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697 A New Automatic Voltage Regulator of Self-Excited Induction Generator using SVC Magnetic Energy Recovery Switch (MERS) Fransisco Danang Wijaya , Takanori Isobe , Kazuhiro Usuki , Jan A. Wiik and Ryuichi Shimada Solution Research Organization Tokyo Institute of Technology, Tokyo, 152-8550 Abstract— In this paper, a new voltage regulator applied to self- excited induction generator using SVC magnetic energy recovery switch (MERS) is proposed. Reactive compensation is required to maintain rated voltage when operating in load varying conditions or variable speeds. The proposed system consists of a bi- directional current switch, dc capacitor and inductor as a filter and operates as a variable reactive compensator. Two types of experiments were conducted to perform voltage control in load varying conditions at constant and variable speed. The proposed system has the following advantages: i) simple and fast control, where only two voltage sensors are required, PI controller for feedback operation gives fast response, ii) low switching losses can be achieved using zero voltage switching and low switching frequency, iii) low harmonic distortion; optimal selection of capacitor and inductor will lead to low distortion. The proposed system is proved to have good performance when being applied as a voltage regulation to induction generator. Further more, rating reduction of SVC MERS of about 60% can be achieved by connecting fixed capacitor in parallel to induction generator terminal. I. I NTRODUCTION Nowadays, the increase of fuel price up to US $ 110/barrel has infected the world economy and lead to energy crisis in some countries. At the same time global warming and environmental concerns become an important issue. These conditions have increased the research in the area of renewable energy. One promising research area is application of self excited induction generator (SEIG) in micro-hydro power, wind power and diesel engine with bio-fuel. For example, in Indonesia as energy supply became a problem, the government projected 500 MW of micro-hydro to be installed, especially in rural areas to develop a green source power system [1]. SEIG consists of an ordinary three phase induction motor excited by a bank of capacitors and driven by a prime mover, such as hydro turbine, wind turbine, flywheel system or diesel engine [2]. Low cost, robustness and low maintenance need are some of the reasons to use this machine. However, there is a problem in the operation of SEIG, with poor voltage regulation in varying load conditions. Various ap- proaches have been proposed for overcoming these problems. Availability of low cost controllable power devices, such as IG- BTs, have made the application of power electronic based VAR compensation possible. Various controllable reactive power Fig. 1. Circuit configuration of the proposed system supplies exist such as TSC (Thyristor Switched Capacitor), TSC-TCR (Thyristor Controlled Reactor), STATCOM, and other variable shunt compensators [3-7]. TSC can only give a variation of capacitance in discrete steps. In transients conditions, charging and discharging of the capacitor will stress the thyristor. To avoid these problems, a combination of TCR and TSC is developed. It has a large reactor in the TCR, in order to have a large continuous control range. Furthermore, variable reactor of TCR will generate harmonic distortion [3,4]. The latest technology of SVC is STATCOM, which uses PWM inverter as voltage source or current source inverter, with high frequency switching to reduce harmonic and more complex control is required. A big dc capacitor to store energy and filter inductors at AC side are needed [6,7]. In this paper, SVC MERS (static VAR compensation mag- netic energy recovery switch) as shown in Fig. 1 is used to control the voltage of SEIG with low switching frequency, low switching losses, simple and fast control. The first section of this paper describes the characteristics of the induction generator. The second section discusses the configuration and operation principle of SVC MERS. This is followed by experimental results, discussion and conclusion. II. C HARACTERISTICS OF I NDUCTION G ENERATOR To generate rated voltage of induction generator, VAR com- pensation is required. If the induction generator is connected to the grid, VAR can be supplied from the grid by other reactive power sources, such as synchronous generator. In Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 698 0 500 1000 1500 100 150 200 Output Power (Watt) Voltage (Volt) 150uF 140uF 130uF 120uF 110uF 160uF Fig. 2. Effect of excitation capacitor isolated or stand-alone condition, capacitor is usually used. This system is called SEIG. A variable capacitor is required in order to realize voltage regulation of SEIG in varying load conditions or for variable speeds. From theoretical calculations [8], a range of fixed shunt capacitor sizes can be calculated to maintain rated voltage under load varying conditions. Fig.2 shows load characteristic curve for a range of fixed capacitor sizes at synchronous speed and unity power factor load for a 200V 1.5 kW induction machine with magnetizing curve given by equation: Ä Ñ ¼Á ¿ Ñ ¿½Á ¾ Ñ ·½¾¿½Á Ñ · ½½¼ (1) The curve shows that variable compensation is needed to maintain rated voltage. III. SVC MERS C ONFIGURATION AND O PERATION P RINCIPLE A. Configuration The configuration of this device is based on 4 IGBTs and a dc capacitor per phase. It is called magnetic energy recovery switch (MERS), and typically inserted in series between AC source and load, as series reactive compensation applied for power factor correction and power flow control [9,10,11,12]. In this paper, this device is used as shunt reactive compensation as shown in Fig.1. MERS is connected with an inductor in series as a filter to reduce the harmonic current flowing in Fig. 3. The operational state condition of SVC MERS. −1 0 1 −1 0 1 0 0.5 1 −1 0 1 I shunt V ln V dc capacitor I IGBT Gate (S1−S3) Time (a) (b) (c) Fig. 4. Typical waveforms of the SVC MERS at discontinuous mode (a) charging time (b) discharging time (c) parallel path time the system and then it is called SVC MERS. For three phase systems there will be one SVC MERS per phase. B. Operation Principle Operational states to control upward current are shown in Fig.3. Two IGBTs are turned on and off in pairs one time each cycle of the ac power source (50-60 Hz) and controlled synchronously. In a half cycle, two switches (S1 and S3) are turned on, the current flowing is charging and discharging the dc capacitor with the same polarity. In this case, current is flowing from ground to line. When the dc capacitor voltage is equal to zero, the current is flowing in parallel. The other half cycle, the other pair (S2 and S4) is turned on, with similar conditions, but with the opposite current flow direction. The waveforms of phase voltage, shunt current, dc capacitor voltage, gate signal and IGBT current are shown in Fig.4. It can be seen that the IGBT always turn on at zero current and turn off at zero voltage, therefore the low switching losses can be achieved. By controlling the swicthes as describe above, three different control modes can be achieved, which are balance mode, dc-offset mode and discontinuous mode. DC offset and discontinuous mode operation are illustrated in Fig.5. In the dc-offset mode, the IGBT will turn off at non zero voltage because a dc voltage (dc-offset) still remain in the dc capacitor. Discontinuous mode occures when there is a dc voltage over the capacitor only part of the time. While balance mode operation is the mode between these two modes, Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 699 02040 −250 0 250 02040 −8 −4 0 4 8 −2 0 2 S1,S3 ON S2,S4 ON V LL δ I shunt Gate V shunt V dc Voltage (V) Time (ms) Voltage (V) Current (A) 02040 −250 0 250 02040 −8 −4 0 4 8 −2 0 2 S1, S3 ON S2, S4 ON V dc V LL V shunt I shunt Voltage (V) Current (A) Time (ms) δ Gate Voltage (V) Fig. 5. SVC MERS operation dc offset and discontinuous mode meaning a sinusoidal injected current. Variable reactive compensation can be achieved by con- trolling the current flowing to the dc capacitor by applying appropriate gate signals. The control is based on performing a phase shift of the gate signals. The control variable called Æ phase is the phase difference between line voltage ( Î ÐÐ ) and the time of switching. The value of Æ phase depends on how much reactive power must be supplied to the induction generator and the load. By controlling Æ phase, balance mode, dc-offset mode and discontinuous mode can be achieved. With a small inductance, balance mode occurres when Æ Fig. 6. Equivalent circuit of SVC MERS. is close to 30 Ó and in this case reactance of SVC MERS ( ×ÚÑÖ× ) is equal to real reactance of MERS capacitor ( ). Dc-offset mode appeares when Æ is less than 30 Ó and will make ×ÚÑÖ× larger than . In discontinuous mode, Æ is larger than 30 Ó , as a result ×ÚÑÖ× will be less than . From another point of view, as illustrated in Fig.6, SVC MERS is a capacitor controlled by semiconductor devices. The reactive current ( Á ×ÚÑÖ× ) and the reactive power ( É ×ÚÑÖ× ), can be represented as follows: Á ×ÚÑÖ× ´ Î ÐÒ ×ÚÑÖ× µ (2) É ×ÚÑÖ× ´ Î ÐÒ ×ÚÑÖ× ¾ µ (3) ×ÚÑÖ× ´ ÑÖ× ´Æ µ Ä µ (4) Õ ´ ×ÚÑÖ× Ä µ (5) C. Characteristics of injected current The characteristic of the injected current to the system is determined by the size of the capacitor and inductor. The selection for the operating area can be based on Fig.2. The minimum injected current should be equal to the magnetizing 20 30 40 50 0.8 0.9 1 1.1 0.8 0.9 1 1.1 δ (deg) Relative reactance Current (pu) balance X eq I injected discontinuousdc−offset Fig. 7. Characteristics of the injected current Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 700 0.8 0.9 1 1.1 0 2 4 6 8 Relative reactance THD current (%) discontinuous dc−offset C=105uF; L=15mH C=110uF; L=10mH C=100uF; L=20mH Fig. 8. Harmonic of the injected current current of the induction generator to generate rated voltage at no load condition. Fig. 7 shows the relationship of the injected current to Æ phase and the relative reactance. The relative reactance Õ is described in equation (5). Inductor should be inserted as a filter because of the injected current contains some harmonics, and as a damper in order to reduce inrush current flowing to the IGBT. The relationship between the inductance and the harmonic of the injected current for various operating points is shown in Fig. 8. Three combinations of capacitor and inductor were simulated. It can be seen that larger inductor will reduce the harmonic of the injected current, on the other hand the size of capacitor can be reduced. IV. E XPERIMENTAL S ETUP A. Hardware Setup The experimental setup consists of inverter, wound type induction motor, squirrel cage type induction generator, SVC MERS and three phase resistive loads. Induction motor, as a prime mover, is supplied by inverter to maintain constant speed and also to make variable speed conditions. Induction motor is coupled to induction generator. The SVC MERS inputs are connected to the induction generator terminals and the outputs are in star connection. The parameters of the power circuit components of the experimental setup are shown in Table 1. TABLE I S PESIFICATION OF EXPERIMENTAL SET - UP Items Parameters Inverter 5-kVA 200V Induction motor 1-kW 200V 50Hz Induction generator 1.5-kW 200V 50Hz - Stator resistance, Ê × 1.337 ª - Rotor resistance, Ê Ö 0.713 ª - Stator and rotor inductance, Ä × , Ä Ö 3.85 mH SVC MERS - dc capacitor, C 110 F - filter inductor, L 10 mH Fig. 9. Induction motor and generator used in the experiment Fig. 10. SVC MERS modules and inverter for speed control B. Control System In order to control the terminal voltage, voltage feedback with PI control is proposed. The control part starts with sensing the line to line voltage. Only two voltage sensors are used, which are fed to the control board. Phase lock loop (PLL) technique is applied to synchronize the gate switching time to the phase of the line voltage. For feedback control, the rms value of the line voltage is compared to the reference voltage. The error is given to the PI controller to determine the Æ phase, and then it is fed to the gate controller to generate the gate signals. A Æ phase limiter is applied to keep Æ phase in the operating area. V. R ESULTS AND D ISCUSSION Two types of experiments were conducted. The first part investigated the system in load varying condition with constant speed and the second part investigated the system in variable speed conditions. The experimental results are represented in Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 701 Fig. 11. Control system of SVC MERS 20 30 40 0 0.2 0.4 0.6 0.8 0 0.5 1 (deg) Output power (pu) Voltage (pu) Voltage δ Output power Fig. 12. Steady state voltage characteristic per unit calculation, which base voltage is 200V, base current is 6.8A and base frequency is 1500 rpm. A. Constant Speed Condition In the case of constant speed, the induction generator speed is kept at 1500 rpm. The load is connected to induction generator after its rated voltage is achieved. When voltage is equal to rated voltage, the load is connected and then increased up to 1.5 kW (0.63 pu) in full load condition. Steady state voltage characteristic in load varying conditions is shown in Fig.12. The voltage is always maintained at 1 pu. It can be seen that three modes operation of SVC MERS can be achieved. Fig.13 shows the characteristics of stator current, shunt or SVC MERS current and load current. On no load condition, stator and SVC MERS current are equal to 0.56 pu. The currents increased when the output power increased. Stator current consists of active and reactive currents, while SVC MERS only supplies reactive current to the system. The maximum stator current is about 1.2 pu and SVC MERS current is 0.9 pu at full load condition. Fig.14 shows the experimental transient response of the voltage, load current, shunt current, and dc capacitor voltage with a step change of resistive load from no load to full load condition. The voltage is recovered within two cycles. Operation mode changed from dc-offset mode to discontinuous mode in order to supply required reactive power. 0 0.2 0.4 0.6 0 0.4 0.8 1.2 I stator I shunt I load Output power (pu) Current (pu) Fig. 13. Steady state current characteristic −200 0 200 −10 0 10 0 0.1 0 100 200 Voltage (V) Current (A)Capacitor voltage (V) Time (s) I shunt I load Fig. 14. Transient response characteristic Total harmonic distortion (THD) voltage and stator current are shown in Fig.15. SVC MERS injected some harmonic currents to the system, which consist of low order harmonics dominated by 5th and 7th harmonic. The size of the inductors are selected to 10 mH (0.18 pu) and connected in series with MERS. They are used as a filter to reduce injected harmonic currents and as a damper to avoid high in-rush charging current flowing into dc capacitor. Larger size of inductor will have better performance to reduce harmonic but it will be more expensive. When the size of the dc capacitor of MERS is changed from 110 Fto120 F, the balance mode is shifted to higher output power point. As a result, THD voltage and Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 702 0 0.2 0.4 0.6 1 2 3 4 Output power (pu) THD Voltage (% ) C=120uF C=110uF 0 0.2 0.4 0.6 2 4 6 8 Output power (pu) THD current (%) C=120 uF C=110 uF Fig. 15. Harmonic voltage and current characteristics current are smaller after balance mode point, it means reducing rms current of the stator at full load condition. Therefore by selecting optimal size of dc capacitor and inductor, the harmonic distortion can be reduced for the main operation area. B. Variable Speed Condition The second experiment investigated the reactive power that must be supplied by SVC MERS in order to maintain rated voltage of induction generator for variable speed conditions. In this experiments, the rotor speed is changed from 0.8 pu to 1.3 pu, while induction generator output power is half of its rated power. From the experimetal results, as shown in Fig.16, SVC MERS can control the voltage at its rated voltage. For higher speed, Æ phase is small, meaning lower reactive power is generated by SVC MERS, while for lower speed, Æ phase is larger, meaning higher reactive power is required. C. Discussion 1) Possibilities of rating reduction: In this experiment, all reactive power supplied to induction generator is generated by SVC MERS. Therefore, the capacity rating of SVC MERS is about 0.9 pu. There is a possibility to reduce its capacity rating by connecting three fixed ac capacitors in parallel to the induction generator terminals. These fixed ac capacitors should be selected in order to supply reactive power that is required on no load condition, which is about 0.56 pu based on the experimental result shown in Fig. 13. By doing this, SVC MERS will only supply reactive power for the varying 20 40 60 0.8 1 1.2 0 0.5 1 (deg) Speed (pu) Voltage (pu) Voltage Speed δ Fig. 16. Effect of variable speed to Æ phase control of SVC MERS at half load condition load conditions, so the capacity rating can be reduced to 0.34 pu. Simulation were performed in a simulation package called PSIM to observe the possibilities of rating reduction of SVC MERS. The selected parameters were 110 F fixed capacitor and for SVC MERS, 15.4 F dc capacitor and 3.18 mH inductor. Magnetizing characteristic of the induction generator is based on curve as mentioned in section II. The simulation system configuration is shown in Fig.17. Resistive load is connected to the system. The simulation result of transient operation from no load to full load is shown in Fig.18. Voltage response is almost the same as Fig. 14 for SVC MERS without fixed capacitor. Fast response can be obtained using similar voltage feedback with PI control. At no load condition, SVC MERS did not inject current. Reactive current is supplied by fixed capacitor. This can be achieved by switching off all IGBTs, it can be shown that in the dc capacitor some voltage was appeared. As load increased, terminal voltage decreased. When voltage is less than 5%, SVC MERS started to inject reactive current. On the other side, when load decrease from full load to light load, Æ phase is controlled to minimum value, so that the voltage regulation is less than 5%. Fig.19 shows transient response from full load to half load condition. Stator and load current have low harmonic distortion, because harmonic of injected current oscillated with fixed capacitor. Injected current from SVC MERS was reduced with about 60% at full load condition. The simulation results indicate that rating reduction of the SVC MERS is possible by still maintaining good performance including simple and fast control. The rating reduction of the SVC MERS means the size of the SVC MERS components can be reduced. The size of dc capacitor was reduced from 110 F to 15.4 F and the inductor from 10 mH to 3.18 mH. VI. C ONCLUSION A new voltage regulator applied to self-excited induc- tion generator using SVC magnetic energy recovery switch (MERS) is proposed. It is shown that proposed system can Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. 703 Fig. 17. Simulation scheme configuration for rating reduction of SVC MERS with fixed capacitor −200 0 200 −8 0 8 2.1 2.2 2.3 0 100 200 −8 0 8 Voltage (V) Current (A) Current (A) Capacitor voltage (V) I stator I load I fixed capacitor I svc mers Time (s) Fig. 18. Simulation result of transient characteristic of the system from no load to full load regulate the output voltage at variable load and speed condi- tions with high performance. This proposed system has the following advantages: i) Simple and fast control, where only two voltage sensors are used. PI controller for feedback operation gives fast response and stable operation. The voltage could be stabilized at rated value within 40ms. ii) Low switching losses, that can be achieved using zero current turn on and zero voltage turn off of the IGBT and low switching frequency same as its fundamental frequency. iii) Low harmonic distortion, the THD voltage and current in the SEIG system are below 4% and 8%. Optimal selection of capacitor and inductor will lead to low distortion for the main operation area. Further more, rating reduction of SVC MERS can be achieved with about 60% by combining fixed ac capacitors connected in parallel to the induction generator terminal. R EFERENCES [1] UNDP Report, ”Connecting micro-hydro power Indonesia to the national grid”, UNDP Report, 2003. −200 0 200 −8 0 8 2.8 2.9 3 0 100 200 −8 0 8 Voltage (V) Current (A) Current (A) Capacitor voltage (V) I stator I load I fixed capacitor I svc mers Time (s) Fig. 19. 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Shimada, ”Power control using magnetic energy recovery switches (mers)”, Power Electronics Technology Exhibition & Conference, 2004. Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply.