一种改进的三电平逆变器SVPWM控制算法

黄晓峰;王小鹏;孙春霞

【摘 要】传统三电平空间矢量脉宽调制(SVPWM)算法中扇区分割多为三角形和六边形,扇区判断计算复杂,且采用七段式开关序列,开关频率较大.为此,提出了一种新的三电平逆变器SVPWM控制算法,利用新的空间矢量类六边形进行扇区分割减少扇区判断的复杂性,采用新的四段式开关序列降低开关频率.仿真结果表明,在开关频率降低的情况下总谐波失真(THD)降低,且该算法的系统运行计算时间较短.%In the traditional three-level space vector pulse width modulation (SVPWM) algorithm,the sector judgment is computationally complex since the sector is divided into triangles and hexagons.In addition,the switching frequency is high because the seven-segment switching sequence is adopted.For this reason,a new SVPWM control algorithm for three-level inverter is

proposed,in which the sector judgment is simplified by dividing the sector into quasi hexagons,and the new four-segment switching sequence is adopted to reduce the switching frequency.Simulation results show that the total harmonic distortion grows down with the switching frequency decreasing,moreover,the algorithm runtime is also decreased. 【期刊名称】《测试科学与仪器》 【年(卷),期】2017(008)003 【总页数】6页(P277-282)

【关键词】三电平逆变;空间矢量脉宽调制(SVPWM);扇区分割;开关序列

【作 者】黄晓峰;王小鹏;孙春霞

【作者单位】兰州交通大学电子与信息工程学院,甘肃兰州730070;兰州交通大学电子与信息工程学院,甘肃兰州730070;兰州交通大学电子与信息工程学院,甘肃兰州730070 【正文语种】中 文 【中图分类】TM464

Abstract： In the traditional three-level space vector pulse width modulation (SVPWM) algorithm, the sector judgment is computationally complex since the sector is divided into triangles and hexagons. In addition, the switching frequency is high because the seven-segment switching sequence is adopted. For this reason, a new SVPWM control algorithm for three-level inverter is proposed, in which the sector

judgment is simplified by dividing the sector into quasi hexagons, and the new four-segment switching sequence is adopted to reduce the switching frequency. Simulation results show that the total harmonic distortion grows down with the switching frequency decreasing, moreover, the algorithm runtime is also decreased.

Key words： three-level inverter; space vector pulse width modulation (SVPWM); sector determination; switching sequence CLD number： TM464 Document code： A

Three-level inverter is widely used in high voltage and high power field. A three-level structure with a small stress to the switch transistor can provide

high safety voltage with less harmonic components compared to a two-level structure[1-3]. At present, the modulation algorithm commonly used in three-level inverter[4,5] includes sinusoidal carrier pulse width

modulation (SPWM), selective harmonic elimination PWM (SHEPWM) and space vector modulation (SVPWM). Owing to prominent advantages[6,7] such as more utilization of input voltage, flexibility in switching the legs of three-phase inverters, easy method for improving spectral performance and easy digital implantation, SVPWM has become the most popular control algorithm for the three-level inverter. There are a number of trigonometric functions and root operations in the traditional SVPWM algorithm[8,9], therefore software programming is relatively complex and implementation efficiency is low. To solve these problems end, many scholars have proposed a variety of improvement methods. In the SVPWM algorithm[10-12] based on 60° coordinate system, the sector is divided into hexagons by coordinate transformation, which reduces the calculation of trigonometric functions. But there is a repetition area on the sector segmentation, and the complexity of the sector judgment is increased. In addition, the above algorithms use symmetrical seven-segment switching sequence, with high switching frequency, which will lead to the total harmonic distortion.

A new SVPWM control algorithm for Three-level inverter by dividing the sector into quasi hexagons is proposed to reduce the complexity for the sector judgment of the traditional SVPWM algorithm. The specific sector in which reference voltage vector is located can be determined, only relying

on three-phase voltage instantaneous value after vector synthesis and coordinate transformation, which reduces the operation of the trigonometric function. And then, this paper builds asymmetric four-segment switching sequence in the new sector structure to lower the switching frequency and reduce total harmonic distortion. 1.1 Three-level inverter

Fig.1 shows a schematic diagram of a three-level neutral clamped inverter. In three-level inverter, each phase has three switching states P, O and N, and its corresponding AC side output voltage is -Vdc/2, 0 and Vdc/2. Hence, three-level inverter has 27 kinds of switching states. Each switching state corresponds to a voltage space vector, therefore, three-level inverter contains 27 voltage space vectors, as shown in Fig.2.

The SVPWM algorithm flowchart is shown in Fig.3. Firstly, dividing the spatial vector map into six quasi-hexagonal sectors according to the location of the reference voltage vector for the sector judgments. And then, calculating the dwelling time of basic vector by vector synthesis and coordinate transformation of the reference voltage in the quasi hexagonal sector. Finally, generating PWM control signal according to the four-segment switching sequence to control the three-level inverter voltage output.

1.2 Sector determination

The segmentation of the quasi hexagon sector proposed in this paper is shown in Fig.4, each of which contains six regions.

A new space vector is got by the space vector Vref minus the small vector

Vt, which can be seen form Fig.3. Eq.(1) is the relation of space vectors. The expression of the three-phase voltage is

In the first sector, the expression of the three-phase voltage can be rewritten as

Combining Eqs.(1) with (3), a new three-phase voltage expression is obtained as

By using the Clark transform [12], the three-phase rotating coordinate system changes to a plane Cartesian coordinate system, that is Substituting Eq.(4) into Eq.

The space vector transformation that remainsing five sectors is similar to that of the first sector.The changes are shown in Table 1.

According to the logical comparison of and , the location of reference voltage vector in the new α-β coordinate system can be determined, namely, and are determined by the positive or negative values of three-phase instantaneous voltage. The relationship between the sector and the positive or negative values of the three-phase instantaneous voltage is shown in Table 2.

During the sampling period, the three adjacent basic vectors could synthesize reference vector Vref. When the end-point of reference vector Vref is located in the first region of the first sector, as shown in Fig.4, the following formula can be obtained based on the volt-second balance theorem[13], namely

where Ts is sampling period, Vz is small vector, V1 is long vector, and V2 is middle vector. The dwelling time of V1 and V2 can be denoted as T1 and

T2, respectively. Tz is the dwelling time of zero vector. Considering the relationship of coordinates transformation, the dwelling time of voltage vector is

Similarly, the dwelling time of voltage vector in other regions can be deduced.

1.3 Switching sequence

For adapting to the new sector structure and reducing the switching frequency, an asymmetric four-segment switching sequence is put forward. Switching sequence of the first sector is listed in Table 3.

The switching sequences of the two regions in sector Ⅰ are as follows. Region 1: PON-POO-PON-PNN; Region 2: PON-POO-PON-OON. The transformation from sequence PNN to sequence PON only changes the B phase, which satisfies the requirement of the smallest change of vector in the process of vector transformation among different regions. The four-segment switching sequence is an asymmetrical switching sequence, and its overall switching number reduces by 1/3 compared to the traditional switching sequence, then the switching frequency shows a downward trend as a whole. The distortion of total harmonic will be decreased with the switching frequency reducing. In the aspect of the vector dwelling time, it is no longer decomposed in the traditional way of 1/2 decomposition, but partially distributed by using the new four-segment switching sequence. Taking the second region of the first sector as an example, only the middle vector PON and the small vector POO, OON function. In this section, simulation model of three-level inverter is built by

Matlab/Simulink to verify the correctness of the control algorithm, as shown in Fig.5.

The parameters of simulation include: DC-bus voltage, 380 V; three-phase output line voltage, 220 V; frequency, 50 Hz; DC capacitance, 4 230 μF; AC load, 15 kW. In the judge sector module, the specific sector in which reference voltage vector is located can be determined, according to the three-phase voltage instantaneous value. In the vector conversion module, the three-phase voltage is converted to new and space vectors. Finally, the PWM control signal is used to control the inverter based on the four-segment switching sequence.

Three-level inverter AB output phase voltage and A-phase line voltage are shown in Figs.6 and 7, respectively.

The three-level inverter voltage has a stable output waveform. Fig.8 is the sector judgment waveform for three-level inventor. Under the force of the three-phase voltage, the three-phase synthetic vector Vref is rotated in the counterclockwise direction, which reduces the calculation time of the judgment mode.

The toltal harmonic distortion (THD) of A-phase output is 0.69%, as shown in Fig.9.

It is proved that the THD decreases as the switching frequency reduces. A new control algorithm is put forward to improve the traditional SVPWM algorithm. In order to solve the problem of complex calculation of the sector judgment in the traditional three-level SVPWM algorithm, a new quasi hexagonal segmentation method is proposed to reduce the

computational complexity of the sector judgment in the case of independent segmentation sector. A new asymmetric four-segment switching sequence is proposed to decrease the total harmonic distortion. Compared to the traditional seven-segment switching sequence, four-segment switching sequence reduces the switching frequency, thereby reducing the total harmonic distortion. The simulation results show that the algorithm can not only reach the voltage output requirement of the three-level inverter, but also reduce the calculation time and the total harmonic distortion.

[1] Nabae A, Takahashi I, Akagi H. A new neutral-point-clamped PWM inverter. IEEE Transactions on Industry Applications, 1981, 17(5)： 518-523. [2] Bernet S. A comparison of three-level converters versus two-level converters for low-voltage drives, traction, and utility applications. IEEE Transactions on Industry Applications, 2005, 41(3)： 855-865. [3] Newton C, Summer M. Multi-level convertors a real solution to medium/high-voltage drives. Power Engineering Journal, 1998, 12(1)： 21-26.

[4] Liu H L, Cho G H. Three-level space vector PWM in low index

modulation region avoiding narrow pulse problem. IEEE Transactions on Power Electronics, 1994, 9(5)： 257-262.

[5] Pou J, Boroyevich D, Pindado R. New feedforward space-vector PWM method to obtain balanced AC output voltages in a three-level neutral-point-clamped converter. IEEE Transactions on Industrial Electronics, 2002, 49(5)： 1026-1034.

[6] Seo J H, Chang H C, Dong S H. A new simplified space-vector PWM method for three-level inverters. IEEE Transactions on Power Electronics, 2001, 16(4)： 545-550.

[7] Celanovic N, Boroyevich D. A comprehensive study of neutral-point voltage balancing problem in three-level neutral-point-clamped voltage source PWM inverters. IEEE Transactions on Power Electronics, 2000, 15(2)： 242.

[8] ZHAO Hui, LI Run, WANG Hong-jun, et al. Study on SVPWM method based on 60° coordinate system for three-level inverter. In： Proceedings of the Chinese Society of Electrical Engineering, 2008, 28(24)： 39-45. [9] Meshram P M, Hanote D, Renge M M. A simplified space-vector PWM for three level inverters applied to passive and motor load. In：

Proceedings of IEEE Conference on Industrial Electronics and Applications, 2009： 3388-3393.

[10] Srinivasan R, Sundararaman K. Digital control of multi level inverters using simplified space vector modulation. In： Proceedings of IEEE International Conference on Control, Automation, Communication and Energy Conservation, 2009： 1-8.

[11] Fan B, Zhao W G, Yang W, et al. A simplified SVPWM algorithm research based on the neutral-point voltage balance for NPC three-level inverter. In： Proceedings of IEEE International Conference on Automation and Logistics, 2012： 150-154.

[12] Beig A R, Narayanan G, Ranganathan V T. Modified SVPWM algorithm for three level VSI with synchronized and symmetrical waveforms. IEEE

Transactions on Industrial Electronics, 2007, 54(1)： 486-494.

[13] Busquets-Monge S, Bordonau J, Boroyevich D, et al. The nearest three virtual space vector PW―a modulation for the comprehensive neutral-point balancing in the three-level NPC inverter. IEEE Power Electronics Letters, 2004, 2(1)： 11-15.