A fast computing algorithm for microgrid fault protection system using communication-assisted digital relays and initially experimental results

This paper presents a fast computing algorithm (FCA) for microgrid (μgrid) fault protection system that can online work with high adaptability and dependability functions and can get a critical fault clearing time shorter than two cycles beginning at fault inception points. Concretely, the FCA uses an overcurrent protection principle to protect branch lines that contain either loads or distributed energy sources in a μgrid system. In addition, in order to protect main lines in the μgrid from fault occurrences, a new fault protection method is developed through using communication-assisted digital relays. The main lines are understood as lines used to interconnect two or more distributed energy sources and not including any load branches along these lines. Two fundamental ways that are used to inspect operation of the H-grid protection system with the FCA are: (i) theoretical calculations from actual results of staged fault tests, motor-starting tests, and tests of μgrid's operation transition between grid-connected and islanded modes which are conducted at a real low-voltage AC μgrid in Taiwan and (ii) doing different fault tests along with FPGA boards to evaluate the FCA from a lab environment. As a result, the FCA can quickly detect and identify different types of grounding fault and optimally isolate faulted sections. Fault clearing time of the FCA for occurrences at the main lines is about 0.5-2 cycles depending on the algorithm's computation time and the communication time among the digital relays, while that at the branch lines is completely dependent on their fault current values.