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

Shunt type static VAr compensators are 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 in the form of a series-connected transformer coupled PWM inverter. The scheme consists of a current source inverter connected to a self controlled DC bus. It presents the following features: (a) instantaneous series injected voltage control through online modulation index control; and (b) steady state modulation index control to a value near unity. The first feature results in fast dynamic response, while the second feature ensures that the harmonic distortion of the injected voltage under steady state operation is kept to a minimum, independently of the operating conditions. Operation of the compensator and of the two loop control scheme are presented. Simulation and experimental results on a prototype unit confirm the feasibility of the proposed structure