Virtual energy storage for frequency and voltage control

The secure and economic operation of the future power system is facing major challenges. These challenges are driven by the increase of the penetration of converter connected and distributed renewable generation and electrified demand. In this thesis, a new smart energy management paradigm, i.e. a Virtual Energy Storage System (VESS), to address these challenges was studied. A VESS aggregates energy storage and flexible demand units into a single entity which performs similarly to a large-capacity conventional energy storage system. A VESS mitigates the uncertainty of the response from flexible demand through coordination with a minimum capacity of costly energy storage systems. Mathematical models of four components of a VESS were developed. Specifically, models of two types of energy storage, i.e. flywheel energy storage and battery energy storage, were developed. Thermodynamic models of two types of flexible demand units, i.e. domestic refrigerators and industrial bitumen tanks, were developed. These models were validated against the performance of similar equipment from the literature. Aggregated models, representing a population of units, for each of flywheels, batteries, refrigerators and bitumen tanks were developed. These aggregated models represent a randomly diversified population of units. These aggregated models were used to establish the frequency and the voltage control schemes of a VESS. A frequency control scheme of the VESS was designed. The control scheme provides low, high and continuous frequency response services to the system operator. The centralised control scheme coordinates models of refrigerators and units of the flywheel energy storage system. Following frequency deviations, the local frequency controllers of refrigerators changed their power consumption. The local frequency controllers of the flywheel units cover the power mismatch between the change in refrigerators power consumption and the required response from the VESS. The required response from the VESS is determined by a droop control. Case studies were conducted to evaluate the frequency control scheme by connecting the VESS to a simplified GB power system. Results showed that the response from the frequency control scheme of the VESS was similar to that of only flywheel energy storage. Based on an economic evaluation, the VESS is estimated to obtain approximately 50% higher return compared with the case II that only uses flywheel energy storage system. These revenues are based on providing frequency response services to the system operator. A voltage control scheme of the VESS was also designed. The control scheme facilitates the integration of distributed renewable energy generation by enhancing the voltage control of the distribution network. The control scheme coordinates models of bitumen tanks and battery energy storage system through different time delay settings of their voltage controllers. The local voltage controllers of bitumen tanks alter their power consumption following significant voltage deviations. If voltage violations continue, the distributed voltage controller of the battery energy storage system charges or discharges the battery using a droop setting obtained from voltage sensitivity factors. A case study was undertaken to assess the voltage control scheme by connecting the VESS, solar panels and wind farms to a UK Generic Distribution System (UKGDS) network. Results showed that the voltage control scheme made a significant improvement in the voltage and reduced tap changing actions of the on-load tap changing transformer and the voltage regulator by approximately 30 % compared with the base case where no VESS was used. Based on an economic evaluation, The VESS is an efficient solution to accommodate distributed renewable energy generation compared with network reinforcement

Benefits of using virtual energy storage system for power system frequency response

This paper forms a Virtual Energy Storage System (VESS) and validates that VESS is an innovative and cost-effective way to provide the function of conventional Energy Storage Systems (ESSs) through the utilization of the present network assets represented by the flexible demand. The VESS is a solution to convert to a low carbon power system and in this paper, is modelled to store and release energy in response to regulation signals by coordinating the Demand Response (DR) from domestic refrigerators in a city and the response from conventional Flywheel Energy Storage Systems (FESSs). The coordination aims to mitigate the impact of uncertainties of DR and to reduce the capacity of the costly FESS. The VESS is integrated with the power system to provide the frequency response service, which contributes to the reduction of carbon emissions through the replacement of spinning reserve capacity of fossil-fuel generators. Case studies were carried out to validate and quantify the capability of VESS to vary the stored energy in response to grid frequency. Economic benefits of using VESS for frequency response services were estimated

Virtual Energy Storage System for Smart Grids

AbstractThis paper forms a Virtual Energy Storage System (VESS) and validates that VESS is a cost-effective way to provide the function of energy storage through the utilization of the present network assets represented by flexible demand. As a solution to convert to low carbon cities, a VESS is firstly modelled to store and release energy in response to regulation signals by coordinating the demand response (DR) from domestic refrigerators in London and the conventional flywheel energy storage systems (FESS). The coordination of DR and FESS mitigates the uncertainties of DR and reduces the capacity of costly FESS. The VESS is applied to provide ancillary services to the power system and contributes to the reduction of carbon emission through the replacement of spinning reserve capacity of fossil fuel generators. Case studies were carried out to validate and quantify the capability of the VESS to vary the stored energy in response to grid frequency. Economic benefits of using VESS for frequency response services were firstly estimated and a potential saving of £91m-£103m is expected

A Modified Strength Pareto Evolutionary Algorithm 2 based Environmental /Economic Power Dispatch

A Strength Pareto Evolutionary Algorithm 2 (SPEA 2) approach for solving the multi-objective Environmental / Economic Power Dispatch (EEPD) problem is presented in this paper. In the past fuel cost consumption minimization was the aim (a single objective function) of economic power dispatch problem. Since the clean air act amendments have been applied to reduce SO2 and NOX emissions from power plants, the utilities change their strategies in order to reduce pollution and atmospheric emission as well, adding emission minimization as other objective function made economic power dispatch (EPD) a multi-objective problem having conflicting objectives. SPEA2 is the improved version of SPEA with better fitness assignment, density estimation, and modified archive truncation. In addition fuzzy set theory is employed to extract the best compromise solution. Several optimization run of the proposed method are carried out on 3-units system and 6-units standard IEEE 30-bus test system. The results demonstrate the capabilities of the proposed method to generate well-distributed Pareto-optimal non-dominated feasible solutions in single run. The comparison with other multi-objective methods demonstrates the superiority of the proposed method

Modelling and control of multi-type grid-scale energy storage for power system frequency response

As a result of the increasing integration of renewable energy sources, power system is changing to a low inertia system with intermittent power supply. Frequency stability is therefore difficult to be maintained. Rather than increasing the spinning reserve capacity from conventional fossil-fuel generators, the use of Energy Storage System (ESS) for frequency response is considered as a technically viable low-carbon solution. To facilitate the grid-level study which aggregates a number of wide-spread small-size ESS, simplified models of multi-type ESSs including batteries and flywheels were developed. A generalized frequency controller was developed and applied to the aforementioned types of ESS. The controller coordinates the response amongst a population of ESS based on the units' State of Charge indicator. An adaptive droop control is combined with the coordinated control to guarantee a linear frequency response provided by a smaller number of ESS units. The number of charging and discharging of each unit is therefore reduced which prolongs the lifetime of the ESS units. Case studies were carried out by connecting a number of multi-type ESSs to a simplified GB power system model. Results show that the grid-scale ESSs are able to provide frequency response similar to but faster than frequency-sensitive generators. Implementation of ESS is therefore technically feasible to support the grid frequency stability with the reduction in the spinning reserve capacity

Virtual energy storage system for voltage control of distribution networks

—Increasing amount of Distributed Generation (DG) connected to distribution networks may lead to the voltage and thermal limits violation. This paper proposes a Virtual Energy Storage System (VESS) to provide voltage control in distribution networks in order to accommodate more DG. A VESS control scheme coordinating the demand response and the energy storage system was developed. The demand response control measures the voltage of the connected bus and changes the power consumption of the demand to eliminate voltage violations. The response of energy storage systems was used to compensate for the uncertainty of demand response. The voltage control of energy storage system is a droop control with droop gain values determined by voltage sensitivity factors. The control strategy of the VESS was applied to a medium-voltage network and results show that the control of VESS not only facilitates the accommodation of higher DG capacity in the distribution network without voltage violations or network reinforcements but also prolongs the lifetime of transformer on-load tap changer

Control of a population of battery energy storage systems for frequency response

The control of multiple battery energy storage systems (BESSs) to provide frequency response will be a challenge in future smart grids. This paper proposes a hierarchical control of BESSs with two decision layers: the aggregator layer and the BESS control layer. The aggregator layer receives the states of charge (SoC) of BESSs and sends a command signal to enable/disable the BESS control layer. The BESS controller was developed to enable the BESSs to respond from the highest to lowest SoC when the frequency drops, and from lowest to highest when it rises. Hence, the BESS’s response is prioritised to reduce the impact on the power system and end-users during the service. The BESS controller works independently when a failure occurs in the communication with the aggregator. The dynamic behaviour of the population of the controllable BESSs was modelled based on a Markov chain. The model demonstrates the value of aggregation of BESSs for providing frequency response and evaluates the effective capacity of the service. The model was demonstrated on the 14-machine South-East Australian power system with a 14.5 GW load. 254 MW of responsive capacity of aggregated batteries was effective in reducing the system frequency deviation below 0.2 Hz following a sequence of disturbances

Potential of demand side response aggregation for the stabilization of the grids frequency

The role of ancillary services related to the frequency control have become increasingly important in the smart grids. Demand Side Response is a competitive resource that can be used to regulate the grid frequency. This paper describes the use of heat pumps and fridges to provide ancillary services of frequency response so that to continuously balance the supply with demand. The power consumption of domestic units is usually small and, therefore, the aggregation of large numbers of small units should be able to provide sufficient capacity for frequency response. In this research, dynamic frequency control was developed to evaluate the capacity that can be gathered from the aggregation of domestic heat pumps and fridges for frequency response. The potential of frequency response was estimated at a particular time during winter and summer days. We also investigated the relationship between both loads (domestic heat pumps and fridges) to provide Firm Frequency Response service. A case study on the simplified Great Britain power system model was developed. Based on this case study, three scenarios of load combination were simulated according to the availability of the load and considering cost savings. It was demonstrated that the aggregation of heat pumps and fridges offered large power capacity and, therefore, an instantaneous frequency response service was achievable. Finally, the economic benefit of using an aggregated load for Firm Frequency Response service was estimated