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  • 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 44 UNIFIED POWER CONTROL UNDER DIFFERENT GRID CONDITIONS FOR PMSG- BASED WECS Anas Abdulqader Khalaf Bharati Vidyapeeth Deemed University, Pune ABSTRACT An integrated power control strategy under conditions of different grids operating permanent magnet synchronous generator-based wind energy conversion system (WECS). Strategy generator- party converter DC-link voltage is used to control and grid-side convert And power in the grid is responsible for control of flow injected. Generators double torsional oscillation controller capacity- side lies the drive-train is due to grid-side attributes control grid code defined in the active and reactive current (power) can satisfy the needs Used by, and at the same time mitigates the current distortion even with unsymmetrical bending apparatus grid fault. Grid generator-party converter automatically during faults, generator DC voltage reduces to maintain current and the resulting acceleration is counteracted by regulation pitch generator. If the proposed scheme is employed, compared with traditional strategy, DC chopper ride through the fault of the WECS is intended for help, can be eliminated. Compared with the control scheme structured variables the proposed strategy faster and more accurate recovery of the grid electrical responses, which is beneficial to check the effectiveness of the proposed strategy simulation results. Index Terms: DFIG, PMSGs, LVRTs, WECS, PCC, STATCOM, GSC, BC, RSC, PWM. I. INTRODUCTION Among various renewable energy sources, wind power generation is one of the fastest- growing energy sources have been concerned as double fed induction generator differently. (DFIG) wind systems, a direct drive permanent magnet synchronous generator wind energy conversion system (PMSGs) based such as a gearbox, high power density, high precision and simple control method, the initial installation costs [1], [2] has a lot of advantages except. Scale of wind farms becomes bigger and bigger, more important the grid connection of wind turbines condition. Recently, a few countries Remove the electric grid [3] [4] to add a dedicated grid wind turbine systems has released the code. Plus micro grid is and smart grid energy efficient management [5], [6] research. However, these systems, grid voltage are a fairly conventional compared fluctuated. Therefore, a ride INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com IJEET © I A E M E
  • 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 45 through wind power generation system for the grid control of unusual situations. Grid codes wind turbine system (LVRT) capacity need low voltage ride through. LVRTs for variable speed wind turbine several solutions proposed systems. For this purpose, during active power grid fault [7], [8] to absorb a crowbar in the rotor system in favor of DFIG (an external resistor) is connected to the common coupling (PCC) or reactive power. Point voltage grid- side converter (GSC) is controlled by the wind turbine to produce power for its active operations. However, in the case of a weak grid and grid fault, the GSC in smaller power capacity and voltage fluctuation due to substantial risk of per No power or voltage support. In addition, a static synchronous compensator (STATCOM) DFIG wind turbine smooth operation during grid faults has been used to guarantee. Reactive power grid has been installed on PCC [9] STATCOM dare injection – [11]. Since this is a grid fault of rotor-side converter (RSC) cannot protect however, DFIG ride through capability alone STATCOM used. On the other hand, this crowbar circuit which protects from over-current SC rotor when the grid is a fault with the Temal. PMSG wind turbine systems, low-cost and simple control with the performance of a braking chopper (BC) LVRT [12] has been applied to-[14]. However, it is BC. Delete since bus power wind turbine systems to improve the quality of the output power is hard to find. The other way, in STATCOM has keep wind turbines grid connected to the grid during the [15] blame the system. With this method, the voltage is regulation many stats And State as transient as well as improved State. As well as being used in PMSG wind turbine generator system is a full scale back pulse width modulated (PWM) converters, of which configuration shown in fig through the grid. Fig. 1: National grid codes [3] Traditional, DC-link voltage is controlled by GSC. However, in the case of GSC grid voltage sags can be out of control on DC-link voltage a grid fault excessive wind turbines to generate electricity, but to generate power grid cannot absorb completely since the enlargement. Fig. 2: PMSG wind power system
  • 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 46 In order to be compliant with the grid codes, DC chopper, such as additional measures, grid disturbances [4], [15] during the system process is required to assist when the grid is at fault, with no more effort to control DC chopper unbalanced power generators and grid can spread among the worst scenario (grid voltage drops to zero),, DC chopper power rating (MW) should be full scale [15]. Such a strategy is hardly some grid codes will be shown in this paper can satisfy the needs of DC Chopper to destroy, a variable-structured controller have been designed [16] and [17]. The controller generator-party power control when grid fault is detected by MPP tracking to produce less electricity. Yet, there is enough torsional damping control thecWECSin some instances marked [18] cannot provide. Fault ride (FRT) capacity to get through enough torsional damping while the reserve ToA novel control strategy is proposed in [3] and [19]. Active power flow control instead, generators-party converter DC-link voltage grid-side converter control, active power grid while maintaining the power controller is used for, the more active a notch filter and a damping loop step kampcTorsional vibration ester, consisting of a positive level is designed to ensure this kind of a strategy successfully during symmetric grid faults can assist WECS FRT [19] as verified by simulation. However, strategy is complex and its performance depends on system parameters. Furthermore, such a solution is hardly unsymmetrical bending apparatus grid fault scenario that is more common in power system can be adaptive. This dissertation work MW class PMSG-based WECS unsymmetrical bending apparatus under various grid fault conditions to operating an integrated power control strategy. Strategy, generators-party converter DC-link voltage, torsional oscillations inherent damping maintains, providing proposed strategy compared with conventional strategy DC chopper over. Comparing the proposed control variable-structured plan strategy to significantly provide torsional damping WECS And a faster and more accurate current (power) during the response grid defect. [19] Compared with the scheme, a system parameter proposed strategy, which results in good simple implementation and robustness is required. In addition, the current in the grid with the perversion of injected tactical can be reduced so that when unsymmetrical bending apparatus is to improve the quality of power grid fault. Analysis and simulation results of the effectiveness of the proposed strategy Check. In fact, they can be divided into two groups: topologies galvanic isolation (fig. 3a) and those with isolation. Lines frequency (LF) transformers (fig. 3 b) over the past decades galvanic isolation for widely used. The main drawbacks LF transformer high weight and high value for these reasons, HF isolation (fig. 3 c) has made photovoltaic applications and topologies for wind power applications particularly popular. Fig 3: Block diagram of interfacing converters
  • 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 47 This section introduces the concept of wind energy; properties of wind energy, emphasizing wind energy extraction by means of PMSG based VSWT. Operation modes of VSWT with fixed blades are analyzed and generator characteristics are given. Need of Permanent Magnet Synchronous Generator in Wind Turbine Grid System Extract the optimum wind power wind turbine generator (WTG) run variable-frequency variable-speed mode is achieved by speed of the rotor tip speed ratios. maximizes the aerodynamic efficiency by maintaining the value of wind speed in sympathy with the ratio allowed for the realization of permanent magnet synchronous generator (PMSG) load line very closely wind turbine generator max power line should be matched to such a In this case, a good generator and system in Milan's best performances, as well as load for maximum utilization of PMSG wind driven between. However, power electronics and control strategies in the recent progression in many different ways to regulate the voltage of PMSG is made possible when the generator torque line can be controlled, Turbine generator as power optimal desired locus loading shaft locus can be made to comply. Theoretical models of wind turbine generator have been developed for producing power. External rotor described in PMSG 20kW CRESTA is used in mathematical models the WECS. Model power voltage and current can be given in terms of mobility (4) and (5) [1]: m λ r ω d i d L r ω q di q L q Ri q v +−+−= dt (4) qiqLrω d di dLdRidv ++−= dt (5) Where, R and L are the machine resistance and inductance per phase. VDS and vq are the 2- axis machine voltages. Id and IQ are the 2-axis machine currents. λM is the amplitude of the flux linkages established by the permanent magnet and ωr is the angular frequency of the stator voltage. The expression for the electromagnetic torque in the rotor is written as: ( )[ ]qimλdiqiqLdL 2 P 2 3 eT −−=             (6) The relationship between ωr and ωm may be expressed as: mω 2 p rω = (7) This equation shows that the generator torque direct quadrature can be controlled by the current element. (4), VD, vq and ID, are stator voltages and currents respectively d-q frame; RS stator resistance; Ld, Lq inductances of d-q are in frame; Be, ωh machine mechanical and electrical; Te, TM are the mechanical turbine electro-magnetic torque and torque the machine respectively; NP machine pole pair number; Permanent magnets are made by Am rotor flux is concerned; J is the total system inertia and associated mechanical drive train for FB friction coefficient.
  • 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 48 The tip speed ratio λ MPPT strategy will keep its optimum turbine price, thus keeping the turbine power coefficient Cp maximum value on the rotor speed control. CP in known and marine current turbine speed reference speed V by the traditional count can be obtained by measuring the flow velocity must be speed-based MPPT opt/R can be expressed in the form of paper low pass filter in case of rotor speed great impact conventional MPPT algorithm calculated reference added to modify the proposed strategy generates the reference speed. opt m _ ref λ1 s 1 V T R ω = ⋅ + (7) Where is the filter time constant T and generator power to reduce volatility caused by great disturbance plays an important role in setting the T to zero (7). Conventional MPPT algorithm (turbine speed fluctuation of sea current terms) leads to. With reference to the speed of conventional MPPT tip-top ratio, the generator under the influence power more than great power turbine seriously will fluctuate this can be explained as follow: when we ignore friction loss of torque equation (4), we can get. m turbine generator m m m e m d dt P P T T J ω ω ω ω− = − = (8) (6), this power differential turbine and generator mainly depends on the system inertia J and rotor speed change rate when a large inertia and low operating speed is. surpassing the current speed is faster when the sea changes the speed of the turbine rotor under the influence, a synchronous change rate by conventional MPPT control; Then there is negligible considering the MCT a 1.5 MW system with large total system inertia ∆P can be huge. Figure 8 shows the power and speed ratio use conventional tip-top MCT generator power MPPT and proposed terms of profiles with optimized filter time constant (T = 7s). More detailed analysis can be found in [6]. Fig. 8: Turbine and generator power responses with conventional MPPT (left) and proposed MPPT (right)
  • 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 49 Fig 9: System performances with different filter time constants (6), this power differential turbine and generator mainly depends on the system inertia J and rotor speed change rate when a large inertia and low operating speed is. surpassing the current speed is faster when the sea changes the speed of the turbine rotor under the influence, a synchronous change rate by conventional MPPT control; Then there is negligible considering the MCT a 1.5 MW system with large total system inertia ∆P can be huge. Figure 8 shows the power and speed ratio use conventional tip-top MCT generator power MPPT and proposed terms of profiles with optimized filter time constant (T = 7s). More detailed analysis can be found in [6] Rotor flux angle of electric generator-party control for the implementation of the angle, need an encoder/resolver or through estimates can be obtained by Stator voltages and currents do not load the machine with the rotor flux by tested and electromotive force (E = ωsψr) can be calculated by measuring.
  • 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 50 Grid-side model Grid-side converter grid has to deal with dips in heterogeneous grid [21] dips. symmetrical components based on the use of three different ways to deal with these three methods have been expanded, vector current feed forward control with the grid voltage (VCCF) negative sequence current work has been used in this method is applied to the control with the positive sequence, And grid-side converter negative-sequence model to develop no need therefore, ideal for grid-side converter is shown in Dc-Link Voltage Balance DC-link neutral point voltage balancing a virtual space vector modulation switching strategy and a Tai-lured voltage control [31], which bounced back topology is used in both the NPC converters balance is achieved through this approach, the DC-link neutral point to include some information about the model No. Modeling of wind turbines Considered in this work by a fixed pitch wind turbines a WECS PMSG are driven; An AC- DC power conversion stage two different perspectives and implemented using a vs. CCI. Shown 2.1 a brief description of each element of the system is given below. II. LITERATURE SURVEY This paper continued operating on wind energy in the grid frequency is a major area of application of the variable-speed generators. They operate and control is a description of this variable-speed wind generator: direct driven permanent magnet synchronous generator (PMSG). This generator is controlled entirely via a frequency converter, which is a pulse-width-modulation (PWM), an intermediate DC circuit consists of a PWM inverter and power is connected to the network. Circumstances under which is different load generator with maximum efficiency incident from the Air Max Power is controlled to obtain. Vector control grid-side inverter power factor regulation allows the windmill they whole system dynamic performance. Various pilot tests a prototype 3 kilowatt system proposed in [1] to verify the benefits of also. Wind turbine generator systems for five different than the purpose of this paper, namely power-flush and permanent magnet direct drive permanent magnet generator system, and induction doubly-fed generator system with three-phase single phase gearbox with Aye cited doubly-fed generator ren system. The cost for a given wind than climate and annual energy yield double-fed induction generator based on. Single phase gearbox with energy costs in terms of yield divided by the most attractive double-fed induction generator. Three-phase gearbox is a cheaper solution is to use standard components. Permanent magnet direct drive generators produce highest energy but this
  • 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 51 gearbox [2] more expensive than generator with the system, even though it is cheaper than direct drive generators, power flush. The goal of this report one of the mapping grid fault types and their frequency is present in different countries. Reports relevant to wind turbines in various countries of low voltage ride through detailed overview capabilities. Quantification of loads for the most relevant study cases Effects on wind turbines? Lifetime is defined [3] Power grid simulation for multi-MW wind power applications for a test bench for the hardware configuration and the control system are described using two variable voltage generated. (Final phase four) medium-voltage, three-stage, three-level voltage source converters insulated gate bipolar transistor with press-pack and a specially designed the step-up transformer system fundamentally different from the known grid simulation test side that almost always use switched inductances. Discussed in MATLAB/Simulink [4] simulation results and measurements is as well as systems are using barriers to. They show the Simulator system for micro-grid with distributed generators. This Simulator, the power amplifiers is a steady state and transient-state operation for a test result and a PV system and wind turbine system design one Yojana two simulation case results the device is delivered including a renewable energy source contained. Simulator generator with a micro-grid was designed for the simulation of the system consists of a power amplifier and a RTDS. deliver a real-time simulation of RTDS generator with a micro-grid models and a renewable energy source The power amplifier for the model of RTDS., which was distributed to each node voltage generator with micro- grid or an indication of renewable energy sources in the production mode was used to produce a simulated the Simulator a test system, distributed generators to PC a dynamic and stable performance are used can be applied to a future study The distributed generator system and model of development performance improvement. [5] III. PROPOSED APPROACH FRAMEWORK AND DESIGN Design of the Unified Power Control for the MW Class PMSG-Based WECS Rising wind energy penetration into the utility grid interconnection requirements leads to a sustainable development. In fact, WECSs in different grids to operate robustly and as a traditional power plant to provide support services to request. PMSG based integrated power control scheme MW class WECS satisfies the requirements of the grid is designed to Described in this section. The scheme, generators party converter is to maintain a constant DC-link voltage and actively controlled torsional oscillation damp. grid-side converter under conditions different grid power systems operator (PSO) as a positive and necessary Negative sequence output power can regulate if a grid is to blame, the DC-link voltage constant, to keep generator-party power converter control generator power and thus minimize the flow to dc link stator begins rotating power surplus large public. Kinetic energy is buffered and generator in acceleration is reflected in the price above its rating when the generator increases the speed of acceleration pitch control counteracted by Can. A. Controller Design of the Generator-Side Converter WECS Jg JH, usually _ _ Max (ts, t _ s, CDC), dynamic responses of WECS can be classified into three time scales. Scale of different response time [24] as quaint country is based on the principle, slow moving and fast mobility will not affect one another. So fast, fast dynamic steady state converges promptly when slow dynamic discussion enough to be should. Similarly, the slow dynamic study of fast dynamic neglected. As a result, based on the following three control WECS sub models can be designed:
  • 9. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 52 In which dc is the reference of the dc-link voltage and the symbols with superscript “-” represent the quasi-steady value of the state variables. Design the reference of the generator torque as proportional and integral coefficients. The slowest model (5) behaves as a first-order system. Fig. 2 shows the typical torque characteristics of the WT. Considering the generator torque as Tg1, the WECS has two possible steady operation points, in which point “B” is stable and “C” is unstable. Regarding ωg = ωg = ωh in (8), dynamics model (5) is stable if kh > ∂Twit/∂ωh. The conclusion can also be identified from the fact that there is unique cross point “B” between torques Characteristic and line OB in Figure. Model (6) is a second-order system and its transfer function with the control input (8) is as follows: Dynamic model (9) will be stable if kh > 0. In (7), responses of TG anoxic are independent. TG responses stably with positive ts. Substituting (8) into (7) and ignoring the high-order items, if z = X∗Dc − Xdc, we have Dynamic model (10) is stable if KP > tski. Based on the perturbation theory, the whole system can be stabilized by controller (8) if all the subsystems are stable or (11) holds true. The proposed scheme has inherent oscillation. Damping capability since the damping torque is provided with the generator speed feed forward
  • 10. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 53 As the first-order dynamic, with vector control techniques [3] PMSG is controlled by stator torque generator and voltage control.-axis rotating reference frame on rotor flux straight into, can be decoupled generator-side converter control diagram Shown in Figure PI regulator DC-link voltage. controller upper and lower limits (11) is to satisfy equation (8) is the input vector controller and i ∗restrictions apply to avoid permanent magnetic of ds, can be set to zero. the rotor flux angle θr frame changes and (8) to the generator speed _ ωg PMSG The proposed master plan [25] electromotive force back the results in the implementation of sensor less based on estimates. B. In the design grid-side converter control Generators and grid are decoupled by back-to-back converter, grid disturbances affecting the operation of generator-party converter. Grid with grid side converter is the WECS-mistakes to take responsibility for running properly. Symmetrical sinusoid currents WECS neckline during power grid faults. And unsymmetrical bending apparatus to improve the quality, positive-sequence active and reactive powers proposed in the strategy are regulated. Control diagram fig. Grid-side converter control rotating reference frame vector control techniques, including positive-sequence is aligned to the grid voltage vector based on positive and negative sequence voltage component grid. Second- order generalized integrator-based phase closed loop [26] with. Angle θ + positive-sequence voltage vector grid to control the frame changes and implemented such a strategy, with positive-sequence. Active and reactive powers grid-side converter, pout, pos, Quota, pos, can be controlled independently-, q-axis, g, wisdom, by living streams. In figs. 4, pout, pos, Quota, pos can be calculated with the following equation: Where · represents the dot product and × denotes the cross product; the symbols with superscript “+” represent the positive sequence variables. Since only positive-sequence powers are controlled in such a strategy, the currents injected into the grid only contain positive sequence components so that the current distortions are mitigated [27]. C. Design of the Pitch Controller Pitch control is activated once your rating above the value increases the speed of the generator and the power extracted from the wind energy reduces later. Aerodynamic characteristics of nonlinear WT, pitch angle control is quite difficult. Conventional linear controller a detailed wind speed range cannot provide satisfactory performance. Controller can be effective with scheduling advantage but the benefits are hard to tune. "And the authors make a strong pitch controller based on inverse system theory proposed. The controller is simple to implement and strengthen the system parameter deviations. Fig. 5 it control diagram shows it to block an inverse system-based control system controls and control parameters by nominal deviation due to error are a strong compensator. Controller is employed in the newspaper and can be found in the details.
  • 11. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 54 Fig. 9: Control diagram of the generator-side converter Fig.10: Control diagram of the grid-side converter Fig. 11: Control diagram of the pitch angle In section IV we are presenting the current state of implementation and results achieved. IV. CONCLUSION Converter-based power system is due to its accelerated power WECS feedback's ability to improve transient stability. However, limits its power equipment rating applications. In order for such systems, advanced control techniques for greater access to power to satisfy the requirements of the systems must be developed. PMSG-based WECS for conventional control strategy is primarily the promise of proper operation of the generator is designed to do. While the grid side in power is regulated indirectly strategy generator torque control (power) is directly responding to unrest will increase the power of the generator. In the grid, which is not desired by the PSO? Under different grid positions is for the first time power system requirements to satisfy the basic idea of the proposed strategy. Strategy, active/reactive current (power) grid-side converter is regulated through enough to provide additional damping oscillation active damping loop generator-party controller is integrated into the. Traditional or variable compared with-structured control strategies the proposed one is the quickest and most precise grid-side current (power) is the answer. Grid during DC chopper or switching, fault controller
  • 12. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 44-56 © IAEME 55 with strategy is required and current distortion can be reduced when unsymmetrical bending apparatus grid fault. Moreover, the proposed strategy a system parameter and it is easy to apply, which makes it attractive for the practice of engineering. However, such a strategy is in response to sacrifice and generator-side variable DC-link voltage fluctuations. It is believed that a large DC-link capacitor if the proposed strategy is employed to improve the system performance can be useful. REFERENCES [1] M. Chinchilla, S. Arnaltes, and J. C. Burgos, “Control of permanent-magnet generators applied to variable-speed wind-energy systems con-nested to the grid, ”IEEE Trans. Energy Convers., vol. 21, no. 1, pp. 130–135, Mar. 2006. [2] H. Polinder, F. F. A van der Pijl, and P. Tavner, “Comparison of direct-drive and geared generator concepts for wind turbines,” IEEE Trans. Energy Convers., vol. 21, no. 3, pp. 543– 550, Sep. 2006. [3] F. Iov, A. D. Hansen, P. Sørensen, and N. A. Catullus, “Mapping of grid faults and grid codes,” Rios Nat. Lab., Tech. Univ. Denmark, Roskilde, Denmark, Tech. Rep. Risø-R- 1617(EN), Jul. 2007. [4] C. Sanitary and J. Janning, “Test bench for grid code simulations for multi-MW wind turbines, design and control,”IEEE Trans. Power Electron., vol. 23, no. 4, pp. 1707–1715, Jul. 2008. [5] J.-H. Jeon, S.-K. Kim, C.-H. Cho, J.-B. Ahn, and E.-S. Kim, “Development of simulator system for micro-grids with renewable energy sources,” J. Electr. Eng. Technol., vol. 1, no. 4, pp. 409–413, Feb. 2006. [6] R. Pollin, H. G. Peltier, and H. Scharber, “Green recovery: A new program to create good jobs and start building a low-carbon economy,” Center for American Progress, Washington, DC, Sep. 2008. Available:http://www.peri.umass.edu/fileadmin/pdf/other_publication_types/green_economic s/peri_report.pdf. [7] F. K. A. Lima, A. Luna, P. Rodriguez, E. H. Watanabe, and F. Blaabjerg, “Rotor voltage dynamics in the doubly fed induction generator during grid faults,” IEEE Trans. Power Electron., vol. 25, no. 1, pp. 118–130, Jan. 2010. [8] L. G. Meegahapola, T. Littler, and D. Flynn, “Decoupled-DFIG fault ride-through strategy for enhanced stability performance during grid faults,” IEEE Trans. Sustainable Energy, no. 3, pp. 152–162, Oct. 2010. [9] Q. Song and W. Liu, “Control of a cascade STATCOM with star con-figuration under unbalanced conditions,” IEEE Trans. Power Electron., vol. 24, no. 1, pp. 45–58, Jan. 2009. [10] W. H. Zhang, S.-J. Lee, and M.-S. Choi, “Setting considerations of distance relay for transmission line with STATCOM,” J. Electr. Eng. Technol., vol. 5, no. 4, pp. 522–529, Jul. 2010. [11] H. M. Pirouzy and M. T. Bina, “Modular multilevel converter based STAT-COM topology suitable for medium-voltage unbalanced systems,” J. Power Electron., vol. 10, no. 5, pp. 572– 578, Sep. 2010. [12] G. Brando, A. Coccia, and R. Rizzo, “Control method of a braking chopper to reduce voltage unbalance in a 3-level chopper,” inProc. IEEE Int. Conf. Ind. Technol., 2004, vol. 2, pp. 975– 978. [13] J. F. Conroy and R. Watson, “Low-voltage ride-through of a full converterwind turbine with permanent magnet generator,”IET Renewable Power.Generation, vol. 1, no. 3, pp. 182–189, Sep. 2007.
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