With the development of the renewable distributed power generation technology, new type s of grid system is attracting more and more attention such as micro grid. Power electronics equipments are needed in the connection between different systems. They introduce lots of harmonic current into the micro grid system, which can decrease the stability of the system. So the detection method of harmonic current is one of hot topics. This paper proposes a novel quadrature harmonic current detection method that is clear, fast and accurate. The simulation and experiment verifies this method. Finally this paper proposes a novel solution of harmonics current for relay protection of micro-grid.