Protection of inverter-based microgrids from ground faults by an innovative directional element

The complicated response of inverter-interfaced distributed generators (IIDG) to faults has been reported, which severely affects all parts of relaying, i.e. fault sensing and polarisation, and faulted phase selection. Given this, the root causes of difficulties in dealing with the protection of inverter-based microgrids are explained. Then, the study describes a directional element using unique features of zero-sequence components that retains satisfactory performance even in IIDG-based microgrids. The zero-sequence component is the only sequence component that can be calculated in the time domain without time delay and thus can cause less delay in the outcome of protective schemes compared with the other two sequences. With this, instantaneous zero-sequence power is defined and one term derived from it, zero-sequence reactive power, is utilised to polarise ground faults. It is proven that the zero-sequence reactive power is negative for forwarding ground faults and positive for reverse ground faults. An interesting feature is that the zero-sequence reactive power is calculated by averaging a new quantity in the time domain over half a power cycle. Hence, the time delay is half that of the conventional phasor-based methods. A sample microgrid is simulated in MATLAB/SIMULINK to evaluate the directional element and the results demonstrate the improvements.