Instantaneous fault current limiter for PWM-controlled Voltage Source Converters

The PWM-controlled Voltage Source Converters (VSCs) are commonly used in industrial and utility applications. In spite of superior features of fast voltage regulation and stable DC-link voltage, PWM-controlled VSCs have the major drawback of being sensitive to the grid disturbances especially the unbalanced conditions and system faults. Unbalanced input voltage generates large negative sequence current flow into the converter which results in oscillations with twice the line frequency on the DC-link voltage. This negative sequence current flow might damage the semiconductor switches. Beside the negative sequence voltage, the input voltage distorted with other harmonics also causes converter performance deterioration by producing harmonics on the DC-link voltage. This paper presents an alternative solution to improve the PWM-controlled VSC performance under unbalanced conditions and system faults and also under distorted input voltage condition caused by other harmonics rather than the negative sequence voltage. This solution is based on direct calculation of the negative sequence (or other harmonics) reference voltage without using any current regulator. This elimination of the current regulator makes the proposed controller very fast and robust. The effectiveness of this solution has been validated by simulation and Hardware-In-the-Loop test.