Variable frequency based relay protection for isolated micro grid constructed with VSCs

For the characteristics of fault current out from VSC (Voltage Source Converter) power source is very different from that of traditional electrical machine, the existing relay protection technology is not suitable for the micro grid (MG), especially for an isolated MG. A method of relay protection for an isolated MG is proposed, where VSC power source will output variable frequency current during the short circuit on transmission line. The amplitude of current is limited to avoid the damage to electronics based switches, and the frequency reflects the fault distance from the relay point. Then, the relay device will receive the current signals in different frequency, and transmit the variable frequency current to a virtual current in normal frequency. Thereafter, the relay system can clear the fault element selectively and automatically according to regular relay rules. The feasibility and effectiveness are verified by simulation.