Fast and Reliable Primary Frequency Reserves From Refrigerators with Decentralized Stochastic Control

Due to increasing shares of renewable energy sources, more frequency reserves are required to maintain power system stability. In this paper, we present a decentralized control scheme that allows a large aggregation of refrigerators to provide Primary Frequency Control (PFC) reserves to the grid based on local frequency measurements and without communication. The control is based on stochastic switching of refrigerators depending on the frequency deviation. We develop methods to account for typical lockout constraints of compressors and increased power consumption during the startup phase. In addition, we propose a procedure to dynamically reset the thermostat temperature limits in order to provide reliable PFC reserves, as well as a corrective temperature feedback loop to build robustness to biased frequency deviations. Furthermore, we introduce an additional randomization layer in the controller to account for thermostat resolution limitations, and finally, we modify the control design to account for refrigerator door openings. Extensive simulations with actual frequency signal data and with different aggregation sizes, load characteristics, and control parameters, demonstrate that the proposed controller outperforms a relevant state-of-the-art controller.Comment: 44 pages, 17 figures, 9 Tables, submitted to IEEE Transactions on Power System

Providing Grid Services With Heat Pumps: A Review

Abstract The integration of variable and intermittent renewable energy generation into the power system is a grand challenge to our efforts to achieve a sustainable future. Flexible demand is one solution to this challenge, where the demand can be controlled to follow energy supply, rather than the conventional way of controlling energy supply to follow demand. Recent research has shown that electric building climate control systems like heat pumps can provide this demand flexibility by effectively storing energy as heat in the thermal mass of the building. While some forms of heat pump demand flexibility have been implemented in the form of peak pricing and utility demand response programs, controlling heat pumps to provide ancillary services like frequency regulation, load following, and reserve have yet to be widely implemented. In this paper, we review the recent advances and remaining challenges in controlling heat pumps to provide these grid services. This analysis includes heat pump and building modeling, control methods both for isolated heat pumps and heat pumps in aggregate, and the potential implications that this concept has on the power system

Load Shifting Versus Manual Frequency Reserve: Which One is More Appealing to Flexible Loads?

This paper investigates how a thermostatically controlled load can deliver flexibility either in form of manual frequency restoration reserves (mFRR) or load shifting, and which one is financially more appealing to such a load. A supermarket freezer is considered as a representative flexible load, and a grey-box model describing its temperature dynamics is developed using real data from a supermarket in Denmark. Taking into account price and activation uncertainties, a two-stage stochastic mixed-integer linear program is formulated to maximize the flexibility value from the freezer. For practical reasons, we propose a linear policy to determine regulating power bids, and then linearize the mFRR activation conditions through the McCormick relaxation approach. For computational ease, we develop a decomposition technique, splitting the problem to a set of smaller subproblems, one per scenario. Examined on an out-of-sample simulation based on real Danish spot and balancing market prices in 2022, load shifting shows to be more profitable than mFRR provision, but is also more consequential for temperature deviations in the freezer

Quantification of flexibility of a district heating system for the power grid

District heating systems (DHS) that generate/consume electricity are increasingly used to provide flexibility to power grids. The quantification of flexibility from a DHS is challenging due to its complex thermal dynamics and time-delay effects. This paper proposes a three-stage methodology to quantify the maximum flexibility of a DHS. The DHS is firstly decomposed into multiple parallel subsystems with simpler topological structures. The maximum flexibility of each subsystem is then formulated as an optimal control problem with time delays in state variables. Finally, the available flexibility from the original DHS is estimated by aggregating the flexibility of all subsystems. Numerical results reveal that a DHS with longer pipelines has more flexibility but using this flexibility may lead to extra actions in equipment such as the opening position adjustment of valves, in order to restore the DHS to normal states after providing flexibility. Impacts of the supply temperature of the heat producer, the heat loss coefficient of buildings and the ambient temperature on the available flexibility were quantified

Enhanced frequency response from industrial heating loads for electric power systems

Increasing penetration of renewable generation results in lower inertia of electric power systems. To maintain the system frequency, system operators have been designing innovative frequency response products. Enhanced Frequency Response (EFR) newly introduced in the UK is an example with higher technical requirements and customized specifications for assets with energy storage capability. In this paper, a method was proposed to estimate the EFR capacity of a population of industrial heating loads, bitumen tanks, and a decentralized control scheme was devised to enable them to deliver EFR. Case study was conducted using real UK frequency data and practical tank parameters. Results showed that bitumen tanks delivered high-quality service when providing service-1-type EFR, but underperformed for service-2-type EFR with much narrower deadband. Bitumen tanks performed well in both high and low frequency scenarios, and had better performance with significantly larger numbers of tanks or in months with higher power system inertia

Demand as Frequency Controlled Reserve

2011-2012 > Academic research: refereed > Publication in refereed journa

Frequency control via demand response in smart grid

In order to have a reliable microgrid (MG) system, we need to keep the frequency within an acceptable range. However, due to disturbances in a MG system (such as a sudden load change), it can experience major or minor deviations in frequency, which needs to be reduced within seconds to provide the system stability. In order to maintain the balance between energy supply and demand, traditionally, generation side controllers are utilized to stabilize the power system frequency. These systems add high operational cost, which is not desired for power system operators. With the introduction of smart grid, more and more renewable energy sources are to be used in the power system. The intermittent behavior of these energy resources, as well as high operation cost of conventional controllers, has led to research for new alternatives. In a smart grid environment, demand response (DR) programs can be considered as a promising alternative to the conventional controllers, to e ciently contribute to the frequency regulation by switching responsive loads on or o . DR programs can reduce the amount of energy reserve required and, hence, are more cost efficient. Moreover, they can act very fast and can provide a wide range of operation time from a few seconds to several minutes. Thermostatically controlled loads (TCLs) are proper candidates to participate in frequency regulation programs. However, individual TCLs do not have a noticeable impact on frequency due to small size. They should be aggregated in order to have a considerable effect on frequency. Nevertheless, there are still many challenges which should be addressed in order to make use of TCLs for frequency control in smart grid. In this regard, proper aggregated load models and control algorithms for TCLs contributing to this service need to be investigated. In this thesis, we present an aggregation model for TCLs and a control strategy to coordinate power provided from DR participants with that of generation side of the MG to keep system frequency within its desired range. For the aggregation model considered in this study, a state space model is used to take into account the interdependency of TCLs' temperature participating in DR programs. The model groups TCLs into clusters, each controlled by an aggregator. A minimum off/on period is considered for individual TCLs to avoid frequent switching of these devices. A control strategy is presented to control frequency by coordinating the generation and demand side regulation service providers. Computer simulation results show that the proposed aggregation model and control strategy can effectively control frequency under various case studies