed by the relevant primary protection system to separate the faulty grid element [6]. Additional support during faults is required in Australia, Germany (E.ON) and the UK. Wind farms have to maximise their reactive current generation for system voltage stabilization. This has to be done with a defined reactive current injection of minimum 2% INominal per %? U at the low voltage side of each wind turbine transformer in Germany and with the greater of Ib Pre-disturbance and 4% INominal per %? U (Automatic access standard) at the PCC in Australia [2] [6]. Further Grid Support shall be given after fault clearance. Active power has to be restored with a minimum gradient of G= 0,2Pr / s (E.ON) or to a defined output level within a specified time [2]. The latter applies to Great Britain and Australia. Requirements are shown below. NGET Dip duration up to 140ms: Restoration of active power output to 90% of PPre-disturbance within 500ms after the voltage returns to U= 90% of rated system voltage. The above discussed requirements of different Grid Codes are examples only. All requirements have to be translated, evaluated (step 1) and compared (step 2). The comparison has shown differences between single demands and their point of reference. Before testing turbine capabilities it has to be ensured that the selected Grid Codes are applicable to wind turbines, that requirements for wind turbines have been determined properly and that the point of reference for requirements has been clearly defined. Success criteria have to be included to an appropriate test plan to verify the turbines capability of Grid Code Compliance. Verification testing: steps 3 and 4 For the verification that a turbine complies with requirements as given above full scale hardware tests have to be performed according to specificly created test plans. The test procedure applied has to be converted in a way that different points within the given voltage-time curves of TSOs will be tested to verify the LVRT capability of a turbine. The same could be done to FACTS (Flexible Alternating Current Transmission System). 3-phase and 2phase voltage dips with different duration have to be performed and accomplished by the turbine. GL Wind– Technical Note 066 [7] describes such a test series. An accredited testing laboratory has to perform testing and measure the test results, which have to be successful accomplished by the turbine. All relevant values shall be measured and reported in a measurement report. Now, an independent certification body shall be involved at the latest. The most appropriate and effective way to prove the turbines conformity to regulations is seeking the assistance of a certification body accredited for the Certification of Grid Connection Compatibility for Wind Turbines according to Grid Codes (DIN EN 45011). The very expensive and time-consuming repetition of tests and measurements can be avoided by a good test plan, which already should be assessed by the certification body. Type Certification (GCC): step 5 The final measurement report will be assessed and results will be compared with the requirements given in Grid Codes. If the results comply with all the requirements a certificate will be issued stating the Grid Connection Compatibility for a single wind turbine type for the respective Grid Codes used as assessment criteria. Project Certification (GCC): step 6 and 7 Simulations of the electrical behaviour of a wind farm will be facilitated by using the measurements of type tests. Most requirements given in Grid Codes refer to the PCC. 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms Results of type test must be converted to this point regarding the local conditions at site. Components such as transformers, filters, FACTS and transmission lines have to be considered, too. For an accurate simulation and the achievement of best possible results the validation of a turbines simulation model is indispensable. This calls for a uniform validation procedure, containing both, testing and simulation. changes. In the long run it will be very helpful having validated simulation models. IV. OUTLOOK TO EXPECTED CHANGES IN THE FUTURE Line-to-line voltage In the past most system operators did not check, whether wind turbines connected to their grid complied with their rules, neither if the wind farms did. Some started with asking for some proving measurement, discussing the results with the wind farm planner during the power purchase agreement. More and more manufacturers of wind turbines started testing and measuring the behaviour of their turbines during full scale tests in the field. A way of testing with medium voltage short circuit testing units became a widely accepted method of proving the ability of wind turbines to ride through low-voltage events without disconnection. Such equipment can be seen in figure 6. The world wide amount of such containers is rising. The most of them (6 in total) are Medium VoltageVoltage Substatio n Literature [1] [2] [3] [4] [5] Time fault occurred Fig. 5. LVRT requirement E.ON in the possession of WINDTEST Kaiser-Wilhelm-Koog, one is in Spain owned by Energy-to-Quality and some more are direct property of several wind turbine manufacturers. The outcome of many measurements is a lot of discussion on the results. This is calling for standardisation. Germanischer Lloyd issued 2 technical notes [7] and [11] with this aim, enabling the Type Certification for Grid Code Compliance as well as Project Certification for Grid Code Compliance internationally. Spanish government started requiring compliance with one aspect of Spanish Grid Code (the part mentioned in P.O.12.3, [13] which is LVRT only). AEE started defining a standard test and measurement procedure to be used for project certificates as required by Spanish government. This PVVC [12] contains also a way showing compliance by using simulation. This simulation needs to use models validated with full scale tests at a wind turbine in the field. PVVC [12] also contains a way of how validation can be done. In Germany a new governmental requirement is on the way, asking for Type Certification concerning more than only LVRT. Furthermore a Project Certificate will be required for each wind farm. This calls for simulation again, but it is also stated clearly, that validation needs to be done. As Grid Codes do not stop changing, simulation is being more and more important. Harmonisation efforts for Grid Codes are starting, e. g. by a group of experts at EWEA. Unfortunately this will lead to additional Grid Codes, being structural harmonised in a first step. First changes in the grid leading to smart grid options, FACTS related to new offshore grids and improved cooperation between national grids will lead to more [6] [7] [8] [9] [10] [11] [12] [13] National Grid “Grid Code”, Issue 3 Rev. 25, National Grid Electricity Transmission, Warwick 2008 E.ON Netz “Grid Code High and extra high voltage“, E.ON Netz GmbH, Bayreuth 2006 Nordel “Nordic Grid Code 2007 (Nordic collection of rules)”, Helsinki 2007 CEPRI “Technical Rule for Connecting Wind Farm to Power System (Recommendations)”, GB/Z 19963-2005, Draft, China 2005 PSE “INSTRUKCJA RUCHU I EKSPLOATACJI SIECI PRZESYLOWEJ”, Wersja 1.0, PSE Operator S.A., Warszawa 2005 AEMC “National Electricity Rules Version 19”, Australian Energy Market Commission, Sydney 2008 GL Wind–Technical Note 066 ”Grid connection compatibility of wind turbines according to Grid Codes (NAR) –Low Voltage Ride Through (LVRT), Test procedure” , Germanischer Lloyd Industrial Services GmbH, Hamburg 2005 VDN “Erzeugungsanlagen am Mittelspannungsnetz - Richtlinie für den Anschluss und Parallelbetrieb von Erzeugungsanlagen am Mittelspannungsnetz“, Entwurf von Juni 2007, Berlin 2007 RD 661/2007 „REAL DECRETO 661/2007, de 25 de mayo, por el que se regula la actividad de producción de energía eléctrica en régimen especial“, MINISTERIO DE INDUSTRIA, TURISMO Y COMERCIO, Spain, 2007 http://www.gl-group.com/pdf/IGCC_list.pdf GL Wind–Technical Note 065 ”Grid connection compatibility of wind turbines according to Grid Codes (NAR) – certification procedure” , Germanischer Lloyd Industrial Services GmbH, Hamburg 2005 AEE “PROCEDURE FOR VERIFICATION VALIDATION AND CERTIFICATION OF THE REQUIREMENTS OF THE PO 12.3 ON THE RESPONSE OF WIND FARMS IN THE EVENT OF VOLTAGE DIPS“ Madrid 2007 PO 12.3: “Requirements regarding wind power facility response to grid voltage dips”, Ministerio de Industria, Turismo y Comercio, Madrid 2006