35kV Transformer Selection Guide: Applications and Key Specifications

A 35kV transformer is used to transfer electrical power between a medium-voltage network and another voltage level, commonly for utility distribution, industrial facilities, renewable energy projects, and infrastructure. To select one, first confirm the system voltage and required secondary voltage, then define capacity, installation conditions, insulation and protection requirements, and applicable project standards. For example, a 35kV-to-0.4kV configuration may suit a low-voltage distribution application, but it is only one possible design. At Huarui, we recommend basing a quotation on the project’s electrical data and site conditions rather than choosing from voltage alone.

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Key Takeaways

• Confirm whether “35kV” refers to the network voltage or the transformer’s rated high-voltage winding, and verify the required insulation level with the project specification.
• Match transformer capacity, secondary voltage, impedance, vector group, cooling, and tap arrangement to the load and protection design.
• Choose oil-immersed or dry-type construction according to location, fire and environmental requirements, maintenance plans, and installation constraints.
• Request a project-specific quotation with the required drawings, standards, test documentation, delivery terms, and accessories clearly defined.

Who This Guide Is For

This guide is intended for electrical contractors, EPC companies, utility procurement teams, industrial facility owners, and project engineers evaluating a 35kV transformer. It is useful during preliminary design, supplier comparison, and preparation of an RFQ. It does not replace a detailed engineering review, because final requirements depend on the network, load profile, local regulations, and installation environment.

What a 35kV Transformer Does

Basic operating role

A transformer changes AC voltage through electromagnetic induction while transferring power between circuits. In a typical distribution arrangement, the high-voltage winding connects to a 35kV network and the low-voltage winding supplies a distribution bus or downstream equipment. The actual ratio, winding connection, and operating duty must match the project design.

Transformers do not generate electricity, and they do not by themselves provide complete system protection. The installation normally requires coordinated switching, surge protection, grounding, metering, and overcurrent protection selected by the project’s electrical designer. Fault levels and protection settings should be checked against the transformer’s impedance and the connected network.

Types and Construction Options

Oil-immersed transformers

Oil-immersed designs use insulating liquid for electrical insulation and heat transfer. They are commonly considered for substations, outdoor installations, and applications where the project can provide suitable containment, clearances, and maintenance access. Buyers should identify the required fluid type, tank arrangement, cooling method, monitoring accessories, and any environmental or fire-safety requirements.

Dry-type transformers

Dry-type transformers use solid insulation rather than an insulating liquid and may be suitable for indoor installations or locations where liquid containment is undesirable. Their suitability depends on the required voltage and capacity, ventilation, enclosure, fire strategy, ambient conditions, and manufacturer design. Do not assume that dry-type construction automatically removes all fire, ventilation, or maintenance considerations.

Construction choice should be based on the complete site specification, not on a general preference for one type. We recommend comparing installation space, access for replacement, operating environment, safety requirements, and expected maintenance arrangements before finalizing the design.

Applications and Voltage Matching

35kV transformers can serve utility substations, industrial plants, mining and processing facilities, commercial infrastructure, and renewable energy collection systems. In each case, the transformer may step voltage down for local distribution or be part of a project-specific voltage conversion arrangement. The application determines the duty cycle, load characteristics, required reliability, and accessory package.

For a distribution example, a project may specify a 35kV primary and a 0.4kV secondary. This is not a universal configuration; other projects may require a different secondary voltage or a higher-voltage interconnection. Confirm the nominal system voltage, maximum operating conditions, grounding method, and required equipment insulation coordination with the electrical designer.

Key Specifications to Define

These values should be stated in the project datasheet rather than inferred from a product label. A frequency of 50Hz or 60Hz, for example, is a system requirement to confirm; it is not a feature that can be assumed for every 35kV unit. Likewise, the requested insulation level and tap range should follow the applicable standard and network conditions.

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A Practical Selection Framework

1. Establish electrical requirements

Record the primary and secondary voltages, rated power, frequency, number of phases, load type, and expected load profile. Identify whether the transformer will supply motors, rectifiers, variable-speed drives, or other loads that may affect harmonics or starting duty. Include any requirement for parallel operation with existing units.

2. Check the site and operating environment

Provide the installation location, indoor or outdoor status, ambient temperature range, altitude, humidity, pollution exposure, available space, and access route. Note fire-safety measures, noise restrictions, seismic or vibration requirements, and any environmental constraints. These details help the supplier assess construction, cooling, enclosure, and accessories.

3. Align protection and system integration

Ask the project engineer to confirm fault levels, grounding, surge protection, relay coordination, and the required transformer impedance. Confirm the vector group and terminal arrangement with connected equipment and the site single-line diagram. If the unit will operate in parallel, verify compatibility requirements before placing an order.

4. Define standards and deliverables

State the required national or international standards, inspection scope, routine and any specified type tests, drawings, manuals, and nameplate information. Requirements vary by market and project, so a supplier should not assume one standard applies to every order. Clarify whether inspection by a third party, special packaging, or export documentation is required.

Pricing, MOQ, and Lead-Time Considerations

The price of a 35kV transformer depends on its rated power, winding materials, insulation and cooling design, efficiency requirements, accessories, testing scope, and delivery destination. Oil-immersed and dry-type designs also differ in construction and installation requirements, so compare quotations only when their specifications are aligned. A lower initial price may exclude items such as protection accessories, special testing, or export packing.

Minimum order quantity and production lead time are supplier- and project-dependent; they can also change with customization, material availability, production scheduling, and inspection requirements. We do not recommend relying on a generic lead-time estimate for a custom unit. Ask for a written quotation that identifies the technical scope, quantity, delivery basis, schedule assumptions, warranty terms, and any exclusions.

Supplier Evaluation Checklist

When comparing suppliers, assess their ability to review the datasheet, explain technical deviations, and provide drawings for approval before production. Confirm the proposed manufacturing scope, quality documentation, inspection and testing plan, packaging method, and after-sales communication process. Request relevant product documentation for the specific design being offered rather than relying on broad capability statements.

• Can the supplier confirm the voltage ratio, capacity, vector group, impedance, and insulation requirements?
• Are cooling, enclosure, terminals, tap arrangement, and accessories itemized in the offer?
• Are applicable standards, tests, inspection points, and documentation clearly stated?
• Does the quotation define delivery terms, schedule, warranty scope, and exclusions?
• Can technical questions be resolved through a documented review before order confirmation?

Conclusion: Choose from the Project Data

The right 35kV transformer is the unit whose electrical ratings, construction, insulation, cooling, protection interfaces, and documentation match the project—not simply one marked “35kV.” Start with the network and load data, verify site conditions, and align the design with the engineer responsible for system protection and compliance. Then compare suppliers using equivalent technical scopes and written commercial terms.

At Huarui, we can review your project requirements and discuss a suitable transformer configuration as part of your equipment sourcing process. To begin an inquiry, prepare the voltage ratio, required kVA or MVA, frequency, installation environment, applicable standards, quantity, and target delivery location. The more complete the specification, the more useful and comparable the technical offer can be.