Battery energy storage systems for the electricity grid: UK research facilities

Grid-connected battery energy storage systems with fast acting control are a key technology for improving power network stability and increasing the penetration of renewable generation. This paper describes two battery energy storage research facilities connected to the UK electricity grid. Their performance is detailed, along with hardware results, and a number of grid support services are demonstrated, again with results presented. The facility operated by The University of Manchester is rated at 236kVA, 180kWh, and connected to the 400V campus power network, The University of Sheffield operates a 2MVA, 1MWh facility connected to an 11kV distribution network

Balancing control for grid-scale battery energy storage systems

Grid-scale battery energy storage systems (BESSs) are becoming increasingly attractive as the connection of a BESS has been shown to improve the dynamic behaviours of the power grid. A key problem with BESSs is the potential for poor utilisation of mismatched cells and reliability issues resulting from the use of a large number of cells in series. This thesis proposes a technique for state-of-charge balancing of many thousands of cells individually (i.e. not in packs) using a tightly integrated power electronic circuit coupled with a new control system design. Cells are organised in a hierarchical structure consisting of modules, sub-banks, banks and phases. The control strategy includes five levels of balancing: balancing of cells within a module, balancing of modules within a sub-bank, sub-banks within a bank, banks in a phase and balancing between phases. The system seeks to maximise the accessible state-of-charge range of each individual cell, thereby enhancing the overall capacity of the system. The system is validated in simulation for a 380 kWh BESS using 2835 lithium-ion cells where charge balancing is demonstrated for mismatched cells. A ‘peak sharing’ concept is implemented to manage voltage constraints so that alternative modules assume a portion of the load when certain modules are not capable of meeting the demand. An experimental validation has been performed to demonstrate the effectiveness of the balancing control. This work is intended to address the challenges of eventual scaling towards a 100 MWh+ BESS, which may be composed of hundreds of thousands of individual cells