Simulation Research of STATCOM System Based on Current Indirect Control

1 Brief description of STATCOM working principle

The use of STATCOM for reactive power compensation has the advantages of a large continuous adjustment range, precise and fast control response, and economical and reliable operation. Its working principle diagram is shown in Figure 1. The main circuit of STATCOM consists of an inverter and a DC side capacitor, which is connected to the power system through a transformer. Ideally, the STATCOM device is equivalent to a "controllable voltage source". Assume that its output voltage is UI and the system voltage is US, and the two are in phase. When UI>US, the current flows from the system to the STATCOM and the current phase leads the system voltage by 90. The device outputs inductive reactive power; conversely, when UI 无功功率" title="Reactive power"> Reactive power

In steady state, the calculation formula of active power and reactive power absorbed by STATCOM from the system is as follows:

Where: US is the system voltage; R is the system equivalent resistance; δ is the phase difference between the system voltage and the device output voltage. When δ<0, Q<0 absorbs capacitive reactive power; when δ>0, Q>0 absorbs inductive reactive power. By adjusting δ, the reactive power of STATCOM can be continuously adjusted.

2 Indirect current control strategy of STATCOM

According to whether the output current is directly controlled, STATCOM can be divided into two control strategies: direct current control and indirect current control. Indirect current control refers to the control of the phase and amplitude of the AC voltage fundamental wave generated by the inverter in the STATCOM device, so as to indirectly control the current on the AC side of the STATCOM. Indirect current control is divided into single δ control and δ and θ coordinated control. Although single δ control is simple and effective, the control of θ is ignored, making it difficult to stabilize the DC side capacitor voltage and increasing losses. In the δ and θ coordinated control, the control of the δ angle is used for reactive power control, while the control of the θ angle can play a role in maintaining the stability of the capacitor voltage. Therefore, the inverse system nonlinear PI method can be used for reactive power control, and the traditional PI control method can be used for the DC side capacitor voltage of STATCOM. The two control loops are independent of each other and do not interfere with each other.

Figure 2 is a PI control block diagram of the inverse system with δ and θ. In the figure, the three-phase instantaneous voltage uA, B, C and instantaneous current iA, B, C are transformed by α, β and instantaneous reactive power calculation to obtain the compensated reactive power Q, which is compared with the reference compensated reactive power Qref. The control quantity δ is obtained through the PI link, and the reference voltage uref is compared with the DC side voltage udc. The control quantity θ is obtained through the PI link. The control quantities δ and θ are input into the STATCOM control system as control parameters.

3 System model construction and simulation results analysis

Through the above analysis of the principle and control strategy of STATCOM, the system-level modeling and simulation will be carried out in Matlab/Simulink environment. Matlab/Simulink is widely used in the modeling and simulation of power systems.

3.1 System model construction

As shown in Figure 3, a system simulation model of STATCOM based on indirect current control is built in Matlab/Simulink. The effective value of the infinite system voltage is used in the figure, the frequency is 50 Hz, and the rated load is S=9 000+j9 000. The main circuit of STATCOM is composed of 48 pulses inverter. Discretization can be used to speed up the simulation speed, and the simulation step size is set to TS=2.5×10-5s.

3.2 Analysis of simulation results

In the simulation, the system load is a three-phase balanced load, so the voltage and current waveforms of phase A can be taken as representatives for observation. Figure 4 is a comparison diagram of the voltage and current phases of phase A of the system before compensation. In the figure, the amplitude of the voltage waveform is larger than that of the current waveform, and the voltage phase leads the current phase by 90°. Figure 5 is the power factor of the system before compensation, and its value remains at 0.707, which is consistent with the theoretical calculated value.

Figure 6 is a comparison diagram of the voltage and current phases of phase A of the system after compensation. The phases of the voltage and current waveforms have basically become consistent, so a higher power factor can be obtained.

This is also proved by the power factor curve after system compensation given in Figure 7. Because the DC side capacitor of the converter needs to be charged, the power factor oscillates in the initial stage. After the capacitor is charged, the oscillation disappears quickly (around 0.06 seconds), and then the STATCOM enters the steady-state working area, and the power factor is close to 1.

4 Conclusion

This paper analyzes the current indirect control strategy and realizes the system simulation of STATCOM based on the current indirect control method. The simulation results verify the correctness and effectiveness of the established model. The advantages of the current indirect control method are that the structure is relatively simple and the technology is relatively mature. However, compared with the current direct control method, the indirect control method has lower control accuracy and slower current response speed. Further research should be conducted on its advantages and disadvantages, and a reasonable choice should be made according to different occasions.