Demonstration of a Large-Scale Energy Storage System for Peak Shaving in the Electric Power System

Abstract

The penetration level of renewable energy (RE) resources used to supply electricity is expected to increase rapidly in the coming years. This growth is strongly encouraged by the government through various policies such as renewable portfolio standards and feed-in tariffs. [1]However, because the performance of a power system is critically constrained by many factors that impact its stability, having a high penetration of RE resources with an unpredictable nature in bulk power systems is very challenging for system operators. [2]To address the stability issues of RE, it has been proposed that large-scale energy storage systems (ESS) be applied to bulk power systems. ESS can be controlled to supply quickly the exact amount of power required in securing the stability of the power system, even with a very high penetration of RE resources that can negatively impact the dynamic performance of the power system. In addition, ESS’s can also be used to provide ancillary services, such as frequency regulation, and bulk energy services, such as peak-shaving and load-leveling, in power systems. Load-leveling and peak-shaving are ESS applications that provide long-term services, wherein the charging and discharging of power takes place within several hours. These services involve charging the ESS during periods when the loads of electric power are low and discharging the stored energy to the power system when the loads are high. [3]In this work, a demonstration of a large-scale, grid-connected 4 MW/8 MWh battery ESS (BESS) using lithium-ion batteries (LiB), has been implemented. The 4 MW/8 MWh BESS is simultaneously connected to a 22.9 kV substation bus and distribution line in the Jocheon substation on Jeju Island. Operation strategies of the BESS have also been developed for peak-shaving (or electric energy time-shift). The peak-shaving operation uses the expected load for the next day. Also, the effects of peak-shaving are also analyzed in this work.In this work, 60Ah(Ampere hour) LiB cell was employed, and 16 cells were assembled to 1.8kWh-class module. A tray consisted of two modules, and one rack was installed using 16 battery trays, a rack BMS(Battery Management System), and a switch gear. One container consisted of 18 racks connected in parallel which summed up to 1MWh capacity. The developed energy storage system was based on a technology of small lithium rechargeable battery. The BESS consisted of three-stage control system as Tray, Rack, and System. The smallest component of BESS is battery cell. The rated capacity of battery cell is 60Ah and 3.7V. BESS is available to change battery capacity by connecting the battery cells in series or parallel. The 1MWh LiB system consisted of eighteen battery racks in parallel. BMS is responsible for monitoring the individual control and protection functions for the entire circuit unit cells. The system BMS is in charge of calculating a state of charge, state of health, power prediction, and internal impedance of the system. This can support the communication protocols of CAN 2.08, RS-485, and MODbus-TCP/IP. The rack BMS is used for monitoring the voltage and the current of the rack and calculating a state of charge, state of health, power prediction, and internal impedance of the rack. In addition, this in charge of the switching control and the cell balancing in the rack. Finally, the tray BMS can measure the voltage and the temperature of each cell, and control the cell balancing in the tray. The ESS of a 4MW PCS and 8MWh batteries installed in Jocheon substation, Jeju Island. There are four 1MW PCS configured to have a total capacity of 4MW. Each 1MW PCS is connected to two containers of 1MWh batteries with a total of 2MWh capacity paired in one PCS. This makes the discharge duration of 2 hours. In addition, the BESS may be charged from the grid and discharge power to the grid thru the PCS as dictated by the PMS. Using a large scale energy storage device can give numerous benefits such as load factor improvement, peak shaving and load leveling, improve quality of distributed renewable energy, support emergency power supply, and offer high-quality power service to customers. The introduction of a large scale energy storage system to the grid can enhance the smart grid to respond efficiently to the demand of electricity from consumers with real-time power output control. Fig. 1 shows the original load and shaved one by ESS operation. From ESS operation for peak shaving, the 5.5 percent and 4.8 percent of peak shaving could be obtained in winter and summer peak time respectively. Figure 1