Harmonic impedance measurement for an islanded microgrid using current injection

Harmonic impedance is the important parameter to analyze the stability of power system. In the microgrid, due to the existence of power converters with constant power characteristics which may result in negative impedance and thus decreases the system stability, harmonic impedance measurement is also of great significance. In this paper, the droop control strategy for parallel inverters in microgrid with virtual impedance to increase its stability is briefly introduced. Three most commonly used methods for harmonic impedance measurement, including the capacitor switching method, the thyristor-controlled sub-circuit switching method and the direct harmonic current injection method, are studied. By comparing advantages and disadvantages of different methods, the harmonic current injection method is chosen to measure the harmonic impedance of the microgrid system. The simulation results under PSCAD demonstrate that the harmonic current injection method is valid to measure the harmonic impedance of microgrid.