A VARIABLE SPEED CONSTANT VOLTAGE CONTROLLER FOR SELF-EXCITED INDUCTION GENERATOR WITH MINIMUM CONTROL REQUIREMENTS

Transcription

1 IEEE 1999 International Conference on Power Electronics and Drive Systems, PEDS 99, July 1999, Hong Kong. A VARIABLE SPEED CONSTANT VOLTAGE CONTROLLER FOR SELF-EXCITED INDUCTION GENERATOR WITH MINIMUM CONTROL REQUIREMENTS Shashank Wekhande Vivek Agarwal Department of Electrical Engineering, Indian Institute of Technology-Bombay, Powai, Mumbai, India agarwai@ee.iitb.ernet.in Abstract: The paper deals with a variable speed, constant voltage controller for induction generator operating in selfexcited mode. A new PWM controller is proposed to regulate the induction generator terminal voltage. The proposed controller regulates three-phase AC output voltage of the selfexcited induction generator with varying rotor speed, transient load conditions and reactive loads. The proposed scheme does not require any real time computations for calculating excitation current, thus mhhizhg the electronic hardware and the cost of the contrdler. A simple over-current protection is incorporated to protect the inverter switches. Computer simulation and experimentd resnlts shaw satisfactory operalion of an indu&generatorwiththe proposed contrd scheme. 1. INTRODUCTION: The squirrel cage induction generator is quite popular in wind turbine systems [l] due to its simpler construction. It requires low initial cost and is not costly to maintain. However, the inductim geaerato: requires a reactive power source for supplying the excitation current. The induction generator can be operated in grid c~~ected or stand-alone self-excited modes. In the selfexcited induction generator, the excitation current is supplied by the capacitors connected across its terminals. The terminal voltage is regulated against changing speed and load conditions, by changing the terminal capacitance. Effective capacitance can be controlled smoothly by varying the firing angle of thyristor controlled reactors, connected in parallel with a ked capacitor [2]. A three-phase PWM inverter may also be used as a static reactive power source. The desired excitation can be controlled by controlling the modulation index and phase of fundamental inverter voltage with respect to the generated voltage. Use of PWM inverter as a reactive power source for induction generator is reported in [3,4]. All these schemes, however, are based on load current sensing. The reactive load current and excitation current required for maintaining constant output voltage of an induction generator have to be supplied by the PWM inverter. Complicated high speed electronic circuits are required to determine reference generator current under varying load and rotor speed conditions. In this paper, a simple control scheme is proposed for self-excitation control of induction generator. The proposed scheme uses a static P W inverter for controlling excitation. This-gives a-- transient response and smooth variation of excitation current. The generated voltage is regulated by controlling the current drawn by the inverter. The voltage is regulated irrespective of varying rotor speed, transient load, and reactive loads. The controller does not require any real time mathematical computation, minimizing hardware and reducing overall cost. A simple over-current protection is incwporated to prevent the inverter switches fiom being damaged The principle of operation of the proposed controller and its block diagram are explained in section 2. The controller has been extensively simulated and the operation is validated by experimental work. The details of simulation and experimental work are presented in sections 3 and 4 respectively followed by conclusions in section PRINCIPLE OF OPERATION: The proposed voltage regulated induction generator controller uses a hysteresis current controkd PWM inverta to supply reactive load current and desired excitation current for induction generator. The proposed scheme is shown in Fig. 1. The reference current is the resultant of two currents viz. the in-phase current and the quadrature current. The inphase current overcomes losses in the converter. The mismatch between the active load current demand and the generator current is reflected in the variation of DC link voltage of the controller. The active current can be controlled by controlling the in-phase current which is obtained by multiplying the DC link error with reference template voltage derived fiom the supply voltage. The quadrature current decides the magnetizing current of the induction generator. This magnetizing current determines the generated voltage. The generated voltage is compared with the reference and the error is multiplied with corresponding cosine template. The variation in generated voltage is reflected in ac error voltage and reference quadrature current. Generation of reference current is shown in Fig. 2. Initially, the controller is kept disabled The indudon gemratur is started in the conventional manner using three capacitors of fixed value. When SUtFcient voltage is generated, the controller is &led. In this scheme, only the inverter current is sensed. The invehter is protected against overcurrent by sensing the mat flowing through it and using this informi#icmto~gatepulsesoftheims /99/$ IEEE 98

2 Fig 1 : Block diagram of proposed Induction generator mtroller 1 I 3. SIMULATION RESULTS: The induction generator system with proposed PWM controller was extensively simulated on the digital computer using SAE%ER software. Various loading conditions were simulated to validate the operation of the PWM controller undm various conditions. These simulation results are now discussed me by one for differenct load conditions: Resistive load The induction generator is loaded with balanced three phase resistive load of loor in each phase. The induction generator is started with 10pF capacitor upto 40 ms while the controller is kept disabled. Subsequently, the controller is enabled and governs the control. Initially, the DC link capacitor is charged to 900V. The induction motor model used in SABER does not have any residual magnetism. The AC capacitors are assigned some initial voltage to start the induction generator. The reference current has two components viz. inphase reference current and quadrature reference current. The in-phase reference current is responsible for regulating the DC link voltage and is derived by multiplying the DC link voltage error with in-phase voltage template derived fiom the generated voltage. The quadrature reference current is responsible for regulation of the generated voltage. This reference current is derived by multiplying the AC error voltage with a voltage templates leading by 90' w.r.t. the generator voltages of respective phases. The AC error voltage is derived by comparing the peak of generated voltage with a reference signal. The actual reference current is derived by adding the quadrature and in-phase reference current components. This is illustrated in Fig. 2. The figure shows the generated voltage, actual reference, quadrature reference and in-phase reference current waveforms. 20 -r (A) i:o (b) -LO I om * Reactive load The induction generator controller regulates the generated voltage for reactive loads also. In case of reactive loads, the controller supplies required excitation current to the induction generator and compensates for reactive Icad current. The operation of the controller is validated with balanced three phase IU load. The RL load comprises of I 99

3 fixed 70S2 resistance in series with 100 mh inductor. Fig. 4 shows the generated voltage, lagging load current and the controller current. The controller supplies the leading current to overcome the lagging load current apart fiom supplying excitation current to the generator. controller current are shown in Fig. 6. The controller current decreases with a reduction in excitation current demand. 10 I I I I I55 I I I TmIE (.) Fig. 5: Response to step haease in load current (a) Controlla current (b) Load current (c) Generator line voltage L = *) Fig, 3: Response to resistive load (a) Geuerated voltage (b) Controller cunem (c) Load current 40 I J O0 40 (A) 00 Fig 4: Response to reactive kad (a) Generated voltage (b) Load current (c) Coobollex current Transient load The induction generator controller is designed to operate with fluctuating load currents. Also, the use of static PWM inverter results in a Mer transient response as compared to the conventional controller with thyristor controlled reactor. The simulation of this induction generator based system, shows a constant voltage irrespective of 33% step increase in the resistive load current. The initial load resistance is 1000 and is changed to at 1.6s. The simulation results for a step increase in load current are shown in Fig. 5. The figure shows that a step increase in load current increases the controller current instantaneously to support the additional excitation current demand which regulates the output voltage to the set value almost instantaneously. The operation of the induction generator is also verified for step decrease in load current. This is realised by suddenly increasing the load resistance fiom 66R to at t = 1.6s. The transient load anrent, supply voltage and the , J I S LO m 0) Fig 6: Respom to step d m in load current (a) Load current (b) Generated voltage (c) Control!a currect With vqing rotor speed The induction generator is normally used in applications such as wind or micro-hydro energy generation with variable rotor shaft speeds. The controller is capable of regulating the generated voltage within specified variation of rotor speed. The simulation results for rotor speed variation fiom -160 rad/= to -200 radsec are shown in Fig. 7. Figure shows a varying rotor speed, the regulated generated voltage and controller current waveforms. -1IO I I 4, o ao..- a, 30-3Q I l Inas (.) Fig 7: Response to variation in rotor specd (a) Rotor speed (b) Generated voltage (c) Controller current (the negative rotor speed mpxsem aaticlockwise rotation of the shaft) 1 100

4 Over-current protection The placement of current sensors directly in series with the controller, enable the controller to be protected against over-current. Operation of the controller against over-current is shown in Fig. 8, where the transient load current reaches the trip current reference at 1.64 sec. A mechanical contactor trips the controller and generator voltage decays as shown in the figure. (A) LO 4111, 4 o J i 0 : 1 ms Fig. 9: Reactive load: (a) Generated voltage (b) Load ament s s TImE (a Fig. 8: Overcurrent protection : (a) controller line current (b)gznerated voltage (c) Overcurrent sensing signal 4. EXPERIMENTAL RESULTS: To veri@ the simulation results presented in section 3, a three phase IGBT based experimental prototype has been developed for a 1 I-P induction generator. The terminal voltage is regulated at 110 V(rms). An armature controlled DC motor is used as a prime mover. The proposed scheme has been experimentally verified for various loads and the results are presented in this section. -- : I..... stw MPor:-4CU#lms ,.. -., :...:...:...:.... T@ +, , Reactive load The reactive load test is performed with a balanced three phase RL load comprising IOOQ resistance in series with 200 mh inductor. The generated voltage and lagging load current waveforms are shown in Fig. 9, while, the generator voltage and went wavefonns are shown in Fig. 10. Trmient load The rotor speed is adjusted to 900 rpm at steady state with 200Q resistance. The three phase balanced resistive load is decreased from 200R to IOOQ using a mechanical contactor. After the sudden change in load, the rotor speed drops to 870 rpm. The increase in load causes transient decrease in the generated voltage as shown in Fig. 11. Similarly, the response of the controller to step decrease in load is shown in Fig. 12. In this experiment, rotor speed is adjusted to 900 rpm at looq load resistance. The per phase balanced load resistance is suddenly increased form looq to 200Q in a step manner. The rotor speed increases to 925 rpm after transient. Fig 10 : Reaaive load (a) Generator voltage (b) Genaaux cumt m%v ai2 tow M e En\ Fig. 11: Response to step increase in load current (a) Generator voltage (b) Load current 101

5 0 stop MPorl48X)mr - 2 C :...:...:...: F...:...: Fig. 12: Response to step deaease in load current (1) Load cumnt (2) Genexator voltage With vqing rotor speed The rotor speed is increased fkom 900 rpm to 1200 rpm by keeping load constant. A three phase balanced load of 100 S2 in each phase is co~ected across the IG. The rotor speed is increased by increasing the armature voltage of the DC motor which is used as a prime mover. The generator voltage remains nearly constant as shown in Fig :... :... :... : I-!. Fig. 13: Response to rarymg rotor speed (a) Generator voltage (b) Generator current Over current protection In this scheme the current flowing in the PWM converter is sensed. This current is used to provide overcurrent protection. The overcurrent limit is set to 1.0 A. The load current is increased above the tripping value and it is found that the controller disables gate pulses to the PWM converter. The IG voltage drops in the absence of controller current and continues to supply the load with the help of fixed capacitors c~nnected for starting purpose. The results during overcurrent condition are shown in Fig. 14. Fig 14: Overcumnt protcaion (a) Generator cumnt (b) Controller current 5. CONCLUSIONS: A new controller for variable speed, constant voltage operation of induction generator, in self-excited mode has been presented in this paper. The proposed controller does not require any on-line computations or any mechanical sensor thereby reducing the complexity and cost of the controller.. The contro!ler has been simulated on digital computer and the operation is experimentally verified. The results of simulation and experimental work follow expected pattern. REFERENCES : [ 11 Bhim Sin& Induction generators-a prospective, Int. J. on Electric machines and power systems, vol. 23, pp A. A. Shaltout and M. A. Abdel-Halim, Solid-state control of a wind driven self-excited induction generator, Int. J. on Electric machines and power systems, vol. 23, 1995, pp N. Ammasaigounden and M. Subbiah, Microprocessor based voltage controller for wind driven Induction Generator, IEEE trans. on I.E., vol. 37, no. 6, Dec. 1990, pp S. Carlsen, Generating fixed voltage and fkequency fkom a generator driven with a variable speed, optimizes the power extraction, The European power Electronics conference EPE 93, pp M. G. Shoes, B. K. Bose and R J. Spigel, Fuzzy logic based intelligent control of a variable speed cage 102

6 machine wind generation system, KEEE trans. on PE vol. 12, no. 1, Jan. 1997, pp [6] S. R Silva and R. 0. C. Lya, PWM convefter for excitation of induction generators, The European Power Electronic Conference EPE- 93, pp [7] C. B. Jacobina, E. R C. da Silva, et. al. Induction generator static systems with a reduced number of components, IEEE IAS annual meeting 1996, pp [8] Y. W. Liao and E. Levi, Modeling and simulation of a stand-alone induction generator with rotor flux oriented control, Journal of Ele. power systems research, vol. 26, 1998, pp S. S. Wekhande and V. Agmal, Static VAR compensator with supply side current sensors and improved transient response using feed-forward control, IEEE-PEDES 98, Australia, pp [IO] S. S. Wekhande and V. Agarwal, A simple wind driven self-excited induction generator with regulated output voltage, Accepted in rntelec 99 conference to be held in Denmark in June [ 111 S. S. Wekhande and V. Aganval, Wind driven selfexcited induction generator with simple de-coupled excitation control, Accepted in Annual IAS 99 conference to be held in USA in Oct