The power conversion system (PCS) is matched with the energy storage battery pack and is connected between the battery pack and the power grid. Unlike inverters such as 2kw inverter or 3000 watt solar inverter, the core function of PCS technology is to convert the AC grid power into DC form and store it in the electrochemical battery pack or convert the energy into AC form and feed back to the grid.
So what are the common functional requirements for PCS? What about the PCS technology? Let's find out in this article.
1. Performance characteristics of PCS
There are various specific PCS technology solutions for battery energy storage. Currently, the energy storage PCS technology of mainstream manufacturers generally use three-phase voltage two-level or three-level PWM rectifiers. Its main advantages are:
- The dynamic characteristics are flexibly controllable with the adjustment of the control algorithm;
- The power can flow in both directions;
- The output current is sinusoidal and has low harmonic content;
- The power factor can be flexibly adjusted between -1 and 1.
At the same time, the performance of the system also has a key impact on the service life of the entire electrochemical battery energy storage unit. The design of the PCS of a battery energy storage power station is of great significance to improving the PCS technology, operational safety and economy of the battery energy storage power station.
The core of the design of the PCS technology is to use the chopping capabilities of fully controlled power electronic switching devices and pulse width modulation (PWM) technology to control the on and off of the switching devices through flexible software algorithms to achieve two-way flow of electrical energy.
The PCS technology mainly consists of dry-type transformers, AC filters (including AC filter inductors and AC filter capacitors), DC capacitors, AC/DC circuit breakers, insulated gate bipolar transistor (IGBT) power modules and their corresponding control systems, etc. In the important part of the PCS technology, the algorithm in the control and protection unit determines the dynamic characteristics and protection action behavior of the energy storage power station on the power grid.
AC LC filtering is mainly used to filter out high-frequency harmonic components caused by switching dynamics in PCSs to prevent harmonic components from being injected into the power grid and causing a decrease in power quality. This type of harmonics has the characteristics of high frequency and wide distribution frequency; The AC contactor is used to control the connection between the energy storage converter and the grid during the grid connection process.
AC EMI filters are mainly used to filter out common-mode interference caused by dv/dt during the dynamic switching process of energy storage converters to prevent high-frequency radiation from affecting the normal operation of electronic components; power semiconductor devices are sensitive to high voltage, which can cause breakdown failure of power semiconductors, so it is necessary to design arresters on the AC side of the PCS to prevent abnormal overvoltage from causing damage to the equipment.
Dry-type transformers are mainly used to convert the low-voltage alternating current output from the energy storage converter in the PCS into a 10kV or 35kV voltage level for connection to the public grid.
At present, the capacity of dry-type transformers used in mainstream large-capacity PCS technology for battery energy storage units is generally 1250kVA. Dry-type transformers have the advantages of strong short-circuit resistance, low maintenance workload, high operating efficiency, small size, and low noise.
2. Functional requirements of PCS
In terms of control functions, the converter is required to have charge and discharge functions, active power control functions, reactive power adjustment functions, parallel and off-grid switching functions, low voltage ride-through functions, frequency/voltage response functions, etc.;
In terms of protection logic, the converter is required to have short-circuit protection, reverse polarity protection, DC over/under-voltage protection, off-grid over-current protection, over-temperature protection, AC line phase sequence error protection, communication fault protection, cooling system failure protection, anti-islanding protection, etc.
In the design of electrochemical battery energy storage power stations, in order to ensure the safe and reliable grid-connected operation of each energy storage unit in the station, the specific PCS technology functional requirements are as follows:
- The PCS technology should realize bidirectional energy conversion between the energy storage 12v battery and the AC grid, and have four-quadrant operation functions with active and reactive power decoupling control.
- The PCS technology should be able to receive control instructions from the monitoring system to charge and discharge the battery.
- The PCS should be able to cooperate with the battery management system (BMS) to ensure battery safety.
- The PCS should be able to perform corresponding actions according to the instructions of the upper-layer management system to achieve closed-loop control of charging and discharging voltage and current.
- The function and performance requirements of the PCS technology should match the needs of the energy storage unit, and should have functions such as grid-connected charging, grid-connected discharge, off-grid discharge, continuously adjustable active power, reactive power regulation, and low voltage ride-through.
- The PCS technology should be able to collect analog quantities such as AC and DC side voltages and currents of the PCS, as well as switch information such as normal operation of the device and fault alarms.
- The PCS should be able to receive analog quantities such as LFP battery voltage, temperature, calculated power, and switch information such as battery normal operation and fault alarms sent from the battery management system.
- The PCS technology should be able to realize the switching and control logic of the device's operating status, and should include the start and stop of the PCS, switching of control methods, conversion of operating status, etc.
- The PCS should be able to operate automatically and display various operating data, fault data, historical fault data, etc. in real time.
- The PCS should be equipped with both hardware fault protection and software protection. The protection function configuration is complete, the protection range overlaps, and there is no dead zone, which can ensure the system safety under various fault conditions.
- The PCS technology should support IEC61850, CAN, and MODBUS communications, and should be able to cooperate with the monitoring system and battery management system to complete the monitoring and protection of the energy storage unit.
3. PCS technologies
- Conversion technology:
The core function of the energy storage converter is to convert DC power into AC power, or convert AC power into DC power. This requires the use of power electronic devices such as full-bridge or half-bridge inverters and bidirectional DC/DC converters to achieve efficient, stable and precise control of electric energy.
- Control technology:
The control PCS technology of energy storage converters includes the detection and control of current, voltage, frequency and other parameters to ensure the stable operation of the converter under various working conditions. Commonly used control strategies include PID control, fuzzy control, neural network control, etc. Through these control strategies, real-time adjustment and optimization of the converter output voltage and current can be achieved.
- Energy management technology:
Energy storage converters need to efficiently manage the storage and release of electrical energy to improve the efficiency and reliability of system operation. Energy management PCS technology include charge and discharge control, predictive maintenance, condition monitoring, etc. of the energy storage systems. Through this PCS technology, refined management of energy storage systems can be achieved and their service life and safety can be improved.
- Power electronic devices and heat dissipation technology:
Power electronic devices used in energy storage converters, such as IGBT, SiCMOSFET, etc., need to withstand high voltage, high current and high temperature conditions. Therefore, the selection and heat dissipation design of power electronic devices is one of the key PCS technology for energy storage converters. Appropriate power electronic devices and good heat dissipation design can ensure the stable operation and long life of the energy storage converter.
- System integration technology:
Energy storage converters usually need to be integrated with other devices (such as batteries, inverters like 1000w inverter, chargers, etc.) to form a complete energy storage system. System integration PCS technology includes the design of electrical connections, communication interfaces, control strategies, etc. Through this PCS technology, the collaborative work of energy storage converters and other equipment can be realized to improve the performance and reliability of the entire system.