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

TL;DR

This paper develops detailed and reduced-order dynamic models of Variable-Speed Refrigeration units to enable fast-frequency response analysis in low-inertia power systems, demonstrating their effectiveness for grid stability.

Contribution

It introduces a detailed dynamic model and a set of reduced-order models for VSR units, facilitating efficient large-scale system analysis and control design.

Findings

Reduced-order models closely match detailed model performance.

VSR units provide significant fast-frequency response.

Simplified models enable scalable system stability studies.

Abstract

In modern power systems, shiftable loads contribute to the flexibility needed to increase robustness and ensure security. Thermal loads are among the most promising candidates for providing such service due to the large thermal storage time constants. This paper demonstrates the use of Variable-Speed Refrigeration (VSR) technology, based on brushless DC motors, for fast-frequency response. First, we derive a detailed dynamic model of a single-phase VSR unit suitable for time-domain and small-signal stability analysis in low-inertia systems. For analysing dynamic interactions with the grid, we consider the aggregated response of multiple devices. However, the high computational cost involved in analysing large-scale systems leads to the need for reduced-order models. Thus, a set of reduced-order models is derived though transfer function fitting of data obtained from time-domain simulations of the detailed model. The modelling requirements and the accuracy versus computational complexity trade-off are discussed. Finally, the time-domain performance and frequency-domain analyses reveal substantial equivalence between the full- and suitable reduced-order models, allowing the application of simplified models in large-scale system studies.