Improved discrete-time switch model for electro-magnetic transient simulation of voltage sourced converters
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Date
2019-08-30
Authors
Liu, Zhiqiang
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Abstract
Switch modeling has always been a challenge in Electro-Magnetic Transient (EMT) type simulation. The commonly used traditional switch models are Resistor Switch Model (RSM) and Inductor Capacitor Switch Model (LCSM). In the RSM, the on-state switch and off-state switch are modeled as a small and large resistor (R) respectively. In the LCSM, the on-state and off-state are represented by specially selected inductances (L) and capacitances (C) respectively. The RSM requires inversion of the system admittance matrix every time a switch operates resulting in a large computational burden. On the other hand, the admittance matrix remains invariant when switches operate for LCSM, thereby avoiding the computationally expensive matrix inversion. Previous versions of the LCSM approach neglected the loss of stored energy in the L or C switch when the switch changed state. This resulted in a spurious switching loss. The purpose of this research is to develop new LCSM model, which permits efficient modelling of VSC topologies with large switch count and operating at high switching frequencies. In order to develop the new model, firstly the spurious switching loss and oscillation are analyzed. Based on the analysis, an energy balance approach is developed to select the post switching current and voltage terms of LCSM so that the stored energy in the switch representation is continuous. This energy balance method helps to mitigate the spurious loss and oscillations in the simulation. In addition to energy balance approach, a new switch model, the RC/RL Switch Model (RC/RLSM) is also proposed. The RC/RL SM uses predictive subroutine and new configuration in order to avoid spurious losses and oscillations. For validation, EMT simulation cases using the proposed energy balance LCSM and RC/RLSM models in various VSC configurations are simulated and compared with the RSM (assumed as the benchmark) for accuracy, losses and computational speed. The agreements of the resulting waveforms confirm that proposed models retain the simulation accuracy, show negligible switching loss and are extremely fast numerically.
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Keywords
Electromagnetic Transient Simulation, Voltage Sourced Converter, Power Electronic Switch Model