Design and Application of Station Power Supply System for Lithium Iron Phosphate Battery Based on Power Exchange Operation

Based on the engineering application design and development of the power supply system of lithium iron phosphate battery pack in the operation and maintenance mode, this paper conducts the application research from four aspects of battery quantity selection, capacity calculation selection, battery management system design, battery pack modular design, etc. The design scheme of the lithium iron phosphate power supply system is formulated, and the matching battery management system is designed. A universal lithium iron phosphate battery module with an “N+1” redundant configuration is developed to improve the maintenance efficiency of the battery pack. It reliably supports the substation power changing operation and maintenance mode and ensures the reliability and operation safety of the substation power supply system.


Introduction
At present, in substations, power plants, and other large power projects, the quality of the DC system has an important impact on the safe operation of the power grid.The DC system of the station is generally composed of a battery group, DC charging device (rectifier device), and a DC feeder screen, to ensure the reliable power supply of the DC system and the uninterrupted DC power supply in the case of accidents.The battery group is the only reliable DC power supply in the substation under the condition of an AC power failure accident.Its operation state determines whether the accident can be accurately removed, whether it can prevent the accident expansion, etc., and the battery pack is the last line of defense for the stable operation of the substation [1][2].
The DC power supply system of the substation mostly uses a valve-controlled sealed lead-acid battery [3][4], whose working current range is small, and heavy handling and maintenance are inconvenient.Hidden defects such as plate corrosion and electrolyte drying are difficult to find, and the single open circuit problem is prominent, which is a major safety hazard of the power grid.Lithium iron phosphate battery has a long life and good temperature characteristics.With the improvement of the battery process, market capacity, and battery quality, it is regarded as an ideal replacement for lead-acid batteries.Some experts and scholars have also done a lot of work in the application of lithium iron phosphate batteries in the power industry and carried out feasibility verification [5][6][7].
This paper focuses on the design and development of lithium iron phosphate batteries.From the perspective of basic theoretical research and engineering application, based on the requirements of the DC system of different voltage levels, the work from four aspects: Research on the characteristics of lithium iron phosphate battery, battery management system (BMS) design, capacity and wiring mode design of lithium iron phosphate battery, research and development of battery pack and supporting device.In this paper, the charge and discharge characteristics of lithium iron phosphate batteries are studied, and the design scheme of the lithium iron phosphate power supply system is formulated.The matching battery management system is designed to ensure the safety of the battery pack during operation.A universal lithium iron phosphate battery module with an "N+1" redundant configuration is developed to improve the maintenance efficiency of the battery pack.

Design of lithium iron phosphate DC system
At present, the series DC power supply system is widely used in substations.The system uses the battery pack to obtain the rated voltage of 110/220 kV, and it is directly connected to the DC bus through the fuse.According to the requirements of the substation DC system, the work design is the required capacity, quantity and wiring mode, and operation mode.The series lithium iron phosphate power supply system is similar to the traditional lead-acid battery power supply system in the substation, with three modes: single charging and single storage, two charging and two storage, and three charging and two storage.The DC system of the 110 kV/220 kV substation of Anyang Power Supply Company is mostly the wiring mode of double charging and double bus.The system structure is shown in Figure 1.The DC load of the substation includes electrical control, signal, measurement and relay protection, automatic device, DC motor of operation mechanism, closing mechanism of electromagnetic operation of a circuit breaker, AC non-stop power supply system in the station, remote operation, accident lighting, etc.According to the requirements of the DC system of the substation and the DL/T 5004-2004 Technical Specification for DC System Design of Power Engineering, the lithium iron phosphate battery required for the project is determined.
The substation with 110 kV and below should be installed with one group of batteries, for the important 110 kV substation can also be installed with two groups of batteries.In order to meet the redundant needs of the unattended DC power supply, two groups of batteries should be configured in the DC power supply system of the 110 kV substation.This project takes the 110 kV substation as an example to calculate the lithium iron phosphate battery.The 110 kV substation has 28 10 kV switchgears.After the renovation and expansion, 25 new 10 kV switchgears are added.According to the statistical data of the new and old equipment, the load calculation is used as the theoretical basis of the battery configuration calculation, and the battery configuration results are verified.The DC load Statistical Table is shown in Table 1.Under normal conditions, the battery pack operates in floating charging mode and automatically switches to balanced charging mode after accident discharge.That is, the charging device mode selection switch is placed in the floating charge-all charge state.

Quantity selection of lithium iron phosphate batteries
According to the DL/T 5004-2004 Technical Regulations for Design of DC System in Power Engineering, for the DC system with control load and power load, the voltage of the DC bus shall not be higher than 110% of the nominal voltage of the DC system.In the case of accident discharge, the voltage at the outlet end of the battery pack shall not be lower than 87.5% of the nominal voltage of the DC system.The number of batteries is determined by the normal floating charging voltage and the DC bus voltage of the single battery.The DC bus voltage is 1.05 times the nominal voltage value of the DC system, which is considered to allow a 5% cable pressure drop of the electrical equipment to ensure that the voltage is not lower than the rated value during normal operation.
where n is the number of batteries in series.n U is the nominal voltage of the DC system, f U is the floating charging voltage of the single battery, and the value is 3.42 V.
According to the calculation, the DC system battery group selects 72 lithium iron phosphate batteries with a nominal voltage of 3.42 V in series.
Voltage check: 1) Generalized state: In the balanced charging operation mode, the DC bus voltage shall not exceed 110% of the rated voltage of the DC system, Uc 1.10 Un/n.The balanced charging voltage of the lithium iron phosphate battery is: 2) Discharge condition: At the end of the accident discharge, for the DC system with combined control load and power load, the voltage at the outlet end of the battery pack is not less than 87.5% of the nominal voltage of the DC system, namely T U 0.875 n U .The battery accident discharge termination voltage is 2.9 V, then, 2.9 72 208.8 ; UT is the battery discharge termination voltage.
208.8 0.949 0.875 220 Through the verification, it can be seen that the voltage of the battery group of 72 lithium iron phosphate batteries in series can meet the requirements under various operation modes.

Lithium iron phosphate battery capacity selection
According to the DL/T 5004-2004 Power Engineering DC System Design Technical Regulations, the battery capacity should be selected according to the following conditions: 1) The discharge capacity should be within the total outage time of the accident of the whole plant (station); 2) the discharge capacity of the starting current of the DC motor and other shock load currents in the initial accident (1 min); 3) the discharge capacity of the random (5 s) impact load current during the continuous discharge time of the battery group; 4) the DC bus voltage level shall be calculated in the most serious accident discharge stage.
The battery capacity selection of this project is calculated according to the voltage control method (capacity conversion method).
According to Table 1, the maximum accident discharge current in the accident discharge stage is 44.92 A.
The accident discharge time of the battery is 2 h, the discharge termination voltage is 2.9 V, and the 10 h battery discharge capacity is calculated as: 10 1.4 44.92 120.94 0.52 where 10 C is the discharge capacity of the battery for 10 h; K K is the reliability coefficient, which is 1.4.The sx C is the discharge capacity of the corresponding continuous discharge time under the accident total power failure state, and the value is 44.92.CC K is the capacity coefficient.Lithium iron phosphate battery has no standard capacity coefficient for reference, but the experiment proves that its capacity coefficient is higher than that of a lead-acid battery.According to regulation DL/T 5044-2004, the capacity coefficient of valve-controlled sealed lead acid battery (lean liquid) (monomer 2 V) at the discharge termination voltage of 1.87 V and the discharge time of 1 h is 0.52.This data is conservatively estimated as the capacity coefficient of lithium iron phosphate battery.Therefore, the 10 h discharge capacity of the battery is selected as 200 Ah to meet the requirements.In order to avoid the 72 single lithium iron phosphate batteries in series, this project adopts the design of a modular lithium iron phosphate battery pack.One lithium iron phosphate battery pack contains 18 single cells with a nominal voltage of 57.6 V.In order to ensure the timely detection of the fault of the battery pack, it is planned to adopt the mode of "n1 thermal backup battery depth management", and reserve one group of lithium iron phosphate battery packs to ensure the safe and reliable operation of  The structure can realize the intermittent charging of lithium iron phosphate battery during normal operation and quickly access the DC bus to ensure the normal operation of the system in case of an accident.

Design of the battery management system based on balanced charging
The battery management system shall be able to monitor and manage the charge and discharge of the individual battery and the battery pack and can realize the data analysis and data upload of the battery pack to ensure the safe and stable operation of the battery pack.The BMS controls the shunt and equalization of the resistor network.In the process of charging the battery pack, when a battery is charged faster, the voltage is higher than other batteries.The system controls the conduction and diversion of the balanced resistance by controlling the switch on and off.It reduces the charging speed of the battery so as to achieve the purpose of balanced charging of each battery.
The BMS system adopts the master-slave architecture, consisting of the battery module monitoring unit (BMU) and the master control unit (BCU).It communicates with the central control unit, photovoltaic controller, charger, and so on through the can bus, as shown in Figure 3.The BMS designed in this project consists of one BCU and multiple BMUs (depending on the form and quantity of battery connection).The BMU detects the voltage and temperature of the battery module and manages the imbalance in the module.

The modular design of lithium iron phosphate battery pack
The existing wiring mode of lead-acid batteries in the substation is series type, and a single failure cannot use the whole group.The performance uniformity of a single battery in the whole battery cannot be managed, and different batches of batteries from different manufacturers cannot be connected in series.This project develops the modular lithium iron phosphate battery pack and standardizes the interface software and hardware protocol to leverage the advantages of the lithium iron phosphate battery itself.
Considering the design of a modular lithium iron phosphate battery pack, N+1 modular thermal backup battery depth management" for batteries, standardized interface hardware, and software protocol are used.A 57.6 V 100 Ah battery pack with 18 single cells in series is built.Figure 4 shows the electrical structure diagram of the battery pack.The lithium iron phosphate battery pack adopts the integrated design, and the power interface is connected by the cable to effectively ensure the dustproof and moisture-proof oxidation of the power contact.The air plug interface has a design to prevent terminal damage caused by reverse power connections.The panel led indicates the display of battery soc in real time to help the operation and maintenance personnel quickly grasp the operating status of the battery.The communication air plug can send real-time battery data to the remote power distribution main station.
Performance testing: The mechanical performance and safety performance test for the developed lithium iron phosphate battery module are conducted.The specific test conditions and requirements are shown in Table 3 and Table 4  The battery was placed in a vacuum chamber and heated at 5ႏ per minute, knowing 130ႏ, for 30 minutes.
No fire, no smoke

Conclusion
According to the operation situation of the DC system in a substation in Henan province, the DC system of lithium iron phosphate battery pack is designed.The single lithium iron phosphate battery is connected in series to form a modular lithium iron phosphate battery pack, and the internal structure design and appearance design are designed.According to the operation requirements of the DC system, the battery management system matching the lithium iron phosphate battery pack is designed.In substations with different voltage levels, the modular number of lithium iron phosphate battery packs can be configured according to the needs.The "N+1" redundant configuration can realize the backup, which can improve the maintenance efficiency of the battery pack and ensure the reliability of the DC system.At the same time, the lithium iron phosphate battery is equipped with a battery management system to balance the single battery to ensure the safety of the battery (group) in operation.Due to the monitoring needs, the battery management system can also upload the data to the main control room to facilitate the operator to monitor the operation status and data of the equipment at any time.The application of lithium iron phosphate batteries in substations is less.The implementation of this paper in a substation in Henan province can provide a reference value for the subsequent promotion.

Figure 1 .
Figure 1.Wiring diagram of two charging and two storage systems of Physics: Conference Series 2636 (2023) 012021the DC system when other battery packs are faulty or maintained.The specific topology of the DC system of lithium iron phosphate battery is shown in Figure2.

Figure 2 .
Figure 2. Topology of the DC system of lithium iron phosphate battery

Figure 4 .
Figure 4. Electrical structure diagram of the battery pack

Table 1 .
220 V DC load statistics table

Table 2 .
Basic parameters of lithium iron phosphate battery

Table 3 .
Mechanical performance testing

Table 4
Safety performance test