Real-time Grid Impedance Measurement of Three-Phase Micro-grid System in d-q Frame using Quadratic Residue Ternary Sequence (QRTS)

Thee-phase micro-grid systems with tightly regulated power converters are prone to interactions and causes small signal instability. The regulated converters behave like constant power loads (CPLs), and cause instability. Adaptive controllers can be used to ensure stable operation of the system, and the controllers require real-time grid impedance measurement. In this paper, a computationally effective impedance measurement technique using the Quadratic Residue Ternary Sequence (QRTS) has been proposed. The QRTS signal is a class of Pseudo Random Binary Sequence (PRBS), which is used for system identification in control engineering. The proposed signal has low crest factor compared to the conventional Newman's multi-sine signal. The effectiveness of the proposed impedance measurement technique using QRTS signal has been tested in MATLAB/Simulink simulation environment for a three-phase micro-grid system. The results have been validated with the conventional Newman's multi-sine perturbation, and results shows that the QRTS is an effective perturbation method to measure real-time grid impedance.