Fault-Tolerant Control of Active Power Filter With Balanced Three-Phase Line Currents

Transformer isolated three-phase three-wire two-level active power filters (APF) using pulse width modulated (PWM) control are widely used for shunt compensation of harmonic currents. In a situation when only two of the converter legs remain connected to the grid due to an open circuit fault in one of the converter legs or its internal lines or in one of its line reactors, the existing fault-tolerant schemes cannot continue operation with balanced grid side line currents, resulting in improper harmonic compensation. A new control scheme is proposed in this article by which APF can seamlessly continue fault-tolerant operation during single line open-circuit fault and inject balanced three-phase harmonic currents into the line for proper compensation, without requiring any redundant legs or switches to change the internal power configuration. Unlike conventional APF schemes, a simple control scheme is proposed to adjust the phase shift between the composite harmonic currents in the two operative legs of the converter connected to the grid via a full rated delta/star isolation transformer (or an existing delta-star distribution transformer) with neutral connected to dc bus midpoint to inject balanced harmonic currents into the grid. Simulation and experimental results are provided to validate the effectiveness of proposed concept.