Negative sequence current cancellation with DSP and space vector controlled PWM modulated inverter

Abstract

Single-phase conversion to three-phase power conversion is a special case of an unbalanced three-phase power compensator. An unbalanced three-phase current can be split into its symmetrical components in classical power system theory. A single-phase supply or load is an extreme case of an unbalanced system. In the case of a three-wire system, the current components are made up of a positive current sequence and balanced negative phase sequence current. In this paper it is shown how the negative sequence current can be eliminated without the use of active power. The opposite negative sequence is injected into the three lines by a three-phase inverter. The inverter has a space vector modulated PWM switching function. A digital signal processor (DSP) produces the control signal for the inverter semiconductor switches. The inverter is a full bridge IGBT (insulated gate bipolar transistor) inverter running in a space vector modulation scheme (SVM). The space vector, which represents the switching states of the six switches of the inverter, consists of the sum of two circular trajectories. One trajectory runs clockwise and the other anticlockwise. The result is an elliptical trajectory. The invertor voltage space vector is calculated on line from the single-phase current and therefore requires a fast processor. A DSP is used to achieve this.