Thermostatically Controllable Loads for Primary Frequency Control in Isolated Microgrids

Abstract

In the last few decades, governments, private investors, and non-governmental organizations have gradually incentivized the integration of Renewable Energy Sources (RESs) and efficient Energy Storage System (ESS) technologies into remote microgrids, in good part because of commitments to counter the harmful effects of greenhouse gas emissions and to reduce dependence on fossil fuels. As a result, hybrid RESs-diesel systems are now being considered as an economic, attractive and reliable option to improve remote microgrids and to offset diesel consumption in isolated communities by displacing generation from conventional units; however, system security and stability is a challenge as the penetration of RESs increases. In this context, it is necessary to explore new mechanisms and control strategies for the provision of frequency support services in order to meet the increased ramp and capacity requirements to integrate RESs into the system effectively. From this standpoint, Demand Response (DR) can be used to increase grid flexibility, improve efficiency, and facilitate the penetration of RESs, by controlling loads in response to predefined control strategies, manipulating the demand profile to provide a service to the system. In particular, DR strategies are well-suited for northern remote communities, where the demand for electricity and heating is much higher than the Canadian residential average consumption due to the harsh climate and poor building energy efficiency. Thermostatically Controllable Loads (TCLs), i.e., Electric Water Heaters (EWHs), Air Conditioners (ACs), and Ground Source Heat Pumps (GSHPs), are ideal candidates to participate in a DR strategy, since they have a high thermal inertia, high-power consumption (among the appliances in a household), and are continuously operating over the day. Therefore, their power consumption can be shifted, or controlled by switching operations without significantly affecting consumer comfort. This is due to the thermal inertia of TCLs, which allow a smooth temperature variation that, in many cases, would be unnoticed by customers. Thus, TCLs provide energy storage capabilities, which are more significant with GSHP systems, and hence, a DR control strategy can use TCLs as thermal energy batteries with similar features than other ESS technologies. Therefore, the research presented in this thesis focus on developing a DR strategy for the provision of primary frequency control in hybrid isolated microgrids using TCLs, i.e., EWHs, ACs, and GSHPs.

A comprehensive review of the components, operation, and restrictions of these TCLs is performed in order to develop computationally efficient, simple, and accurate models, which are able to capture the relevant thermodynamic phenomena in dynamic studies. Based on the thermo-electrical characteristics of the developed models, a decentralized DR strategy is designed, which uses local frequency measurements to control the power consumption of the TCLs according to system frequency deviations while considering consumer comfort. The control logic includes ON/OFF commands to modulate the duty cycle of the TCLs to provide primary frequency control and facilitate the integration of RESs.

The proposed DR strategy along with the developed thermo-electrical models of TCLs, i.e., EWHs, ACs, and GSHPs, are evaluated using a benchmark hybrid microgrid, which has a significant share of PV generation. In order to resemble realistic conditions, power and hot water consumption simulators are used to generate random demand profiles, while considering a droop control for the diesel genset and a solar PV with a Maximum Power Point Tracking (MPPT) control. Different study cases are conducted to analyze the system frequency response and determine the adequacy of the proposed DR strategy, demonstrating the effectiveness of the proposed TCL controls to provide primary frequency control, and thus facilitate higher penetration of RESs, while reducing costs and maximizing fuel savings.