The Control Strategy Of A Cascade H-bridge Based Medium Voltage SVG

With the development of the power electronic technology, the increasement ofnonlinear, impact load severely reduces the power quality. Rapid local reactivepower compensation has important practical significance to improve power factor,reduce the capacity of powered devices, and improve the stability of the grid. StaticVar Generator (SVG) has the advantage of fast dynamic response, low loss, smallsize and low harmonic content, and has become an important development directionof the dynamic reactive power compensation equipment.In the high-voltage and high-power electric field, there are irreconcilablecontradictions between power rating and switching frequency of the existingswitching device. In this paper, the cascade H-bridge multilevel inverter is used toconstitute the SVG main circuit, and the modular structure makes it easy to controland encapsulate. Carrier phase-shifted PWM (CPS-SPWM) modulation is used inthis system to gain higher equivalent switching frequency by a much lowerswitching frequency in switching devices, which has a fine harmonic characteristic.This paper first developed a mathematical model of SVG in the dq coordinatesystem. Dual closed-loop control of voltage and current is used in the control system,and feed-forward decoupling is added to make current PI control of d-q axisdecoupled from each other. Then the power exchange between the SVG and thepower is regulated by controlling the dq current of SVG. This paper also pointed outthe delay brought by the current regulator, and built another mathematical modelbased on the duty cycle. Using differential part to replace the current rate of change,and proposed the current predictive control to compensate the delay brought bycurrent control and improve the dynamic response of this system.DC capacitors of cascade H-bridge are independent of each other, which makesit avoiding the phase shifting transformer and rectifier as well as bring the problemof capacitor voltage imbalance. This paper analyzed the reasons for fluctuations andimbalance of the capacitor voltage. Based on the mathematical model previouslyestablished, fine-tuning of the modulation wave signal or the duty cycle signal canbalance their voltage to guarantee the normal and reliable operation of this system.High power cascade system requires fault-tolerant strategies to improve thereliability of this device. In order to ensure that SVG can still offer the same reactivepower after the failure of one unit, unit redundancy is used in this system. As soonas the unit failure is detected, bypass of this unit is needed. Then adjust the voltageof other capacitors and the modulation strategy to keep the output of SVG remainingunchanged, ensuring the reactive power exchanged with the grid. On the basis of the theoretical analysis, a medium-voltage SVG Simulinkmodel and real device is built. The experimental and simulation results show thatthis system has a good reactive compensation characteristics and fast dynamicresponse, and verifies the correctness of the capacitor voltage balancing controlstrategy and fault-tolerant modulation strategy.