State variable decoupling and power flow control in PWM current source rectifiers

Current source PWM rectifiers among others are gradually replacing thyristor line commutated rectifiers as a source of variable DC power. Advantages include reduced line current harmonic distortion and complete power factor control, including unity power factor operation. However, due to intrinsic nonlinearities and stability problems, the control of the rectifier has usually been achieved using off-line patterns or direct line current control in the abc frame. This paper proposes a state variable control technique that introduces more flexibility in the control of the rectifier and a more straightforward approach to controller design. The proposed technique consists in linearizing the state variable model of the system in the dq frame, decoupling and independently controlling the two components of the line current (active and reactive), eliminating the input damping resistors, and rejecting the effect of supply voltage variations. Also, a space vector modulation technique is used to maximize the current gain and switch utilization of the rectifier. The paper includes a complete formulation of the equations of the decoupled system and a controller design procedure. Experimental results on a 2 kVA DSP based prototype confirm the theoretical considerations