Active Power and Flux Control of a Self-Excited Induction Generator for a Variable-Speed Wind Turbine Generation

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1 2017 Ninth Annual IEEE Green Technologies Conference Active Power and Flux Control of a Self-Excited Induction Generator for a Variable-Speed Wind Turbine Generation Woonki Na 1, Edurad Muljadi 2, Bill Leighty, and Jonghoon Kim 4 1:wkna@csufresno.edu, 2:Edurad.muljadi@nrel.gov, :wleighty@earthlink.net, and 4: whdgns0422@cnu.ac.kr 1 Department of Electrical and Computer Engineering, California State University, Fresno, CA, USA 2 Power System Engineering Center, National Renewable Energy Laboratory, Golden, CO, USA The Leighty Foundation, Juneau, AK, USA 4 Department of Electrical Engineering, Chungnam National University, Daejon, Korea Abstract A Self-Excited Induction Generation (SEIG) for a variable speed wind turbine generation(vs-wg) is normally considered to be a good candidate for implementation in standalone applications such as battery charging, hydrogenation, water pumping, water purification, water desalination, and etc. In this study, we have examined a study on active power and flux control strategies for a SEIG for a variable speed wind turbine generation. The control analysis for the proposed system is carried out by using PSCAD software. In the process, we can optimize the control design of the system, thereby enhancing and expediting the control design procedure for this application. With this study, this control design for a SEIG for VS-WG can become the industry standard for analysis and development in terms of SEIG. Keywords Self-Excited Induction Generation, Active Power and Flux control solid state excitation. Likewise, in an isolated operation, the conservation of real and reactive powers of a SEIG is to be preserved[1]. Thus, in order to operate a SEIG in an isolated operation, the total admittance or impedance of the SEIG is to be zero in terms of the active and reactive power control. In addition to these power controls, the flux control can be realized by using capacitor tap change so that the SEIG can be operated with a high efficiency. And for a simple implementation, a non-isolated DC/DC converter is considered in this study. The paper is organized as follows. In the section II, an overall system is explained. Control strategies are described in the section III. The results and analysis and conclusion are addressed in in the sections IV and V respectively. I. INTRODUCTION Today, the wind energy is the fastest growing energy resource in the world with other renewable and alternative energy sources such as solar, biomass and fuel cells. For a long time, a Self-Excited Induction Generation (SEIG) has been considered for wind energy system applications because of its low maintenance, robust, ruggedness, small size, and low capital cost as compared to a permanent magnet generator[1]. A SEIG is very suitable for standalone applications with the energy captured capability through a battery during wind variation. Although several control studies regarding SEIG s voltage and frequency had been conducted in [2,,4,5], there are not much studies addressed in terms of power and flux controls. In this paper, the proposed work is to develop an active power control and flux control algorithm for a SEIG for a Variable Speed Wind Generation (VS-WG) using a DC/DC converter. A SEIG can be operated as an isolated generator without connection to the utility supply. If an utility is connected to a SEIG, the reactive power for the SEIG can be supplied from the utility. Otherwise, the reactive power for the SEIG is to be compensated by a three-phase AC capacitor or Fig. 1 System Configuration II. OVERALL SYSTEM The overall system schematic is built based on PSCAD [6] as seen in Fig. 1. In Fig.1, IM motor which is an induction generator having three inputs, W, S, and T. The IM motor, i.e., the SEIG can be operated in either 'speed control' using the input, W or 'torque control' mode using the input, T. The input, S is for switching from the speed control mode to the torque control mode. In this study, the torque control mode is mainly enforced instead of the speed control in order to design active power and flux controllers under the speed variations. The SEIG generates a three-phase power which is fed to a threephase diode rectifier. The three-phase diode rectifier is /17 $ IEEE DOI /GreenTech

2 connected to a non-isolated DC/DC boost converter. Throughout the non-isolated boost converter, a capacitor with the resistive load can be charged. Between the SEIG and the diode rectifier, a three-phase capacitor is connected in parallel with a Y connection for the self excitation. The capacitor values would be varied based on the flux controller s estimation. Aforementioned, not only the active power is controlled, but also the flux for the self excitation is estimated by measuring the Root Mean Square(RMS) value and frequency of the output voltages in the SEIG. Fig. 2 Per Phase equivalent circuit of a SEIG Fig. 2 shows a per phase equivalent circuit of a SEIG. Where R L: Equivalent effective load resistance R s: Stator resistance L ls: Stator reactance L lr : Rotor reactance (Referred to the stator side) R r : Rotor resistance (Referred to the stator side) L m: Magnetization reactance E: Generator airgap voltage ω e: Operation frequency s: Slip The governing equations for balancing of equilibrium of the real and reactive powers are as follows: In this section, active power control and flux control strategies are described. Before addressing the active power and flux control strategies, the basic wind turbine characteristic is to be understood. In order to operate a wind turbine at its optimum condition under different wind speeds, the wind turbine must be operated at its maximum power coefficient (C p_optimum =0.~0.5 ) with a constant tip-speed ratio, which is proportional to the ratio of the rotor speed to the wind speed [1]. The aerodynamic power, P air,[w] generated by a wind turbine is a function of the specific density of the air, ρ [kg/m ], the sweep area of the blade, A [m 2 ], a power coefficient, C p, and the cube of the wind speed, V as seen in the equation (). 1 Pair = ρac pv () 2 As the wind speed increases or decreases, the rotor speed is followed with the same rate of the wind speed variation to operate the wind turbine at the optimum power efficient, C p_optimum. With the constant tip-speed ratio, the ideal power is expressed as a cube of function of the rotor rpm seen in equation (4). Pideal = K prpm (4) Where K p is the computed wind turbine data[1]. To simplify the analysis, the ideal power is calculated from (4) by assuming that the rotor speed changes proportionally as the wind speed changes. (i)active Power Controller According to the (4), the ideal active power is calculated. The ideal power can be compared with the measured power output from the generator as seen in Fig. 2. Re( Z total =0 )for the real power balance (1) And Im( Z total=0) for the reactive power balance (2) Where Re: Real part, and Im: Imaginary part Z total: Total impedance of the per phase equivalent circuit in Fig. 2 For details of the active power control and flux control strategies are explained in the following section III. III. CONTROL STRATEGIES Fig. Active Power Controller Fig. shows the schematic of the active power controller in PSCAD. P measured is the measured power from the generator. P ref is the reference power based on (4). In Fig., a PI controller is used, and then the output of the PI controller is compared with a triangular signal generator. The triangular signal generator generates a 1.5kHz of PWM signal to the DC/DC boost converter for the active power control by considering high power applications over 100kW. The gains of the PI controller are chosen through trial and error approaches. Fig. 4 shows the reference power generator block based on (4). By controlling the duty ratio of the PWM signal, 178

3 the DC/DC converter can vary the generator power in order to transfer the reference power to the load through the capacitor. Since this DC/DC converter is capable of increasing or decreasing the input power, the maximum energy capture will be possible under the wind speed variations. Fig.6 Reference flux estimator Fig. 4 Reference power generator The actual flux can be estimated from the measured voltage of the generator and the frequency of the three-phase diode rectifier through low pass filters as shown in the PSCAD block of Fig. 7. (ii) Flux Controller In order to control the flux of the SEIG, most of all, an appropriate capacitor value is to be selected for the self excitation, otherwise the runaway of a SEIG can happen due to the under or over excitation of the SEIG. Throughout the steady state operation that the wind speed varies 1~0.7 pu, the optimal values of capacitors for self excitation can be determined. Fig. 5 shows that as the speed increases, the optimal capacitor value for the self excitaton has to be proportionally decreased. C(uF) Fig.7 Flux estimator The relationship between the magnetizing reactance, X m and the airgap voltage, E with the operating frequency of 60 Hz is given in Fig. 8 from the SEIG open and short circuit tests. It is shown that how the value of X m decreases as the inductor generator saturates in which the based RMS phase volage is set to 277[V] and its base RMS voltage is set to 10[A]. Xm(pu) Speed(pu) Fig. 5. Characteristic of speed vs capacitor In practice, a discrete tap for the capacitor value change would be enforced based on the speed variation. Once identifying the capacitor values, the approximate polynominal can be constructed based on (5). The approximate polynominal is y = 980x x (5) Where x is Speed (pu) and y is Capacitor value. 1pu speed is 1800 rpm. The reference flux is determined by the reference input voltage, 480V and the reference frequency, 60Hz, seen in the PSCAD block of Fig. 6. Fig. 8 Magnetizing Curve of the SEIG IV. RESULTS AND ANALYSIS E(pu) For the initialization, the SEIG is connected to the grid before 0.5 sec, and then the breaker connected to the grid is disconnected, the SEIG is solely operated after 0.5sec. Fig. 9 shows the overall system schematic including all the breakers connections for the flux control. Based on the speed variations, each breaker is controlled to connect an appropriate capacitor for the self excitation. After the breaker, named as BRK in Fig. 9 is disconnected from the grid, the scheduled torque change is enforced over 500 sec. The active power controller and flux controller are tested based on the torque variations profile shown in Fig. 10. Fig. 11 shows the wind speed variation profile from the given torque variation. In 179

4 Fig. 11, the wind speed varies between 1 ~0.4 (pu) over 500 sec based on the torque variation profile of Fig.10. For example, between the wind speeds 1.1 and 1.0 pu, the breaker1, named as BRK1 is connected to the capacitor whose value is of 965uF that is relevant to the speed 1.0 pu for the self excitation, and between the wind speeds 1.0 and 0.9 pu, the breaker 2, BRK2 is connected to 126uF capacitor and etc. Although each breaker(1~4) can be in active based on the wind speed variations, it is difficult to control the flux continuously as observed in Fig. 12 because the continuous capacitor variation under the wind speed change is almost impossible practically by simply having used the 4-tap change of the capacitors for self excitation. However, according to Fig. 1, the active power controller performs better than the flux controller because there is a very minimum deviation between the reference power and measured power. Fig. 14 shows the effective load calculation blocks in PSCAD. In Fig. 14, first the power generation from the generator is measured and filtered, and by using the filtered power and the measured voltage the effective load resistance value can be also calculated. Fig. 9 Overall system schematic with breakers Main : Graphs 0.50 TS Main,Generator : Graphs TM Flux Torque (pu) sec Fig. 10 Torque profile over 500 sec Fig. 12 Flux variation over 500 sec 1.10 W Generator : Graphs 0.40 Pref Main,Generator : Graphs Pmeas Speed (pu) (pu) Fig.11 Wind speed profile over 500 sec sec Fig. 1 Power variation over 500 sec 180

5 Fig. 14 The effective load calculation block With the effective load resistance value in Fig. 14 and the corresponding capacitor for each wind speed, Fig. 15 is drawn by using SEIG parameters calculated based on Mathcad from the SEIG testing data. Each intersection between the ideal power curve, P ideal and each different curve shows the corresponding the wind speed. There are four intersections A,B,C, and D in Fig. 15 between the ideal power and different wind speeds with the corresponding capacitors. Power (pu), : Intersections Pideal tested. The results show that the proposed active power controller can successfully control the power, but flux cannot be solely controlled continuously by using the 4-tap change of the capacitors due to the discrete nature of 4-tap changing capacitor. Having more taps for the self excitation capacitors could improve the flux control performance, but which is not realistic in practice applications. For the future plan, the more detailed analysis regarding the calculation of the self excitation capacitor using Mathcad and MATLAB are planned by comparing a simplified equivalent circuit of the SEIG with the original detailed equivalent circuit of the SEIG. And this analysis would be applied to a SEIG with an active load system such as a hydrogen generation system for a hybrid power system applications. For the hybrid power system, an electrolyzer could be modeled and simulated with the SEIG used in this research with the proposed active power and flux controller. Also, once the proof of the concept of the hybrid power is established, a prototype system could be built for validation of the proposed active power and flux controller. ACKNOWLEDGMENT This research was conducted by the support of the 2016 Department of Energy summer visiting faculty program at National Renewable Energy Laboratory, Golden, CO, USA B A REFERENCES [1] E. Muljadi, B. Gregory, and D., Broad, Self- Excited Induction Generator for Variable-Speed Wind Turbine Generation, NREL/CP , 1996 C D Fig. 15 Speed and Power variation For the wind speed 1.0, 0.9, and 0.8 pu, most of them show close correlation between PSCAD and Mathcad results, but at the wind speed 1.1 pu, there is 10~20% the wind speed deviation because of a mismatch of Mathcad and PSCAD calculations. V. CONCLUSION Wind Speed(pu) In this research, an active power and flux controller for a SEIG have been designed. To validate the proposed control algorithm, a 2.7kW SEIG PSCAD model was built and [2] S. Drouilhet, E. Muljadi, and R. Holz and V. Gevorgian, Optimizing Small Wind Turbine Performance in Battery Charging Applications, NREL/TP , 1995 [] E. Muljadi, S. Drouilhet, R. Holz and V. Gevorgian, Analysis of Wind Power for Battery Charging, 1996, Wind Energy Book VIII: Conference Papers - Proceedings from Energy Week '96, 29 January - 2 February 1996, Houston, Texas. Vol. I: pp [4] P. J. Chauhan, J. K. Chatterjee, Haresh Bhere, B. V. Perumal and Dipankar Sarkar, Synchronized Operation of DSP-Based Generalized Impedance Controller With Variable-Speed Isolated SEIG for Novel Voltage and Frequency Control, IEEE Transactions on Industry Applications, Year: 2015, Volume: 51, Issue: 2, Pages: [5] Hua Geng, Dewei Xu, Bin Wu and Wei Huang, Direct Voltage Control for a Stand-Alond Wind-Driven Self Exicted Induction Generator with Improved Power Quality, IEEE Transactions on Power Electronics, Year: 2011, Volume: 26, Issue: 8, Pages: [6] 181