A Simplified Approach based on Dual Angle-Controlled STATCOMs under System Faults

Voltage source converter (VSC) based Synchronous Static Compensator (STATCOM) is used for voltage regulation in transmission and distribution systems. VSC based on their control mechanism can be categorized as line-frequency switched and pulse width modulation converters. The line-frequency switched VSCs are based on angle-control method and hence also referred as angle-controlled VSCs. Over the past few years, angle-controlled STATCOMs have been deployed by the utilities for the purpose of voltage regulation and stability and to improve transmission capacity. Apart from the advantages offered to increase voltage transient, the on-site STATCOMs suffer from over-current trips and DC bus voltage oscillations under system faults. The negative sequence current during the faults tend to trip the overcurrent protection and possibly saturation of the interfacing transformer. It also leads to second order harmonic oscillations of the DC bus voltage which can trigger over-voltage protection. Dual angle control (DAC) is a control methodology that generates angle oscillations control signal during system faults which reduces the negative sequence current flow and dc-link oscillations compared to conventional angle controlled STATCOMs. The procedure was effective but deals with complexity of calculating the reference signal for angle oscillation. This paper proposes a simplified dual angle control based on DC-link voltage oscillation. The main advantage over the DAC is its simplified control structure, without compromising the performance in reduction of the negative sequence current and DC-link oscillations. Since the dual angle control is based on DClink voltage oscillation and hence it is called DC-link based dual angle control (DC-DAC). PSCAD/EMTDC results for 48-pulse STATCOM connected to a three-bus power system verify the validity of proposed DC-DAC control under unbalanced system condition and faults.