Design and test of isolated inverter power supply based on Matlab

introduction

With the development of renewable energy such as solar energy and wind energy, distributed power generation has gradually become a hot topic for research in various countries due to its advantages of less environmental pollution, high comprehensive energy utilization rate, and reliable power supply. In countries and regions with mature technology such as the United States and Europe, it is widely used in microgrids. Inverter power supply, as an effective power supply source, has become an important part of microgrids and has been widely used in the research and implementation of microgrids.

The control model of the isolated inverter power supply based on PWM designed in this paper adopts the dual-loop control strategy of voltage outer loop and current inner loop, and both the voltage outer loop and the current inner loop adopt PI control mode. The experimental model is established by Matlab software for simulation. The rationality of the control system design scheme and the application effect of the dual-loop control strategy are verified through simulation. The analysis of the simulation results proves the rationality and effectiveness of the system design scheme.

Circuit structure and working principle of PWM inverter

In AC-DC-AC inverters, the DC circuit is usually required to use a thyristor rectifier circuit, as shown in Figure 1(a). The magnitude of the inverter output voltage Uo can be controlled by changing the magnitude of Ud. The frequency of the output voltage Uo can be changed by controlling the frequency of the inverter trigger circuit. However, this frequency conversion circuit has defects: if the output AC voltage is a rectangular wave containing many harmonics, it is not good for the load or the AC power grid; if the output power is adjusted by phase control, the input power factor will be reduced. At the same time, due to the presence of a large filter capacitor in the intermediate DC loop, the inertia is large when adjusting the input power, and the system responds slowly.

To solve the above defects, a frequency conversion circuit as shown in Figure 1(b) can be used. This circuit is usually called a PWM (Pulse Width Modulation) type frequency conversion circuit. Its basic working principle is to regularly control the on and off of the switching devices in the inverter circuit so that the output end obtains a pulse train with equal amplitude and unequal width, and uses these pulse trains to replace the sine wave. By modulating the width of each pulse in the pulse train according to the required rules, the size of the circuit output voltage can be changed, and the frequency of the output voltage can also be changed.

Dual-loop control strategy for isolated inverter source

As shown in Figure 2, it is the principle diagram of the voltage and current dual-loop control based on the PWM isolated inverter source. The outer control loop is the voltage control loop. The feedback value of the voltage Vabc is measured by the measurement module 2 and compared with the given reference value. The error signal is adjusted by the PI controller as the reference of the inner current loop; the inner control loop is the current control loop. The error signal generated by comparing the feedback current value Iabc1 measured by the measurement module 1 with the current reference generates a PWM control signal after being discretized by the PWM generator.

The PI controller is a controller with a proportional-integral control law. Its block diagram is shown in Figure 3. Its control law means that the output signal u(t) of the controller reflects both the input signal e(t) and the integral of e(t) with respect to t, that is:

Where: kP is the adjustable proportional coefficient, TI is the adjustable integral time constant.

In control engineering practice, PI controllers are mainly used to improve the steady-state performance of control systems. Accurate setting of PI parameters is crucial to the control effect. The adjustable integral time constant TI will affect the time and stability of the system to reach stability, and the adjustable proportional coefficient kP will affect the response time of the system. In the isolated inverter source designed in this paper, the PI regulator that adjusts the outer loop voltage feedback value vabc is set to kP = 0.25, TI = 300 by using the engineering setting method; the PI regulator that adjusts the inner loop current feedback Iabc1 has a parameter setting value of kP = 1.25, TI = 1. Simulation results

According to the control scheme, the modeling and simulation of the designed isolated inverter source is completed using the Matlab-Simulink-SimPowerSystems software platform. The simulation time is set to 0.3 s, and the simulation data are all in per-unit value. The simulation model is shown in Figure 5. The PI control models of the designed voltage outer loop and current inner loop are shown in Figure 6 and Figure 7 respectively. [page]

The main parameters of the model simulation are shown in Table 1.

1. Inverter source simulation results

Modeling and simulation are performed according to the parameter settings in Table 1. After the simulation starts, the inverter power supply reaches steady-state operation in a very short time. The output current Iabc and voltage Vabc are measured by measurement module 2, and the output current Iabc1 is measured by measurement module 1. The output waveforms of the modulation coefficient m are shown in Figures 7 and 8.

After the inverter reaches steady state, the output voltage and current waveforms in Figure 7 show that the inverter is very stable and has achieved the expected effect. Figure 8 shows that the modulation coefficient m converges to a stable range of 0.85~0.9 after a short period of oscillation, indicating the stability of the modulation control. [page]

2. Voltage Control PI Simulation Results

After the inverter power supply reaches a steady state, the input signal of the PI regulation of the voltage outer loop control module and the output signal after PI regulation are shown in Figure 9. From the output waveform of Figure 9, it can be seen that the Vd and Vq signals input to the PI converge to 0 after a short fluctuation and are input to the PI regulator. After being regulated by the PI regulator, a relatively stable error signal is output as the reference signal of the current inner loop control, which ensures the stability of the current inner loop control.

3. Current control PI simulation results

After the inverter power supply reaches a steady state, the waveforms of the output Vd, Vq and voltage Uabc of the current inner loop control module after PI regulation are shown in Figure 10.

In the current inner loop regulation, the signal obtained by the current dq transformation is compared with the reference signal input after the voltage outer loop control. The comparison result is used as the PI regulation input signal of the current control loop. After PI regulation, the stable control signal Vd, Vq is output. As shown in Figure 10, the output signal converges to a stable state after a short oscillation, indicating the stability of the current inner loop control system. The output voltage Uabc is used as the input signal of the PWM generator. After the PWM generator is discretized, a PWM control signal is generated to form a closed-loop control system, which ensures the stable operation of the entire control system.

Conclusion

This paper analyzes distributed power generation as an efficient and clean way of power generation, with its advantages of low investment and compatibility with the environment, and its wide application in microgrids. As an important part of the microgrid, the stability, effectiveness and feasibility of the design and operation of the inverter power supply will directly affect the power quality of the entire microgrid. The inverter power supply with dual-loop control of voltage outer loop and current inner loop designed in the scheme, the voltage outer loop can increase the stability of the system and eliminate static errors, and the current inner loop can improve the rapidity and dynamic characteristics of the system. The PI control strategy is adopted, and the experimental simulation platform is established using Matlab software. The simulation results show that the inverter power supply designed in this scheme has good steady-state performance and dynamic performance, the control system design is reasonable and stable, and the parameter selection is reasonable and effective.