Stage-based protection scheme via harmonic undervoltage in power electronic converters interfaced distribution networks

The conventional overcurrent protection fails to isolate the permanent short-circuit fault with the power electronic converters interface, hindering the utilization of various converters for power supply restoration. A novel protection scheme based on the harmonic undervoltage is proposed in this paper to realize accurate and rapid fault isolation. From the system-level perspective, a three-stage harmonic undervoltage protection method is proposed, comprising instantaneous, time-delayed, and inverse-time harmonic undervoltage protection, enabling coordinated operation between primary and backup protection. The operation principle, parameter design, and extension scheme for complex networks of this novel protection method are introduced, enhancing the computational efficiency and adaptability. From the converter-level perspective, the transient overcurrent limited strategy based on voltage feedforward dynamic gain adjustment is presented for avoiding undesired device shutdown during faults. Besides, the active harmonic injection loop is added to the control system, assisting the operation of the proposed protection method. The simulation and the experimental results indicate that the above method can rapidly locate various short-circuit faults in networks with converters, facilitating flexible control of the distribution networks.