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This article mainly studies three different schemes for lithium batteries to be connected to the distribution network and performs related calculations and selections.
This article mainly studies three different schemes for lithium batteries to be connected to the distribution network and performs related calculations and selections. In addition, the constant current and constant voltage control strategies of lithium batteries in charging mode, the V/f control double closed-loop control strategy, and switching control strategy in discharging mode are studied.
Finally, the transient process of the grid after the energy storage system is connected to the grid is studied. The simulation analysis is carried out, which has a certain reference value for the design of the distribution system of important users.
System main wiring and load configuration
The design objective of this energy storage power supply scheme is mainly for important users, so this paper chooses to connect the lithium battery energy storage system to a data center.
For important users, the primary and secondary loads account for a large proportion, and the corresponding tertiary loads account for a relatively small proportion. When setting up the lithium battery energy storage access system, it can be considered that the system will automatically disconnect from the tertiary load after the system loses power.
The connected switch and the energy storage system only need to ensure the power supply reliability of the relatively important primary and secondary loads.
The important user power provided by the lithium battery energy storage system in this paper is 500 kW in total. Therefore, the actual capacity of the lithium battery energy storage system is calculated according to the active power of the primary and secondary total loads of 400 kW for the calculation of the battery capacity and the overall design of the energy storage system.
The choice of the energy storage battery
The structure of the lithium battery energy storage system is basically composed of one or more energy storage units. The battery stack (BP), the energy storage inverter (PCS), and the battery management unit (BMS) constitute one energy storage unit. An energy storage branch is composed of an energy storage unit and a low-voltage access switch.
Generally, for large-capacity megawatt-level energy storage systems, considering the characteristics of economy and operation and maintenance, the capacity of a single energy storage unit is generally set to 1/2 or 1/4 of the total load power, and then according to the capacity of a single energy storage unit.
The capacity is equipped with the corresponding battery management unit and energy storage inverter. Because the electricity requirement of 400 kW/400 kWh belongs to the small-capacity energy storage system, 400 kW can be connected separately and divided into two 200 kW energy storage units connected to the low-voltage bus.
Selection of battery energy storage system access scheme
Considering the voltage level of the busbar and the power supply requirements for important user loads, the low-voltage 400 V side and the high-voltage 10 KV side of the main wiring of the system are both single-bus segmented wiring and combined with the configuration of its active load power, the following three lithium Access to the battery energy storage system.
The first plan is to connect the lithium battery energy storage system to the 10 KV busbar (section II) on the standby power supply side through a 10 KV/0.6 KV transformer, and equip with corresponding circuit breakers and isolating switches, and at the same time configure the corresponding 10 KV bus tie for automatic backup.
The protection measurement monitoring device and the transformer protection device are used to ensure secondary coordination with the original system.
The second plan is to connect all lithium battery parts as a whole energy storage system to the 400 V busbar (section II) on the standby power supply side, equipped with corresponding low-voltage switches, and equipped with corresponding 400 V low-voltage bus-connected backup self-switching protection equipment, and the rest of the wiring constant.
In scheme 3, the lithium battery energy storage system is divided into two parts according to its capacity and connected to two sections of 400 V bus, respectively, and equipped with two sets of switches and secondary equipment of the same type as scheme 2.
Scheme 1 For the energy storage system connected to the 10 KV voltage level grid, due to its high access voltage level, it is easy to cause problems of protection configuration and insulation, which threatens the power supply reliability of important users.
And because it has a great impact on the quality of the power grid, it is stipulated that the data of its power quality should be able to be transmitted remotely to meet the requirements of power grid enterprises for power quality detection, so the requirements for the wiring of the energy storage system are relatively high; when the energy storage battery when connecting to the 10 KV high-voltage side, because the rated voltage of the energy storage battery is relatively low, it must be connected to the 10 KV high-voltage bus through a step-up transformer, so the equipment investment is large.
In scheme 2, because the energy storage system connected to the 400 V voltage level has a small capacity and can store the electric energy for the corresponding time for the use of important users, the reliability of the power supply is high. Its rated voltage is the same as the bus voltage, so no transformer is needed, which reduces the equipment investment cost and is beneficial to the maintenance of the energy storage power supply by the staff in the later stage.
Although the third scheme divides the energy storage system into two parts and connects to two sections of 400 V busbars, which further improves the reliability of the system, the demand for the protection devices of the energy storage system is large, and the investment is relatively large, which causes the later maintenance staff to overhaul and maintenance workload.
Taking into account factors such as power supply reliability, economy, installation, operation, and maintenance, it is more reasonable to use option three as the access option. If you want to know more about lithium battery energy storage systems after reading the above, RENON is happy to share relevant knowledge with you.
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