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A new variable speed constant voltage controller for self-excited induction generator
Electric Power Systems Research 59 (2001) 157– 164 www.elsevier.com/locate/epsr
A new variable speed constant voltage controller for self-excited induction generator Shashank Wekhande, Vivek Agarwal * Department of Electrical Engineering, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India Received 25 August 2000; accepted 5 July 2001
Abstract A new variable speed, constant voltage controller for self-excited induction generator (SEIG) is presented in this paper. The proposed PWM controller regulates the induction generator (IG) terminal voltage against varying rotor speed and changing load conditions. This scheme does not require any real time computations and information regarding rotor speed for calculating the excitation current, thereby minimizing the electronic hardware and the cost of the controller. A simple, over-current protection is incorporated to protect the inverter switches. Computer simulation and experimental results show satisfactory operation of an induction generator with the proposed control scheme. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Induction generator control; PWM control; Reactive power compensator
1. Introduction The squirrel cage induction generators are widely used for low and medium power generation. These are ideally suited for non-conventional energy generation, such as wind turbine systems [1], due to their simple and rugged construction. It requires a low initial cost and is not costly to maintain. However, the induction generator requires a reactive power source for supplying the excitation current. The generator voltage can be regulated by varying the excitation current. The induction generator can be operated in grid connected or stand-alone self-excited modes. In the self-excited induction generator, the excitation current is supplied by the capacitors connected across its terminals. The terminal voltage is regulated by changing the terminal capacitance. Effective capacitance can be controlled by switching capacitor banks or using a thyristor controlled reactor. A three-phase PWM inverter may also be used as a static reactive power source. The inverter can supply This paper is a revised version of the paper presented at IEEE PEDS’99 conference held in Hong Kong in July 1999. * Corresponding author. E-mail address: [email protected] (V. Agarwal).
leading or lagging reactive current. The excitation current can be controlled by controlling the modulation index and phase of fundamental inverter voltage with respect to the generator voltage. The use of a static PWM inverter for controlling excitation gives a better transient response and smooth variation of excitation current. The PWM controller can be placed in series or shunt with induction generator (IG). The series connected controller is used as a PWM controlled ac –dc converter [2]. For supplying ac loads, an additional inverter of full rating is required. Thus, the two-stage controller has to handle active load current and excitation current of the IG and compensate for reactive load current. The active load current is supplied directly by the IG. The shunt connected PWM controller has to supply only the reactive load current and excitation current of the IG. Thus the shunt connected controller requires devices of lower power ratings. Various schemes have been reported for shunt operation of self-excited IG controller [3–10]. A simple V/F control with sinusoidal PWM control is presented in [3]. Stator flux oriented vector control [4] and rotor flux oriented control [5,6] provide better performance. 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
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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. A new IG controller based on indirect estimation of reference current is reported in [7– 9]. This simple control circuit can be used for variable speed, constant voltage operation of IG. The controller is suitable for all sorts of practical loads including reactive, harmonic and unbalanced loads. However, protection of semiconductor switches, in case of a fault, is difficult in this scheme, as only the supply current is being sensed. In this paper, a simple control scheme is proposed for self-excitation control of induction generator. In this scheme, only the controller current is sensed and forced to track the reference current. The generator voltage is regulated by controlling the excitation current supplied by the controller. The voltage is regulated irrespective of varying rotor speed and changing loads. The controller does not require any real time mathematical computation. The control technique also does not require information about rotor speed and position. This eliminates need for mechanical sensors, minimizing
hardware and reducing overall cost. Direct sensing of the controller current enables protection of the controller from over-current. Low rating current sensors are required for this scheme as compared to [7], as the sensors have to sense only the controller current. The remaining paper is organized as follows. 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 verified by experimental work. The details of simulation and experimental work are presented in Sections 3 and 4, respectively, followed by main conclusions in Section 5.
2. Principle of operation The proposed controller uses a hysteresis current controlled, voltage source PWM inverter to supply the reactive load current and desired excitation current for induction generator. The scheme is shown in Fig. 1. As shown in the figure, there are two control loops. The inner current control loop forces the actual inverter
Fig. 1. Block diagram of proposed induction generator controller.
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The sinusoidal template voltages are derived by simply stepping down the generator voltages. The three phase reference cosine templates required in ac control path are generated from reference sinusoidal templates using simple trigonometric relations as shown below: cos(A)=0.58[sin(C)− sin(B)] cos(B)= 0.58[sin(A)− sin(C)] cos(C) =0.58[sin(B)− sin(A)]
Initially, the controller is kept disabled. The induction generator is started in the conventional manner using three capacitors of fixed value. When sufficient voltage is generated, the controller is enabled. In this scheme, only the controller current is sensed. The controller is protected against over-current by sensing the current flowing through it and using this information to shut-off the gate pulses of the IGBTs.
Fig. 2. Reference current generation.
current to follow the reference current generated by the outer loop. The reference current (Iref) is derived by adding two current components, viz. the in-phase current (Iractive) and the quadrature current (Irexcitation). Iref = Iractive + Irexcitation
(1)
The in-phase active current component overcomes losses in the converter. The difference in active load current demand and the active component of generator current flows through the converter. This mismatch in current is reflected in the variation of dc link voltage of the controller. Thus, the active current component of the generator can be controlled by controlling the in-phase component of inverter current. The reference in-phase inverter current is obtained by multiplying the dc link error with reference template voltage derived from the supply voltage. Active reference current component for phase A is given by: Iractive = Vdcerr sin( t)
(2)
Similarly, active current components for the other two phases are derived by multiplying the respective phase voltages. The quadrature reference current is decided by the net excitation current of the induction generator and reactive load current demand. This excitation current of IG determines the generator voltage. The quadrature reference current is obtained by comparing the generator voltage with a reference level and the resultant ac voltage error is multiplied with corresponding cosine template. Generation of reference current is illustrated in Fig. 2. The quadrature reference current component for phase A is: Irexcitation =Vacerr cos( t)
(4)
(3)
3. Simulation results The induction generator system with proposed PWM controller was extensively simulated on the digital computer using SABER software. The generator ac voltage is set to 230 V rms and the dc link voltage is set to 900 V. A 5000 mF capacitor is connected across the dc link of controller. The simulation set-up used in SABER simulation is shown in Fig. 3. The controller has been simulated under various load conditions to validate the operation of the PWM controller. These simulation results are now discussed one by one for different load conditions:
3.1. Resisti6e load The induction generator is loaded with balanced three phase resistive load of 100 V in each phase. The induction generator is started with 10 mF capacitor up to 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 900 V. 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. Fig. 4. Shows the leading controller current. This current is proportional to the excitation current of IG. Balanced three phase generator voltages are shown in Fig. 5. In steady-state condition, the dc link voltage is stable. This is shown in Fig. 6. The harmonic contents in the generator voltage are almost negligible as shown in Fig. 7. The figure also shows frequency spectrum of controller current.
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Fig. 3. Simulation set-up.
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Fig. 4. Response to resistive load: generator voltage, controller and load currents.
3.2. Reacti6e load The induction generator controller regulates the generator voltage for reactive loads also. In case of reactive loads, the controller supplies required excitation current to the induction generator and compensates for reactive load current. The operation of the controller is validated with balanced three-phase RL load. The RL load comprises of fixed 70 V resistance in series with 100 mH inductor. Fig. 8 shows the generator voltage, lagging load current and the controller current. The controller supplies the leading current to overcome the lagging load current apart from supplying excitation current to the generator.
3.3. Transient load The induction generator controller is designed to operate with fluctuating load currents. Also, the use of static PWM inverter results in a faster 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
Fig. 5. Three phase generator voltages with resistive load.
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Fig. 6. DC link voltage.
load current. The initial load resistance is 100 V and is changed to 66 V at 1.6 s. The simulation results for a step increase in load current are shown in Fig. 9. The figure shows that a step increase in load current increases the controller current promptly to support the additional excitation current demand which regulates the output voltage to the set value in about 40–45 cycles. It is, however, not possible to accommodate all these cycles in one figure (as the figure would appear crowded). Hence, only a few transient cycles are shown in the figure, in which the various quantities have not yet settled to their steady-state. However, it was observed that after about one second the waveforms settled to their steady-state. The operation of the induction generator is also verified for step decrease in load current. This is realized by suddenly increasing the load resistance from 66 to 100 V at t= 1.6 s. The transient load current, supply voltage and the controller current are shown in Fig. 10. The controller current decreases with a reduction in excitation current demand and settles to the new steady-state value in about 40–45 cycles. Once again,
Fig. 7. Harmonic contents in generator voltage and controller current.
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Fig. 8. Response to reactive load. Fig. 10. Response to step decrease in load current.
due to the problem cited for Fig. 9, only a few transient cycles are shown in Fig. 10. In this case too, the waveforms settled to their steady-state values in about one second.
3.4. With 6arying 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 generator voltage within specified variation of rotor speed. The simulation result for rotor speed variation from −160 to −200 rad/s is shown in Fig. 11. Figure shows a varying rotor speed, the regulated generator voltage and controller current waveforms.
4. Experimental results To verify the simulation results presented in Section 3, a three-phase IGBT based experimental prototype has been developed for a 1 HP 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. Tecktronix made TDS 220 DSO was used for capturing the results. The currents are sensed using LEM current sensors. The DC link voltage is sensed using L&T made isolation amplifier having a ratio of 25:1. The supply voltage is sensed across the reference voltage transformers having 230:6 ratio.
3.5. O6er-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. 12, 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.
Fig. 9. Response to step increase in load current.
4.1. Reacti6e load The reactive load test is performed with a balanced three phase RL load comprising 100 V resistance in series with 200 mH inductor. The generator voltage and lagging load current waveforms are shown in Fig. 13,
Fig. 11. Response to variation in rotor speed.
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Fig. 14. Reactive load: generator voltage and generator current. Fig. 12. Over-current protection.
while the generator voltage and current waveforms are shown in Fig. 14.
4.2. Transient load The rotor speed is adjusted to 900 rpm at steadystate with 200 V resistance. The three-phase balanced resistive load is decreased from 200 to 100 V 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 generator voltage as shown in Fig. 15. Similarly, the response of the controller to step decrease in load is shown in Fig. 16. In this experiment, rotor speed is adjusted to 900 rpm at 100 V load resistance. The per phase balanced load resistance is suddenly increased form 100 to 200 V in a step manner. The rotor speed increases to 925 rpm after transient.
mover. The generator voltage remains nearly constant as shown in Fig. 17. The waveform is captured over the entire 5 s duration, due to which the figure appears crowded. However, it was noted, with the help of a true RMS voltmeter, that the voltage remains fairly constant during the transient period.
4.4. O6er current protection In this scheme, the current flowing in the PWM converter is sensed. This current is used to provide over-current protection. The over-current 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 connected for starting purpose. The results during overcurrent condition are shown in Fig. 18.
4.3. With 6arying rotor speed The rotor speed is increased in 5 s from 900 to 1200 rpm by keeping load constant. A three-phase balanced load of 100 V in each phase is connected across the IG. The rotor speed is increased by increasing the armature voltage of the dc motor, which is used as a prime
Fig. 13. Reactive load: generator voltage and load current.
5. Conclusions Induction generators are extensively used in low and medium power generation. IGs require an external excitation source. The excitation may be supplied from the grid or locally in self-excited mode.
Fig. 15. Response to step increase in load current.
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Fig. 18. Over-current protection.
Fig. 16. Response to step decrease in load current.
References
Fig. 17. Response to varying rotor speed.
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 controller has been simulated on digital computer and the operation is experimentally verified. The results of simulation and experimental work follow expected pattern.
[1] M. Ermis, H.B. Ertan, et al., Various induction generator schemes for wind-electricity generation, J. Electric Power Syst. Res. 23 (1992) 71 – 83. [2] S. Carlsen, Generating fixed voltage and frequency from a generator driven with a variable speed, optimizes the power extraction, The European Power Electronics Conference EPE ’93, Brighton, UK, pp. 272 – 275. [3] S.N. Bhadra, K. Venkata Ratnam, A. Manjunath, Study of voltage built up in a self-excited, variable speed induction generator/static inverter system with dc side capacitor, IEEE PEDES’96 Conf. Proc., 1996, New Delhi, India, pp. 964 –970. [4] S.R. Silva, R.O.C. Lyra, PWM converter for excitation of induction generators, The European Power Electronic Conference EPE-’93, Brighton, UK pp. 174 – 178. [5] 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, San Diego, USA, pp. 432 – 439. [6] Y.W. Liao, E. Levi, Modeling and simulation of a stand-alone induction generator with rotor flux oriented control, J. Electric Power Syst. Res. 26 (1998) 141 – 152. [7] S.S. Wekhande, V. Agarwal, A simple wind driven self-excited induction generator with regulated output voltage, INTELEC’99, Denmark, June 99, pp. 1 – 19. [8] S.S. Wekhande, V. Agarwal, Wind driven self-excited induction generator with simple de-coupled excitation control, IAS’99, USA, Oct. 1999, pp. 2077 – 2083. [9] S.S. Wekhande, V. Agarwal, A new induction generator controller for stand-alone application with unbalanced load, EECTD’99, Belgium, Sep. 1999. [10] S.S. Wekhande, V. Agarwal, A variable speed constant voltage controller for self-excited induction generator with minimum control requirements, PEDS’99, Hong Kong, July 1999, pp. 98 – 103.
A new variable speed constant voltage controller for self-excited induction generator Shashank Wekhande, Vivek Agarwal * Department of Electrical Engineering, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India Received 25 August 2000; accepted 5 July 2001
Abstract A new variable speed, constant voltage controller for self-excited induction generator (SEIG) is presented in this paper. The proposed PWM controller regulates the induction generator (IG) terminal voltage against varying rotor speed and changing load conditions. This scheme does not require any real time computations and information regarding rotor speed for calculating the excitation current, thereby minimizing the electronic hardware and the cost of the controller. A simple, over-current protection is incorporated to protect the inverter switches. Computer simulation and experimental results show satisfactory operation of an induction generator with the proposed control scheme. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Induction generator control; PWM control; Reactive power compensator
1. Introduction The squirrel cage induction generators are widely used for low and medium power generation. These are ideally suited for non-conventional energy generation, such as wind turbine systems [1], due to their simple and rugged construction. It requires a low initial cost and is not costly to maintain. However, the induction generator requires a reactive power source for supplying the excitation current. The generator voltage can be regulated by varying the excitation current. The induction generator can be operated in grid connected or stand-alone self-excited modes. In the self-excited induction generator, the excitation current is supplied by the capacitors connected across its terminals. The terminal voltage is regulated by changing the terminal capacitance. Effective capacitance can be controlled by switching capacitor banks or using a thyristor controlled reactor. A three-phase PWM inverter may also be used as a static reactive power source. The inverter can supply This paper is a revised version of the paper presented at IEEE PEDS’99 conference held in Hong Kong in July 1999. * Corresponding author. E-mail address: [email protected] (V. Agarwal).
leading or lagging reactive current. The excitation current can be controlled by controlling the modulation index and phase of fundamental inverter voltage with respect to the generator voltage. The use of a static PWM inverter for controlling excitation gives a better transient response and smooth variation of excitation current. The PWM controller can be placed in series or shunt with induction generator (IG). The series connected controller is used as a PWM controlled ac –dc converter [2]. For supplying ac loads, an additional inverter of full rating is required. Thus, the two-stage controller has to handle active load current and excitation current of the IG and compensate for reactive load current. The active load current is supplied directly by the IG. The shunt connected PWM controller has to supply only the reactive load current and excitation current of the IG. Thus the shunt connected controller requires devices of lower power ratings. Various schemes have been reported for shunt operation of self-excited IG controller [3–10]. A simple V/F control with sinusoidal PWM control is presented in [3]. Stator flux oriented vector control [4] and rotor flux oriented control [5,6] provide better performance. 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
0378-7796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 9 6 ( 0 1 ) 0 0 1 5 7 - 2
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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. A new IG controller based on indirect estimation of reference current is reported in [7– 9]. This simple control circuit can be used for variable speed, constant voltage operation of IG. The controller is suitable for all sorts of practical loads including reactive, harmonic and unbalanced loads. However, protection of semiconductor switches, in case of a fault, is difficult in this scheme, as only the supply current is being sensed. In this paper, a simple control scheme is proposed for self-excitation control of induction generator. In this scheme, only the controller current is sensed and forced to track the reference current. The generator voltage is regulated by controlling the excitation current supplied by the controller. The voltage is regulated irrespective of varying rotor speed and changing loads. The controller does not require any real time mathematical computation. The control technique also does not require information about rotor speed and position. This eliminates need for mechanical sensors, minimizing
hardware and reducing overall cost. Direct sensing of the controller current enables protection of the controller from over-current. Low rating current sensors are required for this scheme as compared to [7], as the sensors have to sense only the controller current. The remaining paper is organized as follows. 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 verified by experimental work. The details of simulation and experimental work are presented in Sections 3 and 4, respectively, followed by main conclusions in Section 5.
2. Principle of operation The proposed controller uses a hysteresis current controlled, voltage source PWM inverter to supply the reactive load current and desired excitation current for induction generator. The scheme is shown in Fig. 1. As shown in the figure, there are two control loops. The inner current control loop forces the actual inverter
Fig. 1. Block diagram of proposed induction generator controller.
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The sinusoidal template voltages are derived by simply stepping down the generator voltages. The three phase reference cosine templates required in ac control path are generated from reference sinusoidal templates using simple trigonometric relations as shown below: cos(A)=0.58[sin(C)− sin(B)] cos(B)= 0.58[sin(A)− sin(C)] cos(C) =0.58[sin(B)− sin(A)]
Initially, the controller is kept disabled. The induction generator is started in the conventional manner using three capacitors of fixed value. When sufficient voltage is generated, the controller is enabled. In this scheme, only the controller current is sensed. The controller is protected against over-current by sensing the current flowing through it and using this information to shut-off the gate pulses of the IGBTs.
Fig. 2. Reference current generation.
current to follow the reference current generated by the outer loop. The reference current (Iref) is derived by adding two current components, viz. the in-phase current (Iractive) and the quadrature current (Irexcitation). Iref = Iractive + Irexcitation
(1)
The in-phase active current component overcomes losses in the converter. The difference in active load current demand and the active component of generator current flows through the converter. This mismatch in current is reflected in the variation of dc link voltage of the controller. Thus, the active current component of the generator can be controlled by controlling the in-phase component of inverter current. The reference in-phase inverter current is obtained by multiplying the dc link error with reference template voltage derived from the supply voltage. Active reference current component for phase A is given by: Iractive = Vdcerr sin( t)
(2)
Similarly, active current components for the other two phases are derived by multiplying the respective phase voltages. The quadrature reference current is decided by the net excitation current of the induction generator and reactive load current demand. This excitation current of IG determines the generator voltage. The quadrature reference current is obtained by comparing the generator voltage with a reference level and the resultant ac voltage error is multiplied with corresponding cosine template. Generation of reference current is illustrated in Fig. 2. The quadrature reference current component for phase A is: Irexcitation =Vacerr cos( t)
(4)
(3)
3. Simulation results The induction generator system with proposed PWM controller was extensively simulated on the digital computer using SABER software. The generator ac voltage is set to 230 V rms and the dc link voltage is set to 900 V. A 5000 mF capacitor is connected across the dc link of controller. The simulation set-up used in SABER simulation is shown in Fig. 3. The controller has been simulated under various load conditions to validate the operation of the PWM controller. These simulation results are now discussed one by one for different load conditions:
3.1. Resisti6e load The induction generator is loaded with balanced three phase resistive load of 100 V in each phase. The induction generator is started with 10 mF capacitor up to 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 900 V. 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. Fig. 4. Shows the leading controller current. This current is proportional to the excitation current of IG. Balanced three phase generator voltages are shown in Fig. 5. In steady-state condition, the dc link voltage is stable. This is shown in Fig. 6. The harmonic contents in the generator voltage are almost negligible as shown in Fig. 7. The figure also shows frequency spectrum of controller current.
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Fig. 3. Simulation set-up.
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Fig. 4. Response to resistive load: generator voltage, controller and load currents.
3.2. Reacti6e load The induction generator controller regulates the generator voltage for reactive loads also. In case of reactive loads, the controller supplies required excitation current to the induction generator and compensates for reactive load current. The operation of the controller is validated with balanced three-phase RL load. The RL load comprises of fixed 70 V resistance in series with 100 mH inductor. Fig. 8 shows the generator voltage, lagging load current and the controller current. The controller supplies the leading current to overcome the lagging load current apart from supplying excitation current to the generator.
3.3. Transient load The induction generator controller is designed to operate with fluctuating load currents. Also, the use of static PWM inverter results in a faster 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
Fig. 5. Three phase generator voltages with resistive load.
161
Fig. 6. DC link voltage.
load current. The initial load resistance is 100 V and is changed to 66 V at 1.6 s. The simulation results for a step increase in load current are shown in Fig. 9. The figure shows that a step increase in load current increases the controller current promptly to support the additional excitation current demand which regulates the output voltage to the set value in about 40–45 cycles. It is, however, not possible to accommodate all these cycles in one figure (as the figure would appear crowded). Hence, only a few transient cycles are shown in the figure, in which the various quantities have not yet settled to their steady-state. However, it was observed that after about one second the waveforms settled to their steady-state. The operation of the induction generator is also verified for step decrease in load current. This is realized by suddenly increasing the load resistance from 66 to 100 V at t= 1.6 s. The transient load current, supply voltage and the controller current are shown in Fig. 10. The controller current decreases with a reduction in excitation current demand and settles to the new steady-state value in about 40–45 cycles. Once again,
Fig. 7. Harmonic contents in generator voltage and controller current.
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Fig. 8. Response to reactive load. Fig. 10. Response to step decrease in load current.
due to the problem cited for Fig. 9, only a few transient cycles are shown in Fig. 10. In this case too, the waveforms settled to their steady-state values in about one second.
3.4. With 6arying 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 generator voltage within specified variation of rotor speed. The simulation result for rotor speed variation from −160 to −200 rad/s is shown in Fig. 11. Figure shows a varying rotor speed, the regulated generator voltage and controller current waveforms.
4. Experimental results To verify the simulation results presented in Section 3, a three-phase IGBT based experimental prototype has been developed for a 1 HP 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. Tecktronix made TDS 220 DSO was used for capturing the results. The currents are sensed using LEM current sensors. The DC link voltage is sensed using L&T made isolation amplifier having a ratio of 25:1. The supply voltage is sensed across the reference voltage transformers having 230:6 ratio.
3.5. O6er-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. 12, 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.
Fig. 9. Response to step increase in load current.
4.1. Reacti6e load The reactive load test is performed with a balanced three phase RL load comprising 100 V resistance in series with 200 mH inductor. The generator voltage and lagging load current waveforms are shown in Fig. 13,
Fig. 11. Response to variation in rotor speed.
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Fig. 14. Reactive load: generator voltage and generator current. Fig. 12. Over-current protection.
while the generator voltage and current waveforms are shown in Fig. 14.
4.2. Transient load The rotor speed is adjusted to 900 rpm at steadystate with 200 V resistance. The three-phase balanced resistive load is decreased from 200 to 100 V 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 generator voltage as shown in Fig. 15. Similarly, the response of the controller to step decrease in load is shown in Fig. 16. In this experiment, rotor speed is adjusted to 900 rpm at 100 V load resistance. The per phase balanced load resistance is suddenly increased form 100 to 200 V in a step manner. The rotor speed increases to 925 rpm after transient.
mover. The generator voltage remains nearly constant as shown in Fig. 17. The waveform is captured over the entire 5 s duration, due to which the figure appears crowded. However, it was noted, with the help of a true RMS voltmeter, that the voltage remains fairly constant during the transient period.
4.4. O6er current protection In this scheme, the current flowing in the PWM converter is sensed. This current is used to provide over-current protection. The over-current 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 connected for starting purpose. The results during overcurrent condition are shown in Fig. 18.
4.3. With 6arying rotor speed The rotor speed is increased in 5 s from 900 to 1200 rpm by keeping load constant. A three-phase balanced load of 100 V in each phase is connected across the IG. The rotor speed is increased by increasing the armature voltage of the dc motor, which is used as a prime
Fig. 13. Reactive load: generator voltage and load current.
5. Conclusions Induction generators are extensively used in low and medium power generation. IGs require an external excitation source. The excitation may be supplied from the grid or locally in self-excited mode.
Fig. 15. Response to step increase in load current.
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Fig. 18. Over-current protection.
Fig. 16. Response to step decrease in load current.
References
Fig. 17. Response to varying rotor speed.
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 controller has been simulated on digital computer and the operation is experimentally verified. The results of simulation and experimental work follow expected pattern.
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