Abstract

This paper proposes an active front end (AFE) converter for medium voltage current source drives. The converter comprises a 12-pulse isolation transformer and two PNVM current source rectifiers (CSR) with GTO thyristors. The converter operates with a switching frequency of 100 Hz (2 p.u.), thus reduces its commutation losses by 66% compared to previous PWM approaches. Also, the converter independently controls its active and reactive power flow, thus being able to operate as a Var compensator to the power system. Under this scheme, unity power factor is achieved by setting the reactive power reference to zero. The converter's control system is built over an inner d-q axes synchronous frame current loop, and a dc link current and reactive power outer loops to control the d and q axes respectively. Linear controllers are used in all loops but in the de link. A nonlinear controller is employed to linearize this loop, hence avoiding instabilities during regenerative operation. All these features were achieved using staggered sampling space vector modulation (SVM), which assures harmonic cancellation between both PWM-CSR through the feeding transformer. Moreover, the use of SVM avoids all complications associated to selective harmonic elimination techniques (usually employed in these converters). Specifically, solving nonlinear equations and distributing shorting pulses. Simulation results obtained under different operating conditions validate the proposed AFE converter for medium voltage current source drives.