With the growth of electricity demand increasing year by year, the problem of low load factor (Load Factor) generally exists in the distribution network of large load centers. For example, during the day, there is a big difference in the need for electricity between day and night. Newly built power plants and transmission systems that can support the peak demand of the load are not ideal in terms of cost or operating efficiency. The electric energy storage system using high-energy density and high-efficiency battery packs can reduce this time-varying energy demand fluctuation, thereby improving the operating efficiency of the power station, reducing operating costs, and achieving energy storage.
PCS controls the working mode of the battery energy storage system, so that it can absorb electric energy into the battery at night when the power consumption is low at night, and release the electric energy during the daytime peak power consumption, so as to reduce the impact on the grid. Power requirements, applied to the user side, can also save users’ electricity bills. According to the level of energy provided by the battery to the load, it can be divided into load leveling (LL) mode and peak shaving (PS) mode. According to the ultimate goal of the actual distribution network, which operation mode is actually selected in PS and LL, and then the relationship between the sodium-sulfur battery energy storage capacity and the typical load power fluctuation range of the distribution network can be determined.
Among them, the rated output power Pbs of the battery adopting the LL mode should meet: Pbs≥KmaxPI, where PD is the power capacity of the distribution network, and Kmax is the load power fluctuation range.
The battery energy storage system uses a two-way power converter to realize the two-way flow of energy between the battery and the grid. In the PS operation mode, it can be divided into 3 operation stages:
1) Charging the battery: In this mode, the grid provides energy for the battery pack and load. The charging start time is usually selected after the peak power consumption, and the end charging time is selected before the peak power consumption the next day. The charging time is selected during the low power consumption period after 11:00 in the evening until 7:00 in the next morning (the specific time can be set according to the actual operation of the power grid). During this period, the PCS works in the rectification state, which is equivalent to a For high-power chargers, PCS controls the charging method according to the charging characteristics of the battery to make it comply with relevant industrial standards. When it has the power quality (PQ) control function at the same time, an auxiliary charging stage can be added to optimize the PQ capacity provided.
2) Standby: This mode actually has two parts, one is the period from the end of charging to the beginning of the PS phase, and the other is the period from the end of PS to the beginning of the next charging phase. When in the standby phase, the PCS blocks the control pulse signals of the converter, the battery energy storage system does not participate in the energy exchange with the outside world, the electric energy only flows from the grid side to the load, and the system remains offline until the next working phase arrives.
3) Battery discharge stage (Peak Shaving): In this mode, the PCS controls the energy storage system to realize the PS function, and the battery discharges the load. The start and end time of the discharge can be selected and set by the user, which is generally realized through a local man-machine interface or a remote monitoring system. PCS requires to control the inverter output of the converter so that the output voltage keeps track of the grid-side voltage to achieve stable power output. The PCS of this mode has the characteristics of distributed grid-connected power generation, so its specifications should meet a series of grid-connected operation standards. In order to achieve an effective PS effect in response to load changes, the PCS output power status can be controlled through man-machine interface and other methods.