Balancing control for grid-scale battery energy storage system

Abstract

Ampere-hour (Ah) efficiency: The quantity of electricity measured in Ampere-hours which may be delivered by a cell or battery under specified conditions. Ampere-hour capacity: The total number of Ampere-hours or watt-hours that can be withdrawn from a fully charged cell, indicated by Ah or mAh. Battery: Two or more electrochemical cells connected together electrically in series, parallel, or both, to provide the required operating voltage and current levels. Crate: Charge or discharge current, in Ampere, expressed in multiples of the rated capacity. For example, C/10 charge current for a cell rated at 20 Ah is: 20 Ah/10 = 2 A. Capacity: See Ampere-hour capacity. Cell: The smallest electrochemical unit of a battery used to generate or store electrical energy. Coulombic efficiency: See Ampere-hour efficiency. Cutoff voltage: The cell voltage at which the discharge process is terminated (it is generally a function of discharge rate). Cycle life: The number of times a cell can be discharged and recharged until the cell capacity drops to a specified minimum value usually 80 % of rated capacity. Depth of discharge: The quantity of electricity (Ampere-hours) removed from a fully charged cell, expressed as a percentage of its rated Ampere-hour capacity. Energy density: The ratio of the energy available from a cell to its volume (Wh/L) or mass (Wh/kg). xxi Internal resistance: Expressed in ohms, the total DC resistance to the flow of current through internal components (grids, active materials, separators, electrolyte, straps, and terminal) of a cell. Module: The smallest modular unit, consisting of a number of individual cells connected together electrically in series, parallel, or both. Nominal voltage: The average voltage of the cell. The operating voltage of the system may go above or below this value. Open circuit voltage (OCV): The difference in potential between the terminals of a cell when no load is applied. Pack: Two or more modules connected in series, parallel or both. Power density: The ratio of the available power from a cell to its volume (W / L). Round-trip efficiency: The ratio of energy put in (in MWh) to energy retrieved from storage (in MWh). Self-discharge: The loss of useful capacity of a cell on storage due to internal chemical action (local action) and parasitic currents. State-of-charge (SoC): The present cell capacity in relation to maximum capacity. Terminal voltage: The difference in potential between the terminals of a cell when a load is applied.

Cell SoC Balancing Using a Cascaded Full-Bridge Multilevel Converter in Battery Energy Storage Systems

“…In the cascaded multilevel converter, an output voltage close to a sinusoidal reference is produced by switching on and off the converter modules to create a stepped output voltage. In Fig In [20] a balancing strategy is presented where the above property of the multilevel converter is used in order to enforce different cell currents by controlling the position of each cell in the priority list L, referred to as cell position. A voltage matching algorithm is then used in order to achieve a converter output voltage close to the sinusoidal reference by switching on the appropriate number of cells, according to the priority list.…”

“…The key challenges are circuit complexity and management of power losses in the converter. The control complexity of such a system is addressed in [20] where the balancing of 2835 cells is achieved by using a hierarchical control strategy. In the CHB multilevel circuit switching losses are essentially zero because the MOSFETs are switched at only twice the fundamental frequency.…”

Hierarchical Distributed Balancing Control for Large-Scale Reconfigurable AC Battery Packs

“…A modular BMS following a hierarchical control structure such as that presented in [20] goes some way to addressing this problem. Here, the elements of the CHB are organised conceptually into control layers, labelled banks, modules and elements.…”

“…In [20], balancing of an intermediate layer is performed by distributing the voltage reference for each object of the layer in proportion to that object's SoC deviation from the average layer SoC. A disadvantage of this method is that once balancing is achieved, the voltage reference is equally distributed to the layers such that multiple H-bridges are switched simultaneously.…”

Editorial: Energy storage

“…Liquid air energy storage is one of a number of potentially exciting technologies being developed for large-scale storage. A final paper, by Ooi et al (2015), describes a study of the connection and balancing of very large numbers of lithium-ion batteries. A key problem of a number of battery technologies is that the capacity of each individual cell is small and so means must be found of combining them in such a way that failure or degradation of one cell does not result in poor performance of the entire system.…”