With the acceleration of the global energy transition, energy storage systems are playing an increasingly important role in the power grid. As the key hub connecting energy storage batteries to the grid, the transformer that connects energy storage equipment serves an indispensable function in the entire energy transmission chain. This article provides a comprehensive analysis of its working principle, technical characteristics, selection guidelines, and future trends.
An energy storage system typically consists of core components such as batteries, a power conversion system (PCS), and a transformer. Its main function is to convert the direct current (DC) from the battery into alternating current (AC) and then step up the voltage for grid connection or direct supply to loads. In this process, the transformer undertakes the dual tasks of voltage conversion and electrical isolation – the PCS converts battery DC into low-voltage AC (e.g., 400V, 690V, 800V), and the transformer steps it up to medium or high voltage (e.g., 10kV, 35kV or even 110kV) to meet grid requirements. XINHONG ELECTRICAL focuses on technology accumulation and product development in the field of energy storage transformers, providing high-performance solutions in this area.
From a system architecture perspective, a PCS step-up system (integrated storage step-up system) consists of four core modules: PCS, step-up transformer, switchgear and protection system, and monitoring/dispatching system. Among them, the transformer is the “energy hub” – its performance directly determines the system’s conversion efficiency and operational stability.
The working principle of a transformer connecting energy storage equipment is based on Faraday’s law of electromagnetic induction. The basic mechanism is: when an AC voltage is applied to the primary winding, a changing magnetic flux is generated in the core. This flux passes through the secondary winding, inducing an electromotive force (voltage) according to the principle of electromagnetic induction, thereby transforming the voltage level.
This can be expressed as: U₁/U₂ = N₁/N₂, where U₁ and U₂ are the primary and secondary voltages, and N₁ and N₂ are the number of turns of the primary and secondary windings. By precisely designing the turn ratio, the transformer can step up low voltage to high voltage or step down high voltage to low voltage, flexibly adapting to different voltage requirements in the energy storage system.
Another critical function of the energy storage transformer is electrical isolation. In the transformer structure, there is no direct electrical connection between the primary and secondary windings – energy is transferred only through magnetic coupling in the core. This physical isolation provides multiple safety benefits.
First, it effectively blocks surges, lightning strikes, harmonics, and high‑frequency noise from the grid side from coupling into the battery side, protecting energy storage equipment from external electromagnetic interference. Second, when an inverter fault causes abnormal DC component output, the isolation transformer prevents DC components from entering the grid, avoiding damage to grid equipment. Moreover, in high‑voltage environments, this isolation mechanism prevents electric shock risks caused by insulation faults. XINHONG ELECTRICAL offers a range of high‑performance dry‑type isolation transformers that fully meet industry safety requirements. In industrial areas with severe voltage fluctuations, this isolation mechanism can significantly reduce system downtime.
During operation, especially when the inverter performs PWM modulation, the output current of energy storage systems often contains high‑order harmonics (mainly 3rd, 5th, and 7th), with total harmonic distortion (THD) possibly exceeding 5%. Through optimised winding structures and the use of low‑loss silicon steel cores, transformers can suppress harmonics to a certain extent and improve grid power quality. Additionally, in complex application scenarios such as PV‑storage‑charging systems, transformers can achieve impedance matching between different modules through winding ratios and magnetic circuit design, ensuring smooth system operation.
In energy storage systems, the two‑winding transformer is currently the most widely used type. Its core structure includes a primary winding (high‑voltage side) and a secondary winding (low‑voltage side), magnetically coupled through the core. Two‑winding transformers are favored in the energy storage field for three main reasons:
Efficient bidirectional energy flow: Energy storage systems need to frequently switch between charging (grid to battery) and discharging (battery to grid). The low impedance of two‑winding transformers reduces energy transmission losses and improves system efficiency.
Compact structure: Energy storage power stations typically use a centralised design. Two‑winding transformers have a simple structure, effectively reducing footprint and lowering construction costs.
Flexible voltage matching: By adjusting the turn ratio, the transformer can flexibly adapt to the voltage difference between the battery pack and the grid.
In terms of efficiency, a typical two‑winding transformer can achieve efficiency above 98.5%, with annual maintenance costs around 0.5%–1% of the initial equipment value – combining high efficiency with low operating expenses.
The isolation transformer, with its unique construction of no direct electrical connection between primary and secondary windings, plays an irreplaceable role in ensuring the safety of energy storage systems. It can:
Block system interference: Prevent common‑mode interference, surge voltages, and other disturbances from propagating (interference attenuation rate >90%).
Isolate DC components: Prevent core saturation and heating caused by inverter DC bias, protecting downstream equipment.
Provide safety protection: Achieve high‑voltage to low‑voltage safety isolation through reinforced insulation (insulation resistance ≥1000MΩ, withstand voltage 3kV/1min), complying with safety standards such as GB/T 20046‑2006.
In addition to line‑frequency transformers, high‑frequency transformers and solid‑state transformers (SSTs) represent the frontier of energy storage transformer technology. High‑frequency transformers operate at higher frequencies, significantly reducing volume and weight – a key path to miniaturisation and lightweight energy storage equipment. SSTs go a step further, using a combination of power electronic converters and high‑frequency transformers, with wide‑bandgap devices such as SiC for high‑frequency power conversion. They can reduce size and weight by more than 50% while also enabling active, intelligent control of power flow. SSTs offer functions such as stepless voltage regulation, instantaneous fault isolation, active harmonic filtering, and DC output – making them an important direction for the future evolution of distribution systems toward higher efficiency, digitalisation, and flexibility.
Power matching is the primary principle when selecting an energy storage transformer. The rated capacity of the transformer should be greater than or equal to the rated output power of the PCS. Typically, a 10%–15% capacity margin is recommended to handle system transients and capacity degradation over time. For example, a 10MW PCS should be paired with a step‑up transformer rated at 11–12 MVA.
In practical engineering, the matching coefficient between transformer capacity and PCS power is usually around 0.8, ensuring efficient operation within a safe margin.
The installation environment of an energy storage transformer imposes different protection requirements. For indoor switchgear rooms, IP54 (dust‑protected and splash‑proof) may be sufficient. For outdoor open‑air installation, IP65 or higher is required for reliable operation under rain, snow, and dusty conditions. Additionally, high‑temperature environments require Class H insulation and forced air cooling; plateau areas require increased creepage distances; and sites with severe harmonic pollution should use low‑loss silicon steel cores and sectionalised winding structures.
Energy efficiency class is a key indicator of transformer economy. According to the Chinese national standard “Minimum allowable values of energy efficiency and energy efficiency grades for power transformers”, transformers of Grade 1 have the lowest no‑load and load losses, offering the best long‑term energy performance. During selection, high‑efficiency products are preferred, targeting an efficiency of no less than 98% at 70%–80% load. XINHONG ELECTRICAL specialises in the R&D and manufacturing of high‑efficiency dry‑type energy storage transformers. By optimising material selection and structural design, we help customers achieve lower operating costs over the entire lifecycle.
Furthermore, long‑term operational indicators such as design life (recommended ≥20 years) and average annual failure rate (recommended ≤0.5%) should be considered to achieve the best balance between procurement cost and operating cost.
Energy storage transformers are widely used in commercial & industrial (C&I) storage, grid‑side storage, and renewable energy power stations. In C&I scenarios, integrated storage cabinets or centralised storage systems use transformers to achieve peak shaving, valley filling, demand management, and other functions, helping enterprises reduce electricity costs. In grid‑side applications, large‑capacity storage stations connect to the distribution grid via centralised transformers to provide ancillary services such as peak shaving, frequency regulation, and voltage support, improving grid power quality. In renewable energy stations (e.g., wind farms, PV plants), storage systems work with transformers to smooth out the intermittency and fluctuation of renewable generation, improving grid friendliness. In PV‑storage‑charging integrated scenarios, the transformer serves as an impedance‑matching hub between different modules, ensuring efficient power flow from PV to storage while meeting the instantaneous high‑power demands of EV fast charging.
With the emergence of new models such as DC microgrids with PV and storage, the application prospects for solid‑state transformers (SSTs) are expanding. Such devices can directly convert medium‑voltage AC into high‑voltage DC, integrating PV, storage, and charging equipment on a common DC bus, enabling efficient energy dispatch and flexible management.
Energy storage transformer technology is currently evolving along three directions: first, efficiency upgrades – higher‑efficiency transformers are gradually replacing conventional ones, with ever‑lower no‑load and load losses; second, intelligent operation and maintenance – by embedding sensors and IoT modules, transformers can achieve real‑time status monitoring, fault warning, and remote management, greatly improving O&M efficiency; third, high‑frequency and integration – the commercialisation of high‑frequency transformers and solid‑state transformers is accelerating, offering more possibilities for high‑power‑density design and system integration of energy storage systems.
At the industry level, continued growth in renewable energy installations is driving rising demand for energy storage transformers. As the energy transition accelerates and power systems become more intelligent, high‑performance, high‑reliability, and intelligent energy storage dedicated transformers will become key equipment supporting the construction of new‑type power systems.
The working principle of a transformer connecting energy storage equipment can be summarised as: based on the law of electromagnetic induction, it performs voltage conversion and electrical isolation between the energy storage battery and the grid. It is both an indispensable energy conversion medium in the energy storage system and a critical barrier for safe system operation. When selecting a transformer, factors such as power matching, voltage level, environmental adaptability, and energy efficiency class must be comprehensively balanced to ensure safe, efficient, and stable operation of the energy storage system throughout its life cycle.
About XINHONG ELECTRICAL
XINHONG ELECTRICAL is dedicated to the R&D and manufacturing of energy storage transformers and power electronic electrical equipment. Our product portfolio includes dry‑type isolation transformers, energy storage step‑up transformers, high‑frequency transformers, and supporting reactors. These products are widely used in C&I energy storage, grid‑side storage, renewable energy stations, and PV‑storage‑charging integration scenarios. With solid technical expertise and a comprehensive quality management system, XINHONG ELECTRICAL continues to provide high‑efficiency, high‑reliability power conversion solutions for energy storage applications to global customers. For any needs regarding energy storage transformer selection or customised development, please contact us for professional technical support and service.
