Performance of a Wind-Diesel Hybrid Power System with STATCOM as a Reactive Power Compensator

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1 Smart Grid and Renewable Energy, 00,, doi:0.436/sgre ublished Online November 00 ( 53 erformance of a Wind-Diesel Hybrid ower System with STATOM as a Reactive ower ompensator awan Sharma, Trilochen Singh Bhatti, Kondapi Seha Srinivasa Ramakrishna entre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India; The Energy and Resource Institute Delhi, New Delhi, India. bitt979@gmail.com Received October 3 st, 00; revised November 5 th, 00; accepted November 30 th, 00 ABSTRAT The paper deals with automatic reactive power control of an isolated wind-diesel hybrid power system. The power is generated by diesel engine and wind turbine as prime movers with electrical power conversion by permanent-magnet synchronous generator (MSG) and permanent-magnet induction generator () respectively. The mathematical model of the system developed is based on reactive power flow euations. The paper investigates the dynamic performance of the hybrid system for % step increase in reactive power load with % step increase in input wind power. Keywords: ermanent-magnet Induction Generator, ermanent-magnet Synchronous Generator, Wind-Diesel Hybrid ower System. Introduction arge interconnected power systems supply power to a locality at large for the optimum utilization of the power sources. Due to the non-availability of constraints on the right of way of providing additional transmission lines, sufficient funds, and rapid growth in load with the developments, the demand to provide power to all by large interconnected systems, especially in developing countries like India, does not met. On the other side, the gap between demand and supply is also increasing day by day that has introduced the use of renewable energy sources in addition to conventional energy sources. The sources such as solar, wind, mini/micro hydro, etc. have been introduced extensively as renewable energy sources which contribute to environmental protection also. These types of systems are called as isolated hybrid power systems. Normally diesel sets and wind turbines are connected with synchronous generators and induction generators (IGs), respectively [,]. There are several advantages of induction generators [3,4], but the performance of an induction generator is poor in terms of voltage regulation as it needs a magnetizing current from the source for the purpose of excitation which reduces both the power factor and efficiency of the induction generator [5]. The application of improves the power factor, voltage regulation and efficiency [5]. In, permanent-magnets are inserted in the conventional rotor of an induction generator. arious models and their applications [6-] have been discussed to analyze the steady state performance of the. A d- reference model for a has been described in [7]. The impedance model based on a conventional per phase euivalent circuit with per unit parameters has also been investigated [5,9,0] to know the performance of the. The reactive power flow euations based modeling of is very essential as it can easily be extended for the multi -machine systems. The MSG is connected with the diesel set to avoid the need of automatic voltage regulator (AR). The diesel generator acts as a local grid. arious Flexible A Transmission System (FATS) devices, such as switched capacitors, S, and STAT- OM, are available which can supply fast and continuous reactive power [-6]. A switched capacitor bank is a good alternate to provide reactive power reuired by the induction generators in the grid connected systems. If there is an excess reactive power generated by the capacitors, it will be absorbed by the grid but in an isolated system, the load and the induction generators need the reactive power. Under varying condition, the mismatch

2 54 between generation and consumption of reactive power may not lead to surplus of reactive power. The surplus of reactive power may cause high voltage spikes in the system, that can cause damage to the euipments connected and on at that instant. The switching capacitors may be an option if under any conditions of generation/load of the hybrid system, the problem of the surplus of reactive power may not arise. In general, a variable source of reactive power is needed to eliminate mismatch between generation and consumption of reactive power. The STAT- OM [] employs a voltage source inverter (S), which internally induces inductive or capacitive reactive power as reuired and is also advantageous [] than that of S. First the system state euations are derived from real and reactive power balance euations of the system. A voltage deviation signal is used by STATOM controller to eliminate the reactive power mismatch in the system. The integral suare error (ISE) criterion is used to evaluate the optimum gain settings of the controller parameters. Finally, the transient responses of the hybrid system are shown for % step increase in reactive power load with % step increase in input wind power.. Mathematical Modeling of the Wind-Diesel Hybrid ower System A wind-diesel hybrid power system with STATOM considered for study is shown in Figure. The real and reactive power balance euations of the system under steady state conditions are given by MSG () QMSG Qcom Q Q () Due to disturbance in load reactive power Q, the system voltage can change that results an incremental change in reactive power of other components. The net reactive power surplus is QMSG Qcom Q Q, and it will change the system bus voltage which will govern by the following transfer function euation, see Euation (3) [7]. Where, K and T are the system gain and time constant. All the connected loads experience an increase with the increase in voltage due to load voltage characteristics as shown below: Q D (4) The composite loads can be expressed in exponential voltage form as Q The load voltage characteristics empirically as c (5) D can be found 0 Q Q D (6) 0 In Euation (3) K D and T is the time constant of the system which is proportional to the ratio of electromagnetic energy stored in the winding to the reactive power absorbed by the system. The small change in MSG reactive power, QMSG s, connected with the diesel engines, in terms of state variables is given by: QMSG s K s (7) Eos K (8) X d X d Similarly Q s can be expressed in terms of system state variables as given below. The euivalent circuit of the is shown in Figure. The reactive power absorbed, Q, in terms of generator terminal voltage, slip and generator parameters can be written as where and K K K Q s K s K s (9) IW 3 X R e Y Y e Y IW coreloss R X R 3 3 Kc 3 X 3X a b c 3 /3 (0) () X a b c d () X RR (3) e Y c RY Xe RY Xe RY IW coreloss If X is infinity, then behaves like an IG. In Euation (), the values of a, b, c and d are selected such that the effect of X is euivalent to the permanent-magnets in providing reactive power at different voltages. The STATOM is based on a solid state synchronous voltage source that is analogous to an ideal synchronous K s QMSG s Qcom s Q s Q s st (3)

3 55 Figure. Single line diagram of the isolated wind-diesel hybrid power system. I Re jx e jx M jx E r' R s ( s) Figure. Euivalent ircuit of. machine which generates a balanced set of three sinusoidal voltages, at the fundamental freuency, with rapidly controllable amplitude and phase angle. The con- figuration of a STATOM is shown in Figure 3. It con- sists of a voltage source converter (S), a coupling transformer, and a D.. capacitor as shown in Figure 3(a). The magnitude of the fundamental component of the converter output voltage is k dc, where dc repre- sent the voltage across the capacitor. The reactive power injection to the system bus has the form: [8]. sin Q k Bk Bcos k G (4) com dc dc dc The system bus voltage is taken as reference voltage, therefore the angle,, is zero. Also G, is negligible as G jb represent the step down transformer admittance. Therefore, considering the value of G and to be zero, Euation (4) can be written as Q k B k Bcos (5) com dc dc The flow of reactive power depends upon the variables and, therefore for small perturbation the linearized STATOM euation can be Q Q Qcom (6) Substituting the value of partial derivatives from Euation (5) in Euation (6), we get (a) (b) Figure 3. STATOM (a) schematic diagram and (b) euivalent circuit. where, com 4 5 Q s K K s (7) K4 kdcb sin (8) K5 kdcbcos (9) p 6 where, k and p, is the pulse number of the 6 inverter with modulation index unity [8]. Using these euations, a simulation model of the system has been developed and the transfer function block diagram of the system is shown in Figure 4. All data of the system are given in the appendix.

4 56 Figure 4. Transfer function block diagram of the wind-diesel hybrid power system. 3. Results and Discussions The simulation results of the hybrid system have been carried out and the dynamic performance is presented in this section. The data of the hybrid power system are given in the Appendix. The performance index,, used in the parameter optimization for the isolated hybrid power system problem is based on of the ISE, which is given dt (0) 0 By focusing on the suare of the error function, the performance index penalizes both positive and negative values of the error. Therefore, the minimization of performance index with respect to the known parameter will provide the best voltage performance of the system. The proportional and integral gains K and K I of the STAT- OM were optimized using Integral Suare Error techniue. The proportional and integral gains, K and K I of the STATOM were optimized and the optimized values of K and K I were 7, and 800, respectively. The transient responses of the wind-diesel hybrid power system for % step increase in reactive power load with % step increase in input wind power are shown in Figure 5. It is observed from Figure 5 that initially as the load increases, the system voltage decreases. The decrease in voltage results in increase in the reactive power of the MSG connected with the diesel set and STATOM when is coupled with wind turbine. The decrease in voltage results in increase in reactive power of synchronous generator and STATOM. The reactive power supplied by the MSG and reduces as the system voltage deviation reduces as shown in Figures 5(b), (c). It has also been observed that the reactive power reuired by the load is finally fulfilled by STATOM under steady state condition as shown in Figure 5(d). 4. onclusions The reactive power control of an isolated wind-diesel hybrid power system has been shown in this paper. A has been considered for the generation of electric power from wind turbine and MSG is connected with the diesel set. STATOM is used for providing variable reactive power reuired by the system. A complete dynamic model of the system has been derived to study the effect of load disturbances and input wind power disturbances. The dynamic performance of the system was

5 57 (a) (b) (a) (b) Figure 5. Transient responses of the wind-diesel system for % step increase in reactive power load with % step increase in input wind power, time vs (a), (b) Q, (c) Q MSG, and (d) Q com. investigated by optimizing the controller gains of the STATOM using ISE techniue. It has been observed that during dynamic condition the STATOM eliminate the oscillations effectively with in 0.0 sec. caused by disturbances. As steady state condition reaches the STAT- OM provides the additional reactive power reuired by the load. Some of the dynamic responses of the hybrid system have also been shown in the paper. REFERENES [] R. Hunter and G. Elliot, Wind-Diesel Systems, a Guide to the Technology and Its Implementation, University ress, ambridge, 994. [] H. Nacfaire, Wind-Diesel and Wind Autonomous Energy Systems, Elsevier Applied Science, ondon, 989. [3] A. A. F. A-Ademi, oad-freuency ontrol of Stand- Alone Hybrid ower Systems Based on Renewable Energy Sources, h. D Dissertation, entre for Energy Studies, Indian Institute of Technology, Delhi, India, 996. [4] O. I. Elgerd, Electric Energy System Theory: An Introduction, Tata Mcgraw Hill, New Delhi, India, 98, pp [5] T. Fukami, K. Nakagawa, Y. Kanamaru and T. Miyamoto, A Techniue for the Steady-State Analysis of a Grid- onnected ermanent-magnet Induction Generator, IEEE Transaction on Energy onversion, ol. 9, No., 004, pp [6] T. Fukami, K. Nakagawa, Y. Kanamaru and T. Miyamoto, erformance Analysis of the ermanent-magnet Induction Generator under Unbalanced Grid oltages, Translated from Denki Gakkai Ronbunshi, ol. 6, No., 006, pp [7] J. F. H. Douglas, haracteristics of Induction Motors

6 58 with ermanent-magnet Excitation, Trans AIEE (AS), ol. l78, 959, pp. -5. [8] B. Ackermann, Single hase Induction Motor with ermanent-magnet Excitation, IEEE Transaction on Magnetic, ol. 36, No. 5, 000, pp [9] T. Fukami, K. Nakagawa, Y. Kanamaru and T. Miyamoto, Effects of the Built-in ermanent Magnet Rotor on the Euivalent ircuit arameters of a ermanent Magnet In- Duction Generator, IEEE Transaction on Energy onversion, ol., No. 3, 007, pp [0] T. Fukami, K. Nakagawa, R. Hanaoka, S. Takata and T. Miyamoto, Nonlinear Modeling of a ermanent-magnet Induction Machine, Transactions on Institute of Electrical Engineers of Japan, ol., No. 7, 00, pp [] J. H. J. otgieter, A. N. ombard, R. J. Wang and M. J. Kamper, Evaluation of ermanent-magnet Excited In- Duction Generator for Renewable Energy Applications. eter.pdf [] N. G. Hingorani and. Gyugyi, Understanding FATs: oncepts and technology of Flexible A Transmission Systems, IEEE ower & Energy Society, New York, 000. [3] S. E. Haue, N. H. Malik and W. Shepherd, Operation of a Fixed apacitor Thyristor ontrolled Reactor (Fc-Tcr) ower Factor ompensator, IEEE Transaction on ower Apparatus and Systems, ol. 04, No. 6, 985, pp [4] A. E. Hammad, Analysis of ower System Stability En- Hancement by Static ar ompensators, IEEE Transactions on ower System, ol., No. 4, 986, pp. -7. [5] E. G. Marra, et al., Self-Excited Induction Generator ontrolled by a S-WM onverter roviding High ower- Factor urrent to a Single-hase Grid, roceedings of Industrial Electronics Society onferences, Aachen, 998, pp [6] S.. Kuo and. Wang, Analysis of oltage ontrol for a Self-Excited Induction Generator Using a urrent- ontrolled oltage Source Inverter (-SI), IEEE roceedings of Generation, Transmission and Distribution, ol. 48, No. 5, 00, pp [7] R.. Bansal, Automatic Reactive ower ontrol of Isolated Wind-Diesel Hybrid ower Systems, IEEE Transactions on Industrial Electronics, ol. 53, No. 4, 006, pp [8] B. Kouadri and Y. Tahir, ower Flow and Transient Stability Modeling of a -ulse Statcom, Journal of ybernetic and Informatics, ol. 7, 008, pp. 9-5.

7 59 Nomenclature E M = Electro-magnetic energy stored in the. E M = Small change in the stored electromagnetic energy of. K, K I = roportional and integral controller gain of the STATOM regulator, respectively = erformance index., Q = Real and reactive power generated by wind turbine with System arameters MSG (p. u. ) SG Q SG (p.u. ) E (p.u.) -.36 in.67 X (p.u.) d -.0 IG (p.u. ) Q (p.u. ) IG 0 - % 90 - r r (p.u.) x x (p.u.) s % (p.u. ) - - Q (p.u.) - - ower Factor MSG, Q MSG = Real and reactive power generated by diesel set with synchronous generator., Q = Real and reactive power of load demand. S = Apparent power delivered by the IG and. Q STATOM = Reactive power generated by STATOM. = Incremental change in the voltage. = Incremental change in the phase angle at a static synchronous compensator. Re, Xe, X m = Euivalent resistance, euivalent reactance, and magnetizing reactance of the respecttively s = Slip of = System terminal voltage. E ER jei = Real and Imaginary parts of the E = erformance index = Exponent in the reactive power load voltage characteristic 0 0, Q = Nominal values of the system voltage and reactive power of the load, respectively. Appendix The data of the wind-diesel hybrid power system are given below: Wind apacity(kw) Diesel apacity(kw) oad apacity(kw) The data of the wind-diesel system are given as follows: K.3, K 0.444, K , K4 3.36, K5.5.