Digital multi-relay protection for micro-grid systems

This paper presents the development, implementation and performances of a digital multi-relay protection for micro-grids. The developed digital protection is based on extracting the second level high frequency sub-bands present in d-q-axis components of the currents flowing through each bus of a micro-grid. The desired frequency sub-bands are extracted using the wavelet packet transform (WPT), and are considered signatures for identifying fault currents and fault locations. The protective functions in the developed digital multi-relay protection are structured without adaptive features or relay-to-relay communications. This structure is established to accomplish fast response, simple implementation, independent functions, and minor sensitivity to levels of fault currents. The digital multi-relay protection is implemented for experimental testing on a laboratory micro-grid for various operating conditions. Experimental performances demonstrate high reliability, accuracy, and speed, along with negligible sensitivity to parameters of protected components and mode of operating the micro-grid.