Abstract
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.
Highlights
The rapid increase in grid-connected power electronics has disrupted the dynamics of most power systems and exposed challenges in the system compatibility and stability [1]
Impedance measurements are important in the design and analysis of power-electric systems
Significant research effort has been directed to the design of perturbation sequences, as the measurement quality is strongly dependent on the applied perturbation signal
Summary
The rapid increase in grid-connected power electronics has disrupted the dynamics of most power systems and exposed challenges in the system compatibility and stability [1]. The resulting responses in the system output voltages and currents are measured, and Fourier techniques are applied to extract the impedance information. In many grid-connected systems, especially the low-frequency impedance (admittance) can be small resulting in a very weak response to the applied perturbation. Such systems include inductive transmission grids [22] and converter systems where control loops shape the terminal characteristics [23]. The frequency-specific excitation power can be drastically increased without increasing the signal time-domain amplitude, and the system impedance(s) can be measured significantly more accurately compared to a similar-amplitude MLBS. The independent measurements can be performed by, for example, sequentially injecting the perturbation to first d-channel and to q-channel
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