Basic analysis of battery energy storage system

1. The structure of the power grid system
2. The structure of the energy storage subsystem
3. Function and structure of energy storage BMS system
4. Safety considerations for battery energy storage system

The capacity of the battery energy storage system is very large, reaching MWH-hundreds of MWH, and the conversion power reaches hundreds of MW. It is generally composed of multi-cluster battery pack, multiple PCS (power conversion systems), and energy management and dispatching systems EMS.

The energy storage management unit ESMU and the power conversion system PCS are connected by CAN communication and RS485. At the same time, it communicates with the local energy management unit LEMS through the Ethernet interface, and performs IEC61850 protocol processing. The video monitoring system uses the Ethernet interface to communicate with LEMS. The system connection is displayed to the system data.

The structure of the power grid system

The power grid system is a dynamic balance system, and there is always a phenomenon of fluctuation and imbalance between power generation and power consumption. The power grid system after the addition of the battery energy storage system can adjust the dynamic balance, allowing a variety of power sources and power consumption.

The loads can form a reliable system together, and the battery energy storage system can adjust the new energy power generation and improve the supply and demand imbalance of new energy power generation.

For each battery pack, it is composed of battery pack management module ESBCM, current sensor module CSM, battery acquisition and balance module ESBMM, etc. Each subsystem of the battery management system and the energy storage management unit are connected by CAN communication.

At the same time, there is also a dynamic ring monitoring system related to the safety monitoring part, which has various sensor signal monitoring, sound and light alarm, fire protection, and air conditioning functions.

The structure of the energy storage subsystem

The main component of the energy storage subsystem is the battery, which is the component for energy storage and the basis of the battery energy storage system. A large-scale energy storage system consists of multiple cluster battery packs, and each battery cluster consists of multiple battery insert boxes. Each battery box is composed of multiple battery packs, and the capacity can reach hundreds of MWH.

The type of battery may be lithium iron phosphate battery, ternary lithium battery, lithium titanate battery, etc. From the perspective of safety, capacity and life, lithium iron phosphate is the current mainstream choice. From the perspective of the composition of the battery, it is composed of cells, single cells, multi-cell battery packs, battery clusters, and battery stacks.

PCS (Power Conversion System) is another important part of the energy storage subsystem, through which the alternating current can be converted into a DC bus voltage up to 1200V, thereby storing the energy in the battery, when the power is needed, the energy in the battery It is converted into AC power by PCS and supplied to the grid or load for use.

BMS (Battery Management System) is also a subsystem of the battery energy storage system. It realizes the calculation of the battery state SoC by monitoring the battery operating state quantities, such as voltage, current, temperature, etc., and realizes balanced management of the battery on this basis. A system of fault alarm, control, protection, and communication. It is a very important core component.

EMS (Energy Management System) is a very important part of the battery energy storage system. Its main function is to determine the control strategy of the battery energy storage system according to the grid load, battery status, and electricity price rules.

Through the control of PCS and BMS, the energy can be reasonably scheduling use. Structurally, it consists of a three-layer structure, a local energy management unit, a regional energy management unit, and a cloud energy management unit.

Function and structure of energy storage BMS system

The main function of BMS, in short, is to monitor, measure, diagnose, protect, equalize, communicate, etc. to the battery system.

For monitoring and measurement, there are many quantities involved, such as battery cell voltage, total battery voltage, cluster current, battery and module temperature, etc. For fault protection, it is mainly to diagnose and alarm battery faults, such as overcharge, overdischarge, overcurrent, overtemperature and other protection functions, as well as anti-circulation current in parallel between battery clusters, etc. Functional safety is an important aspect to be considered.

For communication, it is mainly the communication function of BMS and PCS, EMS and other monitoring devices, in which data recording and storage are the basis for communication. For high-power capacity energy storage systems, the battery voltage is generally high, up to about 1200V, and the cluster current will reach 100-300A. In order to make full use of the battery capacity, the general discharge depth will be more than 80%.

Balance is a very important function of the battery energy storage system. Due to the consistency of the batteries in the battery cluster, there will be many differences in the capacity consistency of the batteries at the end of discharge. In addition, the battery energy storage system needs more than 80% of the depth of charge and discharge.

Therefore, the battery energy storage system must have a very strong battery balancing ability. For example, the balancing current needs to reach 0.5-5A, which is difficult to achieve in general passive balancing. Active balancing can avoid the impact on the usable capacity of the battery pack and improve the overall efficiency of the battery energy storage system.

Temperature has a great impact on the life of a battery energy storage system, and the life of the battery energy storage system needs to be as long as 15 years. Therefore, careful thermal design and thermal management of the battery energy storage system must be carried out, especially for applications such as frequency modulation and peak regulation. The large-rate discharge leads to serious and unbalanced battery heating. Therefore, the cooling system, cooling channel, thermal design, thermal management, etc. are very important.

Data processing capability is also a very important requirement of the battery energy storage system. Due to the complexity of the system, the amount of data is very large, which requires the system control unit to have strong data processing capability and response speed. This requirement puts forward higher requirements on system processor capability, code structure and quality.

Safety considerations for battery energy storage system

As we all know, safety is a very important aspect of high-power energy storage systems, not only involving property safety, but also personal safety. Based on this, battery safety status analysis and early warning are very important capabilities of BMS. First of all, in terms of design indicators, a design life index of 15 years is proposed for the battery energy storage system, so the design life of BMS is also more than 15 years.

Secondly, in the design of BMS, it is necessary to standardize the communication control, such as the communication between BMS and PCS, BMS and EMS, etc., not only the standardization of the protocol, but also the standardization of the control strategy should be considered to improve the reliability of the system.

In terms of algorithms, such as the estimation of battery state requires high precision, high convergence, high robustness, etc., which requires a relatively large effort. In addition, it is particularly important that the battery safety early warning algorithm directly determines the early warning ability in the early stage of failure, and more work needs to be done.

Finally, the safety of the battery energy storage system depends to a certain extent on the improvement of the safety and reliability of the battery itself, so battery manufacturers need to continuously improve in this regard.

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