Power Transformer Manufacturer for EV Charging Hubs and Fleet Depots

A few EV chargers can often be added to an existing commercial electrical system without changing the entire power distribution strategy. A large charging hub or fleet depot is different.

When dozens of chargers are concentrated at one site, the power transformer may need to support several megawatts of connected load, repeated periods of high charging demand, power-electronic loads, and future expansion. The installed charger capacity is important, but it is only the starting point.

For these projects, we need to know how the chargers will actually operate: how many may run together, how long high-load periods last, what harmonic characteristics the charging equipment has, and how much additional charging capacity may be added later.

At HONLE, we review these conditions together with voltage ratio, transformer capacity, impedance, cooling, installation environment, terminals, accessories, and the surrounding substation arrangement before the transformer specification is finalized.

EV Charging Hubs Create Concentrated Electrical Loads

Large charging hubs bring substantial electrical demand into a relatively small part of the distribution system. This becomes especially relevant when a project includes multiple high-power DC chargers.

Adding all charger ratings together gives the installed charging capacity, but that figure does not necessarily represent the load the transformer will carry in normal operation.

A site with 20 chargers rated at 240kW each, for example, has 4800kW of installed charger output. Actual simultaneous demand may be lower depending on vehicle arrival patterns, charger utilization, power sharing, battery state of charge, and the site’s charging management strategy.

Table 1. Illustrative Simultaneous Charging Load for 20 × 240kW Chargers

These figures illustrate charger output only. They are not transformer ratings. Transformer selection also needs to consider charger AC input characteristics, efficiency, power factor, auxiliary loads, operating profile, and the required design margin.

Public Charging Hubs Can Have Irregular Demand Peaks

A public fast-charging hub serves vehicles arriving at different times and with different battery states.

One charger may begin near a high output level and then reduce charging power as the vehicle battery fills, while another vehicle arrives and begins charging. The total site load therefore changes continuously rather than remaining at one fixed level.

Location also affects the load profile. Highway charging hubs, urban charging centers, shopping destinations, and transport terminals can have very different demand patterns during the day.

For these sites, expected peak coincident demand and realistic charger operating data should be included in the transformer load calculation.

Fleet Depots Can Produce Concentrated Charging Windows

Fleet depots often have a different operating pattern.

Buses, delivery vans, logistics vehicles, municipal vehicles, and other commercial fleets may return to the same site within a relatively short period. If many vehicles need to recharge before the next operating shift, charging demand can remain high for several hours.

The schedule may be more predictable than at a public charging hub, but the peak load can still be substantial.

Some fleet operators stagger charging times, prioritize particular vehicles, or use load-management systems to keep site demand within a defined limit. These operating rules should be reflected in the transformer sizing process.

Transformer Capacity Should Follow the Real Charging Demand

Selecting a power transformer only from the sum of charger nameplate ratings can create two problems.

The transformer may be oversized for a condition that rarely occurs, or it may be sized around an unrealistically low simultaneous load that leaves insufficient capacity as charger utilization increases.

A more useful load calculation includes:

• charger quantity;
• charger AC input rating;
• expected maximum simultaneous demand;
• charger efficiency and power factor;
• site auxiliary loads;
• load-management strategy;
• expected duration of high-load periods;
• present and future utilization;
• required operating margin.

Auxiliary loads also need to be included. Lighting, HVAC or ventilation, monitoring equipment, payment systems, communications, workshops, offices, pumps, and other supporting facilities may share the same electrical infrastructure.

For large EV projects, a realistic load schedule gives us a much better basis for transformer selection than charger nameplate ratings alone.

Charger Harmonics Need to Be Reviewed Before the Transformer Design Is Fixed

DC fast chargers use power-electronic conversion between the AC supply and the vehicle battery. Their input current may therefore contain harmonic components that should be considered in larger charging installations.

This does not mean every EV charging hub needs a special transformer.

Charger designs, power-factor-correction methods, filtering, operating conditions, and system configurations vary. The transformer specification should be based on actual charger harmonic data or the results of a project harmonic study where required.

Harmonic current can increase winding eddy-current losses and other stray losses, which may contribute to additional heating. The load-current spectrum therefore becomes relevant when the thermal performance of the transformer is evaluated.

Harmonic Data Is More Useful Than a General “EV Load” Label

Useful information can include:

• charger manufacturer and model;
• rated AC input;
• input power factor;
• total harmonic distortion data;
• individual harmonic spectrum when available;
• number of chargers connected to the transformer;
• expected operating combinations;
• filtering or other power-quality equipment in the system.

With this data, we can review the power transformer against the actual electrical conditions of the charging equipment rather than treating all EV charging loads as the same.

Long Charging Periods Also Affect Thermal Design

A short demand peak and several hours of sustained high loading place different demands on the transformer.

Fleet depots are a good example. If many vehicles charge through the night, the transformer may operate at a relatively high load for long periods. A busy public charging hub may instead experience repeated peaks throughout the day.

Ambient temperature, transformer-room ventilation, cooling method, allowable temperature rise, and expected loading should therefore be confirmed together.

Where harmonic loading is significant, the thermal review should also use the harmonic characteristics provided by the charger supplier or system study.

Impedance Must Coordinate With the Charging System

Transformer percentage impedance affects both secondary fault current and voltage behavior under load.

A lower impedance can reduce voltage drop during heavy loading, but it may also increase prospective short-circuit current. A higher impedance can help limit fault current but may result in a larger voltage drop as load increases.

The required value therefore needs to coordinate with:

• switchgear short-circuit ratings;
• protection settings;
• cables and busbars;
• secondary voltage requirements;
• charger operating limits;
• utility or project specifications.

There is no single impedance value that fits every EV charging hub.

For larger projects, the required impedance should be confirmed from the system study before the transformer winding design is finalized.

Future Expansion Should Be Included in the First-Stage Plan

Many charging hubs are built in phases.

A site may open with only part of its planned charging area while parking spaces, cable routes, switchgear positions, and electrical infrastructure are reserved for later expansion. Fleet operators may also electrify vehicles gradually rather than replacing an entire fleet at once.

That future load should be discussed during the first transformer selection.

Planning for expansion, however, does not mean simply choosing the largest transformer possible. Present demand, final site capacity, utility limits, available space, and the planned substation arrangement all need to be considered.

Phase 1 and Final Build-Out Should Both Be Defined

A useful project plan may include:

• Phase 1 charger quantity and power;
• expected Phase 1 coincident demand;
• Phase 2 charger quantity;
• possible future higher-power chargers;
• final planned charging capacity;
• utility connection limit;
• available substation space;
• switchgear and busbar ratings.

This helps avoid designing the first stage in a way that makes later expansion unnecessarily difficult.

A Second Transformer May Be Better Than One Oversized Unit

Some sites can reserve additional capacity in the first transformer. Others may be better served by adding another power transformer when the next charging phase is built.

The choice depends on site load, grid capacity, redundancy requirements, physical layout, investment schedule, protection scheme, and maintenance strategy.

At a large fleet depot, separate transformers may also divide different charging zones or operating areas. At a public charging hub, another transformer can be added as additional charging bays are commissioned.

Whether the first order represents the finished site or only the first construction phase should therefore be clear from the beginning.

Oil-Immersed and Dry-Type Transformers Fit Different Charging Sites

There is no single transformer construction that fits every EV charging project.

HONLE manufactures both oil-immersed and dry-type transformers, together with 35kV and 110kV oil-immersed power transformers. The power transformer type can therefore be selected according to the installation environment, capacity, voltage class, cooling requirements, and overall substation arrangement.

Outdoor Substations Often Use Oil-Immersed Transformers

Highway charging hubs, logistics yards, bus depots, and other open sites may place the power transformer in a dedicated outdoor electrical area.

Oil-immersed transformers are commonly used for outdoor distribution and higher-capacity power applications. Their insulating liquid provides both electrical insulation and heat transfer from the active parts.

For these projects, we can review oil-immersed distribution transformers as well as higher-voltage transformer solutions according to the required capacity and utility connection.

The final selection also depends on site environment, fire and safety requirements, local standards, and substation layout.

Indoor Electrical Rooms May Favor Dry-Type Designs

Some charging facilities place transformers inside buildings, parking structures, service areas, or dedicated indoor electrical rooms.

Dry-type transformers can be considered where the building arrangement and project requirements favor this construction.

Ventilation, ambient temperature, clearances, enclosure requirements, noise expectations, cable routing, and fire-safety requirements should all be confirmed before the transformer arrangement is fixed.

Because our power transformer range includes both dry-type and oil-immersed designs, the transformer construction can be matched to the actual installation rather than selected only by capacity.

Larger Charging Projects May Require Higher-Voltage Connections

Not every EV charging hub needs a 35kV or 110kV power transformer.

The required voltage level depends on local utility infrastructure, total site demand, grid connection arrangements, and the role of the transformer within the electrical system.

As charging capacity grows, however, some projects move beyond a conventional local distribution transformer. A large fleet charging center or multi-megawatt charging hub may require a dedicated substation or a higher-voltage utility connection.

HONLE’s range includes oil-immersed distribution transformers as well as 35kV and 110kV oil-immersed power transformers. For larger projects, we can review rated capacity, voltage ratio, impedance, cooling method, insulation requirements, tap-changer arrangement, installation conditions, testing scope, and project documentation according to the actual system design.

The voltage level should always follow the grid connection and power-system requirements rather than the charger count alone.

What Should Be Sent to the Power Transformer Manufacturer?

A useful transformer inquiry should contain more than a capacity and voltage ratio.

For example, a request that only states “EV charging station transformer, 2500kVA” leaves several important points unresolved.

Table 2. Key Data for an EV Charging Hub Transformer Inquiry

For replacement projects or sites where switchgear has already been selected, existing drawings are also useful.

Terminal position, cable-box requirements, busbar arrangement, footprint limitations, and installation access can all affect the final transformer configuration.

Manufacturing Details Become More Important as Transformer Size Increases

Once the main electrical parameters are confirmed, the transformer still needs to be designed around the required current, insulation level, cooling, mechanical strength, short-circuit duty, and installation conditions.

Core and winding design influence transformer losses and thermal performance. Terminal and bushing arrangements need to match the project connection. Tank or enclosure design must suit cooling and site installation requirements.

These details become more important as a project moves from a conventional distribution transformer into larger 35kV or 110kV equipment.

HONLE manufactures distribution-level oil-immersed and dry-type transformers as well as 35kV and 110kV oil-immersed power transformers. For larger units, we can coordinate project-specific requirements including rated capacity, voltage ratio, impedance, cooling, insulation, tap changing, installation conditions, testing, and documentation.

For EV charging projects, the transformer design should always return to the same set of questions: how much load the site will actually carry, how that load behaves, where the transformer will be installed, and how the site is expected to grow.

A Large Charging Hub Needs a Transformer Designed Around the Site

EV charging hubs and fleet depots can place substantial and highly concentrated demand on the electrical system.

Transformer capacity should be based on actual simultaneous demand rather than charger nameplate totals alone. Charger harmonic characteristics should be reviewed where relevant. Long charging periods need to be included in the thermal assessment. Transformer impedance must coordinate with the surrounding electrical system, and future expansion should be considered before the first phase is built.

As a Power Transformer Manufacturer, we can review the project from voltage, capacity, charging profile, harmonic data, impedance, cooling, installation environment, terminals, testing requirements, and future expansion.

The final transformer should not simply match the total charger kW. It should match how the charging hub or fleet depot is expected to operate throughout the life of the project.

Feel free to contact us.

FAQ

Do EV charging hubs need special power transformers?

Not automatically. The transformer should be selected according to the actual charger load, harmonic characteristics, operating profile, voltage level, installation environment, and system requirements.

How should transformer capacity be calculated for multiple EV chargers?

Start with the chargers’ AC input requirements and expected maximum simultaneous demand, then include auxiliary loads, power factor, operating margin, and future expansion. Adding all charger output ratings together does not provide the complete transformer requirement.

Do DC fast chargers affect transformer harmonics?

DC fast chargers use power-electronic conversion and may introduce nonsinusoidal current. The actual harmonic level depends on charger design and operating conditions, so larger projects should use charger data or a system harmonic study where required.

Should a fleet depot size its transformer for every vehicle charging at once?

Only if that operating condition is required. If charging is scheduled, staggered, or actively load-managed, the expected maximum coincident demand can be considered together with the required design margin and future charging plan.

Can HONLE supply power transformers for larger EV charging substations?

Yes. Our range includes oil-immersed and dry-type distribution transformers as well as 35kV and 110kV oil-immersed power transformers for projects with different voltage, capacity, installation, and substation requirements.