Certification of advanced Electrical Characteristics based on Validation of WEC Models and Simulation of Wind Farms

Certification of advanced Electrical Characteristics based on Validation of WEC Models and Simulation of Wind Farms Bernhard Schowe-von der Brelie, Dr. Hendrik Vennegeerts, FGH e.v., Martin Schellschmidt, ENERCON GmbH Abstract-- This paper describes the technical solutions ENERCON has implemented at their wind turbines in order to meet advanced requirement on dynamic voltage support. An overview on the status quo of certification regulations and implementation approaches in selected countries both on single WEC types and on wind farms is provided. Furthermore, the model validation and subsequent certification procedure FGH Certification Office has developed and applied to first wind farm certifications will be presented. A focus will be given to the validation of the WEC model and the simulation of the wind farm s LVRT behaviour. Finally, potentials for harmonisation of the certification and, subsequently, on the underlying testing and validation schemes will be sketched. Index Terms-- Power system stability, power system dynamic stability, reactive power, voltage control, wind power generation, certification, model validation I. INTRODUCTION Wind power remains one of the main drivers for ongoing restructuring of German and European power systems. As the number of Wind Energy Converters (WECs) being integrated in the transmission and distribution system is increasing and thus also their share of the overall energy production, it is necessary for WECs to participate in ensuring grid stability like conventional power plants do. For this purpose, grid codes have been introduced by system operators or respective associations in many countries that list the requested electrical characteristics at the point of common coupling to the power grid (medium voltage and high voltage level). The requirements stipulated in current national and international grid codes call for sophisticated WEC technologies. Today, generation and consumption of reactive current during faults depending on residual voltage are essential qualities requested from the WEC. ENERCON has been a forerunner in serving these new requirements by innovative control strategies since many years. At the same time, more and more grid codes imply specific schemes of proving the gride code compliance. In some countries certificates have to be provided not only for single WEC types but also for the system behaviour of entire wind farms. Corresponding certification schemes are highly in focus of all stakeholders worldwide and have been foremost established in Europe. Respective testing directives have been incorporated into the IEC standards and on national levels. For the LVRT test a unique on-site testing facility has been developed at FGH which is today included in the IEC 61400-21 standard. However, the behaviour of wind farms in terms of their aggregated connection to the power grid at the PCC can not be predicted based solely on the aggregation of measurements results of single WECs as interactions have to be accounted for. Neither a measurement at the PCC for the entire wind farm is applicable due to the high rated power of wind farms at the PCC with usually more than 10/20 MVA. Hence, new approaches have to be established based on simulation of the wind farm s behaviour. The validity of the WEC model has to be ensured by model verification with existing field test results taking into account appropriate inherent inaccuracies. II. REQUIREMENTS OF GRID CODES Along with the expansion of installed wind power capacities the requirements of Grid Codes have increased in terms of a demand for advanced capabilities of the WECs. Typical requirements today are: Active Power Control (point setting; frequency and voltage variable) Reactive Power Control in normal system operation (power factor setting) during system faults (Q-U-Mode; increased reactive current injection during voltage dips) Low-Voltage-Ride-Through (LVRT) capabilities to stay connected the power system in case of grid-wise voltage dips System Disturbances acc. to IEC EN 61400-21 (flicker, switching operations, (inter)harmonics) The LVRT capability provides an important contribution to the system stability as an additional loss of gigawatts of wind power in the system would increase serious blackout potentials. In an extension, the reactive current injection depending on the depth of the voltage dip provides a further step to support the system recovering from the fault in terms of backing the voltage. Modern WECs are capable of

providing this service by means of various options. The requirements to provide reactive power during a grid fault were first stipulated in the E.ON Netz GmbH Grid Code issued 1 April 2006 [1]. (See Fig.2) In the following years, these requirements were incorporated into further grid codes. The principle of current injection during grid faults has also been adapted in the BDEW Directive for Medium Voltage of June 2008. As this directive is explicitly asking for a certification of both single WEC units and WEC clusters, e.g. wind farms, for all new installation as from 2009, there is an urgent need for respective certification schemes in terms of this strongly dynamic behavior of WECs. Generally the current injection is required for threephase failures as well as for asymmetrical ones. immediately. This mode is used for riding through faults in radial distribution systems without fault current contribution. The next stage was the development of the PHI Mode. In selecting this mode a constant reactive current can be set for the event of a dip in voltage. Last years ENERCON focused on the development of the QU-Mode as an enhanced UVRT (under-voltage-ridethrough) Mode. The WEC feeds in reactive current depending on residual voltage according to the requirements of the Grid Codes. A series of measurements were taken with four different voltage dips in field tests on a test turbine with a UVRT container (Tests A to D, see Fig. 3). The validation of this feature is demonstrated in Fig. 6. Fig. 1 Different LVRT requirements from selected grid codes [FGH] Current Grid Codes in Germany state that in the event of a voltage dip of more than 10% of the generator's effective voltage value, reactive current of a minimum of 2% of the rated current must be provided per percentage of voltage dip. Furthermore, once voltage has returned, voltage support must be maintained for another 500ms as shown in Fig. 2 [1]. Fig. 3: Validation of the requirements for reactive current injection during under- or overvoltage In another trial run (Test E), the WEC was tested during overvoltage. At an approximate rated voltage of 119% where voltage is situated above the deadband, the WEC injects reactive current into the grid in order to reduce voltage. This trail run is illustrated by Test E (Fig. 4). Another Grid Code requirement states that after voltage has returned to the deadband range, voltage support must be maintained for a further 500ms in accordance with the specified characteristic. (Fig. 1) Fig. 2 Principle of voltage support in the event of a grid fault [1] Fig. 4: Results of the overvoltage test III. ENERCON SOLUTIONS Since 2002 ENERCON has done extensive work to develop the power plant characteristics of its WECs in thousands of tests. The advantage of ENERCON WECs with fault ride through performance is the high flexibility to adapt to the technical and formal needs of the system. ZERO POWER Mode (ZPM) was developed in the first stage. During a fault the WEC immediately stops feeding in current but stays in full operation (rotor keeps rotating). If voltage returns, the WEC recommences current injection Fig. 5: Maintaining voltage support for another 500ms after returning to normal voltage band

Fig. 6: Results of the undervoltage tests As shown in Fig. 5, after a voltage dip, higher grid voltage was determined by the test set up. After fault clearance, voltage increases to 100% above rated voltage, so that the WEC switches from generation mode (voltage support) for undervoltage, to absorption mode for overvoltage. This operating point is maintained for 500ms before the WEC continues in normal operation. The WEC s behaviour during grid faults was commented by the FGH Certification Office as follows in early 2009: Generation and consumption of reactive current during three-phase faults is one of the crucial requirements in the present German Grid Codes A steady dependency of the reactive current on the extent of a voltage dip is an essential condition for a stable reaction of a wind turbine in the power system The QU-Mode solution presented by ENERCON is the most promising alternative to fulfil these requirements FGH Certification Office will examine the conformity of the WEC performance with the Grid Code requirements in detail, when the final test report is available New international specifications require flexible reaction to faults. Voltage may dip even further during a grid fault and the WEC then has to react to this new dip. In this case, ENERCON provides a solution, as shown in Fig. 7. ZERO POWER Mode can be underplayed with an adjustable trigger threshold between 0 and 45% Urated. If voltage drops below the set threshold, the WEC stops injection. If voltage recovers without reaching the rated value, it is supported through reactive current injection. For varying voltage dips, this allows the WEC to react accordingly in the event of a fault. Fig. 7: UVRT-Mode with underplayed ZP-Mode IV. CERTIFICATION Since 2003 ENERCON validates the power plant characteristics of its WECs. ENERCON has ordered FGH to certify all of its new types to minimize the effort for commissioning and compliance tests on site and to provide reliable planning data to system operators. FGH, an renowned research association on power systems in Germany, has established a certification office in 2004 for Power Generation Characteristics of dispersed generation units. This accredited office certifies type specific electrical characteristics of power generation units according to the respective Grid Codes. In the past so called product certificates have been issued by FGH Certification Office. These certificates are based on a conformity check of the results of field tests at a single representative of that WEC type to be performed by accredited testing institutes [7]. As of January 2009 the BDEW Directive for Medium Voltage stipulates that dispersed power generation units and plants connected to the medium voltage grid have to be certified. Furthermore, a newly published directive to the German Renewable Energy Act (EEG) of 2008 is asking for a certification of WECs as well. Hence, a new approach had to be developed as a purely assessment of the clusters behaviour at point of common coupling (PCC) based on an aggregation of individual characteristics as proved by type certificate would totally neglect the various dynamical feedbacks between WECs and other assets in the generation cluster. Therefore the grid codes are asking for a stationary and dynamical power flow calculation based on validated models of the single generation units. In the following, different working groups have been set up under the patronage of the German Association for Wind Power Promotion FGW to elaborate the respective technical guidelines: for field measurement, for model validation and for the certification process itself. FGH has chaired the working group on defining the certification process [9]. Modelling WECs electrical characteristics with highly transient dynamics under failure conditions in terms of a reliable validation and certification process posed a challenge to all stakeholders involved. The envisaged schemes have to be i) valid for different kind of WECs and their correspondent models,

ii) reliable in terms of the utilized simulation software and finally iii) trustful as some internal information on the control algorithms have to be provided. The now developed scheme provides two types of certification: a) a unit certificate likely to the product certificate based on a conformity test of a single unit but enhanced with a model validation of that respective unit type. b) a cluster certificate for the entire generation cluster, e.g. wind farm, consisting of the same or different kind of generation units, which is based on a simulation of the cluster s behaviour taking into account the validated models of each unit involved. While the unit certificate proves the electrical characteristics of that single unit under laboratory conditions, i.e. at the artificial reference point to the cluster internal grid right behind the testing equipment, the cluster certificate provides information about the cluster s behaviour at the PCC as requested by most Grid Code. Under the chair of FGH the technical guideline for power production units, Section 8 (FGW TR8), has elaborated the certification scheme for the unit certification in summer 2009 [8]. FGH Certification Office has positively concluded an accreditation audit in August 2009 in order to be entitled to carry out inspections for unit and plant certification. In contrast to cluster certification which has to be requested by e.g. the wind farm owner, unit certification is in the responsibility of the WEC manufacturer. V. MODEL VALIDATION The model validation can be seen as the most crucial step in the certification scheme. On the one hand the certifier has to be provided with the most detailed and comprehensive information about the WEC control in order to verify the accurate mapping of the control processes onto the model. On the hand the manufactures naturally want to hide as much internal information about the WEC control as possible in order to keep competitive advantages. The following process is foreseen in the TR8: The manufacturer has to provide a comprehensive, computer-based model of the generation unit, which may be and presumably will be in most cases encapsulated as a black box model. The model must be executable in commercial grid analysis applications and thus be capable of describing the electrical characteristics upon which the certificate is based on by simulating the testing procedures as performed in the field test, hence enabling a verification of simulation results against real field test results. In addition, an open, where necessary simplified, model of the unit must be provided. The open model must allow the certifier to follow the logical links between control loops in the relevant system controls. The degree of detail of the open model may be clarified in advance between the certification authority and the manufacturer. In some cases it may be sufficient to present block diagrams. All models are confidential and are not passed on by the certification authority. Generation Unit Model measurement Validation Fig. 8: Model validation via verification t hrough measurements Validated Model unit Simulation Due to the fact that most manufacturers can only provide models to describe the positive sequences in these days, these models are not capable to display the WECs reaction to asymmetrical failures yet. Therefore, it is necessary to comprehensively reveal the unit s fault detection scheme in order to at least describe the correct switching into different performance modes in the respective fault situations. The model may be subdivided into several models specifically for verifying particular characteristics. The models must be capable of modelling all characteristics relevant to power station operation for normal grid operation and for a fault situation in a network analysis program. It has been agreed that models capable to root square means calculations are sufficient with regard to te characteristics to be validated. Model validation is performed by the certification office based on the comprehensive, computer-based unit model i) by comparing simulation results to the measured data given in the test report, ii) as well as on the basis of simulation results for test specifications for a variety of defined setpoint and/or grid conditions. In terms of the comparison a complex scheme of tolerated deviations between the measurement and the simulation has been developed. The validated model will be clearly identified with the unit certificate. It will be stored at the certification office to provide highest confidentiality for its further application in wind farm simulation in the course of cluster certification. VI. WIND FARM SIMULATION FGH has set up a certification scheme for generation clusters like wind farms, that will apply the results of the unit certification in order to evaluate the cluster s electrical characteristics at the PCC. The evaluation is principally based on four different kinds of examinations: i) Stationary power flow calculations to verify: a. reactive power supply in normal operation according to the prerequisites of the Grid Code b. current carrying capacity with respect to thermal stress of assets c. impact of cable capacitances d. impact of additional assets in the cluster s internal grid

grid e. relative voltage fluctuations at the PCC and adherence to admissible voltage bands (EN 50160) within the cluster s internal grid (to avoid unintentional protection releases) ii) Dynamical simulations of the cluster s behaviour in failure situations (symmetrical and asymmetrical) to evaluate the LVRT capabilities based on the validated unit s model. The reactive current injection has to be calculated with respect to the PCC taking into account all the cluster s assets in terms of inductive or capacitive interaction as well as the system s protection measures. iii) Further straight forward calculation with respect to a. characteristics for System Pertubations of single Units must be transferred to cluster according to IEC 61400-21 b. flicker c. harmonics d. fast voltage fluctuations iv) Finally, the application of fixed characteristics of the single units like the active power control by setpoint or frequency-dependent leads to a consideration of the respective behaviour of the entire cluster by simple aggregation. For calculation purposes the public grid shall be represented by its short-circuit power and the impedance angle, which has to be provided by grid operator. All cluster grid internal data must be provided by the plant s operator. Based on these calculations the yielded electrical characteristics of the wind farm will undergo a conformity check with respect to the underlying grid code, thus providing a vote for or against issuing a cluster certificate. VII. SUMMARY AND CONCLUSIONS The requirements stipulated in national and international Grid Codes require sophisticated WEC technology. Generation and consumption of reactive current during three-phase faults depending on residual voltage are the essential qualities required of modern power plants. Certification of WECs by independent and accredited third parties like FGH Certification Office is essential to provide reliable planning data for system operators and customers. The requirements for wind farm certification includes the validation of WEC models for dynamic or transient power system studies. The newly developed certification schemes at FGH form a reliable basis to meet this challenges. VIII. REFERENCES [1] Grid Connection Regulations for High and Extra High Voltage, E.ON Netz GmbH, Apr. 2006 [2] TransmissionCode 2007 Netz- und Systemregeln der deutschen Übertragungsnetzbetreiber, VDN, Aug. 2007 [3] Technische Richtlinie Erzeugungsanlagen am Mittelspannungsnetz, BDEW, Ausgabe Juni 2008 [4] S. Wachtel, S. Adloff, J. Marques, M. Schellschmidt, Certification of Wind Energy Converters with FACTS Capabilities in Proc. European Wind Energy Conference; Brussels, 2008 [5] S. Wachtel, J. Marques, E. Quittmann, M. Schellschmidt, Wind Energy Converters with FACTS Capabilities and the benefits for the integration of wind power into power systems, 7th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Farms, Madrid, Spain, 2008 [6] M. Schellschmidt, M. Kruse, Dr.-Ing. K-H Weck, Certification of Wind Energy Converters with enhanced UVRT options, 9 th German Wind Energy Conference, Bremen, 2008 [7] T. Smolka,, et. Alt: Grid Integration of Wind Energy Converter Experiences of Measurements and Status Quo of Certification Procedures; IEEE-CIGRE Conference, Calgary, Canada, 2009 [8] B. Schowe-von der Brelie, Th. Smolka, P. Siemes, H. Vennegeerts, A.Schnettler.: Certification of wind farms with respect to their power generation unit characteristics first experiences, obstacles and challenges of model validation and testing procedures; EWEC 2009, Marseilles, France, 2009 [9] FGW: Technische Richtlinie Teil 8: Zertifizierung der elektrischen Eigenschaften von Erzeugungs-einheiten und anlagen am Mittel- Hoch- und Höchstspannungsnetz, Revision 0; FGW, Kiel, Germany, 2009 (english version soon available) IX. BIOGRAPHIES Bernhard Schowe-von der Brelie (1971) graduated in physics at Philipps-Universität Marburg in 1997. Since 2006 he is responsible for the business development at the Institute for High Voltage Engineering at RWTH Aachen University and for the Research Association for Power Systems and Power Economics (FGH) e.v. in terms of acquisition and project management. Since 2008 Mr. Schowe-von der Brelie is the deputy Head of FGH Certification Office. He is active in German national working groups on certification schemes for proofing the conformity of single wind energy converters and entire wind farms with respect to the new German grid codes. Dr. Hendrik Vennegeerts (1973) is with Forschungsgemeinschaft für Elektrische Anlagen und Stromwirtschaft e.v. (FGH) since August 2004, first as consultant in and then head of the system technique section. Since August 2006 he is head of the department system studies / software development / training. Here, among other things, various studies and research projects on the electric power supply networks are performed and the software product INTEGRAL, a network calculation tool, is further developed. Mr. Vennegeerts is actively involved in many national and international advisory and steering bodies. He is heading the institutes studies of WEC model implementation and validation. Martin Schellschmidt was born 1974 in Hannoversch Münden, Germany. He finalized his studies in electrical engineering with a degree as a Dipl.-Ing. in electrical engineering at the University of Applied Sciences of Friedberg/Hessen, Germany. Since 2001 he is working with ENERCON in the R&D division. He manages there the R&D Group of validation and funtional testing at the ENERCON head office in Aurich, Germany. Focus of these workings is the validation of wind turbine generators and especially their power plant capabilities. Mr Schellschmidt is active in German national working groups on measuring and certification schemes for wind energy converters and customer generation power plants.