Direct Interfacing of Average-Value Models of VSCs in PSCAD/EMTDC

Voltage-source converters (VSCs) are widely utilized in power systems. Due to their high-frequency switching, discrete detailed models of VSCs are computationally expensive in system-level simulations, and their average-value models (AVMs) have proven indispensable for fast/efficient studies. Conventional AVMs of VSCs use dependent current/voltage sources to interface with external circuits. In PSCAD/EMTDC, with a non-iterative solution, the interfacing variables are computed based on the values of the input voltages/currents from the previous time-step. This one-time-step delay can make the results numerically inaccurate/unstable when large time-steps are used in simulations. In this paper, an AVM is developed for VSCs that is directly interfaced with external circuits without delays to allow large time-step. This is done by formulating the equivalent conductance matrix of the VSC AVM which is merged into (and solved simultaneously with) the rest of the network nodal equations. The new directly-interfaced AVM of VSCs is verified in PSCAD/EMTDC against the classic dependent-source-based AVM and is demonstrated to outperform the existing approach in terms of numerical accuracy at large time-steps.