Three-phase series VAr compensation based on a voltage-controlled current source inverter with supplemental modulation index control

Shunt-type static VAr compensators have been shown to be an effective means of increasing the transmission capability of power systems and of increasing the power factor of industrial loads. This paper presents an alternative to the shunt connection consisting of a series-connected transformer-coupled pulsewidth modulation (PWM) inverter. The scheme consists of a current source inverter (CSI) fed from a self-controlled DC current bus and presents the following features: (1) instantaneous series injected voltage control through online PWM of the DC link current; and (2) steady-state modulation index of the PWM pattern near unity through DC link current minimization. The first feature results in fast dynamic response, while the second feature ensures low and fixed injected voltage harmonic distortion and reduced steady-state compensator losses. Operation of the compensator and of the two-loop control scheme are presented. Simulation and experimental results on a 2 kVA laboratory prototype unit confirm the feasibility of the proposed VAr compensator structure.