MIMO-Identification Techniques for Rapid Impedance-Based Stability Assessment of Three-Phase Systems in DQ Domain

Grid impedance and the output impedance of grid-connected inverter are important parameters for the operation of grid-connected systems, such as solar, wind, and other distributed-generation resource systems. The impedance mismatch between the grid and the interfacing circuit often generates harmonic resonances that lead to reduced power quality and even instability. Since the impedances usually vary over time as a function of many parameters, online measurements are required for stability assessment and adaptive control of the inverters. Several methods have been proposed for quick, accurate measurements of impedances, but the use of multiple-input multiple-output (MIMO) identification techniques have not been considered. Applying the MIMO techniques, different components of the inverter output impedance or grid impedance can be simultaneously measured during a single measurement cycle. Therefore, the operating conditions of the system can be kept constant during the measurements, and the overall measurement time is significantly reduced. This paper shows the use of orthogonal binary sequences to simultaneously measure the “d” and “q” components of grid-connected inverter output impedance and/or grid impedance. Experimental results based on a three-phase grid-connected inverter are presented and used to demonstrate the effectiveness of the proposed methods.

I. Introduction

Climate change and limited fossil fuel reserves have recently accelerated the utilization of grid-connected renewable sources such as solar and wind. Due to large-scale and increased utilization of such distributed resources, the dynamics of the power grid have started to change [1]. One of the main issues is the harmonic resonance between the inverter and the grid, which occurs due to mismatch between the inverter output impedance and the grid impedance. The harmonic resonance indicates a lack of system stability margin and may lead to instability and undesired inverter operation [2].