Combination of Orthogonal Injections in Impedance Measurements of Grid-Connected Systems

Impedance-based stability criterion has become a popular method in determining the stability of grid-connected systems. Recent studies have presented the utilization of various pseudo-random-sequence (PRS) perturbations for rapidly and accurately obtaining the impedances required for the stability analysis. A major drawback of the PRS is, however, that the signal power is linearly distributed over many harmonic frequencies. As the injection amplitude must be kept small to avoid too strong nonlinear distortions, it becomes challenging to provide enough power to the whole frequency band of interest.This work proposes a novel perturbation that is synthesized by summing up several independently designed orthogonal PRS injections. As the orthogonal sequences do not have power at common frequencies, the resulting combined signal can be tailored to have a specific spectral-power distribution at each frequency band of interest. As a consequence, the system impedances can be accurately measured over a wide frequency band. The performance of the method is verified through experimental measurements of a 2.7 kW grid-connected system, where grid impedance measurements and terminal inverter output admittance measurements are performed.