Adaptive Multi-scale Transient Modeling and Efficient Simulation of Wide-area AC/DC Power Systems
Final Report Abstract
Power and energy systems cover very far-reaching scales that are unique in engineering. Regarding varying time scales, two distinct types of tools have emerged for analysis. Tools based on phasors consider the sinusoidal AC voltages and currents by tracking their envelopes. Envelope tracking requires fewer sampling points compared with tracking the naturally occurring AC waveform of carrier frequency fc. This makes phasors popular for studying phenomena such as electromechanical transients where envelopes are illustrative. For tracking natural waveforms, instead, programs of EMT (electromagnetic transients) type are applicable. For holistic considerations, it is desirable to have the opportunity to analyze diverse transients without the need to switch between different tools. The key to such a solution of a multiscale simulation is the introduction of the shift frequency. In the method of Frequency-Adaptive Simulation of Transients (FAST), the shift frequency fs sets the reference for other frequencies to relate to. In general, to support frequency shifting in multiscale simulation of AC power systems, all waveforms are modeled through analytic signals. The imaginary part of an analytic signal relates to the real part through the Hilbert transform. Since the spectra of analytic signals only extends to positive frequencies, shifting of the analytic signal toward the left by the shift frequency is then possible and meaningful. Depending on the situation observed, locational and temporal modifications of the shift frequency are possible. In this project, it was a main objective to extend the capabilities of FAST toward the modeling of nonlinear devices such as power electronic circuits. A second objective was concerned with the co-simulation of electrical and control systems. Finally, a third objective looked into the validation of FAST for AC-DC power systems. Regarding the first objective, the focus was put on the voltage sourced converter (VSC) as a key element of power conversion when connecting DC sources to the AC grid. The multi-scale VSC model supports a seamless transition between acting through switching functions and average value signals. While the switching functions are of interest when tracking natural waveforms, the average values support envelope waveforms. To allow the usage of analytic signals for voltages and currents inside the converter, complex representations of the modulating control signals were introduced. The VSC model was integrated into a novel model of a wind energy conversion system (WECS) where electric power generation is facilitated via a doubly fed induction generator (DFIG). Full multi-scale modeling of the WECS was accomplished. For this contribution, the investigators were named as the recipients of the prestigious 2023 IEEE PES (Power and Energy Society) Prize Paper Award. Regarding the second objective, generalized interfaces were developed to interface multiscale electric power system with their associated control system model counterparts. The interfaces are distinguished by their ability to support stability of the simulation process by compensating time step delays in the interfacing. Regarding the third objective of validation, beyond the WECS, realistic-size AC-DC power systems with more than 10000 nodes were considered.
Publications
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Multi-Scale Modeling and Simulation of DFIG-Based Wind Energy Conversion System. IEEE Transactions on Energy Conversion, 35(1), 560-572.
Xia, Yue; Chen, Ying; Song, Yankan & Strunz, Kai
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Bridging Scales With the Shift Frequency: Frequency-adaptive simulation of multiscale transients in power systems. IEEE Electrification Magazine, 11(4), 29-37.
Strunz, Kai; Chen, Ying & Xia, Yue
