| Traditional reactive power compensation and harmonic control method of substation high-voltage side is complex and expensive because of the high voltage level. The local compensator of low voltage side is scattered, inefficiency, uncoordinated and easy to bring new power quality problems. Distribution transformer STATCOM(DT – STATCOM) considered distribution transformer winding as access point of compensation devices, which reduced the withstand voltage of the compensation device and realized the effect of concentrated compensation. But further research is necessary in topology structure, connection type and control method. This paper presents a new integrated compensation system that combined cascaded H-bridge multilevel inverter with distribution transformer from the high voltage side split winding, and works in the control method of reactive, harmonic and unbalanced current compensation, in order to improve power quality of the distribution network.First, the cascaded type DT- STATCOM system was designed. According to the actual circumstance of distribution network, the integration through distribution transformer high-voltage winding tap, cascaded H bridge topology and LC output filter was adopted to build DT- STATCOM system. Mathematical model of each part was Established to analyzes the compensation current influence of transformer winding current and the voltage and current of transformer both sides. Cascade multilevel converter state equation is established and the compact LCL structure filter composed of LC output filter and transformer equivalent leakage reactance was analyzed to determine the effectiveness of compensation effect.Second, this paper presents a new control method of reactive and harmonic current compensation. Based on instantaneous reactive power theory, reactive and harmonic current detection method was improved to fit the application of across port compensation. A new comprehensive control strategy, which combined with PI control and PR control, was put forward to realize reactive and harmonic current compensation. Carrier phase shifting SPWM modulation was adopted to improve the instruction current tracking. Its principle, mathematical model and transmission characteristics was analyzed to meet the requirement of reactive and harmonic compensation. Matlab/Simulink simulation platform was built to complete the simulation experiments, which have verified the effectiveness of the current detection and control method.Finally, the unbalanced load compensation control method is proposed. The influence of unbalanced compensation on cascade inverter dc capacitor voltage was researched through power analysis method to get the effective range of unbalanced compensation. A separation detection and control method of positive and negative sequence current was researched, and a balance control method of dc capacitor voltage. It is effective to realize the comprehensive compensation of reactive and negative sequence current, and ensure the dc voltage capacitor balance, which is necessary to the safe and stable operation of the system. Matlab/Simulink simulation experiment proved the effectiveness of the control method. |
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