Accurate High Frequency Modelling of the Input Admittance of PWM Grid-Connected VSCs

This paper addresses high frequency admittance modelling of current-controlled voltage source converters (VSCs). Recent studies have shown that harmonic instability may also occur at frequencies above the Nyquist frequency. To form an accurate multiple-frequency model in this frequency range, sidebands that originate from modulation and sampling must be examined. In this paper, an accurate small-signal model is developed taking into account an adequate digital pulse-width modulator (DPWM) representation, which allows to predict dependence of the frequency response on the steady-state DC (SSDC) operating point. It is shown that, when center-pulse sampling is implemented, PWM sidebands do not create additional loops leaving only sampling sidebands to be considered. Using the same approach as for SS-DC operation, a model which accurately represents admittance measurements during sinusoidal AC (S-AC) operation is developed. Its basis is a novel DPWM model suitable for S-AC regime, which allows to predict dependence of the VSC’s input admittance on the grid voltage magnitude. Experimental and simulated admittance measurements, performed on a single-phase two-level VSC during various SS-DC and S-AC regimes, show an excellent match with the proposed models up to twice the sampling frequency