Dynamic grid impedance calculation in D-Q frame for micro-grids

This paper presents a method for online dynamic measurement of the grid impedance that may prevail in an electrical network at any instant of time. Here the grid resistance and inductance are measured during the operation using a non-characteristic frequency current continuously injected into the grid, and subsequently calculating the impedance using discrete Fourier transforms. The proposed measurement is implemented in d-q reference frame so that the computational overhead on the microcontroller is relatively reduced. The continuous injection of non-characteristic current at 75 Hz avoids the injection of inter-harmonics into the grid during measurements. The resulting grid impedance value can be used for many purposes viz. knowing the status of the independent generations in micro-grids, controller tuning in converter systems, islanding detection etc. The proposed dynamic measurement of grid impedance is tested through simulation studies and experimentally. The experiments are conducted with the proposed controller on a scaled down laboratory model of micro-grid with a 1 kVA solar inverter, and the consolidated results of both simulation and experiments are presented.