To achieve the goals of peaking carbon emissions by 2030 and realizing carbon neutrality before 2060, the proportion of new types of power generation in the installed capacity of the power market is continuously increasing. By the end of 2022, the electricity generation from wind and photovoltaic (PV) sources in China reached 1.19 trillion kilowatt-hours, an increase of 207.3 billion kilowatt-hours compared to 2021, representing a year-on-year growth of 21%. This accounted for 13.8% of the total electricity consumption in society, a 2-percentage-point increase from the previous year, and was close to the total electricity consumption of urban and rural residents nationwide.

With the gradual maturation of wind and solar power generation models, in order to enhance the overall reliability of the power system and to coordinate the flexible use and stable consumption of resources, the market has begun to gradually give rise to the demand for energy storage. In 2022, China added 13.30 GW of new energy storage installations, a year-on-year increase of 26.67%, bringing the cumulative installed capacity to 59.40 GW. Similar to the global energy storage type structure, pumped-storage hydropower still dominates in China, while the development of new types of energy storage is the fastest. In 2022, China added 6.90 GW of new installations of new types of energy storage, a year-on-year increase of 182.07%, with a cumulative installed capacity reaching 12.70 GW.

Figure 1: The newly added installed capacity of new-type energy storage in China

It is evident that, with the transformation of the global energy structure and the large-scale deployment of renewable energy, energy storage technology has become a key component of power systems. The Power Conversion System (PCS), as the core equipment of an energy storage system, directly affects the economic benefits and technical feasibility of the entire energy storage system through its performance and efficiency.

The Power Conversion System (PCS) is a device used in electrochemical energy storage systems to achieve bidirectional power conversion between the battery system and the grid (and/or load). It controls the charging and discharging processes of the batteries, performs AC/DC conversion, and can directly supply power to AC loads in the absence of a grid. The PCS consists of a DC/AC bidirectional converter and a control unit. The PCS controller receives control commands from the background control system via communication and controls the converter to charge or discharge the batteries based on the sign and magnitude of the power command, thereby regulating the active and reactive power of the grid. Additionally, the PCS can communicate with the Battery Management System (BMS) through a CAN interface or via dry contact signals to obtain information on the state of the battery pack. This enables protective charging and discharging of the batteries, ensuring their safe operation.

Figure 2: Block Diagram of Energy Storage Equipment System

The topology of the Power Conversion System (PCS) determines its conversion efficiency and reliability. The structure of the PCS is divided into single-stage and dual-stage configurations.

The structure of the single-stage energy storage converter is shown in Figure 3, which consists of only one DC/AC stage (PWM converter). Its working principle is as follows: When the energy storage battery pack discharges, the stored energy is converted from DC to AC through the PWM inverter, and the DC power stored in the battery pack is transformed into AC power and fed back to the grid. When the energy storage battery pack is charging, the AC power from the grid is rectified into DC power through the PWM converter and stored in the battery pack. The PWM converter operates in either rectification or inversion mode to achieve bidirectional energy flow.

Typically, individual energy storage batteries are connected in series and parallel to form a battery pack to ensure the normal operation of the converter. The single-stage topology has high efficiency, a simple structure, low losses, and easy control. However, in practical applications, the single-stage topology still has some drawbacks: the capacity configuration of the energy storage system is not flexible enough, and the voltage operating range of the energy storage battery is relatively narrow.

Figure 3: Single-stage energy storage converter topology

The topology of the dual-stage energy storage converter is shown in Figure 4, which mainly consists of a DC/DC converter and a PWM converter. Its working principle is as follows: When the energy storage battery pack discharges, the DC power from the battery pack is boosted by the DC/DC converter and then supplied to the PWM converter. After being inverted into AC power by the PWM converter, it is fed into the grid. When the energy storage battery pack is charging, the AC power from the grid is rectified into DC power by the PWM converter and then enters the DC/DC converter. The DC/DC converter steps down the DC voltage to charge the energy storage battery pack.

Figure 4: Dual-stage energy storage converter topology

For battery cell configurations involving series connection and parallel-then-series connection, a single-stage converter is more suitable. For battery packs configured in a series-then-parallel manner, a dual-stage design is often employed. In this design, each series-connected battery group is connected through an individual bidirectional DC/DC converter to the intermediate DC link of the DC/AC converter, which is then connected to the grid via the DC/AC converter, as shown in Figure 5.

Figure 5: Extended Diagram of Dual-Stage Converter

This dual-stage converter topology is suitable for large-capacity energy storage systems, where multiple battery groups can be connected. Each battery group is controlled through an independent DC/DC stage, enabling independent charging and discharging control of multiple battery groups. This topology offers a wide voltage operating range for the battery groups, eliminates circulating currents between battery groups, and allows flexible capacity configuration and flexible switching of the entire battery energy storage system, facilitating operation and management. However, due to the two-stage energy conversion, the dual-stage converter topology increases system losses and reduces the overall energy conversion efficiency. The large number of DC/DC converters makes the system more complex. The two-stage converters need to work closely together, and the coordination methods for charging and discharging conditions are different, which increases the difficulty of system control and reduces operational reliability. Based on the number of voltage levels, the topologies of energy storage converters can be divided into two types: two-level circuit topology and multi-level circuit topology, with the three-level circuit topology being a major representative of the multi-level circuit topology.

As shown in Figure 6, the classic three-phase bridge two-level circuit topology has been widely applied in the industry. By controlling the conduction and cutoff of the power electronic devices, specifically IGBTs (Insulated Gate Bipolar Transistors), the AC phase voltage can achieve two levels: +Ud and -Ud. However, the waveform quality of the phase voltage in these two states is not satisfactory. To improve the voltage waveform quality, it is necessary to increase the switching frequency of the devices. But this, in turn, leads to an increase in the switching losses of the devices, thereby reducing the overall efficiency of the converter. Therefore, in order to enhance the utilization rate of the DC voltage, multi-level circuit topologies have attracted significant attention.

Figure 6: Two-level three-phase bridge circuit

In the high-voltage domain, the application of multi-level circuit topologies is more extensive, with the three-level circuit topology being the primary representative. This is mainly due to its simple structure, which is convenient and practical. Compared to traditional two-level circuits, the three-level circuit topology includes an additional neutral point at 0 potential. Compared to traditional two-level circuit topologies, the advantages of three-level circuits are as follows: higher voltage utilization, lower harmonic content, better voltage quality, and reduced filter size. Additionally, the lower switching frequency results in reduced electromagnetic interference and improved system efficiency.

Taking the Diode-Clamped Neutral Point Clamped (NPC) three-level circuit topology as an example, its topology is shown in Figure 7. The middle DC-side capacitor of the three-level circuit is composed of C1 and C2. Each bridge consists of 4 IGBTs, 4 freewheeling diodes, and 2 clamping diodes. The clamping diodes ensure that the voltage across the two IGBTs is the same. The neutral point of the capacitor is connected to the midpoint of the clamping diodes in each phase, allowing the capacitor neutral point to output a zero voltage level. As a result, each phase voltage can achieve three levels: +Ud/2, 0, and -Ud/2.

Figure 7: Diode-Clamped Neutral Point Clamped (NPC) topology

Current detection is one of the key technologies in the Power Conversion System (PCS), affecting the control accuracy and stability of the system. Common current detection techniques include resistor sampling and Hall sensor sampling. In practical applications, the CSM series current sensors from Zhejiang Jucimagnetics Company, based on their proprietary iFluxgate® technology, feature high precision, low temperature drift, low heat generation, fast response time, and modular design. Certified by CE and RoHS, these sensors can accurately obtain charging and discharging currents, effectively optimize traditional charging and discharging methods, extend battery life, and save energy. The series of current sensors can be widely applied in battery management (SOC, SOE, SOF, etc.) and other applications that require precise current measurement, as well as in pure electric vehicles, plug-in hybrid vehicles, and energy storage equipment, such as PACK, BMS, BDU, PDU, etc., in new energy electric vehicles. We sincerely welcome all our esteemed users to widely and deeply understand our products.

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