Energy storage systems (ESS) are increasingly becoming an important part of modern power systems. As more and more solar and wind energy projects are connected to the grid, energy storage technology helps balance power generation and electricity demand. In many energy storage projects, transformers are a key component of electrical systems because they connect battery systems, power conversion equipment, and medium voltage power grids.
Unlike traditional power distribution projects, the application of energy storage systems has special requirements for the selection of transformers. Transformers need to work with battery energy storage systems (BESS), power conversion systems (PCS), switchgear, and protection devices. Choosing the right transformer at the beginning of the project will help reduce losses and avoid problems during long-term operation.
What is the role of transformers in energy storage systems?
In a typical battery energy storage system, the battery stores electrical energy in the form of direct current (DC). When transmitting electrical energy to the power grid, PCS converts direct current into AC (AC) electricity; during the charging process, the process is the opposite.
Energy storage system transformers are usually installed between PCS and medium voltage switchgear. Its main function is to boost the low-voltage alternating current output by PCS to the voltage level required by the power grid.
For example, the operating voltage of PCS may be 400 V, 690 V, or other low voltage levels, while the grid connection requirements may be 10 kV, 20 kV, 22 kV, or 35 kV. The transformer provides the necessary voltage conversion and electrical isolation between the two sides
Key points of transformer selection for energy storage system
Rated capacity
The transformer capacity should match the output capacity of the PCS and the operating requirements of the energy storage system. The calculation needs to consider the continuous power, charge and discharge cycle, overload requirements, and possible future expansion requirements.
Transformers with too small capacity may have the problem of excessive temperature rise, while transformers with too large capacity will increase the initial investment and may lead to low operating efficiency under light load conditions.
01
Voltage and wiring group
The primary and secondary voltages should be selected according to the requirements of PCS and the power grid. The wiring group (connection method) of the transformer is also very important, because it will affect the grounding, phase relationship and harmonic performance of the system.
For large-scale battery energy storage projects, transformers are usually specially designed according to the actual PCS configuration, rather than directly using standard distribution transformer specifications.
02
Cooling and temperature
Transformers in energy storage systems (ESS) may need to operate under frequent charge and discharge cycles. Compared with traditional power distribution applications, this will produce different load characteristics.
The installation environment is also critical. Outdoor energy storage projects may face high temperature, dust, humidity or corrosive environments. Therefore, the cooling method and housing design of the transformer should be selected according to local environmental conditions.
03
Harmonics and power quality
Energy storage converter (PCS) equipment uses power electronic components, so harmonic current is a factor that needs to be considered. Transformer design should fully consider the characteristics of PCS.
In some projects, engineers may need to pay special attention to stray loss, temperature rise, impedance, and heat generation caused by harmonics. These factors should be confirmed before the transformer design is finalized.
04
Oil-immersed or dry-type transformer?
Both oil-immersed and dry transformers can be used in energy storage projects.
Dry transformers are usually suitable for indoor environments or places with high requirements for fire safety and low maintenance. They do not use insulating oil, and can be installed adjacent to electrical equipment if the project layout allows.
Oil-immersed transformers are widely used in outdoor energy storage projects, especially in large-capacity scenarios. They usually have good cooling performance and can be configured according to different voltage and capacity requirements.
The final choice depends on capacity, installation location, fire protection requirements, project standards and local regulations.
Why transformer design is essential for energy storage projects
Project engineers and buyers should also verify the voltage ratio, impedance, cooling method, insulation level, temperature rise, loss requirements, harmonic operating conditions, protection cooperation and applicable standards.
For international energy storage projects, it is necessary to confirm whether they meet IEC or other relevant standards before production.
Summary and consideration
With the continuous development of the energy storage industry, transformers will continue to be an important connection link between the battery system and the power grid. Suitable energy storage transformers should be comprehensively selected in combination with PCS, switchgear and grid-connected requirements, and should not be regarded as isolated components.
For energy storage project developers and EPC (design, procurement, construction) parties, for contractors, providing detailed technical information during the quotation phase helps to select transformers more accurately. When formulating transformer plans for specific projects, information such as voltage level, rated capacity, PCS parameters, installation conditions and applicable standards are particularly important.
