Tài liệu tiếng anh Điện tử công suất mạch MERS Improved wind power conversion system using magnetic energy recovery switch MERS

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Tài liệu tiếng anh Điện tử công suất mạch MERS Improved wind power conversion system using magnetic energy recovery switch MERS

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Improved Wind Power Conversion System Using Magnetic Energy Recovery Switch (MERS) Taku Takaku ∗ , Gen Homma ∗ , Takanori Isobe ∗ , Seiki Igarashi † , Yoshiyuki Uchida † Ryuichi Shimada ∗ ∗ Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology N1-33, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan Telephone: +81-3-5734-3328, Fax: +81-3-5734-2959 E-mail: ttakaku@nr.titech.ac.jp † Fuji Electric Device Technology Co., Ltd. Gate City Ohsaki, East Tower, 11-2, Osaki 1-chome, Shinagawa-ku, Tokyo, 141-0032, Japan Telephone: +81-3-5435-7153, Fax: +81-3-5435-7466 E-mail: igarashi-seiki@fujielectric.co.jp Abstract— This paper presents experimental results on an innovative power conversion technology using magnetic energy recovery switch (MERS) of a wind turbine system with a synchronous generator to improve the output power and the efficiency. An output voltage of the synchronous generator decreases with the increase of current because of synchronous reactance. The MERS, which consists of four MOSFET or IGBT elements and one small DC capacitor just like as a full bridge configuration, is inserted in series between the generator and ac- dc converter. The capacitor absorbs the magnetic energy stored in the synchronous inductance by forced LC resonance. Since MERS compensates the reactance voltage of the synchronous generator by the capacitor voltage, the output voltage of the generator increases and the excitation current of the generator can be extremely reduced. Also, the switching loss of converter in the MERS system is very small because the MERS is not required PWM control and the switching frequency is the same as the generator, and the downsizing of converter is realized. The effect of MERS is verified in a small scale experimental set-up of wind power generation with a permanent magnet type synchronous generator and a dc-excitation type synchronous generator. The data indicate a great potential of the new power conversion technology to make the actual wind turbine system compact and to improve the efficiency. I. I NTRODUCTION Wind turbine system is rapidly developing as one of the most promising renewable energy resources over the world. The penetration of this system is very important to solve the global warming and the exhaustion of fossil fuel. In order to optimize the wind turbine system, many kinds of power conversion systems to connect between the generator and the grid line have been proposed and utilized. The wind power is always fluctuating, and the maximum power far exceeds the ratings value, and it often reaches 1.5 times the ratings value. Therefore, a new electric power conversion technology different from a conventional power generation technology is necessary for the wind power generation system. We do believe that there is still a room to improve the power conversion system to obtain more economical systems. We have proposed a bi-directional magnetic recovery switch (MERS)[1]. The MERS is a quite useful switch having a S 1 S 2 S 3S 4 D C D C M E R S L o a d P o w e r S o u r c e L R C I V M O S F E T o r I G B T Fig. 1. Circuit diagram of bi-directional magnetic energy recovery current switch (MERS). The MERS is inserted in series between power source and load. bridge configuration with four IGBTs or MOSFETs and a small capacitor. A power factor correction is possible regard- less of the impedance and power frequency of the load by the automatic synchronized switching[2]. We are intending to apply the MERS to the power con- version of wind turbine system for improving the system performance. As the first step of this project this paper presents, for the first time, a wonderful effect of MERS in a small scale experimental set-up of wind power generation with a permanent magnet type and a dc-exciting type synchronous generator. II. M AGNETIC E NERGY R ECOVERY S WITCH (MERS) The basic configuration of MERS is shown in Figure 1. Four IGBTs (or MOSFETs) are connected in two parallel arms. Each arm consists of two IGBTs connected in series. Four IAS 2005 2007 0-7803-9208-6/05/$20.00 © 2005 IEEE S G C o n v e r t e r I n v e r t e r G r i d L i n e W i n d T u r b i n e G e n e r a t o r (a) Conventional back-to-back system. M E R S S G D i o d e B r i d g e I n v e r t e r G r i d L i n e W i n d T u r b i n e G e n e r a t o r (b) Newly proposed MERS system. 3-phase MERS is inserted in series between the generator and converter. Fig. 2. Wind turbine power conversion system with synchronous generator. IGBTs are connected in reverse direction each other both in series and parallel connection. The middle points of series are connected to a capacitor. The MERS is inserted in series between ac power source and load. In the case when S1 and S3 are turned on, the current flows in the positive direction. When S1 and S3 are turned off, the magnetic energy which has been stored in the inductance is regenerated into the capacitor. Next, the case when S2 and S4 are turned off, the capacitor discharges the energy to the load and the current flows in the negative direction. The phase of current can be controlled by changing a switching phase angle of MERS. MERS itself generates voltage and compensates for the inductance voltage unlike a conventional series capacitor, so that another dc power supply is not needed. Therefore, by advancing the switching phase angle of S1 and S3 by 90 degrees, the inductive reactance voltage is compensated by the capacitor voltage and the power factor of the circuit becomes unity. III. W IND POWER CONVERSION SYSTEMS WITH MERS Figure 2 (a) is a conventional wind power system widely used. A variable speed synchronous generator with many poles is connected to a grid through an ac-dc converter and a dc-ac inverter. Gearless is possible for this system, and it is more efficient than a induction generator. The disadvantage of this system is that an output voltage of the synchronous generator decreases with the increase of current because of synchronous reactance. Since the overload capacity of generator is small, - H a 8 1 N s 8 0 M E R S N s 8 m e r s 1 Fig. 3. Equivalent circuit of synchronous generator with MERS. E is induced electromotive force (EMF), V 0 is output voltage of generator, V is output terminal voltage. - 8 H a 1 N s 1 8 m e r s 1 8 0 Fig. 4. Phasor diagram of synchronous generator with MERS. An output voltage V is increased because an output current I u leads generator output voltage V 0 . an instantaneous strong wind power energy cannot be taken out and it is not efficient. Figure 2 (b) is a wind power conversion system with MERS that we are proposing in this paper. The MERS can improve a power factor regardless of the impedance and power frequency of the load, and it generates voltage and compensates for the synchronous reactance voltage. So, the output voltage of the generator increases, and it becomes possible to improve limited output power and efficiency of the wind power system. Therefore, it is expected that the efficiency improvement of the power conversion system and the miniaturization of wind turbine generator can be expected. Moreover, the MERS can be used as an ac breaker. Figure 3 is an equivalent circuit of synchronous generator with MERS. The equivalent circuit of synchronous generator is shown by series circuit of induced electromotive force ˙ E and synchronous reactance x s and armature winding resistance r a . When power factor of the load is assumed unity, the voltage of the generator output terminal ˙ V 0 when MERS is not used is shown by the following equation. ˙ V 0 = ˙ E − (r a + jx s ) ˙ I (1) ˙ V 0 decreases more than ˙ E as the output current ˙ I increases, and the phase of ˙ I is delayed than ˙ E. MERS can control the phase of the current by an easy control, and correct the power IAS 2005 2008 0-7803-9208-6/05/$20.00 © 2005 IEEE I M S G 1 . 0 - k W S G I N V M E R S x 3 u v w 3 . 7 k W I n d u c t i o n M o t o r D i o d e B r i d g e G a t e D r i v e r x 1 2 A m p . P u l s e G e n e r a t o r / P h a s e S h i f t e r R o t a r y E n c o d e r T o r q u e M e t e r A c t i v e P o w e r L o a d G r i d S 1 S 2 S 3 S 4 V C C o n t r o l C i r c u i t P r o t e c t i o n C i r c u i t P i n P o u t V d c I u 1 . 5 - k W P M S G o r I f Fig. 5. Experimental configuration of a power conversion system with MERS for wind turbine system. factor of induced electromotive force ˙ E. When MERS is inserted series, because ˙ V mers is equal to x s ˙ I the output voltage ˙ V is given by the following equation ˙ V = ˙ E − r a ˙ I. (2) Figure 4 shows the phase diagram of synchronous generator with MERS. The phase of induced electromotive force ˙ E is equal to the phase of output current ˙ I. The output voltage ˙ V 0 is increased compared with a system without MERS because the leading current flows to the generator. Therefore, the voltage descent of output voltage ˙ V is only the voltage of winding resistance r a ˙ I, and the output voltage ˙ V in a system with MERS is increased compared with a system without MERS. In other words, the output voltage of a generator is recovered by controlling the phase of output current and compensating the synchronous reactance voltage by MERS. IV. E XPERIMENTAL SETUP Before comparing the MERS system with the conventional back-to-back system, we will experimentally investigate ef- fects of MERS in the newly proposed system shown in Figure 2 (b). We will compare the output of the newly proposed system between with MERS and w/o MERS. A. Wind turbine simulator Figure 5 shows a schematic diagram of our small-scale experimental system of wind power generation with a syn- chronous generator. This system consists of a M-G set and an active power load. The M-G set consists of a 1.5-kW interior permanent magnet type synchronous generator (PMSG) or a 1.0-kW dc-excitation type synchronous generator (DCSG) coupled with a 3.7-kW squirrel cage induction motor. The rotation speed of the induction machine is controlled by an inverter to simulate fluctuations of a wind. Fig. 6. 1.0-kW dc-exciting type synchronous generator and 3.7-kW induction motor (M-G set). The mechanical input power of the generator is measured with a torque meter. Fig. 7. Overview of 3-phase MERS modules. Each MERS module consists of four IGBTs (1MB20D-060) and 390 µF electrolytic capacitor. IAS 2005 2009 0-7803-9208-6/05/$20.00 © 2005 IEEE TABLE I S PECIFICATIONS OF 1.5- K W PERMANENT MAGNET TYPE SYNCHRONOUS GENERATOR Rated voltage V n 130.6 V Rated current I n 5.5 A Frequency f n 87.5 Hz Pole 6 Armature resistance r a 1.55 Ω (0.11 p.u.) Synchronous reactance X s 8.12 Ω (0.59 p.u.) Short circuit ratio K 1.66 TABLE I I S PECIFICATIONS OF 1.0- K W DC - EXCITATION TYPE SYNCHRONOUS GENERATOR Rated voltage V n 200 V Rated current I n 2.88 A Frequency f n 50 Hz Pole 4 Armature resistance r a 1.76 Ω (0.044 p.u.) Synchronous reactance X s 29.1 Ω (0.73 p.u.) Excitation resistance R f 62.4 Ω Overviews of the M-G set and MERS modules used in this experimental system are shown in Figure 6 and Figure 7. Specifications of synchronous generators used for these experiments are shown in table I and table II. MERSs are inserted in series between the generator and the active power load. The MERS is composed of four IGBTs (1MB20D-060) and 390 µF dc electrolytic capacitor. B. Gate signal control circuit The phase of MERSs’ gate signals are made to advance 90 electrical degrees from an induced electromotive force E. However, the induced EMF E lags no load induced EMF E 0 by power angle δ, the gate signal is shifted by δ as shown in Figure 8. The phase angle of the rotor is detected by a rotary encoder installed in the generator. And gate signals are given to each IGBT element through amplifiers and gate drive circuits. Because the switching frequency of MERS is the same as the armature voltage frequency, switching loss of IGBT is so small that it can be disregarded. @ S 1 , S 3 S 2 , S 4 E 0 E O N O F F O N O F F Fig. 8. The phase of gate signals lead E by 90 electrical degrees. Where E 0 is induced EMF at no load, and E is induced EMF at heavy load. 02040 −200 0 200 −10 0 10 time (ms) Voltage (V) Phase current (A) I u V dc 140.3 V V uv (a) Without MERS. I u =5.7 A 02040 −200 0 200 −10 0 10 0 5 10 15 time (ms) Voltage (V) Current (A) S1, S3 Gate signal (V) V dc 180.3 V V uv I u V Cu (b) With MERS. I u =5.7 A Fig. 9. Experimental waveforms of output voltage and current. V uv is line-to- line voltage of the generator, V dc is dc-link voltage, V Cu is capacitor voltage of MERS (u-phase). V. E XPERIMENTAL RESULTS A. Permanent magnet type synchronous generator The measured voltage and current waveforms are shown in Figure 9. Though the same output current I u of 5.7 A is observed for with and w/o MERS, the higher dc output voltage V dc of 180.3 V is obtained in the system with MERS, while V dc is 143.0 V in the system w/o MERS. The average value of capacitor voltage V C is 51.3 V. The voltage drop across the synchronous reactance is compensated by this capacitor voltage and the output voltage V dc is increased. IAS 2005 2010 0-7803-9208-6/05/$20.00 © 2005 IEEE 048 0 100 200 Ohase current I u (A) Output voltage V dc (V) with MERS w/o MERS (a) Dc output voltage 048 0 1 2 Output current I u (A) Output power P out (kW) with MERS w/o MERS (b) Dc output power Fig. 10. Comparison between (a) dc output voltage and (b) output power of PMSG with and without MERS as a function of output phase current I u . Figure 10 (a) shows the comparison of the dc output voltage of PMSG with MERS and without MERS. The output voltage V dc decreased with an increase of output current I u in both system. However, the voltage reduction by synchronous reac- tance was recovered by MERS and output voltage increased in the system with MERS. It becomes only a voltage drop by the resistance, and the characteristic of the synchronous generator is the same as a direct current generator. Figure 10 (b) shows experimental results of dc output power as a function of output current I u . The maximum output is 1.2 kW in the system without MERS, while the maximum output is 2 kW or more in the system with MERS. These data indicate that an instantaneous strong wind power can be caught by a generator with the MERS system, and thereby the generator with a small ratings can be used for wind power 01 0 1 2 3 Output Power (p.u.) Excitation power (p.u.) with MERS w/o MERS (a) x s =0.73(p.u.) 01 0 10 Output Power (p.u.) Excitation power (p.u.) with MERS w/o MERS (b) x s =2.76(p.u.) Fig. 11. Comparison of normalized excitation power of DCSG system with MERS and without MERS. generation. Moreover, even when the output power is kept constant, the output current can be decreased in the MERS system because the output voltage increases. As a result, a copper loss and an amount of produced heat of the generator can be suppressed, and a miniaturization of the generator can be expected by applying the MERS to the power conversion system. B. Dc excitation type synchronous generator Figure 11 shows the comparison of the excitation power of experimental DCSG system with MERS and without MERS. The excitation power is normalized by the no-load excitation power. The necessary excitation power of the generation system with MERS to maintain the dc output voltage of 270 V was smaller than that of without MERS. In the case of the synchronous reactance of 0.73 p.u., the necessary excitation IAS 2005 2011 0-7803-9208-6/05/$20.00 © 2005 IEEE M E R S S G 0 . 2 m H ( 0 . 1 p . u . ) 1 6 8 6 9 0 V D C V 6 9 0 V , 6 2 7 A X s = 0 . 4 4 W i n d T u r b i n e D i o d e R e c t i f i e r P W M I n v e r t e r F i l t e r G r i d L i n e T r a n s f o r m e r S G W i n d T u r b i n e 0 . 2 m H ( 0 . 1 p . u . ) 1 6 8 6 9 0 V 0 . 2 m H ( 0 . 1 p . u . ) 1 6 8 6 9 0 V D C 1 0 0 0 V 6 9 0 V , 6 2 7 A X s = 0 . 4 4 F i l t e r P W M C o n v e r t e r P W M I n v e r t e r G r i d L i n e F i l t e r ( a ) P W M c o n v e r t e r s y s t e m ( b ) M E R S c o n v e r t e r s y s t e m Fig. 12. 750-kW wind power conversion system with PWM converter and with MERS. Input filter is not required in the MERS system. power with MERS is about 65% of that without MERS at the output power of 1 p.u In the case of 2.74 p.u., it is reduced to only 21%. The effect of compensating the reactance voltage by MERS is extremely large in the latter case due to the very high reactance voltage. VI. C ONVERTER LOSS The experimental results indicate that the newly proposed power conversion system with MERS has a great potential to improve the performance compared with the conventional system. The remarkable advantage of this new system is a big improvement in the output voltage and the overload output capacity of the generator. The latter advantage suggests that the wind turbine generator can be designed much smaller than in the conventional system, which may be much cost effective. Figure 12 shows 750-kW wind power conversion system with PWM converter and with MERS. An input filter is not required in the system with MERS because of low switching frequency. To compare the loss of these power conversion systems, suppose that the line voltage is 690 V and rated output power of wind turbine generator is 750 kW, and the output current and voltage of the generator was calculated from the experiment results. Figure 13 shows the result of comparing loss of power conversion system with MERS, and PWM converter. The rated output power of the wind turbine used for this calculation is 750 kW. The conduction losses of IGBTs increase in the MERS system. However, turn-on and turn-off switching losses of MERS are so small that it can be disregarded because the switching frequency is the same as the generator. Moreover, there is no loss of the filter because a filter on the converter input side is unnecessary. The total loss of the power conversion system with MERS is 39.7 kW, while that of PWM converter is 54.2 kW, it is reduced to 73%. 3 9 . 7 k W 5 4 . 2 k W M E R S C o n v e r t e r ( 5 0 H z ) P W M C o n v e r t e r ( 5 k H z ) I G B T O N F W D R e v e r s e R e c o v e r y D i o d e R e c t i f i e r F i l t e r F i l t e r I G B T O F F F W D O N P W M I n v e r t e r C o n d u c t i o n l o s s o f I G B T s Fig. 13. Comparison of converter losses. Rated power of wind turbine is 750 kW. A low speed switching IGBT can be used for the MERS system because the switching frequency of MERS is the same as a frequency of the generator. Further improvement in efficiency of power conversion system with MERS would be realized if the combination of V CE and switching frequency were optimized. VII. C ONCLUSION This paper presents the experimental results to improve the output power and efficiency of a wind turbine system with a synchronous generator by applying magnetic energy recovery switch (MERS) to the power conversion system. The wind power conversion system with MERS compensates the voltage drop across a synchronous reactance in a synchronous generator. The MERS system improves the overload capacity of the generator and the necessary excitation power is greatly reduced. Also, the loss of converter is reduced to 70% or less and a miniaturization of power conversion system becomes possible. The experimental results indicate a great potential of the newly developed power conversion system using MERS to make the actual wind turbine system with a synchronous generator more compact and to improve the system efficiency compared with the conventional systems. R EFERENCES [1] K. Shimada et al: “Bi-directional current switch with snubber regen- eration using P-MOSFETs,” in Proc. International Power Electronics Conference, Apr. 2000, no. 3, pp. 1519–1524. [2] T. Takaku et al: “Automatic Power Factor Correction Using Magnetic Energy Recovery Switch,” in Proc. I. E. E. Japan, vol. 125-D, no. 4, 2005 (in Japanese). [3] T. Takaku et al: “Power Supply for Pulsed Magnets with Magnetic Energy Recovery Current Switch,” in IEEE Transactions on Applied Superconductivity, vol. 14, no. 2, pp. 1794–1797, 2004. IAS 2005 2012 0-7803-9208-6/05/$20.00 © 2005 IEEE

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