How Compact Substation Selection Affects EV Charging Station Profitability

TECHNICAL GUIDE | EV CHARGING INFRASTRUCTURE

Practical sizing logic, application scenarios, and implementation guidance

Prepared by HENTG POWER

Power Transformation · Smarter Distribution

This guide explains the key selection principles, capacity calculations, application scenarios, and implementation procedures for EV charging substations. At HENTG POWER, we approach compact substation selection as part of an integrated power-distribution solution—not simply as an individual equipment purchase.

As urban charging networks expand and EV ownership continues to rise, privately developed charging stations are becoming an attractive investment opportunity. However, many investors initially focus on land rental, charger procurement, and traffic projections while overlooking the compact substation—the less visible but essential component behind the entire operation.

Problems often emerge only after construction begins: insufficient transformer capacity, enclosure dimensions that do not fit the site, or utility approval delays. The resulting redesign, equipment replacement, and schedule disruption can quickly undermine the original financial model.

Unlike ordinary building-distribution equipment, an EV charging substation connects three critical systems:

• The utility grid
• The transformer and MV/LV distribution system
• The EV charging equipment

Therefore, selection should not be based only on the total nameplate power of the chargers. Grid connection limits, charging patterns, site conditions, available space, operating costs, future expansion, and local standards must all be considered.

1. Actual Load Is the Basis of Transformer Selection

A common mistake is to multiply the rated power of one charging point by the total number of chargers and select a transformer based directly on the result. This method rarely reflects actual charging-station operation.

Private vehicles, buses, delivery fleets, and heavy-duty trucks have different charging patterns. A station normally does not operate with every charger continuously delivering full rated power. A simultaneity or diversity factor should therefore be applied when estimating the real peak demand.

Illustrative Example: Community Charging Station

Consider a community charging station equipped with twelve 120 kW DC fast chargers:

Because all vehicles are unlikely to charge simultaneously at full output during the evening peak, the station does not automatically require a 1,600 kVA transformer.

The final transformer rating should also consider power factor, auxiliary loads, charger efficiency, harmonic characteristics, ambient temperature, expected load growth, and the connection capacity approved by the utility.

Passenger-Car Stations and Heavy-Duty Charging Hubs Are Different

Fleet and heavy-truck charging stations generally have much higher simultaneity factors than community passenger-car stations.

Logistics vehicles often arrive in batches during the evening or early morning and may remain connected to high-power chargers for extended periods. In these applications, the simultaneity factor may reach 0.80–0.90. Two stations with the same number of chargers may therefore require very different transformer capacities depending on the vehicle type and operating schedule.

The Utility Connection Limit Comes First

The distribution network operator normally approves a maximum connection capacity. Even if sufficient space is available on site, the developer cannot install unlimited transformer capacity without grid approval. Obtaining the official connection-capacity approval and confirming the incoming voltage level should therefore be completed before finalizing the compact substation.

Avoid Both Oversizing and Undersizing

Transformer capacity is expressed in kVA, while charger output is normally expressed in kW. The conversion must account for power factor and overall system design margins.

An excessively oversized transformer increases:

• Initial equipment cost
• Civil-foundation requirements
• Switchgear and cable costs
• No-load losses
• Routine maintenance expenses

An undersized transformer may force the chargers to reduce output during peak periods. Longer charging times can reduce station turnover and lead to customer loss. The objective is not to select the largest possible transformer, but to identify the most economical rating that can safely support the actual operating profile.

2. Two Main Compact Substation Configurations

EV charging projects commonly use either an IEC-style prefabricated compact substation—often called a European-style compact substation—or an IEEE/ANSI-style pad-mounted solution.

The most suitable configuration depends on the local grid standard, utility requirements, site layout, maintenance strategy, and budget. HENTG POWER can provide project-specific IEC compact substations and IEEE/ANSI pad-mounted transformer solutions with coordinated MV switching, transformer, protection, and LV distribution arrangements.

European-Style Compact Substation

A European-style compact substation normally contains three clearly separated sections:

• Medium-voltage switchgear compartment
• Transformer compartment
• Low-voltage distribution compartment

The compartmentalized design provides clear operating and maintenance areas and allows MV, transformer, and LV equipment to be inspected separately.

Its advantages include:

• Clear functional separation
• Convenient maintenance access
• Flexible MV and LV configurations
• Easier replacement of standardized components
• Higher adaptability for complex protection and metering requirements

The main disadvantages are a larger footprint, more extensive civil works, and usually a higher initial cost than a compact pad-mounted arrangement of a similar rating.

This configuration is often suitable for:

• Urban roadside charging stations
• Commercial complexes
• Underground parking facilities
• Public charging hubs
• Projects requiring multiple LV feeder circuits
• Sites with strict maintenance-access requirements

For underground installations, local fire regulations may require a dry-type transformer, special ventilation, fire-resistant construction, or additional protection measures.

American-Style Pad-Mounted Solution

An American-style pad-mounted transformer integrates the transformer and primary switching or protection components into a compact, tamper-resistant enclosure. Depending on the project configuration, its footprint may be considerably smaller than that of a conventional European-style compact substation. It can also reduce foundation requirements and simplify site installation.

Its advantages include:

• Compact footprint
• Lower civil-construction requirements
• Shorter installation time
• Suitable outdoor configuration
• Cost-effective standardized design
• Compatibility with loop-feed or radial-feed systems

It is commonly applied in:

• Logistics parks
• Highway service areas
• Outdoor fleet-charging hubs
• Industrial facilities
• North American and Central American distribution networks

Maintenance space is generally more limited, so fault inspection and component replacement may require more careful planning. For locations with dense public access, appropriate safety clearances, barriers, dead-front interfaces, and tamper-resistant construction should be incorporated.

Enclosure Protection Level

The required enclosure rating depends on local standards, utility specifications, installation conditions, and internal equipment. For outdoor applications, IP33 is often treated as a basic reference level, while IP44 or a higher project-specific rating may be preferred in areas with heavy rainfall, high humidity, dust, salt spray, or coastal exposure.

HENTG POWER can configure enclosure protection, ventilation, anticorrosion treatment, and environmental design according to the project location rather than applying one standard enclosure to every site.

3. Capacity Calculations for Typical Charging Scenarios

Case 1: Community Passenger-Car Charging Station

The site is planned for eight 120 kW DC chargers, and the utility has approved a maximum connection capacity of 1,000 kVA.

Because 657 kVA is not a common standard rating, an 800 kVA European-style compact substation may be selected.

This configuration provides a reasonable operating margin. If sufficient LV feeder circuits and an intelligent load-management system are included, limited future charger expansion may be possible without replacing the complete substation, subject to actual operating data and utility approval.

Case 2: Suburban Logistics and Heavy-Truck Charging Hub

The project includes ten 250 kW high-power DC chargers. Trucks are expected to charge intensively at night, so a simultaneity factor of 0.85 is applied.

One possible configuration is two 1,250 kVA pad-mounted transformers or compact substations.

Compared with one large transformer, a dual-transformer arrangement may offer:

• Easier transportation and lifting
• More flexible maintenance
• Better phased-load management
• Partial station operation while one unit is under maintenance
• Reduced risk of complete revenue interruption

If the transformers operate in parallel on a common bus, their voltage ratio, vector group, impedance, tap position, protection, and load-sharing characteristics must be properly coordinated. A sectionalized LV bus may also be considered where operational continuity is more important than full parallel operation.

Case 3: Underground Charging Station in a Commercial Complex

The underground parking facility is planned for six 75 kW DC chargers. Vehicle arrivals are relatively dispersed, and a simultaneity factor of 0.50 is applied.

A 315 kVA compact substation may be suitable, subject to local regulations and the required transformer type. A customized low-profile enclosure can accommodate restricted ceiling height, while separated compartments and coordinated ventilation can reduce thermal risks in enclosed spaces.

After determining the transformer rating, the number and capacity of LV outgoing circuits must also be checked. Adequate spare feeders can make future charger additions significantly easier and reduce the need to replace the complete LV switchboard.

4. Details That Affect Long-Term Operation

Thermal Management

Transformer life and charging-station availability are directly related to thermal performance. A transformer operating near full load continuously generates heat. If ventilation is insufficient, high-temperature protection may operate and cause intermittent charger shutdowns.

Outdoor substations may use natural ventilation louvers, while forced-air ventilation can be added for hot-climate projects. For underground or enclosed installations, the substation ventilation design must be coordinated with the building or parking-garage ventilation system.

HENTG POWER evaluates ambient temperature, solar radiation, altitude, enclosure dimensions, transformer losses, and ventilation requirements when developing a project-specific thermal design.

Cables, Busbars, and Switching Devices

Cable sizes, busbars, circuit breakers, and protection devices must be selected according to the actual transformer and charger load.

Reducing costs by using undersized LV breakers or busbars may create long-term overheating and insulation-aging problems. These issues may not appear during initial commissioning but can result in frequent tripping and increased maintenance costs after several years of operation.

Project specifications should therefore require:

• Components from reputable, project-approved manufacturers
• Compliance with applicable IEC, IEEE, ANSI, UL, CSA, or local standards
• Verified short-circuit withstand capability
• Proper breaker selectivity and protection coordination
• Sufficient busbar current capacity
• Temperature-rise verification
• Adequate cable-termination space

Future Expansion

Many charging stations add more chargers after three to five years of operation. Where grid capacity permits, reserving approximately 15%–20% capacity and sufficient LV outgoing circuits can extend the service life of the station.

The additional initial cost is normally much lower than the combined expense of:

• Replacing the complete transformer or substation
• Rebuilding the civil foundation
• Replacing MV and LV cables
• Modifying switchgear
• Repeating utility approval procedures
• Suspending station operation during reconstruction

Distance from the Grid Connection Point

The distance between the compact substation and the utility connection point also affects project cost and efficiency. Excessive cable length increases cable procurement costs, installation and trenching costs, voltage drop, power losses, and protection-coordination complexity.

Where practical, the compact substation should be positioned close to the approved MV connection point while maintaining safe access, vehicle clearances, drainage, ventilation, and maintenance space.

5. Implementation Process: From Grid Application to Energization

1. Confirm utility connection conditions — Submit the preliminary charging-station layout and estimated demand to the local utility or distribution network operator. Confirm the approved connection capacity, incoming voltage, connection method, metering requirements, protection requirements, and utility interface point. The applicable voltage may be 10 kV, 11 kV, 13.8 kV, 22 kV, 33 kV, or another local distribution voltage.
2. Conduct a site survey — Verify the available installation area, access and lifting conditions, ceiling height, drainage, flood risk, ventilation, cable-routing distance, maintenance clearances, and environmental conditions. The survey determines whether an IEC compact substation, pad-mounted solution, dry-type arrangement, or customized enclosure is most appropriate.
3. Calculate the real charging load — Review the charger layout, rated power, vehicle type, charging schedule, simultaneity factor, auxiliary loads, power factor, harmonics, and expected future expansion. Use these factors to determine the transformer rating, number of transformers, MV switchgear, LV circuits, enclosure rating, ventilation method, protection, and metering arrangement.
4. Prepare and submit technical drawings — The manufacturer should prepare the general arrangement drawing, single-line diagram, foundation drawing, cable-entry plan, equipment data sheets, and protection configuration. Submit these documents to the utility and project consultant before production begins. HENTG POWER supports project teams with technical data confirmation, SLD and GA development, component selection, document review, and customized substation engineering.
5. Install, test, and energize — After delivery, lift the equipment into position, complete the cable connections, and perform testing and commissioning.

Typical commissioning activities include:

• Mechanical inspection
• Cable termination
• Insulation testing
• Protection setting verification
• Transformer testing
• Functional checks
• Communication-system testing
• Utility inspection
• Final grid energization

Among all project stages, utility connection approval and technical drawing approval are the two most important milestones. Sharing the proposed transformer and substation parameters with the utility at an early stage can significantly shorten the approval cycle and support on-time commissioning.

Conclusion

The long-term profitability of an EV charging station is supported by many engineering details. As the core power-supply asset, a compact substation cannot be selected by matching only one transformer rating to the total charger capacity.

The correct solution requires a balance between:

• Utility connection conditions
• Actual peak demand
• Vehicle and charging patterns
• Site restrictions
• Environmental conditions
• Maintenance strategy
• Future expansion requirements
• Applicable technical standards

The recommended approach is to determine the realistic peak demand first, select the appropriate substation configuration for the site, and reserve a reasonable expansion margin. This helps control the initial investment while reducing hidden power losses, charger derating, unplanned shutdowns, and expensive future reconstruction.

HENTG POWER provides integrated transformer and power-distribution solutions for EV charging infrastructure, including IEC compact substations, IEEE/ANSI pad-mounted transformers, MV switchgear integration, LV distribution systems, protection coordination, and project-specific engineering support.