The Control Strategy Of DSTATCOM Using Direct Current Control Method

The development of modern industry, On the one hand become increasingly demanding for the high quality of power system, such as the making of delicate semiconductors.on the other hand contaminate the power system more and more seriously and even make the power system unstable,such as the electrified railways which bring about a great deal of negative-sequence current. The traditional compensation devices with the characteristic of long response time, low precision and large size, can't meet the demand of modern power system. A new type of reactive-load compensation equipment is urgently in need.STATCOM(Static Synchronous Compensator) is a new type of static reactive power compensation device developed recent years,with the good characteristic of short response time, high precision and small size. When STATCOM is used in the distribution network,it is called DSTATCOM(Distribution Static Synchronous Compensator),which can compensate quickly the reactive power and part of harmonic current generated by different kinds of load.This paper compares the two means of controlling the current. Through comparison,this paper thinks that the direct current control strategy is more suitble for DSTATCOM. This paper builds the model of DSTATCOM using switch function method. This paper adopts the PARK Transformation to get the averaging model equivalent circuit under the synchronized rotation reference frame and employs dq decoupling control. It analyse the model of DSTATCOM using AC Small Signal Analysis method to get AC Small Signal equivalent circuit. Based on the circuit, this paper builds the voltage- and current double-loop control system. The engineering design method is adopted to design the adjustor, which makes the design more clear.The unbalance compensation control is very important in this paper. This paper compares the unbalance compensation control between the indirect current control method and the direct current control method, reveal that in the direct current control method the load-unbalance can be compensated by DSTATCOM,while the voltage-unbalance brings about no damage to the device,only makes the power factor less than 1. Severe voltage-unbalance should be treated as a fault,which can be compensated by series FACTS devices.This paper simulates the start course of the DSTATCOM,compensation of different kinds of load and compensation of unbalance load. The simulation results show that the direct current control method proposed in this paper is right and effective.