The traditional method for modelling the semiconductor switch in a real-time digital simulator is the ‘R’ approach, where the switch is modeled as a variable resistor, taking on a small value for conducting and a large value for the non-conducting state. This value change requires a time-costly admittance matrix re-formulation, which, particularly when simulating Voltage Source Converters (VSC) operating at high switching frequencies, makes it difficult to run in real-time. Thereby, an alternative as ‘LC’ approach is proposed, which models the switch as a small inductor for the ‘ON’ state and a small capacitor in series with a resistance for the ‘OFF’ state.
This paper further investigates this issue by conducting impedance frequency scans on an electromagnetic transients (EMT) simulation model of a system containing a VSC converter modeled with the ‘R’ and ‘LC’ approaches in rtds, and comparing it with the analytical impedance scan obtained from a small signal model of the same system. To conduct the frequency scan, a harmonic current containing several equi-magnitude frequency components is injected at the node at which the impedance of the system is to be measured. A discrete Fourier Transform of the measured resultant voltage is carried out to yield the voltage components at the various injected frequencies. The impedance scan is then obtained by dividing the harmonic components of voltage by those of the current. By the comparison between different impedance results, it is shown, that with a proper choice of parameters and a sufficiently small time-step, the ‘LC’ approach leads to a highly accurate, yet high speed simulation suitable for real-time simulation.
Yi Qi, Ani Gole ~ University of Manitoba