Core Technology empowers China's new infrastructure and talks about the black technology behind energy storage

In 2022, under the dual stimulation of policy support and market demand, energy storage has suddenly become one of the hottest industries. The essence behind this is the continuous increase in the installed capacity of renewable energy . Unlike thermal power and other units that can actively control power generation, renewable energy power generation is affected by the natural environment, has strong uncertainty, and is even incompatible with the power grid. Therefore, it needs to be equipped with an energy storage system to solve the problems of consumption, peak shaving, Various issues such as frequency regulation and power grid stabilization.

According to data from the National Energy Administration, between 2016 and 2021, China's average wind and light curtailment rates have dropped from 17.0% and 10.3% to 3.1% and 2.0% respectively . In addition to power grid operation optimization, one of the reasons is technological progress. In addition, it is also inseparable from the development of energy storage. Moreover, Chen Yongchong, leader of the energy storage technology research group of the Institute of Electrical Engineering of the Chinese Academy of Sciences, recently stated that although my country’s energy storage installed capacity ranks first in the world, the ratio of energy storage to wind power and photovoltaic new energy installed capacity (referred to as the “new storage ratio”) is less than 7 %; relatively speaking, the average storage-to-new ratio in other countries and regions has reached 15.8%. With the rapid increase in the scale of new energy power generation, there is still a lot of room for growth in my country's new energy storage ratio.

In terms of energy storage technology, although pumped hydropower storage currently occupies the absolute dominant market, this century-old technology is giving way to electrochemical energy storage (BESS) that uses batteries as buffer storage media. The latter Because it is not restricted by natural conditions and has the characteristics of rapid response and flexible deployment, it is fully consistent with the characteristics of renewable energy with small capacity, dispersed layout and large quantity. It can be deployed flexibly on the household side, industrial and commercial side and grid side. According to statistical data from the "2022 Energy Storage Industry Application Research Report" released by the Energy Storage Application Branch of the China Chemical and Physical Power Supply Industry Association, there will be 146 new energy storage projects in China in 2021, and there will be 131 electrochemical energy storage projects. Among them, there are 120 lithium-ion battery energy storage projects. At the same time, in addition to large-scale projects, more small and micro energy storage stations represented by industrial, commercial and household applications are blooming.

Although in the long run, the development of BESS is the general trend, at this stage, issues such as cost, safety, service life, and environmental protection still hinder the industry to a certain extent, and technological development has become a very important factor. In short, BESS is the power conversion between DC and AC, battery charging and discharging, and the control process of the entire industrial system. Naturally, it is inseparable from power electronics technology to ensure the safety and high efficiency of the energy storage system. Therefore, generally speaking, a typical BESS contains the following parts: PCS (power conversion system) is responsible for power conversion between the battery DC link and the inverter AC bus (i.e. battery and grid), BMS (battery Management system) is responsible for monitoring key information of the battery, and EMS (energy management system) is responsible for operating and controlling the entire system.

No matter any part of the energy storage system, it is inseparable from the support of chips. This article will use specific products and solutions to let everyone understand the key components of the BESS system and how to overcome the challenges faced. In this field, Texas Instruments (TI), one of the major suppliers of power electronics, has decades of relevant technology accumulation. At the same time, TI also has rich product and system experience in battery management and industrial control, and can build a complete system for the entire ESS system. signal and energy transfer links, so this article selects several representative products and solutions from TI.

Bidirectional power conversion system

In the past, when there was no supporting energy storage, the power flow from renewable energy sources was often transmitted directly to the grid side in one direction. Adding an energy storage system to this architecture makes it obvious that a two-way system is better than two one-way systems. Therefore, bidirectional DC/DC charge and discharge management needs to be added to the energy storage side. At the same time, the AC/DC side also needs to be changed to a bidirectional architecture and integrate PFC (power factor correction) and inverter. This more flexible two-way structure helps achieve better peak shaving and valley filling, storing electric energy when the electricity price is cheap or generating a lot of electricity, and releasing it in time when there is demand.

Generally speaking, this bidirectional architecture means two independent conversion systems, including power, control, protection, etc. , which brings higher system cost, more complex layout and wiring, and larger volume.

Bidirectional PFC and inverter stage

Since the power devices in the topology are essentially the same, the two can be combined to achieve high efficiency and small size of the system. Among them, the bidirectional DC/DC power stage is specially built for energy storage inverters. The common topologies of bidirectional DC/DC power stage are CLLLC and DAB. For the off-grid/grid-connected bidirectional inverter/PFC power stage, no special topology is required to implement it, because the inverter power stages commonly used in standard string inverters, such as two-stage H-bridge, HERIC, three-level TNPC, three-level NPC and three-level ANPC can all achieve bidirectional conversion.

These complex topologies bring many challenges to power conversion and control. At the same time, for high-power inverters, there may be several parallel connections, so synchronization requirements with each other and with the grid are also required.

At the same time, in order to achieve higher conversion efficiency and energy storage density, energy storage systems are increasingly introducing wide bandgap semiconductors as their power devices. Compared with traditional silicon devices, wide bandgap semiconductors can achieve higher switching frequencies. , thereby improving conversion efficiency and reducing size, but this higher switching frequency also brings challenges from driver design to layout and wiring, EMI, thermal management, etc.

Efficiency, volume and cost are important, but reliability and safety always come first. For example, a grid-connected bidirectional inverter needs to have detection and isolation functions. When the power grid fails (such as power outage, power outage, overvoltage, etc.), it should be disconnected in time.

Just like the above challenges, no matter what type of power topology, design is never easy. Therefore, the best solution is to choose an optimized reference design solution and a one-stop supplier as much as possible. TI provides a variety of topology reference solutions to meet the needs of various power conversion systems.

TIDA-010210 Reference Design

Taking TI's GaN-based 11kW bidirectional three-phase ANPC reference design TIDA-010210 as an example, this reference design provides a design template for implementing a three-level three-phase ANPC inverter power stage based on gallium nitride (GaN). The use of fast-switching power devices enables higher switching frequencies of 100kHz, which not only reduces the size of the filter's magnetic components, but also increases the power density of the power stage. The multi-level topology allows the use of 600V rated power devices at higher DC bus voltages up to 1000V, which is not possible in other topologies. At the same time, lower switching voltage stress can reduce switching losses, resulting in a peak efficiency of 98.5%, and also improve system reliability.

The solution integrates a number of TI's star products, including C2000 32-bit MCU, GaN FET with integrated driver, protection and temperature reporting, gate drivers, switching converters and other power products, as well as digital isolators, amplifiers and other signal chain products.

Multilevel topologies allow the use of low-voltage switching devices, but also mean that more switches need to be driven and overvoltages need to be avoided even during abnormal operation. In this reference design, TI uses only one C2000 to control 18 power components under limited PWM, and implements hardware-based chain protection through the integrated CLB, eliminating the need for an external FPGA or CPLD. And only software control is needed to realize the reversing function.

High reliability battery management system

Similar to the power conversion system, the battery management system must first ensure sufficient safety and reliability, and then optimize it in terms of efficiency, volume and cost, and BMS is the most important system to ensure battery safety.

Typical energy storage BMS systems can be roughly divided into BMU, BDU and BCU . The BMU implements independent battery information collection, the BDU manages the battery cluster, and the BCU is responsible for the overall control and communication.

Since the battery management system pays more attention to battery charge and discharge management, it needs to collect various types of information in detail, including battery information storage, collection, equalization control, charge and discharge management, etc. Therefore, a high-performance AFE is the key to achieving a high-level BMS and should solve customer pain points in terms of accuracy, robustness, safety, and system cost.