Research on PWM Control Strategy Based on H-bridge Cascade Inverter

1 Introduction

The H-bridge cascade multilevel converter uses a method of connecting multiple power units in series to achieve high voltage output. Its output mostly uses a multi-level phase-shifted PWM control method to achieve lower output voltage harmonics, smaller dv/dt and common-mode voltage and smaller torque pulsation. To achieve high voltage, it is only necessary to simply increase the number of units. This implementation method has low technical difficulty. Each power unit is a separate DC power supply, which is independent of each other. The control of one unit will not affect other units. The main difference between the implementation method of the H-bridge cascade inverter and the single-bridge inverter lies in the PWM control method. This article discusses the PWM control method of the H-bridge cascade inverter.

2 H-bridge cascade inverter structure

Each power unit is an independent DC power supply, and its design method is shown in Figure 1 below:

Figure 1 Power unit structure diagram

According to the description of the power unit in the above figure, the power unit of this implementation can generate three levels, namely +Udc, 0, and -Udc. When S1 and S4 are turned on, and S2 and S3 are turned off, the load obtains a voltage of +Udc; when S2 and S3 are turned on, and S1 and S4 are turned off, the load obtains a voltage of -Udc; when S1 and S3 (or S2 and S4) are turned on, and S2 and S4 (or S1 and S3) are turned off, the load obtains a voltage of 0 (Note: In the control process, it is necessary to strictly avoid the two power devices in the same bridge arm being turned on at the same time, that is, the two control signals of the same bridge arm are required to be opposite). Therefore, it can be seen that when different PWM control strategies are used, different PWM waveforms can be generated.

3 Carrier phase shift control theory

Generally speaking, N-1 triangular carriers are required for inverter modulation of the N level. In the carrier phase shift modulation method, all triangular waves have the same frequency and amplitude, but the phases of any two adjacent carriers must have a certain phase shift, and its value is

The modulation signal is usually a three-phase sinusoidal signal with adjustable amplitude and frequency. By comparing the modulation wave with the carrier, the required driving signal of the switching device can be generated [1].

4 PWM control strategy

The inverter usually outputs in the form of a sinusoidal wave. For a single-phase bridge, its output can usually be divided into two modes: unipolar modulation and bipolar modulation (due to space limitations, the specific implementation method can be found in the references). The inverter based on the H-bridge method can also output a waveform similar to the single-phase bridge output, and its PWM control strategy should be slightly adjusted. The waveforms output by unipolar modulation and bipolar modulation differ in performance. Since unipolar modulation can output three levels, while bipolar modulation can only output two levels, the dv/dt of bipolar modulation is larger, which has a greater impact on the insulation of the motor. In the product design process, a unipolar modulation waveform is usually used as the final output waveform. This paper takes the structure of the H-bridge cascade inverter and the SPWM generated by the carrier phase shift method as the control signal of each power unit to achieve the output of a unipolar SPWM waveform. Several PWM control strategies are discussed and studied below:

1) Single-arm chopping: The so-called single-arm chopping method is that S1 and S2 are used as half-cycle control signals. When the positive half cycle is, S1 is turned on and S2 is turned off; when the negative half cycle is, S1 is turned off and S2 is turned on; the control signal of S3 is SPWM signal.

Figure 2 S3 control signal waveform

Figure 3 S1 control signal waveform

Figure 4 Power unit output waveform

The control signal of S4 is opposite to the control signal of S3. Through such control, the waveform shown in Figure 4 can be output. Although its output waveform is similar to the waveform of the unipolar modulation output of the single-phase bridge, and the dv/dt is small, this method causes the power of the two bridge arms to be unbalanced. 2) Bipolar modulation: The bipolar modulation of the H-type inverter is the same as the bipolar modulation of the single-phase bridge, and the control signal is generated in the same way. The difference is that one is a single bridge arm and the other is a double bridge arm. In order to solve this problem, the control signal shown in Figure 2 is input to S1 and S4, and S2 and S3 are opposite to the signal of S1 and S4. This control method can only produce a combination of two switching states, that is, S1 and S4 are turned on at the same time, S2 and S3 are turned off at the same time; S1 and S4 are turned off at the same time, and S2 and S3 are turned on at the same time. A waveform similar to the bipolar modulation of the single-phase bridge can be output. Although the dv/dt of the output waveform of this method is large, high-order harmonics will be generated, and the impact on the system will increase, but the power of the two bridge arms of the power unit is balanced, and its control method is simple and easy to implement. Since there is a distance between the output unit of the control signal and the power unit in the high-voltage inverter system, they are connected together through optical fiber. This method can reduce the use of optical fiber, reduce the cost of the product, and also reduce the difficulty of field wiring.

3) Unipolar modulation: Although the single bridge arm chopping method can achieve an output waveform similar to the single-phase bridge unipolar modulation, this control method has inherent defects. Here we introduce another control method. As shown in Figure 1, S1 is controlled by the control signal shown in Figure 5, S3 is controlled by the control signal shown in Figure 6, and S2 and S4 are the reverse signals of the control signals of S1 and S2 respectively. The control signals shown in Figures 5 and 6 are symmetrical SPWM signals with a fundamental phase difference of 180 degrees. Since the fundamental phase difference is 180 degrees, the duty cycle of the carrier cycle corresponding to the two control signals is 1, that is, complementary. There are four combinations of output waveforms: S1 is on, S3 is off, output +Udc; S1 is on, S3 is on, output 0; S1 is off, S3 is off, output 0; S1 is off, S3 is on, output -Udc. See the left side of the dotted line in the figure, the first three switch combinations will appear, and the right side of the dotted line will appear the last three switch combinations, that is, the PWM wave with a duty cycle that satisfies the sinusoidal change as shown in Figure 7 can be output.

Figure 5 Left bridge arm control signal

Figure 6 Right bridge arm control signal

Figure 7 H-bridge unipolar modulation output waveform

The PWM control implemented in this way realizes the transformation from unipolar SPWM to bipolar SPWM, and realizes the power balance of the left and right bridge walls. At the same time, the inverter output voltage harmonics obtained in this way are very low, and the output does not need to use a filter, which is called a perfect harmonic-free inverter. In the inverter control, DSP control is usually used. Since DSP can only output two levels, it cannot directly realize unipolar SPWM and requires the assistance of external devices. This method uses the combination logic relationship of the power unit (the logic relationship is shown in Table 1) to replace the function of the external device, saving devices, reducing development costs and development difficulties, and is simple to control and easy to implement.

Table 1 Logical relationship

5 Conclusion

In high-power and high-voltage inverter technology, PWM control technology is one of its core technologies. A good PWM control strategy is the guarantee of product performance. In this paper, the PWM control method of H-bridge cascade high-power and high-voltage inverter is mainly discussed, three implementation methods are given, and their implementation methods and performance are analyzed and compared.