Five major integration technologies for energy storage power stations

1. Classification of large energy storage systems

According to electrical structure, large energy storage systems can be divided into:

● Centralized: Low-voltage, high-power boost-type centralized grid-connected energy storage system, with multiple clusters of batteries connected in parallel and then connected to the PCS. The PCS pursues high power and high efficiency, and is currently promoting a 1500V solution.

● Distributed: Low-voltage and low-power distributed boost grid-connected energy storage system, each cluster of batteries is linked to a PCS unit, and the PCS adopts a low-power, distributed arrangement.

● Intelligent string type: Based on the distributed energy storage system architecture, innovative technologies such as battery module level energy optimization, battery single cluster energy control, digital intelligent management, and full modular design are adopted to achieve more efficient application of energy storage systems.

● High-voltage cascaded high-power energy storage system: single-cluster battery inverter, directly connected to the power grid with a voltage level above 6/10/35kv without a transformer. The capacity of a single unit can reach 5MW/10MWh.

● Centralized distributed: Multiple branches on the DC side are connected in parallel, a DC/DC converter is added at the battery cluster outlet to isolate the battery cluster, and the DC/DC converters are connected to the DC side of the centralized PCS after collection.

2. Energy storage integration technology routes comparison

● Centralized solution: 1500V replaces 1000V becomes the trend

With the development of centralized wind power plants and energy storage to larger capacity, DC high voltage has become the main technical solution to reduce costs and increase efficiency, and the energy storage system with DC side voltage increased to 1500V has gradually become a trend. Compared with the traditional 1000V system, the 1500V system increases the withstand voltage of cables, BMS hardware modules, PCS and other components from no more than 1000V to no more than 1500V. According to CPIA statistics, in 2021 China's photovoltaic system market with a DC voltage level of 1500V will account for about 49.4%, and it is expected to gradually increase to nearly 80% in the future.

The 1500V energy storage system will help improve the compatibility with the photovoltaic system. The voltage on the DC side is 1500V, and through higher input and output voltage levels, the line loss of the AC and DC side and the loss of the low-voltage side winding of the transformer can be reduced, and the efficiency of the power station system can be improved. The power density of the equipment (inverter, transformer) is improved, the volume is reduced, and the workload of transportation and maintenance is also reduced, which is conducive to reducing the system cost.

Compared with the 1500V energy storage system solution, the 1000V solution has also improved in performance. According to estimates, compared with traditional solutions, the initial investment cost of the 1500V energy storage system is reduced by more than 10%. But at the same time, after the voltage of the 1500V energy storage system rises, the number of batteries connected in series increases, making its consistency control more difficult, and the requirements for DC arc risk prevention and protection and electrical insulation design are also higher.

● Distributed solution: high efficiency and mature solution

The distributed solution is also called multi-branch parallel connection on the AC side. Compared with the centralized technical scheme, the distributed scheme transforms the parallel connection of the DC side of the battery cluster into the parallel connection of the AC side through the distributed string inverter. It avoids the risk of parallel circulating current, capacity loss, and DC arcing caused by parallel connection on the DC side, and improves operational safety. At the same time, the control accuracy is changed from multiple battery clusters to a single battery cluster, and the control efficiency is higher.

The distributed solution has the highest efficiency and limited cost increase, and its market share will gradually increase in the future. At present, the 100-megawatt power station in operation chooses the equipment of CATL and Sineng. Compared with the centralized solution, it is necessary to replace the 630kw or 1.725MW centralized inverter with a low-power string inverter. For power conversion system companies, if they have string inverter products and strong research and development capabilities, they can quickly cut into distributed solutions.

● Intelligent string solution: one package one optimization, one cluster one management

The intelligent string solution proposed by Huawei solves three main problems in the centralized solution:

• Capacity decay.
• Consistency.
• Capacity mismatch.

The intelligent string solution solves the above three problems of the centralized solution through the string, intelligent and modular design.

Stringization. The energy optimizer is used to realize battery module-level management, the battery cluster controller is used to achieve inter-cluster balance, and the distributed air conditioner reduces the temperature difference between clusters.

Intelligent. Apply advanced ICT technologies such as AI and cloud BMS to internal short-circuit detection scenarios, apply AI to predict battery status, and adopt multi-model linkage intelligent temperature control strategies to ensure optimal charging and discharging status.

Modular. The modular design of the battery system can separate the faulty module separately without affecting the normal operation of other modules in the cluster. Modular design of PCS, when a single PCS fails, other PCS can continue to work, and when multiple PCS fail, the system can still keep running.

● High-voltage cascade solution: an efficient solution without parallel structure

The high-voltage cascaded energy storage solution is designed through power electronics to achieve a grid-connected voltage of 6-35kv without a transformer. The advantages of the high-voltage cascade solution are as follows:

Safety. There are no cells connected in parallel in the system, some batteries are damaged, the range of replacement is narrow, the range of influence is small, and the maintenance cost is low.

Consistency. The battery packs are not directly connected, but connected after AC/DC, so all battery packs can be controlled by SOC balance through AC/DC. There is only a single battery cluster inside the battery pack, there is no parallel connection of battery clusters, and there will be no current sharing problem. The balance control between the cells is realized through the BMS inside the battery cluster. Therefore, this solution can maximize the use of battery capacity, and in the case of the same grid-connected power on the AC side, fewer batteries can be installed to reduce the initial investment.

High efficiency. Since the system does not have cells/battery clusters running in parallel, there is no short-board effect, and the system life is approximately equal to the life of a single cell, which can maximize the operating economy of the energy storage device. The system does not need a step-up transformer, and the actual system cycle efficiency on site reaches 90%.

● Centralized distributed solution: DC isolation + centralized inverter

The centralized distributed solution is also called multi-branch parallel connection on the DC side. On the basis of the traditional centralized solution, a DC/DC converter is added at the outlet of the battery cluster to isolate the battery cluster, and the DC/DC converters are connected to the DC side of the centralized PCS after collection. 2-4 PCSs are connected in parallel to a local transformer, and connected to the grid after boosted by the transformer.

By adding DC/DC DC isolation in the system, the DC arcing, circulating current and capacity loss caused by DC parallel connection are avoided, which greatly improves the safety of the system and thus improves the system efficiency. However, since the system needs to go through two stages of inversion, it has a negative impact on the system efficiency.

Conclusion

Summary and comparison of the five major integration technology routes of energy storage power stations:

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