Simulation study of a cascade high-voltage inverter with energy feedback function

1 Introduction

As shown in Figure 1, the rectifier part of a general cascade high-voltage inverter uses uncontrollable diodes, so the energy transmission is irreversible. When the motor is in a regenerative power generation state, the feedback energy is transmitted to the DC bus capacitor, generating a pump-up voltage, making the capacitor voltage unstable. Excessive pump-up voltage may damage the switching device, thereby threatening the safe operation of the inverter.

To this end, this paper adopts mature three-phase PWM rectification technology and uses controllable switching devices to form a rectification circuit of a single power unit to achieve bidirectional energy transmission. At the same time, closed-loop control is performed on the DC bus capacitor voltage to stabilize the DC bus capacitor voltage. This method can also achieve a unity power factor on the grid side, making the cascaded high-voltage inverter a true green inverter. Simulation proves that this method is simple and effective.

2 Mathematical modeling and working principle of the rectifier part of a single power unit

From the topological structure of Figure 1 (a), we can see that the cascaded high-voltage inverter is composed of multiple power units. Therefore, we can take a single power unit as the research object, establish its mathematical model and analyze its working principle.

As can be seen from Figure 1 (b), the rectifier part of the power unit is composed of uncontrollable diodes. In order to achieve energy feedback and stabilize the DC bus capacitor voltage, it is necessary to replace the diode with a controllable IGBT to perform PWM rectification control. Figure 2 is the topological structure diagram of the modified power unit.

In Figure 2, lx (x=a, b, c) is the AC side filter inductor, and the resistor rx (x=a, b, c) is the combination of the equivalent resistance of the filter inductor lx and the equivalent resistance of the power switch tube loss.

[page] Assume the three-phase power supply voltage is:

Wherein: ed, eq, id, iq are the components of the power supply voltage vector and input current vector of the rectifier part of the power unit on the dq axis respectively.

It can be seen from equation (3) that the d and q axis variables are coupled to each other, so the d and q axis currents cannot be controlled separately. For this reason, the feedforward decoupling control of id and iq is introduced, and the pi regulator is used as the current loop controller, then the following equation is obtained:

Where: ud*, uq* are the voltage references of the dq axes; kdp and kdi are the proportional and integral coefficients of the d-axis PI regulator, respectively; kqp and kqi are the proportional and integral coefficients of the q-axis PI regulator, respectively.

It can be seen from formula (4) that the voltage command has achieved complete decoupling control, and its system control block diagram is shown in Figure 3. In Figure 3, a voltage-current double closed-loop structure composed of a pi regulator is adopted. The outer voltage loop is used to achieve output voltage stability, and the inner current loop controls the AC input current to be in phase with the input voltage. Its working principle is as follows: the output voltage vdc is compared with the given reference voltage vdc* and sent to the voltage PI controller. The output signal of the voltage controller is used as the given value id* of the active component of the grid-side current. Its size is adjusted according to the active output of the rectifier. In order to achieve unity power factor rectification or inversion, the given value iq* of the reactive component is set to 0. In steady state, the current given signals of the dq axes are both DC quantities. The two given values ​​are compared with the transformed feedback values ​​id and iq on the grid side and sent to the current PI regulator. After decoupling and dq→αβ transformation, the control signal of the three-phase grid-side voltage in the two-phase stationary coordinate system is obtained. After passing through the voltage space vector pulse width modulation module, six SVPWM control signals are output, thereby realizing the control of the power unit rectifier.

[page]3 Simulation system of power unit cascade

According to the mathematical model introduced in Section 2, the power unit simulation model is shown in Figure 4.

Among them, the simulation model of the rectifier part controller is shown in Figure 5.

4 Power unit cascade simulation system

FIG6 is a system simulation model of a cascade-type high-voltage inverter with three power units connected in series per phase.

[page]5 Simulation experiment

The experimental parameters used in the system simulation are as follows: the voltage loop sampling frequency is 2.5kHz; the current loop sampling frequency is 2.5kHz; the three-phase PWM rectifier input voltage effective value VM = 380V; the inductor parasitic resistance value R = 0.5ω; the DC bus voltage is given VDC* = 750V, the initial voltage VDC = 550V; the three-phase input power frequency F = 50Hz; the triangle wave carrier frequency FS = 2.5kHz; the DC bus terminal capacitance C = 3200μF; the grid-side filter inductance L is 0.8MH. The load power is 1MW. The influence of switching loss is not considered in the simulation.

In this simulation experiment, from 0 to 0.25s, the rectifier of the cascade inverter is in an uncontrolled rectification state, and the anti-parallel diode in the IGBT in the rectifier performs uncontrolled rectification; from 0.25s to 0.55s, the rectifier of the cascade inverter is in a controlled rectification state, and the IGBT in the rectifier starts to work; at 0.55s, the inverter suddenly applies load; at 0.8s, the current direction of the inverter's controlled current source is changed, the energy of the inverter begins to be fed back, and the rectifier of the cascade inverter is changed from a rectification state to an inverter state.

Figure 7 is the simulation waveform of the grid-side phase current, phase voltage and DC bus voltage of the power unit of the cascade inverter. As can be seen from Figure 7 (b), when the rectifier of the cascade inverter starts working at 0.25s, VDC rises rapidly from the initial value of 550V to the given value VDC* and quickly stabilizes; at 0.55s, the inverter suddenly loads, and VDC is instantly pulled down, but it can quickly stabilize at the given value again. After stabilization, the voltage fluctuation is very small; at 0.8s, due to the change in the current direction of the controlled current source, the energy of the inverter begins to be fed back, and the rectifier begins to change from the rectification state to the inverter state. The fed-back energy causes VDC to be instantly pulled up at 0.8, but because the response speed of the rectifier of the cascade inverter is very fast, VDC is quickly stabilized at the given value again. At the same time, because of the fast response speed of the rectifier, VDC does not rise much at 0.8, ensuring the safe operation of the system.

As can be seen from Figure 7 (a), when the rectifier of the cascade inverter starts working at 0.25s, the grid-side current fluctuates slightly, but it stabilizes quickly; when the inverter suddenly loads at 0.55s, the grid-side current fluctuates very little and stabilizes quickly. By comparing the phases of the grid-side voltage and current, it can be seen that the two phases almost overlap, and the power factor is close to 1; at 0.8s, the cascade inverter enters the energy feedback state, and the rectifier is in the inverter state. The rectifier makes the phase angle of the grid-side current differ by nearly 180° from the grid-side voltage, and the power factor is close to -1. The three-phase output voltage, current and single-phase output voltage waveforms of the cascade inverter inverter are shown in Figure 8.

6 Conclusion

The waveform of the simulation experiment shows that the improved cascade high-voltage inverter can not only perform bidirectional energy transmission and realize energy feedback, but also the control system has a very fast response speed, which makes the inverter have better dynamic performance. Therefore, the improvement plan is correct and feasible.