A three phase oil immersed transformer is a core component of medium-voltage power distribution systems. It consists of three sets of primary and secondary windings wound around a silicon steel laminated core, all enclosed within a steel tank filled with transformer mineral oil. The insulating oil serves two simultaneous functions: it provides dielectric insulation between windings and metallic components, and it acts as a thermal transfer medium that dissipates heat generated during operation into the external cooling fins or radiators.

Compared to dry-type (cast-resin) transformers, the three phase oil immersed transformer offers higher overload tolerance, longer service life in outdoor environments, and better cost-effectiveness at medium-to-large capacity ratings (above 315kVA). This makes it the dominant transformer type for outdoor substations, pole-mounted rural distribution, and industrial step-down applications globally.

The key distinguishing feature of Redway Electric's three phase oil immersed transformers is the fully-sealed corrugated tank (hermetically sealed) design — eliminating the conservator tank, preventing oil oxidation from atmospheric contact, and removing the need for periodic oil replacement or maintenance inspections.

Key Structural Features & Manufacturing Advantages

Fully Sealed Corrugated Oil Tank Our standard three phase oil immersed transformers use a hermetically sealed corrugated tank that expands and contracts elastically with oil thermal variation — eliminating the conservator tank entirely. This prevents atmospheric moisture and oxygen from contacting the transformer oil, virtually eliminating oil aging and oxidation degradation. Result: maintenance-free operation for 15–20+ years under normal service conditions.

High-Grade Silicon Steel Core with Step-Lap Lamination The three-phase core is constructed from high-permeability, cold-rolled grain-oriented silicon steel (CRGO), cut and stacked using step-lap joint technology. This reduces magnetizing current, minimizes no-load losses, and significantly lowers audible noise levels (typically 45–58 dB depending on rating) compared to butt-joint or non-step-lap construction.

Vacuum Pressure Impregnation (VPI) Winding Process Both high-voltage and low-voltage windings are processed through vacuum pressure impregnation (VPI) with insulating varnish after winding. This process eliminates air voids in the insulation system, enhances dielectric strength, and improves moisture resistance — directly reducing the risk of partial discharge failures in service.

Copper or Aluminum Winding — Selectable Copper windings offer lower resistive losses, higher current density, and longer service life, and are recommended for projects where energy efficiency and long-term reliability are priorities. Aluminum windings reduce unit weight and upfront cost, and are suitable for budget-sensitive projects where load factor is lower. Both options are available across the full capacity range.

Proprietary High-Voltage Wire Fixing Technology Redway Electric holds a granted utility model patent (Patent No.: ZL 2020 2 1536485.3, CNIPA) for our high-voltage wire fixing device — a design innovation that strengthens winding stability under short-circuit impulse forces, directly improving the transformer's ability to withstand fault current events without internal mechanical deformation.

Radiator Design with Anti-Corrosion Coating External cooling radiators are hot-dip galvanized and coated with anti-corrosion primer before final painting. This provides reliable weather protection for outdoor installations in humid, coastal, or industrial environments where atmospheric corrosion is a service-life concern.

Quality Control & Certification

Every three phase oil immersed transformer manufactured at Redway Electric's Shijiazhuang facility undergoes a comprehensive routine test protocol before shipment, in full accordance with IEC 60076-1 Clause 10:

Routine Tests Performed on Every Unit:

• Winding resistance measurement (per IEC 60076-1)
• Voltage ratio and phase displacement verification
• Short-circuit impedance and load loss measurement
• No-load loss and no-load current measurement
• Induced voltage test (dielectric test)
• Separate source voltage withstand test (insulation resistance)
• Oil leakage inspection under pressure

Type Tests Available Upon Request:

• Temperature rise test (IEC 60076-2)
• Lightning impulse test (IEC 60076-3)
• Short-circuit withstand test (IEC 60076-5)
• Sound level measurement (IEC 60076-10)

Certifications & Recognition:

• ISO 9001 Quality Management System
• CE conformity (available for EU-destined units)
• National High-Tech Enterprise designation (China)
• Hebei Province "Specialized, Refined, Distinctive & Innovative" SME (Cert. No. SKX202508A2050001)
• 128+ industry authorized certificates and patents
• CNIPA Utility Model Patent: ZL 2020 2 1536485.3

Full test reports, factory acceptance test (FAT) documentation, and third-party inspection arrangements (SGS, BV, TÜV) are available upon request.

OEM & Customization Capabilities

Beyond our standard S11/S13 catalog, Redway Electric handles full custom engineering for three phase oil immersed transformers with non-standard requirements:

Custom Electrical Specifications Any voltage ratio, any kVA rating within production capability, special impedance values, specific vector group (Yd1, Yd11, YNd11, Yy0, etc.), custom tap changer range (DETC or OLTC).

Custom Mechanical & Structural Requirements Special tank dimensions to fit existing substation bays, cable box vs. bushing termination, ground-level vs. pole-mounting hardware, ONAN/ONAF/OFAN cooling configuration, bi-directional radiator layout for restricted installation spaces.

Special Environment Versions High-altitude (1,000m–4,500m) derated designs with enhanced insulation; tropical-climate versions with reinforced anti-corrosion coating; extreme-cold versions (−40°C rated) for northern-climate markets; dust-proof and sealed-bushing versions for mining and industrial environments.

White-Label / OEM Branding Full white-label service for distributors and EPC contractors — custom nameplate, brand painting, and documentation in your company identity.

Lead Times (Standard Production): 50kVA–315kVA standard catalog: 10–15 working days 400kVA–1,250kVA standard catalog: 15–20 working days 1,600kVA–2,500kVA standard catalog: 20–30 working days Custom-engineered units: 30–50 working days

How to Select the Right Three Phase Oil Immersed Transformer

Procurement engineers evaluating three phase oil immersed transformers should confirm the following parameters before requesting a quotation:

Step 1 — Define Load Requirements Calculate total connected load in kW, apply a demand factor and power factor (typically 0.8–0.9 for industrial loads) to determine required kVA. Apply a 15–20% capacity reserve for future load growth.

Step 2 — Confirm Grid Voltage Levels Identify primary (MV) voltage level: 6kV, 10kV, 11kV, 22kV, 33kV, or 35kV. Identify secondary (LV) distribution voltage required: 380V, 400V, 415V, or 433V.

Step 3 — Specify Installation Environment Indoor or outdoor? Pole-mounted, pad-mounted, or ground-level? Altitude above sea level? Minimum/maximum ambient temperature? Presence of corrosive atmosphere, dust, or explosive gas?

Step 4 — Choose Loss Grade (S11 / S13 / S15) Consider the transformer's annual operating hours and local electricity tariff. For high duty-cycle installations, the long-term energy savings from S13 or S15 units typically outweigh the higher capital cost within 3–5 years.

Step 5 — Confirm Applicable Standard IEC 60076 (most international markets), GB 1094 (China domestic), IEEE C57 (North America), or local national standard? Specify vector group convention (Dyn11 is standard for European-style 3-phase 4-wire distribution; Yyn0 for some Asian markets).

Send us your load calculation sheet or project specification — our engineers will respond within 24 hours with a complete technical proposal and factory-direct quotation.

Request a Quote for Your Three Phase Oil Immersed Transformer Project

Fill in your project requirements below, or contact us directly via the channels listed. Our engineering team responds within 24 hours with a technical proposal and factory pricing.

What to include in your enquiry for fastest response:

• Required kVA rating
• Primary voltage (MV side)
• Secondary voltage (LV side)
• Grid frequency (50Hz or 60Hz)
• Installation type (outdoor pad / pole / indoor substation)
• Applicable standard (IEC / IEEE / local)
• Quantity and delivery location