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. 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.

1. Energy storage system: In order to ensure safety and improve efficiency, multiple technical routes are in full bloom
The electrochemical energy storage system consists of two parts, the DC side and the AC side. The DC side is the battery warehouse, including batteries, temperature control, fire protection, confluence cabinets, containers and other equipment, and the AC side is the electrical warehouse, including energy storage converters, transformers, containers, etc. The battery on the DC side generates DC power. To achieve electrical energy interaction with the grid, AC-DC conversion must be performed through a converter.

Energy storage system classification: centralized, distributed, intelligent string, high-voltage cascade, distributed
According to the electrical structure, large-scale energy storage systems can be divided into:
(1) 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 the 1500V solution is currently being promoted.
(2) 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.
(3) 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 .
(4) 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.
(5) Distributed type: 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. Iteration of energy storage technology route revolves around safety, cost and efficiency
Safety, cost and efficiency are the key issues that need to be solved in the development of energy storage. The core of the iteration of energy storage technology is to improve safety, reduce cost and improve efficiency.

(1) Security
The safety of energy storage power stations is the most concerned issue in the industry. Potential safety hazards in electrochemical energy storage power stations include electrical fires, battery fires, hydrogen explosions in fire, system abnormalities, etc. Tracing the causes of safety problems in energy storage power stations can usually be attributed to the thermal runaway of the battery. The causes of thermal runaway include mechanical abuse, electrical abuse, and thermal abuse. In order to avoid safety problems, it is necessary to strictly monitor the battery status to avoid the occurrence of thermal runaway incentives.
(2) High efficiency
Cell consistency is a key factor affecting system efficiency. The consistency of the battery depends on the quality of the battery, the energy storage technology solution, and the working environment of the battery. As the number of battery cycles increases, the differences between the batteries are gradually reflected. The differences in the actual working environment during the superimposed operation will lead to aggravated differences between multiple batteries, and the consistency problem is prominent, which poses challenges to BMS management and even faces challenges. Security Risk.

Series mismatch between battery modules: the usable capacity of the batteries connected in series can only reach the capacity of the weakest battery module, so that the capacity of other batteries cannot be fully utilized.

Parallel mismatch between battery clusters: The available capacity of the battery clusters on the parallel link can only reach the capacity of the weakest battery cluster, so that the capacity of other batteries cannot be fully utilized.

Differences in battery internal resistance cause circulation: battery circulation increases the temperature of the battery core, accelerates aging, increases system heat dissipation, and reduces system efficiency. In the design and operation scheme of the energy storage power station, the consistency of the battery should be improved as much as possible to improve the system efficiency.

(3) Low cost

The cost of an energy storage system is related to initial investment and cycle life. The aging and decline of battery materials, charge and discharge system, battery operating temperature, and consistency of monomers will all affect the cycle life of the battery. When the battery temperature difference in the container is greater than 10 degrees, the battery life will be shortened by more than 15%. Differences in temperature rise between modules can also lead to a shortened overall system life. The energy storage system should improve the cycle life of the system by optimizing the charging and discharging method, reducing the temperature difference between the systems, and improving the consistency of the battery.

3. Energy storage integration technology route: topology schemes are gradually iterated
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. The 1500V technical solution of the energy storage system comes from the photovoltaic system. According to CPIA statistics, in 2021, the market share of domestic photovoltaic systems with a DC voltage level of 1500V is 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 photovoltaic systems.

Looking back on the development of photovoltaic systems, the voltage on the DC side should be 1500V. Through higher input and output voltage levels, the line loss on the AC and DC side and the winding loss on the low-voltage side of the transformer can be reduced, and the efficiency of the power station system can be improved. Equipment (inverter, transformer) The power density is improved, the volume is reduced, and the workload of transportation and maintenance is also reduced, which is conducive to reducing system costs. Taking the 1500V photovoltaic system solution released by TBEA in 2016 as an example, compared with the traditional 1000V system, the efficiency of the 1500V system is increased by at least 1.7%, the initial investment is reduced by 0.1438 yuan/W, the number of equipment is reduced by 30-50%, and the inspection time 30% shorter.

Compared with the 1500V energy storage system solution, the 1000V solution has also improved in performance. Taking Sungrow's solution as an example, compared with the 1000V system, the energy density and power density of the battery system have increased by more than 35%. The power station with the same capacity requires less equipment, and the cost of equipment such as battery system, PCS, BMS, and cables is greatly reduced. , infrastructure and land investment costs are also reduced simultaneously. 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 solution, the distributed solution converts the parallel connection of the DC side of the battery cluster to the parallel connection of the AC side through the distributed string inverter, which avoids the risks of parallel circulation, capacity loss, and DC arcing caused by the parallel connection of the DC side, and improves operation. 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.

Shandong Huaneng Huangtai Energy Storage Power Station is the world's first 100-megawatt energy storage power station with decentralized control. The Huangtai energy storage power station uses the battery of Ningde era + the PCS system of Shangneng Electric. According to estimates, after the energy storage power station is put into operation, the battery capacity utilization rate of the whole station can reach about 92%, which is 7 percentage points higher than the current industry average. In addition, through the decentralized control of battery clusters, the automatic calibration of the battery state of charge (SOC) can be realized, which significantly reduces the workload of operation and maintenance. The grid-connected test efficiency is up to 87.8%. Judging from the current project quotations, the decentralized system is not more expensive than the centralized system.

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

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

The smart string solution proposed by Huawei solves three main problems in the centralized solution: (1) Capacity attenuation. In the traditional solution, the use of batteries has an obvious "short board effect". The battery modules are connected in parallel. When charging, one battery cell is full, and the charging stops. When discharging, one battery cell is empty, and the discharge stops. The overall life of the system depends on the life. shortest battery. (2) Consistency. In the operation and application of the energy storage system, due to different specific environments, there is a deviation in the consistency of the battery, which leads to an exponential decay of the system capacity. (3) Capacity mismatch. Parallel connection of batteries is likely to cause capacity mismatch, and the actual capacity of the battery is far lower than the standard capacity.
The intelligent string solution solves the above three problems of the centralized solution through string, intelligent and modular design: (1) String. 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. (2) Intelligence. 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. (3) Modularization. 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 cascading scheme: high-efficiency scheme 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. Taking Xinfengfeng's 35kv solution as an example, the single energy storage system is a 12.5MW/25MWh system. The electrical structure of the system is similar to that of high-voltage SVG, consisting of three phases A, B, and C. Each phase consists of 42 H-bridge power cells paired with 42 battery clusters. There are a total of 126 H-bridge power units in three phases, and a total of 126 battery clusters, storing a total of 25.288MWh of electricity. Each battery cluster consists of 224 cells connected in series.

The advantages of the high-voltage cascading scheme are reflected in: (1) 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. (2) 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. (3) 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%.

As a new technical route, the high-voltage cascading solution needs to be verified by operation. (1) In terms of technology, on the one hand, each phase of the high-voltage cascade scheme is 35kv, and the electromagnetic environment is harsh, which puts forward higher requirements for BMS control. On the other hand, the high-voltage cascading scheme is parallel connection on the AC side, and multiple H-bridges are selected for connection. ABC three-phase AC, each phase has multiple H-bridges in series, which reduces reliability. In order to improve reliability, redundant design must be carried out , if an H-bridge fails, it can be switched to a bypass circuit. (2) In terms of operation, the DC side and the AC side of the 35kv energy storage system are placed in the same position, making operation and maintenance more difficult and posing certain safety risks. The penetration rate of the current high-voltage cascading solution is still low, and multiple projects need to verify reliability and stability.

From the project price point of view, the energy storage project quotation of the high-voltage cascading scheme is similar to the price of the traditional project. In April 2022, Jinpan Technology and Tianjin Ruiyuan Electric Consortium won the bid for the CGNPC Hainan Baisha Bangxi 25MW/50MWh energy storage project with a bidding price of 64.999166 million yuan and a unit price of 1.30 yuan/wh.

Distributed solution: DC isolation + centralized inverter

The distributed scheme is also called multi-branch parallel connection on the DC side. On the basis of the traditional centralized scheme, a DC/DC converter is added at the outlet of the battery cluster to isolate the battery cluster, and the DC/DC converter is connected to the centralized PCS DC after collection. On the side, 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, DC arcing, circulating current, and capacity loss caused by DC parallel connection are avoided, which greatly improves system security and improves system efficiency. However, since the system needs to go through two stages of inversion, it has a negative impact on the system efficiency.