Small and Large Signal Impedance Modeling for Stability Analysis of Grid-connected Voltage Source Converters

Abstract

Interactions between grid-connected converters and the networks at their terminals have resulted in stability and resonance problems in converter-based power systems, particularly in applications ranging from wind and PV farms to electric traction and HVDC transmission networks. Impedance-based modeling and analysis methods have found wide acceptance for the evaluation of these resonance problems.

This thesis presents small and large signal impedance modeling of grid-connected single and three phase voltage source converters (VSC) to enable the analysis of resonance conditions involving multiple frequency components, and both the ac and dc power systems at the VSC terminals. A modular impedance modeling approach is proposed by defining the VSC impedance as transfer matrix, which captures the frequency cross-coupling effects and also the coupling between the ac and dc power systems interfaced by the VSC. Ac and dc impedance models are developed for a VSC including the reflection of the network on the other side of the VSC. Signal-flow graphs for linear time-periodic (LTP) systems are proposed to streamline and visually describe the linearization of grid-connected converters including the frequency cross-coupling effects. Relationships between the impedance modeling in dq, sequence, and phasor domains are also developed. The phasor-domain impedance formulation links the impedance methods with the phasor-based state-space modeling approach generally used for bulk power systems. A large-signal impedance based method is developed for predicting the amplitude or severity of resonance under different grid conditions. The small-signal harmonic linearization method is extended for the large-signal impedance modeling of grid-connected converters. It is shown that the large-signal impedance of a converter is predominantly shaped by hard nonlinearities in the converter control system such as PWM saturation and limiters.

This thesis also deals with the problem of synchronizing a generator or microgrid with another power system. A VSC-based synchronizer is proposed for active phase synchronization and a distributed synchronization method is developed for microgrids.