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Modelling of Variable-Speed Refrigeration for Fast-Frequency Control in Low-InertiaSystems

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Document pages: 12 pages

Abstract: In modern power systems, shiftable loads contribute to the flexibility neededto increase robustness and ensure security. Thermal loads are among the mostpromising candidates for providing such service due to the large thermalstorage time constants. This paper demonstrates the use of Variable-SpeedRefrigeration (VSR) technology, based on brushless DC motors, forfast-frequency response. First, we derive a detailed dynamic model of asingle-phase VSR unit suitable for time-domain and small-signal stabilityanalysis in low-inertia systems. For analysing dynamic interactions with thegrid, we consider the aggregated response of multiple devices. However, thehigh computational cost involved in analysing large-scale systems leads to theneed for reduced-order models. Thus, a set of reduced-order models is derivedthough transfer function fitting of data obtained from time-domain simulationsof the detailed model. The modelling requirements and the accuracy versuscomputational complexity trade-off are discussed. Finally, the time-domainperformance and frequency-domain analyses reveal substantial equivalencebetween the full- and suitable reduced-order models, allowing the applicationof simplified models in large-scale system studies.

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