Frequency-Domain Equivalent Model of District Cooling Systems for Primary Frequency Regulation

With increasing adoption of intermittent renewable generation, challenges to maintain stable frequencies for power systems are increasing. Demand-side resources, such as thermo-statically controlled loads (TCLs), have been proven capable of providing regulation services. The district cooling system (DCS), a type of centralized TCL that provides cooling services for a group of buildings, is an ideal resource for this purpose. A DCS usually has significant regulation flexibility because its rated power capacity is large (up to 100 MW) and it can utilize thermal inertia of an aggregation of buildings. However, as a large-scale system with complex thermal dynamics, its effective regulation is nontrivial, and its traditional demand-driven control scheme has difficulty considering regulation signals from power systems. To address these challenges, we propose a bidirectional-driven control scheme in a DCS for the first time. On this basis, we formulate the thermal and electricity model of a DCS in the frequency domain and derive an equivalent model to control it like a traditional generator. Furthermore, we propose a sensitivity-based control strategy for a DCS to allocate mass flow adjustments among buildings, which effectively considers heterogeneity of buildings to balance temperature impacts. Numerical studies illustrate effectiveness of the proposed model and control strategy.