Battery energy storage substations connect batteries, power conversion systems, medium-voltage collection networks, and the utility grid into one coordinated power system. The power transformer sits at a critical point in this arrangement. It must transfer charging and discharging power reliably while supporting voltage conversion, insulation coordination, fault-current control, thermal stability, and grid compliance.
Choosing a power transformer manufacturer for a battery energy storage project therefore requires more than matching a rated capacity and voltage ratio. The transformer specification must reflect how the battery energy storage system will actually operate: how much active and reactive power the PCS can deliver, how long the system remains near full output, how frequently power direction changes, what harmonic currents are present, and what environmental conditions apply at the substation.
A suitable power transformer should be designed around this complete operating profile rather than selected as a standard grid transformer with only a different nameplate.
A BESS Power Transformer Is Defined by Its AC Duty
Battery Capacity Does Not Determine Transformer Capacity
A battery energy storage system is commonly described by both power and energy. A project may be rated at 50MW/100MWh, 100MW/200MWh, or another combination. The MW rating describes how quickly the system can charge or discharge, while the MWh rating describes how long that output can be sustained.
The power transformer is primarily sized around the AC-side power flowing through the PCS, not around the battery’s total stored energy. A larger MWh value may extend the operating period without automatically increasing the required transformer MVA rating.
For example, two projects may both use a 50MVA power transformer even though one stores energy for two hours and the other for four hours. However, the longer-duration system may keep the transformer close to rated load for a longer period, which changes the thermal duty and loss calculation.
The manufacturer should therefore receive the real charging and discharging profile rather than only the battery capacity printed on the project summary.
Charging and Discharging Use the Same Transformer
A conventional AC power transformer can transfer power in either direction. The transformer does not normally need to be marketed as a special “bidirectional transformer” simply because the battery charges from the grid and later discharges back into it.
What changes is the operating duty.
During charging, power flows from the grid through the power transformer and PCS into the batteries. During discharging, the direction reverses. In both cases, the manufacturer must understand the maximum apparent power, power factor range, reactive power requirements, operating duration, expected overloads, and switching frequency between different modes.
The important question is not whether the power transformer can operate bidirectionally. It is whether its electrical and thermal design matches the full range of power conditions produced by the BESS.
PCS Data Must Be Confirmed Before Transformer Design
The PCS Defines the Low-Voltage Interface
The power conversion system converts battery DC power into AC power during discharge and converts AC power back into DC power during charging. Its output voltage, current, frequency, harmonic spectrum, grounding requirements, and overload capability directly affect the power transformer specification.
Before production begins, the technical data should identify:
- PCS rated output voltage
- Number and rating of PCS units
- Maximum continuous active power
- Maximum apparent power
- Charging and discharging power limits
- Power factor and reactive power range
- Short-term overload capability
- Total harmonic current distortion
- Switching technology and harmonic spectrum
- Neutral grounding requirements
- Expected operating modes
This information becomes especially important when several PCS units are connected to one transformer winding. The winding current is determined by their combined AC output, while the physical connection arrangement affects busbar design, cable quantity, terminal layout, phase balance, and protection coordination.
A power transformer manufacturer should review the PCS documentation together with the transformer data sheet rather than treating them as separate equipment packages.
MVA Must Include Reactive Power
A BESS may provide reactive power support even when active power is below its maximum value. In some operating modes, the PCS may continue delivering or absorbing reactive power while the battery is not charging or discharging at full MW.
For this reason, selecting the power transformer only from maximum active power can leave insufficient apparent-power capacity.
The required transformer rating should be checked against the complete PCS capability curve. The calculation must include active power, reactive power, auxiliary load, operating margins, temperature correction, and any permitted overload duty.
Oversizing without analysis is not always the best answer. A significantly oversized power transformer can increase capital cost, physical size, no-load loss, and transport requirements. The better approach is to define the real operating envelope and select a rating that provides sufficient thermal and electrical margin without unnecessary excess.
Voltage Configuration Must Match the Whole Substation
The Voltage Ratio Comes from Both Sides of the System
The low-voltage winding normally matches the PCS output or the collection arrangement connected to the PCS blocks. The high-voltage winding connects to the BESS collection system, project substation, or utility grid.
Depending on the project architecture, a power transformer may perform one of several roles:
| Transformer position | Typical function | Main coordination point |
| PCS block transformer | Raises PCS voltage to the MV collector | PCS output and collector voltage |
| Central step-up transformer | Connects the MV collector to the grid | Substation bus and grid voltage |
| Auxiliary transformer | Supplies cooling, controls and station loads | Auxiliary load profile |
| Main grid transformer | Transfers the complete BESS output | Utility interconnection requirements |
A modular project may use many smaller block transformers connected to a medium-voltage collector. A larger project may then use a central power transformer to connect that collector to a 35kV, 66kV, 110kV, or another grid voltage level.
The manufacturer must know where the transformer sits in the single-line diagram. The same MVA rating can require a very different design depending on whether it connects directly to PCS equipment, an MV collector bus, or the external grid.
Vector Group and Grounding Must Be Coordinated Early
The winding connection affects phase displacement, grounding, zero-sequence current paths, protection, harmonic behavior, and parallel operation. It cannot be chosen independently by the transformer factory.
The required vector group should be coordinated with:
- PCS grounding instructions
- Utility grounding practice
- Protection relay philosophy
- Earth-fault detection
- Zero-sequence requirements
- Neutral availability
- Parallel transformer operation
- Harmonic mitigation strategy
Some PCS suppliers may require a grounded transformer neutral, while other systems use a separate grounding transformer or a different collector arrangement. These interfaces should be resolved before winding design and drawing approval.
Changing the vector group after production has started can affect winding connections, insulation arrangement, terminals, protection settings, and project documentation. It is one of the details that should be fixed at the technical clarification stage.
Harmonics Must Be Included in the Thermal Design
Converter Loads Create More Than Fundamental Current
A PCS is a power-electronic converter rather than a purely sinusoidal AC source. Modern converters can maintain good waveform quality, but harmonic current is not automatically zero.
Harmonic currents can increase winding eddy losses, stray losses, local heating, vibration, and sound. Their effect depends on harmonic order, magnitude, transformer construction, conductor dimensions, leakage field distribution, and the surrounding electrical network.
IEEE 519-2022 establishes waveform-distortion goals at the point of common coupling for electrical systems containing linear and nonlinear equipment. This makes harmonic data a system-level input rather than a value that should be assumed by the transformer manufacturer.
The project should provide either a guaranteed PCS harmonic spectrum or a system study. A single total harmonic distortion value is useful, but it may not fully describe the heating effect because higher-order harmonics influence transformer losses differently from lower-order components.
The power transformer design can then consider conductor arrangement, winding dimensions, transposition, structural shielding, cooling margins, and the applicable temperature-rise limit.
DC Components Require Separate Attention
A power transformer is intended to receive AC current. Under normal PCS operation, battery DC does not pass directly through the transformer. However, converter imbalance, control conditions, grounding arrangements, or grid phenomena may introduce a small DC component or DC bias on the AC side.
IEC 60076-1 includes harmonic-content and DC-current considerations within the general requirements for power transformers. It also covers areas such as transport, liquid preservation, tank testing, monitoring facilities, and environmental considerations.
Where a project study identifies possible DC injection or geomagnetically induced current exposure, the value should be included in the specification. The manufacturer can then assess its possible effect on core saturation, magnetizing current, vibration, noise, and heating.
This issue should be based on project data rather than handled through a generic statement that all BESS transformers require the same special core.
Impedance Controls More Than Fault Current
The Lowest Impedance Is Not Always the Best Choice
Transformer impedance influences short-circuit current, voltage regulation, system stability, protection coordination, and load sharing. A lower impedance can reduce voltage drop during normal operation, but it can also increase fault current on the connected bus.
A higher impedance can help limit fault current, although excessive impedance may create unacceptable voltage variation and reactive power demand.
For a battery energy storage substation, the specified impedance should be coordinated with:
- PCS fault-current contribution
- MV and HV switchgear ratings
- Utility short-circuit level
- Protection settings
- Collector cable impedance
- Voltage-drop limits
- Reactive power control
- Parallel transformer requirements
The power transformer manufacturer should work to the approved impedance value and tolerance. It should not independently select the lowest practical figure simply to improve voltage regulation.
Parallel Units Need Closely Matched Parameters
Many BESS projects use multiple transformer blocks or plan future expansion. When two power transformers are expected to operate in parallel, they need compatible voltage ratios, vector groups, phase sequences, tap positions, impedances, and impedance angles.
Large differences can cause unequal load sharing or circulating current even when the total connected load remains within the combined rating.
For phased projects, interchangeability should be discussed before the first order. A later transformer should not be assumed to operate in parallel with an earlier unit merely because both nameplates show the same MVA and voltage ratio.
Manufacturing tolerances, approved drawings, guaranteed losses, tapping ranges, terminal orientation, protection accessories, and test records should be retained as part of the expansion plan.
Thermal Duty Follows the Operating Schedule
Frequent Cycling Is Not the Main Thermal Question
Charging and discharging cycles are central to battery operation, but the power transformer does not experience battery cycling in the same way as the cells. Its temperature responds mainly to current magnitude, losses, cooling conditions, ambient temperature, and the duration of each operating period.
A BESS used for frequency regulation may change power rapidly but spend much of its time below full load. A peak-shaving system may operate close to maximum output for several continuous hours each day. A renewable-energy shifting project may charge heavily during one part of the day and discharge heavily during another.
These applications can have the same rated MW but different transformer thermal profiles.
The manufacturer should therefore receive an operating schedule showing expected loading over time. Where appropriate, a thermal assessment can consider daily cycling, emergency operation, seasonal ambient conditions, cooling-stage control, and the time available for the transformer to cool between high-load periods.
Site Temperature and Altitude Affect Available Capacity
Ambient temperature, solar radiation, altitude, dust, humidity, salt contamination, wind, and installation clearance all influence cooling and insulation performance.
High altitude reduces air density and can affect external insulation and cooling. High ambient temperature reduces the temperature margin available to the transformer. Restricted airflow around radiators can make a suitable factory design perform poorly after installation.
IEC 62933-4-3:2025 addresses environmental effects on battery-based energy storage systems and includes the complete BESS, its power and communication connections, and points of connection within its scope.
For the power transformer, the project specification should state actual site conditions rather than only referencing “outdoor installation.” A transformer intended for a mild coastal site cannot automatically be applied to a high-altitude desert or a corrosive marine environment without review.
Transformer Losses Affect BESS Revenue
Both No-Load and Load Loss Matter
A BESS power transformer may remain energized even when the battery is neither charging nor discharging. During these periods, no-load loss continues as long as the transformer remains connected.
Load loss rises with current and becomes more important during high-power charging and discharging. Harmonic currents can add further loss beyond the fundamental-frequency calculation.
The economic balance depends on the project duty:
- A transformer energized continuously at low average load gives greater importance to no-load loss.
- A transformer operating near rated output for long periods gives greater importance to load loss.
- A frequently dispatched BESS requires both losses to be evaluated across the expected schedule.
- A redundant transformer that remains energized but lightly loaded may accumulate avoidable no-load energy consumption.
The manufacturer’s guaranteed loss values should be compared using the project’s operating profile, not only by selecting the lowest number in one column.
Lower guaranteed loss may require different core material, flux density, conductor area, winding construction, physical dimensions, and material cost. These trade-offs should be evaluated before the technical offer is approved.
Tap Selection Must Support Grid Conditions
Voltage Range Should Be Defined Before Production
Grid voltage does not remain fixed under every operating condition. Collector voltage may also vary with power flow, cable drop, reactive power, and switching configuration.
A tapping range allows the transformer ratio to be adjusted so that operating voltage remains within the required range. Depending on the project, the transformer may use an off-circuit tap changer or an on-load tap changer.
An off-circuit tap changer is suitable where ratio changes are infrequent and the transformer can be de-energized before adjustment. An on-load tap changer supports voltage adjustment while the transformer remains energized, but it adds cost, controls, maintenance requirements, and design complexity.
The decision should be based on the substation voltage-control philosophy. The power transformer manufacturer needs the required tapping range, step size, number of positions, rated tapping position, control requirements, and any parallel-operation logic.
Adding an on-load tap changer as a precaution is not automatically better. In some BESS projects, the PCS already provides fast reactive power and voltage support, while transformer taps are used only for slower system-level adjustment.
Mechanical Interfaces Should Be Approved with the Electrical Design
Cable and Busbar Layout Can Change the Transformer
A technically correct power transformer can still create installation problems when its terminals do not match the site layout.
The transformer drawings should coordinate:
- HV and LV terminal type
- Bushing or cable-box position
- Bus duct or cable entry direction
- Number and size of parallel cables
- Neutral terminal arrangement
- Surge arrester mounting
- Current-transformer requirements
- Marshalling box location
- Radiator clearance
- Lifting and jacking points
- Rail gauge or wheel arrangement
- Oil containment and foundation dimensions
- Fire separation and maintenance access
BESS sites often contain repeating PCS blocks arranged within tight equipment corridors. Terminal orientation and transformer footprint can affect cable length across dozens of identical units. A small mismatch repeated throughout the project can create significant site work.
Drawings should therefore be reviewed by both electrical and civil teams before production release.
Factory Tests Must Verify the Approved Design
Routine Testing Is the Minimum Release Point
The factory test plan should follow the applicable standard and the approved project specification. Depending on transformer rating and project requirements, the scope may include routine tests, type tests, special tests, and customer-witnessed inspections.
Typical checks can cover:
- Winding resistance
- Voltage ratio and phase displacement
- Short-circuit impedance and load loss
- No-load loss and current
- Applied-voltage withstand
- Induced-voltage withstand
- Insulation resistance
- Tap changer operation
- Oil tests
- Tank pressure, vacuum or leakage checks
- Temperature-rise verification where specified
- Sound measurement where specified
- Lightning impulse testing where applicable
- Accessory, alarm and control checks
- Final dimensional and visual inspection
The test procedure should identify the standard, sequence, acceptance criteria, instruments, report format, and witness points.
A test report has more value when it can be traced to the approved transformer serial number, drawings, nameplate data, guaranteed values, and calibration records.
Documentation Must Continue Through Delivery
Technical approval does not end when the power transformer passes its electrical tests. The project may also require final drawings, data sheets, test reports, material records, oil certificates, spare-parts lists, manuals, packing lists, shipping drawings, preservation instructions, and site assembly procedures.
Large transformers may be shipped with radiators, bushings, conservators, coolers, and other accessories removed. Transport conditions should therefore be considered during design and documentation, not arranged only after production is complete.
Clear marking and packing are particularly important when one project contains several similar units. Every removed component should remain traceable to the corresponding transformer and installation position.
Send the Manufacturer a Complete Operating Package
One Data Sheet Is Not Enough for a BESS Project
A useful enquiry package should connect the power transformer specification with the complete battery energy storage substation.
At minimum, the manufacturer should receive:
| Information | Why it matters |
| Single-line diagram | Shows transformer position and network interfaces |
| PCS data sheet | Defines AC voltage, power, harmonics and grounding |
| Charging and discharging profile | Supports rating and thermal review |
| Active and reactive power range | Confirms required MVA |
| Grid voltage and fault level | Supports insulation and impedance coordination |
| Harmonic study or spectrum | Supports loss and heating assessment |
| Site conditions | Affects cooling, insulation and surface protection |
| Protection philosophy | Coordinates vector group, neutral and accessories |
| Layout and cable schedule | Defines terminals, cable boxes and dimensions |
| Applicable standards | Establishes design and testing basis |
| Loss evaluation method | Supports lifecycle cost comparison |
| Delivery and installation plan | Influences transport and accessory arrangement |
This package allows technical questions to be resolved before they become production changes.
Where some information is not yet final, it should be clearly marked as preliminary. The manufacturer can identify which values must be frozen before design release and which can remain open until the drawing stage.
HONLE Supports Project-Specific Power Transformer Manufacturing
The Transformer Is Matched to the Real Substation
HONLE manufactures oil-immersed distribution and power transformer solutions across multiple voltage levels, together with dry-type transformer series for different installation conditions. Our oil-immersed product range covers distribution applications at 10kV, 20kV, and 35kV, as well as higher-capacity 35kV and 110kV power transformers for substations, industrial power systems, renewable energy projects, and grid connections. Each configuration can be developed around the required capacity, voltage ratio, loss targets, cooling method, short-circuit strength, and site conditions.
For battery energy storage substations, our work can begin with the project single-line diagram, PCS information, grid voltage, required rating, impedance, vector group, tapping arrangement, cooling method, environmental conditions, accessories, and test requirements.
The transformer duty also depends on its position within the BESS. A modular PCS block transformer, a medium-voltage collector transformer, and a 110kV grid interconnection transformer serve different functions even when they belong to the same battery energy storage project.
Manufacturing Details Are Controlled Before Shipment
Once the technical requirements are confirmed, the design and approval process can coordinate winding arrangement, core construction, insulation levels, oil circulation, terminal layout, tank structure, cooling equipment, monitoring devices, protection accessories, and transport conditions.
Factory testing then checks the finished power transformer against the approved technical basis. Final drawings and reports support consultant review, site installation, commissioning, and future maintenance.
This connection between project data, transformer engineering, manufacturing control, testing, and documentation is especially important for overseas BESS projects involving developers, EPC contractors, consultants, PCS suppliers, utilities, and installation teams in different locations.
Conclusion
Choosing a power transformer manufacturer for a battery energy storage substation starts with understanding the complete AC operating duty.
The transformer rating should be based on PCS apparent power, charging and discharging limits, reactive power, operating duration, environmental conditions, and required margin—not simply on the battery’s MWh capacity. Voltage ratio, vector group, grounding, impedance, harmonics, tapping, losses, thermal performance, terminals, protection accessories, and factory tests must be coordinated as one system.
A suitable power transformer for a BESS project should be designed around the actual electrical, thermal, mechanical, and environmental conditions of the energy storage substation. When these operating requirements are confirmed early, the transformer can provide better coordination with the PCS, collection system, protection equipment, and grid connection.
As a power transformer manufacturer, HONLE supports this process through project-specific technical review, customized manufacturing, factory testing, documentation, and delivery coordination. By confirming the PCS and grid interfaces before production, the finished transformer can connect more smoothly with the battery energy storage system and provide stable service throughout repeated charging and discharging operation.
Frequently Asked Questions
What type of power transformer is used in a battery energy storage substation?
The transformer type depends on the BESS architecture, voltage level, installation environment, fire-safety requirements, and required capacity. Oil-immersed power transformers are commonly considered for outdoor substations and higher-capacity applications, while dry-type units may suit selected indoor or auxiliary installations.
What does a power transformer do in a BESS project?
A power transformer raises or lowers the PCS AC voltage so the battery energy storage system can connect to the medium-voltage collector network or utility grid. It also affects insulation coordination, fault current, voltage regulation, grounding, and protection.
How is a BESS power transformer sized?
The rating should be based on maximum PCS apparent power, charging and discharging limits, reactive power, auxiliary demand, overload requirements, site temperature, altitude, and operating duration. Battery capacity in MWh alone is not enough to determine transformer MVA.
What voltage should a BESS power transformer use?
The low-voltage side normally matches the PCS output or PCS collection arrangement, while the high-voltage side matches the project collector bus or grid connection. The final voltage ratio must follow the approved single-line diagram and utility requirements.
Is an oil-immersed or dry-type transformer better for a BESS?
Neither type is automatically better for every project. The decision depends on capacity, installation location, cooling conditions, fire strategy, maintenance access, environmental exposure, footprint, and project standards.
Does a BESS need a special bidirectional power transformer?
A conventional AC power transformer can transfer power in both directions. Its design must still match the BESS charging and discharging power, reactive power, harmonic content, thermal duty, and grid requirements.
Is transformer MVA calculated from battery MWh?
No. Battery MWh determines stored energy and operating duration, while power transformer MVA is mainly determined by AC apparent power, including both active and reactive power.
Can several PCS units connect to one power transformer?
Yes, provided their combined voltage, current, apparent power, harmonics, grounding, protection, and overload duty are included in the transformer and system design. Terminal layout and the number of parallel cables should also be confirmed before production.
Why are PCS harmonics important for the power transformer?
Harmonic currents can increase winding losses, stray losses, local heating, vibration, and sound. The power transformer manufacturer should receive the PCS harmonic spectrum or the relevant power-quality study instead of relying only on a general THD value.
What impedance should a BESS power transformer have?
The impedance must be coordinated with fault-current limits, switchgear ratings, voltage drop, PCS operation, protection settings, and parallel transformer requirements. The lowest possible impedance is not automatically the best option.
Should a BESS power transformer use an on-load tap changer?
Not in every project. The choice depends on grid-voltage variation, the voltage-control strategy, PCS reactive power capability, required adjustment frequency, and whether the transformer must change ratio while energized.
What information should be sent to the power transformer manufacturer?
The enquiry should include the single-line diagram, PCS data, charging and discharging profile, active and reactive power range, grid voltage, fault level, harmonic data, site conditions, vector group, impedance, tap requirements, terminal layout, standards, and factory test requirements.