6545(Print), ISSN (Online) Volume 4, Issue 2, March April (2013), IAEME & TECHNOLOGY (IJEET)

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1 INTERNATIONAL International Journal of JOURNAL Electrical Engineering OF ELECTRICAL and Technology (IJEET), ENGINEERING ISSN 0976 & TECHNOLOGY (IJEET) ISSN (Print) ISSN (Online) Volume 4, Issue 2, March April (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJEET I A E M E APPLICATION OF CROWBAR PROTECTION ON DFIG-BASED WIND TURBINE CONNECTED TO GRID Nadiya G. Mohammed 1 M. Tech. candidate, Electrical Engineering, Department of Bharat Vidyapeeth University College of Engineering Pune, India. ABSTRACT To provide voltage stability support in weak transmission networks for the ability of doubly fed induction generators is investigated in this paper. Here we analyzed the response of wind turbines to voltage dips at the point of common coupling and its effects on system stability. In order to support the grid voltage by injecting reactive power during and after grid fault events, we developed a control strategy for the operation of the grid and rotor side converters. To study performance of strategy, it is analyzed for different voltage dips at the point of common coupling of a wind farm and compared with the case when the converters do not provide any voltage support. The Chilean transmission network model is used for the Simulations, because of its radial Configuration it would be a good example of weak power system. In addition to this we are presenting the theoretical analysis of the fault current for the application of crowbar protection. The basic method for the fault current measurement during the unsymmetrical and symmetrical faults in the grid system is discussed. In this paper we presented the analytical model for crowbar protection rather than doing simulation studies in actual. Keywords- Doubly fed induction generator (DFIG); unbalanced grid fault; fault ridethrough; Crowbar protection; Analytic Hierarchy Process; squirrel cage induction generator SCIG. 81

2 I. INTRODUCTION The trend in wind generation has been the installation of large and concentrated wind farms into electrical power networks in last year s. As a result, wind power has reached in some regions significant penetration levels imposing new problems to the Transmission System Operators (TSO). Wind power injections are already exceeding local demand in some regions like Spain & Northern Germany. In many countries, this problem has demanded the creation of new grid requirements for wind generators. Disconnection of wind turbines in case of disturbance is not admitted anymore, and voltage and transient stability support -during and after grid fault events- are required [1]-[2].During disturbances the risk of losing a significant fraction of wind generators is decreases and grid operators can guarantee a reliable and secure power system operation even by high wind power penetration levels. In variable speed wind turbines, Doubly Fed Induction Generators (DFIG) is the most common technology used. In Europe in 2005 [3] 45% of the medium to large wind turbines installed. The active and reactive power injected into the grid is controlled by using power converters enables DFIG to operate at optimal rotor speed and to maximize power generation in normal grid conditions. In case of voltage dips close to the wind farm, high currents will pass through the stator winding, which will also flow through the rotor winding due to the magnetic coupling between stator and rotor. A protection to system is required because it causes high currents that could damage the converters. By short circuiting the generator rotor through a crowbar and thus blocking the rotor side converter [4] protection is achieved. Once the rotor side converter is blocked, the DFIG operates like a typical induction generator and therefore, the control of active and reactive power through the rotor is inactive. To provide insight and understanding about the effective Fault Ride-Through (FRT) capability of DFIG in weak transmission networks and its effects on system stability is a paper aim. A control strategy allowing the grid and rotor side converters to support the grid voltage by injecting reactive power during and after grid faults is developed. Using the Chilean transmission network simulations are performed, which can be considered a weak power system. The analysis into grid fault ride-through of wind turbines has flourished within the last years and this truth underlines by itself the relevancy of the subject. A large number of papers made by academics are accompanied by advanced hardware and software system solutions adopted within the new wind turbines by makers to assure grid fault ride-through capability. The competition within the field among makers is fierce and this makes it terribly tough to induce careful data on the actual grid fault ride-through technical solutions adopted in industrial wind turbines. This paper deals with the behavior of wind turbines during grid disturbances. Below are major objectives of this paper: - Analyze the short-circuit current fed into the grid by DFIG wind turbines each with crowbar and DC-chopper protection. - Analyze the fault current contribution of wind farms. - Investigate voltage profiles ensuing from faults at sub-transmission and medium voltage networks to know if issues for grid fault ride-through of wind turbines could arise. - Investigate and devise potential ways that to boost voltage support by wind farms in weak networks throughout a grid disturbance. In below sections we are discussing above stated things one by one. 82

3 II. MODELING AND CONTROL OF DFIG A. Wind Turbines with DFIG As many authors have described the modeling of wind turbines with DFIG [5]-[8], therefore, just the main issues will be described here. Figure 1 gives the arrangement of a DFIG. This concept uses a wound rotor induction generator whose stator windings are directly connected to the grid, while the rotor winding is connected to the network via a back-to-back IGBT-based converter. The rotor side converter regulates the active and reactive power injected by the DFIG and the grid side converter controls the voltage at the DC link. Fig. 1: Doubly fed induction generator. The overall structure of the turbine model contains the mechanics model, mechanical model and therefore the electrical model for the generator. The renowned mechanism disc idea [7] is taken under consideration by the mechanics model underneath the assumption of constant wind rate. The drive train is approximated by a 2 mass model considering one giant mass to represent the rotary engine rotor inertia and one little mass representing the generator rotor. The 2 masses are connected by a versatile low speed shaft characterized by stiffness and damping [8]. As was common in fundamental simulations, the generator is diagrammatic by a 3rd order model, whose equations are simplified by neglecting the stator coil transients [9]. A pitch angle management is additionally enforced to limit the generator speed throughout grid disturbances and in traditional operation underneath current of air speeds [5]. Finally, a protection system is enclosed so as to dam the rotor facet convertor once it s safe operation is vulnerable. The protection system monitors the voltage at the purpose of common coupling (PCC), the magnitude of the rotor current and therefore the generator rotor speed. Once a minimum of one in all these variables exceed range of their most and minimum values, the protection system blocks the rotor facet convertor by short circuiting the generator rotor through a crowbar. B. Rotor Side Converter The rotor aspect convertor (RSC) controls severally the reactive and active power injected by the DFIG into the grid in a stator coil flux dq-reference frame. Figure a pair of shows the management scheme of the RSC. The q-axis current part is employed to manage the active power employing a most power pursuit (MPT) strategy to calculate the active power 83

4 reference [10]. The reference price for the active power is compared with its actual price and therefore the error is distributed to a PI controller that generates the reference value for the q- axis current. This signal is compared to its actual price and therefore the error is seasoned a second PI controller determinant the reference voltage for the q-axis component. The d-axis is employed to manage the reactive power changed with the grid, that in traditional operation is about to zero so as to operate with unity power issue just in case of disturbance, if the evoked current within the rotor circuit isn't high enough to trigger the overcurrent protection, the RSC is about to inject reactive power into the grid so as to support the voltage restoration. In such case, the particular voltage at the PCC is compared to its reference price and therefore the error is seasoned a PI controller to come up with the reference signal for the reactive power of the DFIG almost like the management strategy of the q-component, the error between the reactive power reference and its actual price is seasoned a PI controller to work out the reference price for the d-axis current. This signal is compared to the d-axis current price and therefore the error is distributed to a third PI controller that determines the reference voltage for the d-axis part. Finally, the dq-reference voltages area unit passed through the PWM module and therefore the modulation indexes for the management of the RSC area unit determined. C. Grid Side Converter The objective of the grid facet convertor (GSC) is to maintain the voltage at the DC link between each power converters. In traditional operation, the RSC already controls the unity power issue operation and thus the reference price for the changed reactive power between the GSC and therefore the grid is about to zero just in case of disturbance, the GSC is about to inject reactive power into the grid, whether or not the RSC is blocked or is unbroken operative. Figure 3 below shows the management diagram of the GSC. As for the RSC, the management of the GSC is performed exploitation the dq- system, however rather than rotating with the mechanical device flux, the axis rotates with the grid voltage. Fig. 2: Control diagram of the rotor side converter. 84

5 The actual voltage at the DC link is compared with its reference price and therefore the error between each signal is passed through a PI controller that determines the reference signal for the d-axis current. This latter signal is deducted with its current price and therefore the error is shipped to a different PI controller to obtain the reference voltage for the d-axis element. As for the q-axis current, its reference price depends whether the system operates in traditional operation or throughout disturbance. In traditional operation, the GSC is assumed reactive neutral by setting the reference price of the q-axis current to zero just in case of disturbance, the particular AC-side voltage of the GSC is compared with its reference price and therefore the error is passed through a PI controller that generates the reference signal for the q-axis current. This reference signal is compared to its current price and therefore the error is shipped to a second PI controller that establishes the reference voltage for the q-axis component. Finally, each reference voltages in an exceedingly dq-reference frame are sent to the PWM module that generates the modulation indexes for the management of the GSC. The injection of re active and active power by the GSC is limited by its nominal capacity represented by the following equation in per unit base: The present work considers a method that prioritizes the injection of reactive power (q-axis current). The d-axis current is calculated supported equation (1) throughout traditional operation, the strategy doesn't gift limitations with the management of the DC link voltage since the q-axis current is ready to zero and therefore the device capability is merely wont to management the DC link voltage. Fig. 3: Control diagram of the grid side converter. 85

6 III. DFIG USING CROWBAR PROTECTION A DFIG using crowbar protection is considered in this section. The analysis assumes that the crowbar remains connected during the whole duration of the fault, 250 ms in this study. This may not be the case for three-phase faults since the RSC would be re-started as soon as the rotor current decays below a certain predefined value. During symmetrical faults, the analysis here presented is therefore applicable during the period between crowbar insertion and RSC re-starting. However, for unsymmetrical faults, the RSC will most likely not be restarted during the fault since the cause of high rotor current is the negative sequence network voltage which does not decay during the fault period (Semaan 2006(b)). For the most severe unsymmetrical faults, the proposed analysis is therefore applicable during the whole duration of the fault. The method proposed above for calculating the short-circuit current of a SCIG cannot be directly applied to a wind turbine driven DFIG, because of mainly two reasons. The first reason is that the value of the crowbar resistance may be up to 20 times the value of the generator rotor resistance (Akhmatov 2005) and the total resulting rotor resistance can no longer be neglected. In (Morren 2007) it has been proposed a method for calculating the maximum short-circuit current of a DFIG with high crowbar resistance during a symmetrical three phase fault at the generator terminals. The authors of the mentioned reference proposed to include the effects of the crowbar resistance to calculate the maximum short-circuit current of a DFIG in two steps. First, the rotor transient time constant is modified according to: 1 The second step to account for the presence of the high crowbar resistance proposed in (Morren 2007) is to include it in the impedance limiting the short-circuit current. However, this proved to be still a too rough approximation when comparing with the simulations, leading to inaccurate calculations of the DFIG short-circuits current as a function of time. The second reason, that makes the SCIG short-circuit current calculation method inaccurate when applied to a wind turbine driven DFIG, is that a wind turbine driven DFIG may operate in a slip range between -0.3 and The forced stator and rotor fluxes, which for the SCIG have been calculated based on the assumption of zero slip, for a DFIG should be calculated based on its initial rotor speed and delivered active and reactive power. D. Influence of high crowbar resistance on natural stator flux Let us start with the stator transient time constant. For a DFIG with high total rotor resistance, the stator transient time constant needs to be expressed in a slightly different way. The natural stator flux, which is fixed with respect to the stator, generates a voltage in the rotor whose frequency and magnitude in a rotor reference frame are proportional to the rotor speed. A current will flow in the rotor, having the same frequency of the induced voltage and opposite to the rotor speed. 86

7 E. Influence of high crowbar resistance on natural rotor flux Let us denote the natural rotor flux immediately after the fault as rn. In the SCIG case, this flux in a rotor reference frame is a DC component decaying with the rotor transient time constant. This fact is no longer true for a DFIG with high rotor resistance. To explain why this no longer holds, we may find it useful to refer to a simpler analogous situation. F. Influence of high crowbar resistance on negative sequence fluxes The rotor negative sequence current can be obtained with a simple current division between the magnetizing and the rotor circuit branches, as done for a SCIG. G. Wind turbine driven DFIG The initial slip of a wind turbine driven DFIG may be significantly different from zero, thus the initial and post-fault forced components of the rotor flux can no longer be calculated under the assumption of zero rotor current. The pre-fault rotor flux must also be re-calculated taking into account the initial conditions of the DFIG. If the initial apparent power, fed into the grid according to generator convention, and rotor slip of the DFIG are known, the pre-fault rotor flux is calculated. There are different simulations studies carried out for this protection systems which are out of scope of this paper. Based on these studies we are making following summary: H. Summary From the studied simulation results, an approximate method for predicting the symmetrical and unsymmetrical short-circuit current of a SCIG and a DFIG has been proposed. The proposed method gives a good prediction of the short-circuit behavior of a wind farm using a DFIG with crowbar protection, both for symmetrical and unsymmetrical faults in the network. A linear model of the induction machine has been considered and saturation has been neglected. The accuracy of the results obtained with the proposed method may be sufficient to replace the use of simulations in many contexts, e.g. calculation of maximum current, calculation of its DC and AC components and short circuit calculations for protection relays settings. The impedances of step-up transformer and MV line should be added in series with the DFIG stator impedance. Even though these impedances may have a non-negligible resistive part, it has been found that in practical situations this fact does not affect the accuracy of the method. Moreover, the method is capable of accurately reproducing the DFIG fault current even for unsymmetrical faults on the MV side of the step-up transformer. A factor that may limit the accuracy of the proposed method is the delay with which the crowbar resistance is inserted relative to the fault inception instant. Delays below 5 ms result in almost no loss of accuracy. IV. DYNAMIC SIMULATIONS The simulations performed in this work are based on the Chilean transmission system. The voltages in the bulk network are from 110 to 500 kv with nearly 750 busbars and 220 generators. The installed capacity of the system is about MW for a peak load of

8 MW. The system is characterized by long distances between major load centers and generation plants and long transmission lines covering a total length of km. In order to illustrate the structure of the network, a simplified diagram is shown in Figure 4. For this work, a 150-busbar model of the Chilean transmission network is implemented in the power system simulation tool DIgSILENT Power Factory [11]. The model includes 150 synchronous generators representing the existing conventional power plants and around 100 consumption centers distributed throughout the system. The model considers only fundamental frequency components of currents and voltages, which allows a constant impedance representation of the network. The ability of DFIG to provide voltage stability support in weak transmission networks and its effects on system stability are analyzed by including a 100 MW wind farm to the developed 150-busbar model of the Chilean transmission network (green circle in Figure 4). The wind park consists of 20 wind turbines of 5 MVA each. All of them are based on DFIG whose converter is about 40% of the generator capacity. The wind farm is represented by an aggregated model where the 20 wind turbines are modeled as one equivalent generator connected at a 220 kv busbar through two transformers. Fig. 4. Chilean bulk power network. To analyze the ability of DFIG to provide voltage stability support, the control strategies presented in Section II.B and C are developed to allow the RSC and GSC to inject reactive power during and after grid fault events. The strategies are compared with the case when the converters do not provide any voltage stability support. The performance of the strategies is analyzed for two voltage dips with duration of 150 ms applied at the 220 kv busbar where the wind park is connected. 88

9 V. WORK DONE The simulation studies performed over this topic is illustrating here. Based on the Chilean transmission system simulations are performed. The voltages in the bulk network are from 110 to 500 kv with nearly 750 busbars as well 220 generators. The installed capacity of the system is about turbines with power transmission system during and post fault is illustrated and explained. We have studied this model in [13]. The dynamic simulation model is modified by connecting the proposed crowbar protection system at DFIG terminal. As shown in Fig. 5, the dynamic simulation model consists of six 1.5 MW wind turbines connected to a 25-kV distribution system exports power to a 120-kV grid through a 30 km transmission line. The wind turbines operate at 10 m/s wind speed and zero pitch angle, where the maximum turbine output power is 0.55 pu of its rated power. The power characteristics of the wind turbine for different wind speed values are shown in Fig.6. Figure 5: Investigated Crowbar Protection Methods of DFIG. Figure 6: Turbine power characteristic with zero pitch angle 89

10 The simulation scenario is performed for different value of proposed crowbar resistances as shown in Table 1: Table 1: Relations between proposed crowbar resistance (R_terminal) and stator resistance (R_stator). Effect of Terminal Crowbar Resistance Value Figure 7 shows the variations of the rotor current with the proposed terminal crowbar resistance. Figure 7: Rotor current variations of DFIG wind farm during fault for different values of the terminal crowbar resistance. 90

11 VI. CONCLUSION In this study paper, we have presented the analytical method so as to calculate the fault currents of wind turbines with DFIG using the crowbar protection for any symmetrical and unsymmetrical fault at any location within the grid. The projected methodology may be used to calculate the peak fault current, its AC or DC part and it's a sound tool that may replace simulations once managing protection settings or part size. The method has been verified against simulations. The fault currents calculated using the analytical methodology show a high agreement with the results from simulations. The impact of the impedance of cables and transformer, of the GSC current contribution and of the delay with that the crowbar is inserted on the accuracy of the strategy has been analyzed. It s been found that, in sensible practical, only the crowbar insertion time will decrease the accuracy of the proposed methodology. If the lever is inserted at intervals five ms from the fault, the strategy remains sufficiently correct. A radical clarification of the physical method occurring in an exceedingly DFIG throughout a short-circuit is additionally given. The analytical methodology may also be applied to calculate the fault currents of SCIG wind turbines. In addition to this, during this paper we have also shown the possibilities as well as limitations of DFIG concerning Fault Ride-Through capability at the time of grid fault events. This paper is having main motive of control strategies of the grid and rotor side converters in order to provide the reactive power support during grid disturbances. The practical work evaluation we are keeping it for future scope. VII. REFERENCES [1] Grid Code: High and Extra High Voltage, E.ON Netz GmbH Tech. Rep., 2006, Status: 1. [2] Technical Requirements for the Connection of Generation Facilities to the Hydro-Québec Transmission System: Supplementary Requirements for Wind Generation, Hydro-Québec Tech. Rep., [3] R. Pena, J. C. Clare, G. M. Asher, Doubly fed induction generator using back-to-back PWM converters and its application to variable speed wind-energy generation, IEE Proc. Electr. Power Appl., Vol. 143, No. 3, May 1996, pp [4] J.J. Zhao, X. Li, J.T. Hao, Voltage control scheme in distribution network with double feed induction generator wind farm, WSEAS Transactions on Circuits and Systems, Vol. 8, No. 8, 2009, pp [5] H. K. Jafari, Simulation of doubly-fed machine with improved wind turbine, WSEAS Transactions on Circuits and Systems, Vol. 8, No. 2, 2009, pp [6] M. B. C. Salles, K. Hameyer, J. R. Cardoso and W. Freitas, "Dynamic Analysis of Wind Turbines Considering New Grid Requirements," IEEE International Conference on Electrical Machines, Portugal, Sept [7] J. Smajo, Electromagnetic torque analysis of a DFIG for wind turbines, WSEAS Transactions on Systems, Vol. 7, No. 5, 2008, pp [8] J. Morren, S. W. H. De Haan, Short-circuit current of wind turbines with doubly fed induction generator, IEEE Trans. on Energy Conversion. Vol. 22, No. 1, June 2005, pp [9] F. Van Hulle, "Large Scale Integration of Wind Energy in the European Power Supply: Analysis, Issues and Recommendations," EWEA Tech Rep.,

12 [10] N. Rahmat, T. Thiringer and D. Karlsson, "Voltage and Transient Stability Support by Wind Farms Complying with the E.ON Netz Grid Code," IEEE Trans. Power Systems, vol. 22, No. 4, pp , Nov [11] P. Flannery, G. Venkataramanan, Evaluation of voltage sag ride-through of a doubly fed induction generator wind turbine with series grid side converter, Proc. of the 38th Annual Power Electronics Specialists Conference, June 2007, pp [12] B. Singh, V. Emmoji, S. N. Singh, Performance evaluation of series and parallel connected grid side converters of DFIG, IEEE Power and Energy Society General Meeting, July 2008, pp.1-8. [13] Reactive Power Generation by DFIG Based Wind Farms with AC Grid Connection, [14] Haider M. Husen, Laith O. Maheemed and Prof. D.S. Chavan, Enhancement of Power Quality in Grid-Connected Doubly Fed Wind Turbines Induction Generator International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp , ISSN Print : , ISSN Online: [15] Ameer H. Abd and D.S.Chavan, Impact of Wind Farm of Double-Fed Induction Generator (DFIG) on Voltage Quality International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp , ISSN Print : , ISSN Online: AUTHORS 1. Nadiya G. Mohammed M. Tech. candidate, Electrical Engineering, Department of Bharat Vidyapeeth University College of Engineering Pune, India. 92