35kV / 36kV Substation Transformer Selection Guide for Export EPC Projects

Introduction

For overseas EPC and utility projects, 35kV and 36kV transformer selection should start from the grid interface, project specification and local utility approval route. The same transformer rating may be accepted in one market and rejected in another if voltage ratio, impedance, loss level, tapping range, protection interface or documentation is incomplete.

This guide explains how an overseas buyer, EPC contractor or engineering consultant can specify a 35kV / 36kV substation transformer package without relying on a single-country design code. The recommended reference framework is IEC, local utility requirements and the written project specification.

1. Export Selection Workflow

Step	Buyer decision	What to confirm before RFQ
Grid voltage	35kV, 33kV, 34.5kV or 36kV class	Utility nominal voltage, highest system voltage, frequency and insulation level
Secondary voltage	6.3kV, 10.5kV, 11kV, 13.8kV or 0.4kV system	Downstream switchgear rating, neutral arrangement and protection philosophy
Transformer type	Oil-immersed or cast resin dry-type	Indoor/outdoor installation, fire strategy, maintenance access and environmental limits
Loss level	Standard, high-efficiency or tender-specific loss guarantee	No-load loss, load loss, auxiliary loss and evaluation method
Impedance	Typical 6-8% range, project dependent	Short-circuit level, voltage regulation, parallel operation and protection coordination
Documentation	FAT, drawings, datasheets and routine test reports	Submission format required by EPC, consultant or utility reviewer

2. Oil-Immersed vs Dry-Type Selection

Selection factor	Oil-immersed transformer	Cast resin dry-type transformer
Outdoor substations	Common choice for outdoor yards and containerized substations	Possible with enclosure, but cooling and IP rating must be checked
Indoor plant rooms	Needs oil containment and fire protection review	Often preferred where fire load and indoor safety are critical
Large rating and overload	Strong thermal capacity and mature cooling options	Suitable for indoor distribution ratings with forced air where specified
Maintenance	Oil testing and inspection required	Lower routine maintenance, but ventilation must be well designed
Buyer focus	Loss guarantee, oil quality, bushings, protection devices	Partial discharge, temperature rise, enclosure and ventilation design

3. Voltage Ratio and Tap Range

The nameplate voltage ratio must match the local incoming network and the downstream distribution system. For export projects, do not assume that a 35kV label is enough. Many tenders require explicit confirmation of highest system voltage, impulse withstand level, frequency, vector group and tap changer range.

• Confirm whether the project uses 50Hz or 60Hz.
• Confirm primary voltage tolerance and highest system voltage.
• Define off-circuit or on-load tap changer requirements.
• Match secondary voltage with MV switchgear and downstream transformers.
• Confirm vector group for grounding, protection and parallel operation.

4. Capacitor Bank and Power Factor Coordination

Capacitor banks should be coordinated with transformer impedance, harmonic environment and switching equipment. Overseas EPC projects often require the supplier to provide transformer data that allows the consultant to verify power factor correction, inrush current and resonance risk.

RFQ item	Why it matters
Transformer impedance	Affects short-circuit level and capacitor switching duty
No-load and load loss	Used for lifecycle cost and efficiency evaluation
Harmonic study input	Required where VFDs, rectifiers or solar inverters are present
Protection interface	Needed for transformer, capacitor and feeder coordination
Single-line diagram review	Prevents mismatch between transformer, switchgear and capacitor bank

5. Documents Overseas Buyers Should Request

• Technical datasheet with rated power, voltage ratio, impedance, vector group, tapping range and cooling method.
• General arrangement drawing, foundation drawing and terminal arrangement drawing.
• Routine test report and factory acceptance test plan.
• Type test references where required by the tender.
• Oil quality report or dry-type partial discharge report, depending on transformer type.
• Packing list, nameplate drawing and installation manual for site commissioning.

6. Reference Standards and Approval Route

For international procurement, the base technical language should be IEC 60076 for power transformers, IEC 61936-1 for power installations above 1kV, and the local utility or consultant specification for site acceptance. If the project has its own national code or utility standard, that requirement should be listed in the RFQ and checked against the supplier documentation before production release.

Buyer Recommendation

Before placing an order, ask the transformer supplier to submit a complete technical schedule and drawing package for consultant review. The safest purchasing path is not simply choosing a model number, but matching transformer rating, impedance, loss guarantee, protection interface and documentation to the local utility approval process.