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.