Small Signal Stability Analysis of Microgrid System Based on Real-time Grid Impedance Measurement using Quadratic Residue Binary Sequence (QRBS)

Thee-phase micro-grid systems with tightly regulated power converters are prone to interactions and causes small signal instability. The regulated converters behaves like constant power load (CPL), and cause instability. Adaptive controllers can be used to ensure the stable operation of system, and controllers require real-time grid impedance measurement. In this paper, a computationally effective impedance measurement technique using the Quadratic Residue Binary Sequence (QRBS) has been proposed. The QRBS 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. In order to measure the grid impedance, the QRBS sequence is added to the d and q-components of the CPL current. Once the grid impedance is measured, stability of the system is analyzed using the Generalized Nyquist Stability Criteria (GNC). The effectiveness of proposed impedance measurement algorithm using the QRBS signal has been tested in MATLAB/Simulink simulation environment for a three-phase micro-grid system. Simulation results are effectively demonstrated the impedance measurement algorithm using QRBS sequence.