GRID CODE REQUIREMENTS FOR LARGE WIND FARMS: A REVIEW OF TECHNICAL REGULATIONS AND AVAILABLE WIND TURBINE TECHNOLOGIES

Transcription

1 GRID CODE REQUIREMENTS FOR LARGE WIND FARMS: A REVIEW OF TECHNICAL REGULATIONS AND AVAILABLE WIND TURBINE TECHNOLOGIES ABSTRACT M. Tsili Ch. Patsiouras S. Papathanassiou National Technical University of Athens (NTUA) School of Electrical and Computer Engineering 9 Iroon Polytechniou str., Athens, Greece Tel , Fax , st@power.ece.ntua.gr This paper provides an overview of grid codes with regard to the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by Transmission System Operators of countries or regions with high wind penetration and therefore incorporate the experience after several years of system operation with significant penetration levels. The paper focuses on the most important technical requirements for wind farms, present in practically all grid codes, while reference is also made to additional requirements, present in certain codes. In addition, the capability of modern wind turbines to satisfy the above requirements is reviewed, especially their response to grid disturbances, demonstrating that recent developments in wind turbine technology provide to the wind farms stability and regulation capabilities comparable to those of conventional generating plants. Key words: Wind turbine, interconnection, grid codes, technical requirements, LVRT, constant speed, variable speed 1. INTRODUCTION Increasing wind power penetration levels to the power systems of many regions and countries has led to the elaboration of specific technical requirements for the connection of large wind farms, usually as a part of the grid codes issued by the Transmission System Operators (TSOs). These requirements typically refer to large wind farms, connected to the transmission system, rather than smaller stations connected to the distribution network. The new grid codes stipulate that wind farms should contribute to power system control (frequency and also voltage), much as the conventional power stations, and focus on wind farm behavior in case of abnormal operating conditions of the network (such as in case of voltage dips due to network faults). Grid code requirements have been a major driver for the development of WT technology and several relevant publications are already available. The present paper extends the overview to several countries, providing a presentation and comparison of the most recent available editions of the following grid codes: o The German code from E.ON Netz [[1]] that applies to networks with voltage levels 380, 220 and 110kV. Its requirements are often used as a reference for other codes. o The Great Britain code [[2]], where the requirements for wind farms are presented in combination with requirements for other power production units. It applies to networks with voltage levels 400, 275 και 132 kv (32 kv for Scotland). o The Irish code published by ESB National Grid [[3]] (Section WPFS1, Wind Farm Power Station Grid Code Provisions), applying to network voltage levels 400, 220 και 110 kv. o The Νordic Grid code from Nordel [[4]] that applies to all wind farms connecting to Nordic Grid (the interconnected system of Denmark, Sweden, Norway and Finland) o The code of Denmark, [[5]], referring to wind turbines connected to grids with voltages below 100 kv. Some of Eltra s requirements applying to wind turbines connected to networks with voltage levels above 100 kv are also presented, [[6]]. o The grid code of Belgium [[7]] issued by the Belgian TSO, Elia, applying to networks with voltage levels 30 to 70 kv and 150 to 380 kv. 1

2 o The grid codes of two Canadian TSOs, Hydro-Quebec [[8]], applying to networks with voltages above 44 kv and Alberta Electric System Operator (AESO) [[9]], that applies to wind farms with rated capacity above 5 ΜW connected to network voltages 69 to 240 kv. o USA rule for the interconnection of wind generators published by FERC in June 2005 [[10]] that applies to wind farms with rated capacity above 20 MW. o Codes from other countries like Spain [[11]], Italy [[12]] Sweden [[13]] and New Zealand [[14]]. The article focuses on the technical regulations regarding the connection of large wind farms to the high voltage transmission system, whereas requirements concerning small units or dispersed generation connected to the distribution network are not presented. The latter ones usually refer to power quality, contribution to short-circuit level and protection system settings that are not key issues for large wind power stations connected to the transmission system. The main emphasis is placed on the requirements that have been introduced in the last years and concern active and reactive power regulation, voltage regulation and wind farm behavior during grid disturbances. The technical implementation of the above requirements is not part of the grid codes. Several solutions are provided by wind turbine manufacturers, in order to achieve grid code compliance. In the second part of the article, a brief presentation is made of available technologies for modern, commercial wind turbines, in terms of their electrical system configuration, as far as their response to grid disturbances and compliance to grid code requirements is concerned. 2. COMMON GRID CODE REQUIREMENTS The present Section includes the requirements encountered in the majority of grid codes concerning wind farm interconnection. These include fault ride-through, system voltage and frequency limits, active power regulation and frequency control, as well as reactive power/power factor/voltage regulation Fault ride through requirements The large increase in the installed wind capacity in transmission systems necessitates that wind generation remains in operation in the event of network disturbances. For this reason, grid codes issued during the last years invariably demand that wind farms (especially those connected to HV grids) must withstand voltage dips to a certain percentage of the nominal voltage (down to 0% in some cases) and for a specified duration. Such requirements are known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT) and they are described by a voltage vs. time characteristic, denoting the minimum required immunity of the wind power station. The FRT requirements also include fast active and reactive power restoration to the prefault values, after the system voltage returns to normal operation levels. Some codes impose increased reactive power generation by the wind turbines during the disturbance, in order to provide voltage support, a requirement that resembles the behavior of conventional synchronous generators in over-excited operation. Figure 1 presents in the same graph all LVRT requirements cited in the examined grid codes. The requirements depend on the specific characteristics of each power system and the protection employed and they deviate significantly from each other. More demanding appear to be the requirements of the German, UK, Nordic, Danish, Belgian, Hydro-Quebec, Swedish and New Zealand grid codes, which stipulate that wind farms must remain connected during voltage dips down to 0%. However, it must be noted that these requirements apply to the connection point to the network, generally at HV level. Taking into account typical impedance values for the step-up transformers and interconnecting lines, a relatively simple calculation indicates that the corresponding voltage dip at lower voltage levels, near the WT terminals, are likely to be somewhat above 15% [[15]], facilitating compliance to the LVRT requirements. Specifications may vary according to the voltage level or the wind farm power: e.g. wind farms connected to the Danish grid at voltages below 100 kv are required to withstand less severe voltage dips than the ones connected at higher voltages, in terms of 2

3 voltage dip magnitude and duration. Similar differences can be observed in the regulation governing the connection of wind farms below and above 100 MW in the Swedish transmission system. Apart from the FRT curve, the codes of Denmark and Hydro-Quebec define specific kinds of faults (or sequences of faults, in the Danish code) that the wind farm must withstand (including remote faults in the case of the Hydro-Quebec code, cleared by slow protective devices). These more detailed requirements could be attributed to the isolation of the Hydro- Québec transmission system, which has no synchronous link to neighbouring systems, [[16]]. Another important difference lies on the active power restoration rates specified by the German and British/Irish grid codes: while the British code requires immediate restoration (at 90% in 0.5 s after voltage recovery), E.ON Netz requires restoration with a rate at least equal to 20% of the nominal output power (reaching 100% in 5 s after voltage recovery). The less severe requirement of the German code may be attributed to the physical location of the German grid and its strong interconnection to the UCTE system, as opposed to the weakly interconnected British system, where the need for active power restoration to the pre-fault values is more crucial for system stability V (%) time (s) USA-Ireland-AESO Germany Denmark (<100 kv) UK Nordel Belgium (large voltage dips) Belgium (small voltage dips) Sweden (<100 MW) Sweden (>100 MW) New Zealand Hydro-Quebec Italy Spain Figure 1. LVRT requirements of various grid codes Requirements for reactive current supply during voltage dips Some grid codes prescribe that wind farms should support the grid by generating reactive power during a network fault, to support and faster restore the grid voltage. E.ON requires wind farms to support grid voltage with additional reactive current during a voltage dip, or increased reactive power consumption in the event of a voltage swell, as shown in Fig. 2. The voltage control must take place within 20 ms after fault recognition by providing additional reactive current on the low voltage side of the wind turbine transformer amounting to at least 2 % of the rated current for each percent of the voltage dip. A reactive power output of at least 100 % of the rated current must be possible if necessary. The above applies outside a ±10% dead band around nominal voltage. According to the Spanish grid code, wind power plants are required to stop drawing reactive power within 100 ms of a voltage drop and to be able to inject reactive power within 150 ms of grid recovery as shown in Fig. 2. Finally, Great 3

4 Britain and Ireland specify in their grid codes that wind farms must produce their maximum reactive current during a voltage dip caused by a network fault. V (p.u.) Required reactive current/voltage static characteristic I (p.u.) Germany Spain Figure 2. Reactive output current during voltage disturbances, according to the German and Spanish grid codes Active power and frequency control These requirements refer to the ability of wind farms to regulate (usually, but not exclusively, reduce) their power output to a defined level (active power curtailment), either by disconnecting turbines or by pitch control action. In addition wind farms are required to provide frequency response, that is to regulate their active output power according to the frequency deviations. The grid codes of the following countries demand that wind farms have the ability of active power curtailment: Germany, with a ramp rate 10% of grid connection capacity per minute. Ireland, with a ramp rate 1-30 MW per minute. Nordic Grid Code, with a ramp rate 10% of rated power per minute. Denmark, with a ramp rate % of rated power per minute. According to the German code when frequency exceeds the value 50.2 Hz wind farms must reduce their active power with a gradient of 40% of the available power of the wind turbines per Hz. The British code requires that wind farms have a frequency control device capable of supplying primary and secondary frequency control, as well as over-frequency control. It is remarkable that it also prescribes tests, which validate that wind farms indeed have the capability of the demanded frequency response. The Irish code demands a frequency response system, which will control active power according to a prescribed response curve. According to the Hydro-Quebec grid code, wind farms with rated power greater than 10 MW must have a frequency control system that helps reduce large (>0.5 Hz) and short term (<10 s) frequency deviations in the power system. As a general remark, it is clear that most grid codes require wind farms (especially those of high capacity) to provide frequency response, i.e. to contribute to the regulation of system frequency. It should be emphasized that the active power ramp rates must comply with the respective rates applicable to conventional power units. 4

5 2.3. Voltage and frequency operating range Wind farms must be capable of operating continuously within the voltage and frequency variation limits encountered in normal operation of the system. Further, they should remain in operation in case of voltage and frequency excursions outside the normal operation limits, for a limited time and in some cases at reduced output power capability. Figure 3 provides a comparison of operating frequency limits (the scale is only indicative of the duration that wind power plants are required to remain in operation) in countries with 50 Hz power systems (Canada and USA limits are not included, as well as those of New Zealand, where the requirements are different in the northern and southern part of the country). Where possible, voltage limits in relation to frequency limits appear, as well (they are not included in the case of Germany, although they are available, because different % values are specified for the three transmission system nominal voltage levels). It is obvious that the most extreme frequency limits are 47 Hz and 54 Hz. In countries like Ireland, characterized by an isolated power system with weak interconnections, frequency limits are expectedly wider. It is remarkable that New Zealand s grid code prescribes a frequency range of Hz. The strictest continuous operation limits for frequency appear in the British code ( Hz) and for voltage in the Danish code (90-106% nominal voltage). 30 min-linear power reduction (0% at 49 Hz- 15% at 47.5 Hz) (95-105%) (95-105%) 10min at % or >30min at 85-90% with <5% power reduction at % >30min with <5% power reduction at voltage % No connection of WTs above 50.2 Hz is allowed 3 min-power reduction (95-105%) 30 min small power reduction* (90-105%) (95-105%) 10min at % or >30min at 85-90% with <5% power reduction at voltage % >30min reduced power at % Figure 3. Overview of operating frequency limits imposed by grid codes Reactive power control and voltage regulation Reactive power control is important for wind farms, because not all wind turbine technologies have the same capabilities, while wind farms are often installed in remote areas and therefore reactive power has to be transported over long distances resulting in power losses. Recent grid codes demand from wind farms to provide reactive output regulation, often in response to power system voltage variations, much as the conventional power plants. The reactive power control requirements are related to the characteristics of each network, since the influence of the reactive power injection to the voltage level is dependent on the network short-circuit capacity and impedance. Some codes prescribe that the TSO may define a set-point value for voltage or power factor or reactive power at the wind farm s connection point. 5

6 line-to-line voltage per voltage level in kv UK +5% -5% Germany UK UK (under 33 kv) underexcited power factor overexcited Figure 4. Requirements for power factor variation range in relation to the voltage, according to the German and British codes Rated MW (%) A -0.1 E C 0 D B 0.2 UK Ireland MVAr Figure 5. Reactive power in relation to active power according to the British and Irish codes. Figure 4 compares the wind farm power factor range (based on rated power) in relation to grid voltage, according to the German (red line) and British (yellow line) codes. The nominal voltages are 380, 220,110 kv for Germany and 400, 275 kv for Great Britain. The British code refers to wind farms with rated power above 50 MW. The German code specifies that wind farms may function in lagging or leading power factor in case of overvoltages. According to the British code, power plants must be able to provide their full reactive power at voltages ± 5% around the nominal, for voltage levels 400 and 275 kv. The reactive power variation capability according to the British and the Irish codes is shown in Figure 5, where: Point A is equivalent (in MVAr) to: 0.95 leading power factor at rated MW output Point B is equivalent (in MVAr) to: 0.95 lagging power factor at rated MW output Point C is equivalent (in MVAr) to: -5% of rated MW output Point D is equivalent (in MVAr) to: +5% of rated MW output Point E is equivalent (in MVAr) to: -12% of rated MW output 6

7 Active Power (%) Reactive Power (%) Ireland, power factor Ireland, 0.95 power factor Denmark, absorbed Q Denmark, produced Q Canada (AESO) dynamic capability Canada (AESO) continuous capability Figure 6. Comparison of reactive power requirements imposed by grid codes. The above points apply to the British code. The Irish code requires a power factor equal to lagging or leading, at active power output levels below 50% of the rated. A comparison of reactive power requirements is shown in Figure 6, which includes several available active-reactive power curves imposed by national grid codes. 3. WIND TURBINE TECHNOLOGIES AND GRID REQUIREMENTS In this Section a brief review is presented of wind turbine technology aspects, associated with grid code compliance. Wind turbines are generally divided in two main technological categories: 1. Constant speed wind turbines, which are equipped with squirrel cage induction generators directly connected to the grid. The rotational speed of the rotor is practically fixed, since they operate at a slip around 1%. Since the induction machine absorbs reactive power from the grid, connection of compensating capacitor banks at the wind turbine (or wind farm) terminals is necessary. Their aerodynamic control is based on stall, active stall or pitch control. A variation of this scheme utilizes a wound rotor induction generator and electronically controlled external resistors to the rotor terminals, permitting a very variation of speed (typically up to 10% above synchronous). 2. Variable speed wind turbines, the rotor speed of which varies significantly, according to the prevailing wind conditions. Two major types are available: The first is utilizes a Doubly-Fed Induction Generator (DFIG) and a rotor converter cascade of reduced rating, while the second employs a synchronous or induction generator, the stator of which is interfaced to the grid via a full-power converter. The aerodynamic control of variable speed machines is based on blade pitch control (although stall operation is in principle possible, but not preferred in practice). In case of DFIGs the generator s stator is directly connected to the grid while the rotor is connected through a cascade of two voltage source converters (rectifier-inverter, connected back-to-back, as in Fig. 7). Wind turbines with full converter use either a synchronous or an asynchronous generator, whose stator is connected to the grid via an AC/DC/AC converter cascade. In this case, the converter handles the total generator power to the grid and therefore no size economies are possible. 7

8 As described in the previous Sections, the latest grid codes require that wind farms must remain in operation during severe grid disturbances, ensure fast restoration of active power to the prefault levels, as soon as the fault is cleared, and in certain cases produce reactive current in order to support grid voltage during disturbances. Depending on their type and technology, wind turbines can fulfill these requirements to different degrees, as explained in the following. Starting with constant speed wind turbines, their low voltage behavior is dominated by the presence of the grid-connected induction generator. In the event of a voltage dip, the generator torque reduces considerably (roughly by the square of its terminal voltage) resulting in the acceleration of the rotor, which may result in rotor instability, unless the voltage is restored fast or the accelerating mechanical torque is rapidly reduced. Further, operation of the machine at increased slip values results in increased reactive power absorption, particularly after fault clearance and partial restoration of the system voltage. This effectively prevents fast voltage recovery and may affect other neighboring generators, whose terminal voltage remains depressed. Since the dynamic behavior of the induction generator itself cannot be improved, measures that can be taken in order to enhance the fault ride-through capabilities of constant speed wind turbines are the following: Improvement in the response of the wind turbine aerodynamic control system, in order to perform fast limitation of the accelerating mechanical torque, to prevent rotor overspeed. Physical limitations of the blades and the pitch regulation mechanism impose a limit on the effectiveness of such an approach. Supply of reactive power through static compensation devices at the wind turbine or wind farm terminals, such as SVCs or STATCOMs. These device would provide high amounts of reactive power during faults, to effectively support the terminal voltage and therefore limit the magnitude of the voltage dip experienced by the wind turbines. Nevertheless, FACTS are complicated and costly devices, while there is an obvious limitation to the voltage correction they can achieve, particularly in the event of nearby system faults. Variable speed wind turbines, on the other hand, present the distinct advantages of direct generator torque and reactive current control and the possibility to endure large rotor speed variations without stability implications. For this reason, grid disturbances affect much less their operation and, generally speaking, they are capable of meeting stringent requirements. In case of voltage disturbances, rotor overspeed becomes an issue of much smaller significance, since a limited increase of speed is possible (e.g % above rated), the rotor inertia acting as an energy buffer for the surplus accelerating power, until the pitch regulation becomes effective. In case of severe voltage dips, an energy surplus may occur in the electrical part, potentially leading to dc overvoltages. This is dealt with via proper redesign of the converter controllers, increase of the local energy storage capacity (e.g. capacitor size) or even by providing a local power dissipation means. However, even with variable speed wind turbines there still exist LVRT issues affecting their response. In the case of DFIG wind turbines, the direct connection of the generator stator to the grid inevitably results in severe transients in case of large grid disturbances. Hence, the stator contributes a high initial short circuit current, while large currents and voltages are also induced in the rotor windings, as a consequence of the fundamental flux linkage dynamics of the generator. Furthermore, the depressed terminal voltage reduces accordingly the power output of the grid side rotor converter, leading to an increase of the dc bus capacitor voltage. To protect the power converters from overvoltages and overcurrents, DFIGs are always equipped with a device known as a crowbar, that short circuits the rotor terminals as soon as such situations are detected. Once the crowbar is activated, the DFIG behaves like a conventional induction machine, i.e. control is lost over the generator. Notably, crowbar activation is possible not only at the instant of a voltage depression, but also in case of abrupt voltage recovery, after clearance of a fault. Conceptually, two crowbar options are available: 8

9 The passive crowbar, utilizing a diode rectifier or a pair of antiparallel thyristors to short the rotor terminals. The disadvantage of this option is the lack of control on the deactivation of the crowbar, leading to sustained operation with short-circuited rotor. The active crowbar, that uses IGBT switches to short the rotor. This enhances considerably the operation of the device, with a faster elimination of the rotor transients (typically within 100 ms) and therefore faster regain of control. After deactivation of the crowbar, full controllability over the wind turbine behavior is resumed. Hence, although voltage dips inevitably cause torque and power transients in the DFIG wind turbine, which excite the rotor crowbar protection for a limited time interval, the various implementations of the active crowbar can improve the stability of the wind turbine and its response to sudden voltage changes. Figure 7. Typical configuration of a DFIG wind turbine electrical part. Another important improvement can be achieved by the addition of a fast stator switch, as shown in Fig. 7. The stator is disconnected for a short period (Short Term Interruption) through the stator switch and the rotor is demagnetized. After that, the generator side inverter is restarted, the stator is reconnected and operation is resumed. During the stator disconnection the grid side converter stays active and feeds reactive power to the grid. This implementation results in the limitation of transients, both in magnitude and duration, and permits to keep full control of the generator during the greatest part of the disturbance interval. Moreover, with proper converter control strategies, the DFIG can satisfy requirements for reactive power injection during faults. Variable speed wind turbines with full power converters present the distinct advantage that the converter totally decouples the generator from the grid. Hence, grid disturbances have no direct effect on the generator, whose current and torque variations during voltage dips are much lower compared to the DFIG and the respective transients fade out faster, [[24]]. From the point of view of the reactive output power, the grid side converter has the ability to produce reactive current during the voltage dip, up to its rated current. Notably, this wind turbine type may exhibit better voltage control capabilities even than conventional synchronous generators, [[25]]. Another notable advantage of this type against the DFIG-based wind turbines is related with the behaviour of the latter in case of unbalanced disturbances. In such situations, the low negative sequence impedance of the induction generator may give rise to large rotor currents, whose frequency lies outside the controllers bandwidth, resulting in the activation of the crowbar (or the disconnection of the stator) until the disturbance is cleared. Wind turbines can control their active power output by pitch control, while variable speed wind turbines have the additional capability for such control via variation of their rotor speed. Hence, power curtailment, ramp rate limitations and contribution to frequency regulation is possible, even for constant speed machines. In the latter case, however, the grid frequency is directly related to the generator slip and hence a change in frequency will transiently affect the active 9

10 power produced by the wind turbine. In the case of variable speed machines, on the other hand, the generator power is directly controlled and therefore their primary frequency response is entirely adjustable via proper design of the control systems. Up to now, wind turbine compatibility to the various grid code requirements is established only through specific tests or simulations that are performed by the manufacturers or other independent laboratories, upon demand of system operators. Standardized type tests have not been developed yet, due to the diversity of requirements appearing in grid codes and the relatively limited time they have been in force. Moreover, testing the actual behaviour of wind turbines during system faults presents significant difficulties, since on site tests on installed machines are necessary, which would involve variation of power system variables, such as voltage and frequency. A first reference to such type tests appears in the draft of the IEC , 2 nd Ed., [[26]], which is under development. 4. CONCLUSIONS In this paper, the grid code technical requirements were presented for the connection of wind farms to the power systems, basically at the HV level. A comparative overview and analysis of the main requirements was conducted, comprising several national and regional codes from many countries where high wind penetration levels have been achieved or are expected in the future. The objective of these requirements is to provide wind farms with the control and regulation capabilities encountered in conventional power plants and are necessary for the safe, reliable and economic operation of the power system. Current wind turbine technology, particularly its development over the last years, has been heavily influenced by these requirements. Modern wind turbines are indeed capable of meeting all requirements set, with the exception of the constant speed machines, which are practically not marketed anymore for large scale applications. 5. ACKNOWLEDGEMENT This work has been financially supported by the Regulatory Authority for Energy (RAE) of Greece, to which the authors express their gratitude. 6. REFERENCES [1] Grid Code - High and extra high voltage. E.ON Netz GmbH, Bayreuth, Germany, 1 st April [2] The Grid Code, Issue 3, Revision 24. NATIONAL GRID ELECTRICITY TRANSMISSION plc, UK, 19 th November [3] Grid Code Version 3.0. ESB National Grid, Ireland, 28 th September [4] Nordic Grid Code (Eltra a.m.b.a. / Elkraft System a.m.b.a. / Fingrid Oyj / Statnett SF / Affärsverket svenska kraftnät). Nordel, September [5] Grid connection of wind turbines to networks with voltages below 100 kv, Regulation TF Energinet, Denmark, May [6] Grid connection of wind turbines to networks with voltages above 100 kv, Regulation TF Energinet, Denmark, December [7] Grid code for the local transmission system operator (Arrêté du Gouvernement wallon relatif à la révision du règlement technique pour la gestion du réseau de transport local d électricité en Région wallonne et l accès à celui-ci, 24 May 2007). Walloon Energy Commission (Commission Wallone pour l Energie-CWaPE), Wallonia, Belgium. [8] Transmission Provider Technical Requirements for the Connection of Power Plants to the Hydro-Quebec Transmission System. Hydro Quebec Transenergie, March [9] Wind power facility technical requirements. Revision 0. Alberta Electric System Operator, Canada, 15 November

11 [10] Interconnection for Wind Energy, Final Rule. Federal Energy Regulatory Commission, USA, 2 June [11] Requisitos de respuesta frente a huecos de tension de las instalaciones de produccion de regimen especial, PO REE, Spain, November [12] CEI 11/32, Appendice N.6 Normativa impianti di produzione eolica. February 2006 (draft). [13] Affärsverket svenska kraftnäts föreskrifter om driftsäkerhetsteknisk utformning av produktionsanläggningar. Svenska Kraftnät, Stockholm, Sweden, [14] Connection and Dispatch Guide. Transpower New Zealand Limited, March [15] New generation technologies and GB grid codes, Report on Change Proposals to the Grid Codes in England & Wales and in Scotland, Sinclair Knight Merz, December [16] S. Bernard, D. Beaulieu, G. Trudel, Hydro-Québec grid code for wind farm interconnection, Proc. of Power Engineering Society General Meeting, 2005, June 2005, Vol. 2, pp [17] G. Joos, Review of grid codes, Proc. of First International Conference on the Integration of RE and DER, 1-3 Decembre, 2004, Brussels, Belgium. [18] Petitions for Rulemaking or, in the alternative, Request for Clarification of order 2003-A, and Request for Technical Conference of the American Wind Energy Association. American Wind Energy Association, USA, 20 May [19] Willi Christiansen & David T. Johnsen, Analysis of requirements in selected Grid Codes. Section of Electric Power Engineering, Technical University of Denmark (DTU), January [20] J. Matevosyan, T. Ackermann, S. Bolik, L. Söder, Comparison of International Regulations for Connection of Wind Turbines to the Network - Proc. of Nordic Wind Power Conference, NPWC 04, 1-2 March [21] Thomas Ackermann, Wind Power in Power Systems, John Wiley & Sons, [22] A. Morales, X. Robe, J. L. Rodriguez-Amenedo, S. Arnalte, R. Zubiaur, Z. Torbado, Advanced Grid Requirements for the Integration of Wind Farms into the Spanish Transmission System, Proc. of European Wind Energy Conference and Exhibition, EWEC 2007, 7-10 May [23] F. Iov, A. Hansen, P. Soerensen, N. Cutululis, Mapping of Grid Faults and Grid Codes, Risoe National Laboratory, Denmark, July [24] J. Niiranen, Experiences on voltage dip ride through factory testing of synchronous and doubly fed generator drives European Conference on Power Electronics and Applications, 2005, Sept. 2005, p.11. [25] P. Boerre Eriksen, T. Ackermann, H. Abildgaard, P. Smith, W. Winter, J. Rodriguez Garcia, System Operation with High Wind Penetration, IEEE Power & Energy, Vol. 3, Issue 6, Nov./Dec. 2005, pp [26] IEC Ed Wind Turbine Generator Systems Part 21: Measurement and assessment of power quality characteristics of grid connected wind turbines. Committee Draft (CD), July