Open Circuit Fault-Tolerant Two-Level STATCOM with Balanced Line Currents

Three phase Static Compensators (STATCOM) using Pulse Width Modulated (PWM) converters are widely used for shunt compensation in transmission lines. In case only two of the STATCOM legs remain connected to the grid due to an open circuit fault in one of its legs or line reactors, existing fault-tolerant schemes cannot continue operation with balanced grid side line currents. A new scheme is presented in this paper in which the STATCOM can continue operation during single line open circuit fault and inject balanced line currents, without additional redundant leg or switches. The proposed fault-tolerant technique adjusts the phase shift between the current in the two operative leg of the STATCOM and uses a full VA rated delta-star coupling/isolation transformer to inject balanced VAr into the grid. This arrangement provides a simple solution for incorporating fault-tolerant capability during design of 3-leg STATCOM. Simulation and experimental results are presented to validate the proposed scheme.