Battery Energy Storage System (BESS) Electrical Integration Design 1
The electrical integration design of a Battery Energy Storage System (BESS) is based on the application scenario and includes various aspects such as DC, high/low voltage distribution, control power distribution, grounding, lightning protection, and safety standards.
01 BESS Electrical System Overview
The BESS electrical system is generally divided into two parts: the main circuit and the control circuit. The main circuit consists of the DC loop, PCS, and AC grid connection interface. The DC loop starts from the battery cluster output/input, goes through high-voltage boxes, and ends at the Battery Connection Panel (BCP). The system includes DC protection and switches to ensure safe parallel connection, fault isolation, and re-engagement after recovery. The BCP connects to the PCS on the DC side, and centralized PCS systems often use single-port input/output, while modular PCS requires multiple ports.
The AC grid connection interface may connect to either low-voltage (400V/690V) or high-voltage grids (above 6kV), with options for off-grid switch devices. The control circuit powers the BESS system and auxiliary equipment. Automatic transfer switches (ATS) and UPS are recommended for critical loads such as local controllers and fire systems.
02 Electrical Design Key Points
Key Points of Electrical Design for Low-Voltage Switchgear: Typically includes circuit breakers, isolating switches, measuring instruments, surge protectors, and cabinets. Considerations must include voltage insulation, busbars, protective wires, and cables. Use resettable circuit breakers and configure surge protectors in stages.
Transformers: Used to match different DC side voltages with grid voltages while taking into account the efficiency and control methods of the PCS (Power Conversion System). In BESS (Battery Energy Storage Systems), transformers achieve matching between the PCS AC side voltage and the grid voltage. In off-grid scenarios, three-phase three-wire systems can be converted to three-phase four-wire supply, providing a neutral line.
BESS commonly uses dry-type transformers with copper windings, which are non-flammable and non-explosive. The thermal resistance of the insulation materials is categorized into classes Y, A, E, B, F, H, and C, corresponding to operating temperatures of 90, 105, 120, 130, 155, 180, and 220 degrees Celsius. The heat resistance determines the permissible temperature rise limit; increasing the temperature rise allows for a higher permissible current density, reducing volume and cost but also increasing active losses.
Dry transformers can have natural air cooling or forced air cooling. In BESS, a heat dissipation air channel should be designed, with a preference for natural air cooling. Although forced air cooling can temporarily allow transformers to run at overload, it increases losses and affects insulation and lifespan. Generally, at an elevation of 1,000 meters, the operating temperature limit for transformers decreases by 2.5% for natural cooling and by 5% for forced cooling for every 500-meter increase; for every 100-meter increase, insulation voltage requirements increase by 1%. In BESS, the temperature control box for the transformer communicates with the local controller to manage cooling fan operations and monitor the temperature of each winding phase, enabling high-temperature alarms and fault protection.
Disadvantages of Dry Transformers: Limited protection levels, high cost, and difficulty in recycling epoxy resin castings. Oil-immersed transformers, on the other hand, are cost-effective, produce low noise, and can be installed outdoors, but are complex to maintain and have high fire safety requirements, making them less convenient for installation and operation. They are generally used for high voltage and large capacity needs.
Currently, BESS is typically connected to grids at 400V or 6kV–35kV, with transformer winding connection groups that can be selected as Dyn11 or YNd11. The D connection allows zero-sequence currents to circulate, preventing injection into the power supply system. When off-grid, the allowable current in the neutral line can be 75% of the phase current, accommodating greater unbalanced load capabilities. Therefore, for isolation-type PCS, the internal transformer’s PCS-side winding can be selected as D connection, catering to both grid-connected and off-grid modes. On the grid side, Y connection can be selected, and the N line should be determined based on operating mode.
High Voltage Switchgear: Most commonly used are the central-type and ring network cabinets. The selection of high-voltage cables needs to consider the installation method and environment. The voltage levels of high-voltage switchgear correspond to the standard high-voltage levels in power systems, classified into 3.6kV (3kV), 7.2kV (6kV), 12kV (10kV), 24kV (corresponding to 20kV), and 40.5kV (35kV). According to the cabinet structure, they can be categorized into metal-enclosed partitioned, metal-enclosed armored, and metal-enclosed box types. By the internal switchgear, they are divided into load switch cabinets, circuit breaker cabinets, and GIS cabinets; based on the installation method of circuit breakers, they are classified as fixed and withdrawable types; by the position of the circuit breaker truck, they are categorized as floor-mounted and center-mounted types.
In BESS, the central-type and ring network cabinets are commonly seen.
Central-type Cabinet: Known as metal-armored center withdrawable switchgear, primarily used for high-voltage terminal users, enabling large load distribution and providing overload, undervoltage, short-circuit protection, monitoring, and measurement functions for power lines. Depending on the internal configuration, it can serve as an incoming, outgoing, PT, or isolation cabinet. The typical layout includes a bus and instrument room on the upper level, a switch room in the middle, and a cable room below. Key equipment consists of vacuum circuit breakers, current transformers, microprocessor protection devices, and auxiliary position switches. The core of the vacuum circuit breaker is the vacuum extinguishing chamber, known for its high segmentation capability, insulation strength, vacuum level, low tripping value, low re-ignition rate, and low leak rate. The central-type cabinet features five prevention functions to avoid various operational errors and enhance safety.
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Ring Network Cabinet: Refers to load switchgear designed for ring network power supply, which can be categorized into load switch cabinets, load switch + fuse combination cabinets, isolation switch + circuit breaker combination cabinets, or various specialized cabinets based on internal configuration. The structure and protective functions are similar to those of the central-type cabinet. With the addition of microprocessor protection devices, it can implement current instantaneous disconnection, inverse time overcurrent protection, overload protection, zero-sequence and negative-sequence overcurrent protection, overvoltage, undervoltage, and non-electric quantity protection tripping, along with multi-quantity measurement, remote control, and remote signaling capabilities.
Central-type and ring network cabinets can be used in combination. For example, a central-type cabinet can serve as the 10kV feeder cabinet in substations above 110kV, while ring network cabinets are used for downstream distribution, forming a public ring network connected to BESS.
Battery Collection Cabinet (BCP): BCP is the main electrical switchgear closest to the battery cluster, enabling parallel collection of multiple battery clusters, DC line strategy and protection, and electrical connection between batteries and PCS. BCP primarily ensures safe connections on the DC side of the battery and PCS. In the DC collection circuit, it typically includes fuses + isolation switches or DC circuit breakers, forming a break during maintenance, and may also include surge protectors (SPD), power measurement devices, and related status monitoring equipment.
Since isolation switches cannot provide short-circuit or overcurrent protection for DC, they must be used in conjunction with fuses. Additionally, electric-operated circuit breakers can offer short-circuit and overcurrent fault protection and actively disconnect battery outputs in conjunction with PCS and SPD.
On the battery side of the BCP, the voltage between DC positive and negative terminals and ground should not exceed 1.5 times the maximum working voltage, depending on the insulation parameters of different batteries and BMS. SPDs can be installed to quickly disconnect the DC circuit or shut down BESS upon fault signals. SPDs are commonly connected in a common mode, with two SPDs connected to the positive and negative terminals to ground.
To protect against transient overvoltages during the operation of some switching devices on the battery side, overvoltage counters may also be installed. These counters monitor the overvoltage state of the circuit in real-time and accumulate counts, providing insights into SPD maintenance based on the number of overvoltage occurrences.
Control Distribution Design: Control system distribution devices, real-time controllers, and BMS typically integrate within the BCP, reducing the number of cabinets, lowering costs, and facilitating convenient monitoring of overall operational status. The power supply for each control system should be calculated based on the BESS capacity and actual operating conditions.
03 Structure Design Key Points
Battery clusters are categorized into air-cooled and liquid-cooled systems based on their thermal management methods. As the names suggest, air cooling uses the storage system’s airflow channels to direct cooling air to the battery cluster’s inlet, while liquid cooling circulates coolant through pipelines to the liquid cooling plates of the modules, cooling the battery cells via the liquid.
Each battery cluster is typically equipped with a switch box that integrates components such as contactors, fuses, molded case circuit breakers, or isolators, precharge control circuits, current sensors, battery cluster control modules, and power supplies. These provide functions such as voltage and current collection, as well as overcurrent and overvoltage protection for control and safety.
Depending on the wiring requirements, the switch box can be placed either on the upper or lower part of the battery cluster. If the storage system uses a top-placed high-voltage box solution, the top-mounted switch box is selected; conversely, if the wiring is arranged at the bottom, a bottom-mounted switch box is used.
The manufacturing process is influenced by many factors, such as production equipment, batch manufacturability, design, machining precision, heat treatment, and cost. Therefore, structural design must fully consider the impact of these factors on manufacturability. From the perspective of product manufacturability, the structural design has the following basic requirements:
The overall layout and structure of the energy storage system should be as simple as possible and divided into several reasonable parts.
Materials and heat treatment methods should be selected appropriately, and parts should be designed according to the material characteristics and processing technology, including specifying shapes, dimensions, tolerances, and technical requirements.
To improve the reliability and safety of the entire system, the design should use mature, validated structures as much as possible.
The structural design of the battery cluster must provide support for the batteries and other components during use. Therefore, it must have sufficient strength to withstand conditions such as transportation, vibration, and operation.
The structural design of the energy storage system needs to meet several basic design criteria, including manufacturability and assembly, structural strength, environmental adaptability, safety protection, and thermal protection.