The Power Conversion System (PCS) is paired with a battery storage system and connects between the battery pack and the power grid. Its core function is to convert AC power from the grid to DC for storage in the electrochemical battery pack or to convert energy from the battery pack to AC to feed back into the grid. The technical specifications related to grid-connected battery energy storage stations mainly rely on the software control algorithms of the energy storage power conversion system. The specific technical solutions for battery energy storage power conversion systems are diverse. Currently, mainstream manufacturers generally adopt three-phase voltage-type two-level or three-level PWM rectifiers for their energy storage power conversion systems, with the main advantages being:

1. Dynamic characteristics are flexibly controllable with adjustments to the control algorithm;

2. Power can flow in both directions;

3. The output current is sinusoidal with low harmonic content;

4. The power factor can be flexibly adjusted between -1 and 1.

The performance of the power conversion system in a battery energy storage station profoundly influences and determines whether the entire station can operate safely, stably, efficiently, and reliably. Additionally, the system's performance has a key impact on the lifespan of the entire electrochemical battery storage unit. The design scheme of the power conversion system for battery energy storage stations is of significant importance for enhancing the operational safety and economy of the stations. This chapter will explore design-related issues based on the various components of the battery energy storage station's power conversion system.

The core of power conversion system design utilizes the chopping ability of fully controlled power electronic switching devices and Pulse Width Modulation (PWM) technology. Through flexible software algorithms controlling the switching devices' turn-on and turn-off, bidirectional flow of electric energy is achieved. The power conversion system mainly consists of dry-type transformers, AC filters (including AC filtering inductors and capacitors), DC capacitors, AC/DC circuit breakers, Insulated Gate Bipolar Transistor (IGBT) power modules, and their corresponding control systems. The control and protection unit within the power conversion system, through its algorithms, determines the dynamic characteristics and protection behavior that the energy storage station exhibits towards the grid.

The national standard GB/T 34120—2017 "Technical Specification for Energy Storage Converters of Electrochemical Energy Storage Systems" specifies functional requirements for the control algorithms in the power conversion system, such as charging and discharging functions, active power control, reactive power regulation, on-off grid switching, low voltage ride-through, frequency/voltage response, etc.; in terms of protection logic, the converter should have short circuit protection, polarity reversal protection, DC over/under voltage protection, off-grid overcurrent protection, overtemperature protection, AC input phase sequence error protection, communication failure protection, cooling system failure protection, and anti-islanding protection.

AC LC filters are mainly used to filter out the high-frequency harmonic components caused by the switching dynamics in the power conversion system, preventing these harmonics from degrading power quality when injected into the grid. AC contactors control the connection between the energy storage converter and the grid during grid connection processes. AC EMI filters primarily serve to filter out common-mode interference caused by dv/dt during the switching dynamics of the energy storage converter, preventing high-frequency radiation from affecting the normal operation of electronic components.

Power semiconductor devices are sensitive to overvoltages, which can cause power semiconductor breakdown and failure; therefore, surge protectors are necessary at the AC side of the power conversion system to prevent equipment damage due to abnormal overvoltages. Dry-type transformers are mainly used to convert the low-voltage AC output of the energy storage converter to a 10kV or 35kV voltage level for connection to the public grid. Currently, the capacity of dry-type transformers used in mainstream large-capacity battery energy storage units is generally 1250kVA. Dry-type transformers have the advantages of strong short-circuit resistance, low maintenance workload, high operational efficiency, compact size, and low noise.

Functional Requirements

In the design of electrochemical battery energy storage stations, to ensure the safe and reliable grid-connected operation of each storage unit within the station, the design of the power conversion system can refer to the following principles:

(1) The power conversion system should realize bidirectional energy transfer between the energy storage battery and the AC grid, featuring four-quadrant operation capability with decoupled control of active and reactive power.

(2) The power conversion system should be able to receive control commands from the monitoring system for charging and discharging the batteries.

(3) The power conversion system should work with the Battery Management System (BMS) to ensure battery safety.

(4) The power conversion system should execute corresponding actions according to commands from the upper-level management system, achieving closed-loop control of charging and discharging voltage and current.

(5) The functionality and performance requirements of the power conversion system should match the needs of the storage unit, capable of grid-connected charging, grid-connected discharging, off-grid discharging, continuously adjustable active power, reactive power regulation, and low voltage ride-through, among others.

(6) The power conversion system should be able to collect analog quantities such as AC and DC side voltages and currents, and digital quantities such as device normal operation and fault alarm signals.

(7) The power conversion system should be able to receive analog quantities such as battery voltage, temperature, calculated charge, and digital quantities such as battery normal operation and fault alarm signals from the Battery Management System.

(8) The power conversion system should be able to perform device operation status switching and control logic, including start-stop of the power conversion system, control mode switching, and operation status transition, among others.

(9) The power conversion system should be able to operate automatically, displaying real-time operating data, fault data, and historical fault data, among others.

(10) The power conversion system should be equipped with both hardware fault protection and software protection, with comprehensive protection functions, overlapping protection scopes, and no dead zones, ensuring system safety under various fault conditions.

(11) The power conversion system should support IEC 61850, CAN, MODBUS communications, and should work with the monitoring system and the Battery Management System to monitor and protect the storage units.

PCS Fuse Solution

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