The topology of the Power Conversion System (PCS) of electrochemical energy storage system is closely related to the technical route of the electrochemical energy storage system

PCS can operate in the following two states and thus shoulder two important functions:

1. The working state of the rectifier: converts the alternating current of the power grid into direct current when charging the battery of the energy storage system

2. Inverter working state: When discharging the battery of the energy storage system, the direct current of the battery is converted into alternating current and fed into the power grid

Therefore, PCS is an important equipment to realize bidirectional energy transfer between DC cell and AC network.

In recent years, thanks to new power electronic devices including IGBTs (insulated-gate bipolar transistors), insolated gate bipolar transistor) and IGCT (integrated gate commutated thyristor) development and performance improvement, The production and application of high voltage and high power PCS devices have become a reality.

The topology of PCS can be roughly divided into the following types:

1. Contains only DC/AC links

The PWM converter is responsible for converting the direct current into alternating current, the LC filter is responsible for filtering the alternating current, which can reduce harmonics, and the transformer can match the transformed voltage with the parallel AC network voltage, and play the role of electrical isolation between the battery system and the grid. When the battery system is charging, PWM works in the rectifier state; When discharging, PWM works in the inverter state.


Advantages: Suitable for grid distributed independent power grid connection, simple structure, PCS link energy consumption is relatively low.

Disadvantages: large system size and high cost. Lack of flexibility in capacity selection. The short-circuit fault on the power grid side may generate a short time and large current on the DC side of PCS, which will have a great impact on the battery system.


2. Contains DC/DC and DC/AC links

Bi-directional DC/DC converters are responsible for the up-down conversion, thus avoiding the use of transformers in some scenarios. When the battery system is charging, the PWM works in the rectifier state, and the DC/DC converter rectifies the AC voltage on the grid side to the DC voltage suitable for the battery pack. When the battery system is discharged, PWM works in the inverter state, and the DC/DC converter converts the DC voltage of the battery pack to the appropriate DC voltage, and then converts it into the AC voltage suitable for the external AC system through the PWM converter.

Advantages: It makes the battery capacity configuration more flexible and adaptable, and can realize the charge and discharge management of multi-series and parallel battery modules.

Disadvantages: The DC/DC link has energy loss, and the efficiency of the entire system is reduced.

3. Cascade PCS (high voltage direct hanging)

The power unit is the core component of cascaded PCS device, which is responsible for AC/DC conversion and power transmission. The DC side of each power unit is connected to the corresponding battery pack, and the AC side is connected in series to form a converter chain. The output voltage of the in-phase power unit is superimposed to form a high-voltage access grid. The power unit integrates H-bridge converter circuit, DC ripple absorption circuit, charge current limiting circuit, signal isolation conversion module, power unit control board, DC contactor and so on.

The aspects that should be considered when selecting the level of cascade PCS are:

(1) The more cascaded links, the higher the equivalent switching frequency, the lower the output harmonics, but the more difficult the control is, the more links need to be monitored, the more complex the control and detection system is.

(2) The higher the DC voltage of the chain link, it is conducive to reducing the number of chain links and improving the stability of the overall system, but the number of battery cells that need to be connected in series has increased, resulting in more difficult battery voltage balancing.

(3) When the link has the automatic bypass function, once the faulty link is removed, the capacity of the PCS device should not be reduced, so the selection of the DC side voltage of each link should consider the number of redundant links. The more links allowed to bypass, the higher the operating voltage on the DC side should be set.

Advantages: Avoid the use of booster transformer to directly connect to the power grid, improve the dynamic response speed of the device, and reduce the operating loss of the energy storage power station.

Disadvantages: high operation and maintenance costs, more complex technology. The battery management system and power module are integrated, making it difficult to divide responsibilities when problems occur.