Power system frequency control using battery energy storage systems

The penetration of Renewable Energy (RE) sources (e.g., solar, wind) into the power system occurs via the replacement of conventional synchronous generation sources. As a result, power systems are becoming more dynamic, owing to system inertia loss. In case of any imbalance between generation and load, the Rate-of-Change of Frequency (RoCoF) is significantly higher in systems with low-inertia. A higher RoCoF may, in turn, be more likely to result in a violation of a system‚ÄövÑv¥s frequency operating bounds, resulting in under-frequency load-shedding or over-frequency generation-shedding, or even may lead to a system collapse. Therefore, frequency control in high penetration of RE systems needs to be faster than systems with higher inertia. Energy storage systems, such as flywheels, pumped hydro storage systems, compressed air energy storage, Battery Energy Storage Systems (BESS), and supercapacitors, can potentially be used to provide a rapid injection of power into the system via Primary Frequency Control (PFC) to balance between generation and load. This results in mitigating higher RoCoF and avoiding power systems collapse. This thesis investigates the opportunity of BESS for PFC applications in power systems with high RE penetrations. A comparison of power system frequency response is conducted for a simple modelled power system with PFC provided either by Synchronous Generators (SG) or BESS. Mathematical models of conventional governor and turbine are developed, representing conventional SG frequency control, and are used to illustrate system frequency response for a range of typical conventional generating units. A mathematical model of a power-electronics interfaced Li-ion BESS is developed and used to represent a non-synchronous inverter-based generation with PFC capabilities. The simulation results demonstrate that the BESS can be used for PFC providing a faster and better response. BESS is also capable of almost eliminating frequency overshoot and reducing 70% settling time while providing PFC in power system with various types of conventional synchronous generating units and a nonsynchronous generating unit. However, the requirements of PFC arrangements change with RE penetrations in power systems due to low system inertia and characteristics of primary frequency response. Recognising these facts, detailed simulation case studies have been carried out to investigate the application of BESS for PFC in power systems with very high penetration of renewable energy, and consequently, low levels of synchronous generation. By re-creating a major Australian power system separation event and then subsequently simulating the event under low inertia conditions but with BESS providing frequency support, it has been demonstrated that a droop-controlled BESS can significantly improve frequency response, producing both faster reaction and smaller frequency deviation. Furthermore, it is shown via detailed investigation how factors such as available battery capacity and droop values impact the system frequency response characteristics, providing guidance on how best to mitigate the impact of future SG retirements. It is intended that this analysis could be beneficial in determining the optimal BESS capacity and droop value to manage the potential frequency instability risks for future power systems with high RE penetrations. The impact of BESS control strategies and response parameters on PFC performances in power systems with high RE penetration is investigated. MATLAB/Simulink is used to build a simplified Australian interconnected power system model, and simulations are carried out at various RE penetrations in the power system. Simulation results show that the response parameters of Grid-following (GFL) BESS, such as frequency dead band and power response time constant, significantly influence the dynamic performance and stability characteristics difference between GFL and Grid-forming (GFM) BESS. By appropriately selecting the GFL and GFMBESS response parameters and required BESS capacity to maintain frequency within normal operating limits and the dynamic performance and stability characteristics differences can be reduced. Compared to GFL, using GFM would still be beneficial for low-inertia power systems, as using GFM based solution can reduce the BESS capacity required to maintain frequency stability by providing additional damping to the system.