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A High Performance 1MVA Matrix Converter Suitable for Wind Power Systems


A High Performance 1MVA Matrix Converter Suitable for Wind Power Systems Y. Tamai*, Y. Abe*, A. Odaka*, I. Sato*, and A. Sakuma** Fuji Electric Holdings Co., Ltd. **Fuji Electric Systems Co., Ltd. Hino-city, Tokyo, Japan E-mail: tamai-yasuhiro@fujielectric.co.jp Abstract Global warming has been attributed to the increase of the atmospheric gases produced by the burn of fossil fuel. Wind energy is one of the most important alternatives to reduce this problem due to its smaller environmental impact and renewable characteristics that contribute for a sustainable development. Nowadays, an increasing number of researchers and manufacturers have been developing and proposing power converters to set up between wind power generators and the electrical grid. In the converters, matrix converters are paid particular attention since these are capable of converting the variable AC from the generator into constant AC to the grid without bulky DC capacitors or energy storage components. In this paper, a matrix converter with approximately 1MVA rating suitable for a wind power is considered. In the matrix converter, a two series- and two parallel-connected 1.2kV general purpose IGBTs as one arm, is applied as a power device to realize considerable low loss rather than high voltage IGBTs, as well as gaining high power. One of the most critical issues in series-connected switching devices is the presence of unequal voltages especially due to unsynchronized gate signals and different devices electrical characteristics. A well-known method to deal with it is to use a snubber circuit or an active gate-controlled strategy. However these tend to make the converter structures more complicated and device losses increased. To achieve a voltage balancing using a simple and small-sized circuit, a series connected technique with a kind of transformer coupling magnetically all the gate lines is applied. A control strategy for PWM pulse in the matrix converter is based on the virtual indirect control method with virtual rectifier and inverter, which enable the input current and the output voltage to control independently. In this paper, firstly, a basic configuration and its principle of operation for a 1MVA high power matrix converter are discussed. Secondly, IGBT voltage balancing operation is discussed. Finally, conclusions of the paper are given. 一种适用于风力发电系统的 1MVA 高性能矩阵变换器

Y. Tamai*, Y. Abe*, A. Odaka*, I. Sato*, and A. Sakuma** 1 富士电机控股株式会社 2 富士电机系统株式会社 邮箱:tamai-yasuhiro@fujielectric.co.jp 摘要 化石燃料燃烧排放的气体导致全球变暖。由于风能对环境影响很小及其可再生特 点,风能成为可持续发展中最重要的一种能源选择。 现在, 已经有越来越多的研究机构和制造商在建议和开发用于发电机和电网之间的 变换器。在不需要直流母线电容和能量存储装置的情况下,矩阵变换器能够将发电机侧 频率变化的交流电变换成频率固定的交流电。因此矩阵变换器受到了特别关注。本文介 绍了适用于风力发电的 1MVA 矩阵变换器。 在矩阵变换器中,使用四个通用型 1.2kV 的 IGBT 采用两串两并的方式组成一个桥 臂。将此桥臂作为一个功率器件使用在提供了功率的同时,使其损耗也比高压 IGBT 更 低。 串联使用开关器件时最棘手的一个问题就是由于驱动信号不一致或者器件特性不 一致导致的不均压。一种有名的抑制方法是增加吸收电路或者采用有源门极控制方法。 然而,这种方法使得变流器结构复杂和器件损耗增加。为了实现使用一种简单的小型化 电路达到电压平衡的目的,我们采用了一种由变压器电磁耦合所有驱动线的串联技术。 矩阵变流器中采用的 PWM 控制方法是基于虚拟整流器和逆变器的虚拟间接控制。 它 使输入电流和输出电压能够独立控制。 本文首先介绍了 1MVA 矩阵变换器的基本配置和工作原理。 其次, 讨论了 IGBT 工作 电压平衡问题。最后给出结论。

I. INTRODUCTION Wind power generation system is expected as one of the solutions to prevent global warming. Capacity of the wind power generation system increases every year, and capacity of the system has reached MVA-class recently. There are several kinds of wind power generation system. (ex. Permanent Magnet Generator system. ) These systems need ac-ac converter to connect power obtained by wind power generator to grid. A Matrix converter is one of AC-AC converter, and has many advantages over conventional converters, as compactness, long life and so on. Because of those advantages, low capacity matrix converter has been brought into practical use. However, high voltage, large capacity matrix converter has not been brought into practical use due to some problems. One of the problems is that the high voltage, large capacity matrix converter needs high-voltage power semiconductor. In general, switching speed of high voltage power semiconductor is very low. Therefore, bulky-sized cooling fins are required because of large switching losses. Furthermore, switching frequency is limited under 1kHz.

As a result, input filter of the matrix converter becomes large. These factors deteriorate the effect of applying the matrix converter to wind power generation system. In this paper, a two series- and two parallel-connected 1.2kV general purpose IGBTs is applied for one arm as a power device to realize considerable low loss compared with high voltage IGBTs, as well as gaining high power. One of the most critical issues of series-connected switching devices is unequal voltages especially due to unsynchronized gate signals and different electrical characteristics of the devices. A well-known method to deal with it is to use a snubber circuit or an active gate-controlled strategy. However these tend to make the converter structures more complicated and device losses increased. To achieve a voltage balancing using a simple and small-sized circuit, a series connected technique to magnetically couple all gate lines with a kind of transformer is applied. In this paper, firstly general matrix converter configuration and its basic operation are discussed. Secondly, a voltage-balancing technique for series connected IGBTs is proposed. Then, experimental results concerning voltage sharing and the matrix converter operation are shown. Finally, conclusions of this paper are given here. II. Principles of matrix converter Fig.1 shows power converter circuit configuration of the following two converters. (a) conventional voltage source PWM rectifier and inverter, (b) matrix converter. Both of these two converters can realize bi-directional power flow. The conventional PWM system is a well-known device that converts an input AC voltage into a DC voltage by a PWM rectifier, and then PWM inverter converts DC voltage into the desired AC voltage. A voltage smoothing capacitor is required in the DC link circuit, and an electrolytic capacitor is typically used for this purpose. On the other hand, the matrix converter arranges semiconductor switches into a matrix configuration and controls them to convert an input AC voltage directly into the desired AC voltage. Bi-directional switches are necessary as the semiconductor switches, since AC voltage is impressed on it. Performance of those two converters is equal. In other words, though the matrix converter is a single converter unit, the matrix converter is able to provide performance equivalent to that of the conventional PWM system. Additionally, the large DC link electrolytic capacitor is not required for the matrix converter. As a result, smaller size and longer lifespan can be achieved.

power AC/DC DC/AC input filter

power

Grid reactor

PWM rectifier

Motor PWM (Generator) inverter

Grid

AC/AC

Electrolytic capacitor
or
Bi-directional switch

Motor (Generator)
Conventional IGBT + Diode Reverse Blocking IGBT (RB-IGBT)

(a) Conventional PWM system converter Fig.1. Conventional PWM system and matrix converter.

(b) matrix

III. Operation principle of series connection technology for IGBTs In the series connection of IGBTs, one of the most critical problems is the unequal voltage between the IGBTs, which occurs mainly by unsynchronized gate signals and different characteristics of the devices. Voltage balancing methods employing several kinds of snubber circuits or active gate control strategies have been reported and successfully achieved good voltage sharing. However, these methods make the circuit more complicated and do not have enough ability without making a switching time slow to control their transient surge voltage sharing especially during the turn-off, hundreds of nano-second phenomena, due to their delay time. Therefore, these methods increase the switching losses. To deal with this issue, a simple method for voltage balancing by means of synchronizing gate signal using a passive control for IGBTs is developed, and its operation principle is given here. Fig. 2 shows a circuit configuration with series connection of two IGBTs, where Q1 and Q2 represent IGBT, GD1 and GD2 represent gate driver. A gate-balancing core (referred as GBC in the figure) is a two-winding 1:1 turn ratio transformer, which interconnects both gate lines by coupling magnetically each other for the gate signal synchronization. Fig. 3 indicates waveforms with and without the gate-balancing core in the circuit. Assuming that the turn-off operation of DRV1 is conducted faster than that of DRV2 by a time error ?toff. Without GBC, it is clear that collector-emitter voltage Vce1 tends to be larger than Vce2 during turn-off phase due to existing of ?toff as depicted by dotted line in Fig. 3. Whereas, with GBC, the gate waveforms tend to be identical as illustrated by solid line in the figure due to a basic operation of transformer.
Ig1 Ig2 0 Ig1
GD1 Ig1 VGD1 VT1 GD2 VGD2 GBC Ig2 VT2 Vg2 Ic Vg1 Q2 VCE2 Q1 VCE1

Ig2 ΔVg1 , ΔVg2 Vg2

Vg1 Vg2 0 VT1 0 VT2

Vg1

VCE1

Vce1 Vce2 0 Δt off

VCE2

W ith Gate-Balanc ing Core W ithout Gate-Balanc ing Core

Fig.2. Configuration of series connected two IGBTs.

Fig.3. Turn-off waveforms with and without Gate-Balancing Core.

IV. Experimental results A. Experimental system Fig. 4 shows the circuit configuration of the tested matrix converter. Table I shows the parameter of the matrix converter. The power circuit of the matrix converter consists of nine bi-directional switches. Each bi-directional switch is composed of combining two conventional semi-conductor switches, and the conventional semi-conductor switch is composed of two series- and two parallel-connected IGBTs. Fig. 5 shows the picture of the power circuit of the matrix converter. The features of the power circuit are as follows: (1) The bus bar of the power circuit consists of three layers structure to reduce its parasitic inductance. As a result, surge voltage of IGBTs is reduced. (2) No snubber circuit is required for each IGBT module. This contributes to realize low cost and small size matrix converter. (3) A resistor is connected to each IGBT for balanced voltage distribution. Fig. 6 shows the structure of the experimental system including the control circuit. The control method of the matrix converter is usually complex and much different from that of the conventional PWM system. To solve this problem, a virtual indirect control method has been developed and employed. The switching patterns for the matrix converter are obtained by combining the switching pattern for the virtual rectifier and inverter. As a result, the input current and the output voltage can be controlled independently. In addition, since this control method can be implemented as a direct extension of the control of the conventional PWM system, techniques developed in the past can be applied easily.
1-phase

Output

Bi-directional switch

Input filter Input

2 series-, 2parallel connected
Fig. 4. Circuit configuration of the tested matrix converter Table I. the parameter of the tested matrix converter Input voltage Output 800Vrms, 50Hz 1MVA

IGBT rating

1200V, 900A

Fig. 5. Picture of the power circuit of the tested matrix converter

Source

ditto

Motor

ditto
Carrier PWM rectifier controller PWM Rectifier Duty Calculator Virtual DC link Current PWM Inverter Duty Calculator Matrix Converter PWM generator Input Current Command Calculator Inverter controller V/f Control

Input Voltage Speed Detection Command

Fig. 6. Structure of the experimental system

IC VCE1

IC VCE1 VCE2 VCE2

0

0

Fig. 7. Experimental turn-off waveforms, (a) Without GBC, (b) With GBC VCE1,VCE2:200V/div, Ic:100A/div, time:250ns/div B. Basic operation results for voltage sharing Fig. 7 shows experimental turn-off waveforms of the two IGBTs connected in series as depicted in Fig. 2 without and with GBC, where ?toff is set for 40ns. It is apparent that both voltages are effectively balanced by the operation of the gate-balancing core as mentioned before. Furthermore, another feature on these results is a fact that the turn-off time with GBC are not changed compared with that without GBC. It means that switching losses could not be affected by applying the gate-balancing core. The same operation is made during the turn-on phase. C. Motor driving performance Fig. 8 gives measured input and output waveforms of the matrix converter on a light-load condition as shown in table II. Where, Vin and Iin represent input voltage and current, Vo and Io represent output voltage and current. It can be seen that input waveforms is converted directly to output waveforms with different frequency on a high-voltage condition.
2000V Vin 0 100A Iin 0

2000V Vout 0 50A Vout 0 4ms

Fig.8. Measured output and input waveforms of the matrix converter. Table II. Experimental conditions Input voltage Output frequency 800Vrms, 50Hz 30Hz Carrier frequency Load 5kHz Induction motor (No load)

V. Conclusion In this paper, a high voltage high capacity matrix converter suitable for wind power systems is proposed and tested. Summary of the outcomes indicated in this paper is as follows: (1) The IGBTs series connection technology has been developed. It is useful for achieving

high voltage, large capacity matrix converters. (2) Basic performance of the high voltage, large capacity matrix converter applied the IGBTs series connection technologies is evaluated. Furthermore, practical use of a high current reverse blocking IGBT(RB-IGBT) is expected. By using the RB-IGBT, efficiency of matrix converters will be improved. That will accelerate practical use of the matrix converter.


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