Justified Fault-Ride-Through Requirements for Wind ... - IEEE Xplore

IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 26, NO. 3, AUGUST 2011

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Justified Fault-Ride-Through Requirements for Wind Turbines in Power Systems C. Rahmann, H.-J. Haubrich, A. Moser, R. Palma-Behnke, Senior Member, IEEE, L. Vargas, and M. B. C. Salles, Student Member, IEEE

Abstract—In this paper, a novel adaptive strategy to obtain technically justified fault-ride-through requirements for wind turbines (WTs) is proposed. The main objective is to promote an effective integration of wind turbines into power systems with still low penetration levels of wind power based on technical and economical considerations. The level of requirement imposed by the strategy is increased stepwise over time, depending on system characteristics and on wind power penetration level. The idea behind is to introduce stringent requirements only when they are technically needed for a reliable and secure power system operation. Voltage stability support and fault-ride-through requirements are considered in the strategy. Simulations are based on the Chilean transmission network, a midsize isolated power system with still low penetration levels of wind power. Simulations include fixed speed induction generators and doubly fed induction generators. The effects on power system stability of the wind power injections, integrated into the network by adopting the adaptive strategy, are compared with the effects that have the same installed capacity of wind power but only considering WTs able to fulfill stringent requirements (fault-ride-through capability and support voltage stability). Based on simulations and international experience, technically justified requirements for the Chilean case are proposed. Index Terms—Fault-ride-through capability, grid code, transient stability, voltage stability support, wind power, wind turbine.

I. INTRODUCTION HROUGH the years, different grid code requirements for wind turbines (WTs) have been adopted around the world. The differences in the regulations depend on the local needs of the transmission network and the existing wind power penetration level [2]. In a first stage, characterized by low penetra-

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Manuscript received April 20, 2010; revised August 03, 2010 and October 20, 2010; accepted November 10, 2010. Date of publication January 06, 2011; date of current version July 22, 2011. This work was supported in part by the German Academic Exchange Service (DAAD), in part by the Institute of Complex Engineering Systems (Milenio, Chile), in part by the German Research Foundation (DFG), and in part by the Chilean Council of Scientific and Technological Research (CONICYT, Fondecyt Grant # 1080668). Paper no. TPWRS-003102010. C. Rahmann, H.-J. Haubrich, and A. Moser are with the Institute of Power Systems and Power Economics (IAEW), RWTH Aachen University, Aachen, Germany (e-mail: [email protected]; [email protected]; [email protected]). R. Palma-Behnke and L. Vargas are with the University of Chile (DIE, CMM, ISCI), Santiago, Chile (e-mail: [email protected]; [email protected]. cl). M. B. C. Salles is with the Laboratory of Applied Electromagnetism, University of Sao Paulo, Sao Paulo, Brazil (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRS.2010.2093546

tion levels and an initial phase of technological development, the requirements were mainly focused on protecting the turbines themselves without considering the impacts on the power system operation [3], [14]. WTs were allowed/forced to be disconnected in case of disturbances, and no contribution to power system stability was required. Nevertheless, different problems experienced by some transmission system operators (TSOs) in countries with high penetration levels of wind power have led to a general reevaluation of grid requirements around the word. As a consequence, the immediate disconnection of WTs in case of voltage dips is usually not admitted anymore and voltage stability support is in some cases also required [4]–[6]. Although most modern WTs are able to fulfill restrictive technical requirements, it does not mean that such requirements should always be demanded as they have direct implications on WT design, thus resulting in additional costs for manufacturers. For instance, allowing WTs with doubly fed induction generators (DFIGs) to ride through a fault increases the total costs of the WT by up to 5% [7]. Similarly, stringent requirements do not allow the integration of low cost technologies based on fixed speed induction generators (FSIG) without additional investments in dynamic reactive power support devices like static var compensators (SVC) or static synchronous compensators (STATCOM). Hence, if no unnecessary barriers to wind power integration want to be imposed, costly and challenging requirements to WTs design such as voltage stability support or faultride-through (FRT) capability should only be demanded if they are technically needed for a reliable and secure power system operation [7], [8]. In order to determine grid requirements where the real needs of the power system are reflected, independent impact studies must be carried out. By doing this, technically justified requirements allowing a secure wind power network integration can be achieved. Countries with still low amounts of wind power but with high projections of future network integration are now facing the question of which requirements to impose. In many of these cases, the adoption of strict rules reflecting the technical needs of some regions with high penetration levels would not have an appropriate technical justification (at least not in a first stage of wind power development). The application of technically justified requirements becomes even more important in countries without subsidies, where the economic pressures of a competitive environment could make such requirements to be the determinant factor in the realization of some WT projects. For those countries, the competitive conditions leave basically two strategies to follow in order to manage the wind power network integration: 1) to wait until the investment costs of WTs and the

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Fig. 1. FRT requirements for WTs according to E.on Netz.

required equipment for FRT capability and voltage stability support fall into ranges that make them competitive against conventional energy sources, or 2) to generate technically justified requirements without considering financial subsidies but incorporating economical considerations and taking advantage of the power grid flexibility degrees. In this paper, an adaptive strategy to obtain technically justified FRT-requirements for WTs based on system characteristics and on the wind power penetration level is proposed. The objective is to introduce costly and challenging requirements only when they are technically needed for a reliable and secure power system operation. The proposal is specially aimed at countries with still low penetration levels of wind power, where the WT integration is carried out in a competitive environment without any kind of subsidies. This paper is organized as follows. Section II summarizes the requirements for WTs in case of disturbance imposed by E.on Netz. In Section III, an identification of the main features of FSIG and DFIG, regarding voltage stability support and FRT capability, are presented. Section IV presents the proposed strategy to obtain technically justified FRT requirements for WTs. A practical study on the Chilean network and the results obtained by the application of the proposed approach are presented in Section V. Finally, the conclusions are given in Section VI. II. BEHAVIOR OF WTS IN CASE OF DISTURBANCE ACCORDING TO THE E.ON NETZ In order to illustrate how demanding the FRT requirements for WTs can be, the case of the German TSO E.on Netz is presented. Fig. 1 shows the requirement for the fault-ride-through process for WTs in case of disturbance. According to the German code, WTs connected to the transmission system (which includes voltages levels upon 220 kV and upwards) must remain connected to the grid during short circuit faults as long as the voltage at the point of common coupling (PCC), measured at the high voltage level of the grid connected transformer, is above the continuous line defined in Fig. 1 [4]. As can be observed, the turbines have to sustain the operation within the 150 ms after the fault appearance even if the voltage at the PCC decreases to zero. In addition to the FRT capability, requirements regarding to voltage stability support are also imposed. WTs must be able to

Fig. 2. FRT requirements for WTs according to E.on Netz.

provide at least 2% of the rated reactive current for each percent of voltage dip (see Fig. 2). A reactive current of 100% of the rated current must be possible if necessary. The reactive current must be injected into the network within 20 ms after the fault detection on the low voltage side of the connecting transformer. A dead band is also included around the reference voltage, in which the WTs can operate with power factor control. The requirements imposed by E.on in case of disturbance are technology-oriented, i.e., they address WTs manufacturer design requirements while no power system security constraints or limits are explicitly defined. Such levels of requirements are well justified in power systems with high penetration levels of wind power (like the northern area of Germany covered by E.on, which is one of the areas with highest penetration levels of wind power in the world), but not necessary in power systems in an initial stage of wind power development. Requirements based on system security margins are an alternative to such grid codes. In this work, a mixture of both approaches is used in order to achieve technically justified FRT requirements. III. WIND GENERATOR TECHNOLOGIES In this section, a brief description of the dynamic response of FSIG and DFIG during grid fault events is presented. The possibilities and limitations of both technologies regarding voltage stability support and FRT-capability are also summarized. Both technologies have been described in many publications [10]–[13], and therefore, only the main issues are presented here. A. Fixed Speed Induction Generator (FSIG) An FSIG uses a squirrel cage induction generator directly connected to the grid through a step-up transformer [14]. In normal operation, a FSIG operates by drawing reactive power from the network [15]. Switched capacitor banks for reactive power compensation are thus typically installed at the WT terminal in order to ensure a power factor close to unity for some operating points during a year. The compensation is typically

RAHMANN et al.: JUSTIFIED FAULT-RIDE-THROUGH REQUIREMENTS FOR WIND TURBINES IN POWER SYSTEMS

Fig. 3. Basic configuration of an FSIG wind turbine.

Fig. 4. Basic configuration of a DFIG wind turbine.

rated at around 30% of the wind farm capacity [16]. A typical configuration of an FSIG is shown in Fig. 3. During a short circuit fault close to the terminals of the generator, the voltage at the PCC drops rapidly. The reduced terminal voltage leads to a decrease of the electrical torque. If the mechanical torque remains constant, the generator will accelerate [17]. The operation at increased slip values results in high absorption of reactive power by the generator. Especially after the fault clearance, high amounts of reactive power are needed in order to build up the generators magnetic field [9], [18]. As a consequence, the terminal voltage may not be able to fully recover and the generator will continue to accelerate, thus increasing its reactive power consumption [1]. In some circumstances, unless the generator is disconnected, the described situation could lead to voltage instability and in the worst case, to a voltage collapse in the whole system [19]. Wind farms based on induction generators have difficulties in meeting demanding FRT requirements without negatively affecting the system stability [9], [20]. In this context, the use of dynamic reactive power compensation like SVC, STATCOM, or DSTATCOM [1] can reduce the reactive power consumption during the FRT process and considerably improve power system stability [9], [20]. Nevertheless, the voltage correction that can be achieved by these devices in case of nearby faults during the fault is limited. B. Doubly Fed Induction Generator (DFIG) A DFIG uses an induction generator with wound rotor. The stator winding of the generator is coupled directly to the grid while the rotor winding is connected to the network via a back-to-back IGBT-based converter. The rating of the power converter is generally in the range from 25% to 30% of the rated power of the generator. A typical arrangement of a DFIG is shown in Fig. 4. In normal operation, the use of power converters enables DFIGs to operate at optimal rotor speed, thus maximizing the power generation. Independent control of active and reactive

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power is achieved by using a stator voltage-oriented or stator flux-oriented approach for the control of the converters [21]. In case of voltage dips at the PCC, high currents will pass through the stator winding that will also flow through the rotor due to the magnetic coupling between stator and rotor [22]. Furthermore, the reduced terminal voltage will reduce the capability of the grid side converter (GSC) to transfer the power from the rotor side converter (RSC) to the grid. The additional energy charges the DC bus pushing its voltage to higher values. Since such overcurrents/overvoltages could damage the converters, a protection system is always included. A usual protection system is an external rotor impedance known as crowbar circuit. When a fault is detected, the protection system acts by short circuiting the generator rotor through the crowbar [23]. Once the RSC is blocked, DFIG operates like a typical induction generator and the controllability of active and reactive power is lost. When the fault is cleared and the voltage at the PCC is recovered, the crowbar is disconnected and the DFIG can resume its normal operation very fast. DFIGs equipped with crowbar are able to ride through grid disturbances [25]. In case that the induced currents are not high enough to trigger the crowbar protection, the WT can also provide voltage stability support by setting the RSC to inject reactive power into the network [24]. A control strategy allowing both converters to inject reactive power can also be implemented in order to increase the voltage support provided by the DFIG [24]. In case of large disturbances, however, the RSC will be disconnected and the DFIG will exhibit similar characteristics as an FSIG unit [35]. In such circumstances, a control strategy allowing the GSC to inject reactive power can improve the voltage level at the PCC during the fault and contribute to its reestablishment after the fault clearance [25]. Nevertheless, the injected reactive current with this strategy still may not be enough to accomplish severe grid requirements regarding voltage stability support. Further investments in additional protection devices like DC-choppers [35], [36], in larger converter, or in dynamic reactive power compensation are needed if such requirements must be met. IV. ADAPTIVE STRATEGY The idea of having an adaptive strategy to obtain technically justified FRT-requirements (AS-FRT) is to introduce stringent requirements only when they are technically needed for a reliable and secure power system operation. The level of requirement is increased stepwise with the time, depending on system characteristics and on wind power penetration level. The strategy considers the application of different level of requirements at different regions (zones), depending on the particular needs of the system at the specific zone. By doing this, stringent requirements that could reflect the needs of one particular area in the system are not expanded into other ones where such requirements would have no technical justification. A. Considered Requirement Levels The AS-FRT considers three requirement levels for WTs regarding FRT capability and voltage stability support. More requirement levels could also be considered by the strategy if a more gradual and progressive technical adaption is required. For

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TABLE I REQUIREMENT LEVELS DEMANDED BY THE ADAPTIVE STRATEGY

Fig. 5. FRT requirement for WTs according to the AS-FRT.

the purposes of this paper, however, three requirement levels were considered appropriated enough in order to present the adaptive strategy. The initial stage is characterized by requirements based on system security margins while no requirements regarding WT design like FRT capability or voltage stability support are imposed. When the wind power penetration reaches levels that may threaten the system security, additional requirements concerning WT design are imposed. The requirement levels considered are summarized in Table I. Details regarding the times involved in the FRT-process in case of requirement levels B and C are shown in Fig. 5. Voltage dips are measured at the high voltage level of the grid connecting transformer. In case of voltage stability support, WTs must maximize its reactive power injections to the network without exceeding the turbine limits. The maximization of reactive current must continue for at least 150 ms after the fault clearance or until the grid voltage is recovered within the normal operation range. In an initial stage, when the wind power penetration is low, the requirements are those indicated by requirement level A (see Table I). At this level, WTs may be disconnected for their own protection if the voltage drops below a certain limit determined by the protection itself. Two system security margins, according to the Chilean grid code, must be met at this point [NT]: • the voltage level at all buses of the system must not decrease below 0.7 p.u., except during the fault itself; • the voltage level at all buses must be recovered to a band of 10% around the nominal value within 20 s after the fault appearance. If system security margins at a given network bus could be threatened, the requirements to new WT projects at this bus change to those presented in B to finally impose the requirements described in C.

Fig. 6. General diagram to obtain the AS-FRT.

It is assumed that WTs incorporated under a specific requirement level (A, B, or C) must only fulfill the level valid at the time of its network integration, independent of later changes in the level of requirements at the connection point. Therefore, retroactive effects are not applied in this work. Alternatively, a bounded period of time can be stipulated to the adaptation of the WTs to the new grid requirements. B. Proposed Methodology In order to obtain technically justified grid code requirements for WTs, a systematic approach to gradually increase the wind power in the system (reflecting the expected development of wind power over the time) is required. The proposed methodology is shown in Fig. 6. The first step of the methodology is to establish potential places for the installation of the WTs. Since the evaluation of every possible connection point is not viable, the methodology only considers a limited set of places in which the wind potential is especially high. This is justified since the energy produced by a WT is proportional to the cube of the wind speed, meaning that a slight increase in wind speed results in a large increase in electricity generation. Therefore, in a competitive environment, places with higher potential are those that will probably be used for WT installation. Once the potential places for WT installation are chosen, the next step is to determine a criterion to gradually incorporate the turbines into the network. Since the trend followed by WT investors is not predictable, the incorporation of WTs into the network is made according to a list of priorities in which the potential places are ranked according to an economic point of view. Based on this list, WTs are first going to be incorporated


in the place with the higher rank, i.e., the most attractive place for investors. The list must include not only the wind potential but also the installation costs (including transportation, land costs), interconnection costs to the transmission system, transmission charges to be paid by the WTs owners, and others costs that could arise at each particular place. If all relevant costs are considered, the use of the list of priorities should reasonably reflect the trend to be followed by WT investors in a competitive environment. The next step is to determine the set of contingencies to be simulated in order to ensure the fulfillment of the system security margins for all operating conditions of the system. Since in a real power system, this could lead to an intolerable number of dynamic simulations, a method to select the critical contingencies (CC) is applied. In this work, the first step includes the determination of electrical zones within the power system in which the same group of CC can be found. By this way, the amount of dynamic simulations to realize can be significantly reduced. The Chilean transmission network is an isolated power system prone to face voltage stability problems due to its longitudinal configuration [26]. Considering this fact and also the performance of the considered WTs regarding reactive power, the determination of the electrical zones is based on voltage criteria. A voltage control area (VCA) is defined as a group of buses that are coherent in terms of voltage deviation for any set of disturbances in the network [27]. Different approaches have been proposed in the literature for the determination of VCAs. Reference [28] uses, for example, the concept of electrical distances which are a measure of voltage interaction between different buses of the system. A normalized Jacobian matrix where the smaller off-diagonal elements are eliminated can also be used [29]. Once the electrical zones are determined, the establishment of the CC of each VCA is made using the well-known contingency selection methods “generator shift factors” and “line outage distribution factors” [30]. The dynamic simulations of the CC will determine the maximum wind power at a given network bus for each requirement level, thus also indicating when a change of the requirement level must be carried out. The integration process of wind power begins in the first place of the list of priorities with the requirement level A. The WT incorporation at this place continues until no more integration is possible because the system security margins could be threatened. When the limit is reached, a change of the requirement level at the pertinent network bus takes place, meaning that new WTs can be incorporated here only under the requirement level B. Further WT integration continues in the following place according to the list of priorities with the requirement level A. The process of WT incorporation finishes when no more places are available in the network and the requirement level at each electrical zone is the requirement level C. To determine technically justified requirements where the real needs of the power system are reflected, the development of the network, i.e., new generating units, lines, compensation devices, and also the current development of wind power, must be considered. In this context, the requirements obtained with the AS-FRT must be periodically reevaluated in order to incorporate relevant changes related to the network expansion as well as the current wind power penetration level.

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C. Considered WT-Technologies For the requirement level A, it is assumed that only FSIGs are integrated into the network. This is justified since investors are constantly seeking to minimize their investment costs, and FSIGs are the cheapest technology able to fulfill this requirement. For the requirements levels B and C, only DFIGs are integrated into the system. No oversizing of the converters is considered. Integration of FSIG under the requirement levels B and C would be also possible if additional dynamic reactive power compensation is included. In this work, however, it is assumed that if FRT capability and/or voltage support are required, the additional costs of incorporating supplemental reactive power compensation (in order to be able to ride through grid faults and/or to support voltage stability), together with the higher efficiency of DFIG, make more profitable the use of DFIG rather than FSIG.1 Therefore, only DFIGs were integrated into the system in this case. No synchronous generator with full converter was considered since the proposed strategy is aimed at countries where the wind power integration is carried out without any kind of subsidies. This implies that no major incentives to invest in “top technologies” are generated. For the Chilean case, for example, the competitive environment, together with the existing cost difference between WTs with induction generators and synchronous generators with full converter [37], had made that until now, only WTs projects based on induction generators (either SCIG or DFIG) have been registered. No project with synchronous generators with full converter has been presented until now. Due to these reasons, it was assumed that the wind power network integration in such countries is going to be characterized by the connection of the cheapest WT alternatives, and therefore, only WTs based on induction generators were considered. V. CASE STUDY In Chile, the government support of renewable energies is reflected by enforcing all generation companies, starting at year 2010, to supply at least 5% of its contracted energy using renewable sources, increasing with a rate of 0.5% annually until 2024 [31]. In this context, open access is ensured for WTs and based on a merit order dispatch, they can be considered base load generators. Although this meant a major improvement for the integration of the technology into the Chilean network, the law does not introduce any explicit subsidy or compensation, leaving the WTs to participate in a competitive market. Until October 2009, there has been around 1100 MW of 14 approved WT projects for the Chilean Central Interconnected System (SIC), and there are more still under study [32]. Regarding the requirements for wind power plants in case of disturbance, WTs connected to the Chilean network must be able to remain connected to the grid as long as the voltage at the connection point is above 20%. No requirement regarding voltage stability support is demanded until now. 1This depends on the project itself and how demanding the grid requirements are. For each case, independent studies must be carried out, in order to make appropriated assumptions.

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TABLE II PLACES RANKING ACCORDING TO THE LIST OF PRIORITIES

demand of this area is low, containing around 15% of the total demand of the system. B. Developed Model

Fig. 7. Simplified diagram of the Chilean transmission network.

A. Chilean Central Interconnected System The simulations performed in this work are based on the Chilean main transmission system expansion plan for the year 2015 [33]. The system still shows low penetration levels of wind power, having 18 MW of FSIG and 46 MW of DFIG units. The system is a good example of a medium size isolated power system characterized by long distances among major load centers and generation areas. Long transmission lines are also a distinctive of the system covering a total length of 2.200 km. The voltages in the bulk network are from 110 to 500 kV with nearly 750 busbars. The system is composed by hydroelectric and thermal power stations. Hydroelectric power plants are concentrated in the south of the country comprising 50% of the whole installed capacity. Series compensation is available in some 220- and 500-kV transmission lines in the central zone of the system. In order to illustrate the structure of the network, a simplified diagram is shown in Fig. 7. The developed model for 2015 considers additional generation capacity expected to come online until this year as well as the decommissioning plan of conventional power plants. The planned installed capacity is around 15 GW for an estimated peak load of 9 GW. The whole power system can be separated in three main electrical areas. Area X, covering the north of the country, is characterized by thermal generation with around 30% of the total thermal capacity of the system. The power consumption in this area is low, representing no more than 15% of the total demand. This area is characterized by having good wind conditions concentrating around 60% of the approved WT projects for 2015 in the Chilean system. Area X is connected to Area Y through a 220-kV double transmission line of 125 km. Area Y concentrates around 70% of the total demand of the system and includes the major load center located in Santiago. Area Z contains 50% of the total installed capacity of the system. The total

A simplified model of 250 busbars of the Chilean network was implemented in the power system simulation tool DIgSILENT Power Factory [34]. The model includes 170 synchronous generators representing the conventional power plants at the year 2015 and around 100 consumption centers distributed throughout the system. The wind power injections considered at each connection point consist of induction generators of 2 MW. In case of DFIGs, the converter rating was chosen to be 30% of the generator capacity, while for FSIG, the compensation was rated at 40% of the wind farm capacity. The WTs were represented by aggregated models where the turbines are modeled as one equivalent generator. For all DFIGs integrated into the network under the requirement level C, a control strategy allowing both grid and rotor side converters to inject reactive power during and after the fault was developed in order to support the voltage stability. Details regarding the models used in the dynamic simulations can be found in [24]. C. Results In order to obtain technically justified FRT requirements for the Chilean case, the methodology presented in Section IV was applied. The WTs were integrated into the network in four different connection points (network buses) distributed in two of the electrical areas shown in Fig. 7. No wind power network integration was realized in Area Z. Although some interesting places in terms of wind potential were detected in this area, the technical conditions of these locations (like distance to the network) were not appropriate enough to make the related WT projects interesting from an economic point of view (at least not in a competitive environment without subsidies like the Chilean system). The ranking of each place according to the list of priorities as well as the pertinent area where each place belongs is shown in Table II. The installed wind power capacity obtained by adoption of the AS-FRT is shown (in MW) in Table III. In order to compare the effects on power system stability of the wind power integrated by adopting the AS-FRT with the effects that have the same installed capacity of wind power but only considering WTs able to fulfill the requirement level C, the power system shown in Fig. 7 was simulated for two cases: • Case 1: 1256 MW of installed wind power capacity obtained by the application of the AS-FRT (Table III);


TABLE III WT INSTALLED CAPACITY BY ADOPTION OF THE AS-FRT

Fig. 8. Terminal voltage of the wind park connected at bus II.

Fig. 9. Terminal voltage of the wind park connected at bus IV.

• Case 2: with the same installed wind power capacity as before (1256 MW) but only considering DFIGs able to fulfill the requirement level C. The following figures show two extreme cases related to the voltage drop at the connection point of the wind parks, thus presenting different cases regarding the dynamic behavior of the WTs: a large voltage drop at the connection point of the wind farm (Fig. 8) and a distant fault (Fig. 9). Beyond the penetration limits presented in Table III, the system behaves unstably.

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Fig. 8 shows the voltage at the connection point (high voltage level of the grid connecting transformer) of the wind park connected at bus II by applying a three-phase short circuit at the during 150 ms. This case is thus resame bus at lated to the worst case with respect to maintaining voltage stability, since a three-phase short circuit is applied directly at the connection point of the wind farm. In the case of the AS-FRT (dotted line in Fig. 8), the short circuit at bus II leads to the network disconnection of the connected FSIGs just after some milliseconds of the fault appearance due to the action of the protective relay system. The DFIGs connected at this bus remain connected. Nevertheless, the high currents appearing in the stator windings activate its protection system by short circuiting the generator rotor through the crowbar and thus blocking the RSC. The GSC remains in operation. In case of DFIG integrated under the requirement level C, the GSC is activated to maximize its reactive power injections when the fault is detected. The process of reactive power injection continues until the grid voltage is recovered within the normal operation range. As expected, Fig. 8 shows that the voltage restoration to its nominal value is faster when considering just DFIGs able to fulfill the requirement level C due to its reactive power injections supporting the voltage stability. Nevertheless, the difference between both voltage responses is not significant. Although the voltage restoration with the AS-FRT is slower, the response is . No voltage improvequite similar for both cases from ment during the fault is obtained when considering just DFIGs supporting the voltage stability. Fig. 9 shows the voltage at the high voltage level of the grid connecting transformer of the wind park connected at bus IV by in a neighborapplying a short circuit of 150 ms at hood bus. As in the case before, when considering the AS-FRT (dotted line in Fig. 9), the FSIGs connected at bus IV are disconnected when the fault is detected. The major difference with the fault showed in Fig. 8 is that in this case, the induced currents in the rotor circuit of the DFIGs are not high enough to activate the crowbar protection, and therefore, the RSC remains connected during the fault. Both converters are thus able to support the voltage restoration by injecting reactive power during the fault in case of WTs connected under the requirement level C. As can be seen in Fig. 9, the case considering just DFIGs able to fulfill the requirement level C corresponds to the highest level of the terminal voltage during the fault. The voltage improvement with respect to the case of the AS-FRT is around 8%. However, the difference between both voltage responses is important for just the first 1.3 s after the fault clearance. After this time, the voltage response at bus IV for the AS-FRT is as good as in the case of only considering DFIGs able to fulfill the requirement level C. Further dynamic simulations were developed including different operating conditions and fault locations in order to compare the transient response of the system by adopting the AS-FRT with the case where only WTs able to fulfill the requirement level C are considered. All of them show similar results regarding system performance, indicating that there are

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no major differences regarding the effects on the power system stability. VI. CONCLUSIONS In this work, an adaptive strategy to obtain technically justified FRT-requirements for WTs is proposed. The level of requirement imposed by the strategy is increased stepwise over time, depending on system characteristics and on wind power penetration level. The strategy introduces stringent requirements only when they are technically needed for a reliable and secure power system operation. The strategy is based on technical and economical considerations. Dynamic simulations have shown that the system performance by adopting the proposed strategy does not differ significantly when the same installed capacity of wind power but only considering WTs able to ride through grid faults and to support voltage stability are considered. Although the voltage restoration to its nominal value is slower by considering the strategy, the differences are not significant enough to justify stringent requirements in an initial stage of wind power development. The presented strategy may be especially important in countries with still low amounts of wind power but with high projections of future network integration, where no subsides for renewable energies exist. In those cases, when wind power penetration level is still low, imposing less costly and challenging requirements in the initial stage could promote wind power network integration without threatening the system security. REFERENCES [1] F. O. Resende and J. A. Pecas Lopes, “Evaluating the performance of external fault ride-through solutions used in wind farms with fixed speed induction generators when facing unbalanced faults,” in Proc. IEEE Int. PowerTech Conf., Romania, 2009. [2] N. R. Ullah, T. Thiringer, and D. Karlsson, “Voltage and transient stability support by wind farms complying with the E.on Netz grid code,” IEEE Trans. Power Syst., vol. 22, no. 4, pp. 1647–1656, Nov. 2007. [3] I. Erlich and U. Bachmann, “Grid code requirements concerning connection and operation of wind turbines in Germany,” in Proc. IEEE Power Eng. Soc. General Meeting, Jun. 2005, pp. 2230–2234. [4] Grid Code: High and Extra High Voltage, 2006, Status: 1, E.on Netz GmbH Tech. Rep. [5] Wind Turbines Connected to Grids With Voltages Above 100 kV—Technical Regulation for the Properties and the Regulation of Wind Turbines, Elkraft System and Eltra Regulation, 2004, Draft version TF 3.2.5. [6] Red Electrica de España (REE), 2006, Requisitos de Respuesta Frente a Huecos de Tension de las Instalaciones de Produccion de Regimen Especial, PO 12.3. [7] , T. Ackermann, Ed., Wind Power in Power Systems. Chichester, U.K.: Wiley, 2005, ch. 7, p. 135. [8] F. Van Hulle, Large Scale Integration of Wind Energy in the European Power Supply: Analysis, Issues and Recommendations, 2005, EWEA Tech Rep. [9] S. Foster, L. Xu, and B. Fox, “Grid integration of wind farms using SVC and STATCOM,” in Proc. UPEC’2006, Sep. 2006. [10] M. A. Pöller, “Doubly fed induction machine models for stability assessment of wind farms,” in Proc. IEEE Int. PowerTech Conf., Italy, 2003. [11] J. B. Ekanayake, L. Holdsworth, X. G. Wu, and N. Jenkins, “Dynamic modeling of doubly fed induction generator wind turbines,” IEEE Trans. Power Syst., vol. 18, no. 2, pp. 803–809, May 2003.

[12] J. G. Slootweg, H. Polinder, and W. L. Kling, “Representing wind turbine electrical generating systems in fundamental frequency simulations,” IEEE Trans. Energy Convers., vol. 18, no. 4, pp. 516–524, Dec. 2003. [13] S. K. Salman and A. L. J. Teo, “Windmill modeling consideration and factors influencing the stability of a grid-connected wind powerbased embedded generator,” IEEE Trans. Power Syst., vol. 18, no. 2, pp. 793–802, May 2003. [14] H. M. EL-Helw and S. B. Tennakoon, “Evaluation of the suitability of a fixed speed wind turbine for large scale wind farms considering the new UK grid code,” Renew. Energy, vol. 33, no. 1, pp. 1–12, Jan. 2008. [15] S. Probert, Generator Fault Ride Through (FRT) Investigation—Stage 1, Transpower, Feb. 2009, New Zealand. [16] G. Holdsworth L, X. G. Wu, J. B. Ekanayaka, and N. Jenkins, “Comparison of fixed speed and doubly fed induction wind turbines during power system disturbances,” Proc. Inst. Elect. Eng., Gen., Transm., Distrib., vol. 150, no. 3, pp. 343–352, May 2003. [17] T. Sun, Z. Chen, and F. Blaabjerg, “Voltage recovery of grid-connected wind turbines after a short-circuit fault,” in Proc. 29th IECON 2003 Conf., Nov. 2–6, 2003, pp. 2723–2728. [18] O. Samuelsson and S. Lindahl, “On speed stability,” IEEE Trans. Power Syst., vol. 20, no. 2, pp. 1179–1180, May 2005. [19] V. Akhmatov, “Analysis of dynamic behavior of electric power systems with large amount of wind power,” Ph.D. dissertation, Technical Univ. Denmark, Kgs. Lyngby, Denmark, Apr. 2003. [20] S. Foster, X. Lie, and B. Fox, “Coordinated control and operation of DFIG and FSIG based Wind Farms,” presented at the Power Tech Conf., Lausanne, Switzerland, 2007. [21] M. Kayikçi and J. V. Milanovic, “Assessing transient response of DFIG-based wind plants—The influence of model simplifications and parameters,” IEEE Trans. Power Syst., vol. 23, no. 2, pp. 545–554, May 2008. [22] J. Morren and S. W. H. de Haan, “Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip,” IEEE Trans. Energy Convers., vol. 20, no. 2, pp. 435–441, Jun. 2005. [23] A. D. Hansen and G. Michalke, “Fault ride-through capability of DFIG wind turbines,” Renew. Energy, vol. 32, no. 9, pp. 1594–1610, Jul. 2007. [24] C. Rahmann, H.-J. Haubrich, L. Vargas, and M. B. C. Salles, “Investigation of DFIG with fault ride-through capability in weak power systems,” presented at the IPST Conf., Japan, 2008. [25] W. Qiao and R. G. Harley, “Effect of grid-connected DFIG wind turbines on power system transient stability,” in Proc. IEEE PES 2008 General Meeting. [26] L. Vargas, V. H. Quintana, and R. Miranda, “Voltage collapse scenario in the chilean interconnected system,” IEEE Trans. Power Syst., vol. 14, no. 4, pp. 1415–1421, Nov. 1999. [27] R. A. Schlueter, I. Hu, M. W. Chang, J. C. Lo, and A. Costi, “Methods for determining proximity to voltage collapse,” IEEE Trans. Power Syst., vol. 6, no. 1, pp. 285–292, Feb. 1991. [28] J. Zhong, E. Nobile, A. Bose, and K. Bhattacharya, “Localized reactive power markets using the concept of voltage control areas,” IEEE Trans. Power Syst., vol. 19, no. 3, pp. 1555–1561, Aug. 2004. [29] C. A. Aumuller and T. K. Saha, “Determination of power system coherent bus groups by novel sensitivity-based method for voltage stability assessment,” IEEE Trans. Power Syst., vol. 18, no. 3, pp. 1157–1164, Aug. 2003. [30] A. J. Wood and B. F. Wollenberg, Power Generation, Operation and Control, 2nd ed. New York: Wiley, 1996. [31] [Online]. Available: http://www.senado.cl/. [32] Sistema de Evaluación de Impacto Ambiental (Environmental Impact Assessment Office). [Online]. Available: http://www.e-seia.cl/. [33] Comisión Nacional de Energía (National Energy Commission). [Online]. Available: http://www.cne.cl/cnewww/opencms/. [34] DIgSILENT PowerFactory. [Online]. Available: http://www.digsilent.de/. [35] C. Feltes, S. Engelhardt, J. Kretschmann, J. Fortmann, F. Koch, and I. Erlich, “Comparison of the grid support capability of DFIG-based wind farms and conventional power plants with synchronous generators,” in Proc. IEEE Power Eng. Soc. General Meeting, Jul. 2009, pp. 1–7. [36] M. Rasmussen and H. K. Jørgensen, “Current technology for integrating wind farms into weak power grids,” in Proc. IEEE Power Eng. Soc. Transmission and Distribution, Aug. 2009, pp. 2230–2234. [37] E. Hau, Windkraftanlagen. Grundlagen, Technik, Einsatz, Wirtschaftlichkeit. Berlin, Germany: Springer-Verlag, 2008.


C. Rahmann was born in Santiago, Chile, on September 15, 1979. She received the degree in Electrical Engineering from Universidad de Chile, Santiago, in 2005. She is presently pursuing the Ph.D. degree at the Institute of Power Systems and Power Economics (IAEW), RWTH Aachen University, Aachen, Germany. Her main interests are modeling of electrical power systems, power systems dynamics and stability, distributed generation, and integration of wind power into power networks.

H.-J. Haubrich was born in Montabaur, Germany, on March 1, 1941. He received the diploma and the Ph.D. degree in electrical engineering from the Technical University of Darmstadt, Darmstadt, Germany, in 1965 and 1971, respectively. In 1973, he joined the Vereinigte Elektrizitaetswerke Westfalen AG, Dortmund, where he was head of the main department for network planning. Since 1990, he has been head of the Institute of Power Systems and Power Economics and the Forschungsgemeinschaft Energie (FGE). His research is focused on technical and economic aspects of electrical power engineering.

A. Moser was born in Linz, Germany, in 1965. He received the diploma and the Ph.D. degree in electrical engineering from the RWTH Aachen University, Aachen, Germany, in 1991 and 1995, respectively. Since March 2009, he has been head of the Institute of Power Systems and Power Economics. He was in charge of the business development at European Energy Exchange (EEX) in Leipzig, Germany, and head of the clearing and settlement department of EEX’s clearing house. From 1997 to 2000, he was a product developer for TSO applications with Siemens AG in Nuremberg, Germany, and Minneapolis, MN.

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R. Palma-Behnke (SM’04) was born in Antofagasta, Chile. He received the B.Sc. and M.Sc. degrees in electrical engineering from the Pontificia Universidad Católica de Chile, Santiago, and the Dr. Ing. degree from the University of Dortmund, Dortmund, Germany. He is currently a Professor in the Electrical Engineering Department at the University of Chile, Santiago. His research field is the planning and operation of electrical systems in competitive power markets and new technologies.

L. Vargas received the Electrical Engineer diploma and the M.Sc. degree from the Universidad de Chile, Santiago, in 1985 and 1987, respectively, and the Ph.D. degree in electrical engineering from the University of Waterloo, Waterloo, ON, Canada. Since 1994, he has worked at the Universidad de Chile, where currently he is an Associate Professor. His main research interests are in the areas of wind energy, supply and demand forecasting, and expansion planning of energy systems.

M. B. C. Salles (S’02) received the degree in Electrical Engineering at Mackenzie Presbyterian University, Sao Paulo, Brazil, in 1998 and the M.Sc. degree from the University of Campinas (UNICAMP), Sao Paulo, in 2004. He is pursuing the Ph.D. degree at the University of São Paulo (USP). He has experience on computational modeling of power systems and also of electromagnetic devices using the Finite Element Method. His main interests are on distributed generation, power generation, dynamics and stability of power systems, wind turbines, and induction generator.