DEVELOPMENT OF CURRENT SHARING STRATEGIES FOR CONTROLLED CHARGING AND DISCHARGING OF MULTIPLE BATTERIES

Abstract

Rising environmental issues due to the burning of fossil fuels and penetration of new loads such as electric vehicles, requisite an increase in the share of renewable energy sources (RES) in the grid. But their increased penetration into the grid presents various challenges in the grid such as fluctuations in the voltage and frequency, and stability issues due to their intermittent nature. Energy storage systems(ESSs) are promising solutions for mitigating the impact of new loads and the intermittent nature of renewable energy sources. Lead acid batteries, being the most matured battery technology are widely used as storage systems due to their low cost. With the advancement in technology and reducing cost, there are different battery technologies (such as Li-ion, Redox, and Sodium sulphur) available in the market. Thus, the storage systems could be a combination of different battery technology. To ensure reliable operation in such systems batteries needs to be charged and discharged in a controlled manner. Thus, formulation of the current sharing strategy is required for controlled charging and discharging of the batteries. In this work, the current sharing strategy based on State of Charge(SoC), generation availability & load demand, and droop control have been studied. In SoC based current sharing, the instantaneous value of SoCs, determine the currents to be dispatched through the batteries. Based on day ahead forecasted generation and load demand data, currents to be dispatched through the batteries can be determined. In case of current sharing strategy using droop control, SoC based and Incremental Cost(IC) based schemes have been studied. In SoC based and IC based droop control schemes, circulating current flows between the batteries. Thus, in order to overcome this problem an improved control scheme has been formulated and studied. In order to compare SoC and IC based schemes from the economic perspective, the total degradation cost has been calculated and analyzed for both schemes.