A static synchronous compensator using hybrid multi-inverters

A static synchronous compensator (STATCOM) is required to have (1) less harmonic contents in the output, (2) higher compensation VA capacity, and (3) less devices losses. To satisfy these requirements this paper proposes a hybrid configuration STATCOM, which consists of two GTO inverters with square-wave switching and one neutral-point-clamped IGBT inverter with PWM switching. The GTO inverters fulfill the second and third requirements and the IGBT inverter the first requirement. The IGBT inverter generates a voltage difference between the reference output and the sum of the two GTO inverter outputs. Therefore, the IGBT inverter has two functions: cancellation of the harmonic components generated by the GTO inverters and control of the compensation current. One problem of the hybrid STATCOM is that the IGBT inverter is forced to operate in an over-modulation range if required compensation voltage of the IGBT inverter is high. To solve this problem, this paper also proposes a variable DC voltage control strategy, where the DC voltage is adaptively adjusted in accordance with the required STATCOM output voltage. As a result, the proposed STATCOM can compensate for large reactive current almost without harmonic distortion.