Space phasor modulation technology for three-phase four-leg inverter

Abstract: The space phasor modulation technology of three-phase four-leg inverter with symmetrical load and asymmetrical load is introduced.

Keywords: three-phase four-bridge-arm; inverter; space phasor; modulation technology

1 Introduction

There are two types of output types for three-phase inverters: one is three-phase three-wire output, and the other is three-phase four-wire output. The former can only output line voltage, while the latter can output both line voltage and phase voltage. The three-phase four-wire output of three-phase inverters generally adopts the following methods:

1) Use a △/Y output transformer and use the secondary Y connection method to form a neutral point;

2) Use a neutral forming transformer (NFT) to form the neutral point. NFT is actually an autotransformer with a transformation ratio of 1:1.

3) Use the midpoint of the DC input power supply as the neutral point;

4) Connect the midpoints of the same-side bridge arms of three full-bridge inverters with independent power supplies to form a neutral point;

5) Use three-phase four-leg inverter.

The biggest advantage of using a three-phase four-bridge-arm inverter is that it does not require a △/Y output transformer, but forms a neutral point transformer, which greatly reduces the size and weight of the inverter and keeps the voltage stress unchanged.

Figure 1 Main circuit of three-phase four-leg inverter

The main circuit of the three-phase four-bridge-arm inverter is shown in Figure 1. It is composed of a three-phase half-bridge inverter and a bridge arm (composed of S7 and S8) formed by a neutral point. Switches S1, S4, S7 and S8 form a full-bridge inverter for phase A; switches S3, S6, S7 and S8 form a full-bridge inverter for phase B; switches S5, S2, S7 and S8 form a full-bridge inverter for phase C. Since S7 and S8 are common bridge arms forming the neutral point, the triggering of the inverter switches of the three bridge arms A, B and C and the excitation of the output current will be restrained on the common bridge arm, which is a problem that must be solved when designing the control circuit of the three-phase four-bridge-arm inverter.

2 Working Mode and Space Phase of Four-leg Inverter

The main circuit shown in Figure 1 can be simplified into the equivalent circuit shown in Figure 2 if the neutral line inductance LfN is ignored. Each bridge arm has two working modes. For example, for the A-phase bridge arm, when the upper tube is turned on and the lower tube is turned off, the voltage at point A is VAg=E, and SA is defined as 1, which means VAg=E(SA)=1E; when the lower tube is turned on and the upper tube is turned off, the voltage at point A is VAg=0, and SA is defined as 0, which means VAg=E(SA)=0×E=0. According to this definition, the four-bridge-arm inverter has a total of 24=16 switching modes M0 (SA, SB, SC, SN)~M15 (SA, SB, SC, SN), including two zero switching modes M0 (0, 0, 0, 0) and M15 (1, 1, 1, 1) and 14 non-zero switching modes M1 (0, 0, 0, 1)~M14 (1, 1, 1, 0). There are a total of 15 different switching modes as shown in the first column of Table 1. The voltage between the midpoint of each bridge arm and point g in the table can be obtained from the definition of the switching mode of each bridge arm.

VAg=E·(SA);VBg=E·(SB)

VCg=E·(SC);VNg=E·(SN)

The voltage between the midpoint of each bridge arm and the neutral point is:

VAN=VAg－VNg=E(SN)－E(SN)=E(SA－SN)

VBN=VBg－VNg=E(SB)－E(SN)=E(SB－SN)

VCN=VCg－VNg=E(SC－SN)

VNN=VNg－VNg=0

Figure 2 Simplified equivalent main circuit of three-phase four-leg inverter

The four-leg inverter has 16 switching modes, 8 more than the three-leg inverter. When expressed in three-dimensional a-b-y orthogonal coordinates, the three-dimensional space voltage phasors Va, VB and VY corresponding to the 16 switching modes can be calculated as follows and listed in the fourth column of Table 1. Va = E (2SA-SB-SC) = (2VAg-VBg-VCg)

Table 1. Three-dimensional voltage phasors corresponding to 16 switching modes