Decoupled control of grid connected inverter with dynamic online grid impedance measurements for micro grid applications

Abstract

This paper presents a decoupled control of grid connected inverter using dynamic online grid impedance measurements for a micro grid application. The proposed controller is implemented in synchronous reference frame (SRF) and controlled using linear PI controllers. The mutual coupling introduced between the d and q control loops due to the transformation into SRF is accurately decoupled using the dynamically measured grid impedance using a feed-forward control. The decoupling allows independent control of active and reactive powers against step changes in the active/reactive power references. The online measurement of the actual impedance and its use further for decoupling is proposed in this paper for making the decoupling accurate inspite of the network configuration being altered like in micro grids. 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 continuous injection of non-characteristic current at 75 Hz avoids the injection of sub-harmonics into the grid during measurements. The control loop is updated periodically with the estimated grid impedance, thus enabling the independent control of active and reactive powers delivered by the inverter. The proposed decoupled controller with grid impedance measurement is tested through simulation studies and hardware experiments. 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 performances are presented for step changes in the power references and the results are presented.