Single-phase Oil-immersed Transformer
A single phase oil immersed transformer converts voltage in a single-phase AC circuit, with primary and secondary windings wound on a single-limb or cruciform silicon steel core, housed in a hermetically sealed or conservator-type steel tank filled with mineral insulating oil. The oil serves as both dielectric insulation medium and thermal transfer agent, dissipating heat through the tank walls and external cooling fins.
When to specify single phase over three phase:
Single phase distribution transformers are the dominant choice in the following scenarios:
Rural Overhead Distribution (Most Common Global Use Case) In dispersed rural areas, medium-voltage distribution lines often carry single-phase (two-wire) rather than three-phase circuits to minimise line construction cost across long distances with low load density. A single phase oil immersed transformer is the correct and only compatible unit for these single-phase MV lines. This represents the majority of rural electrification infrastructure in Africa, South and Southeast Asia, Latin America, and parts of the Middle East.
North American Overhead Distribution Standard The North American radial distribution model predominantly uses single-phase 7.2kV, 7.96kV, 13.2kV, or 14.4kV lines with individual pole-mounted single phase oil immersed transformers (commonly called "overhead distribution transformers") feeding residential and small commercial loads. ANSI C57.12.20 governs this design. Honway Power produces ANSI-configuration units for this market.
Three-Unit Bank for Three-Phase Supply from Single-Phase Infrastructure Three individual single phase oil immersed transformers can be connected in a bank (delta-delta, delta-star, or open-delta) to provide three-phase supply where a single three-phase unit is too large, or where modular replacement of failed phases is operationally preferable.
Railway Traction Power Supply Single-phase 25kV or 27.5kV traction transformers feed overhead catenary systems. The single-phase configuration of the MV/HV input mirrors the traction supply system design.
Isolated Industrial Single-Phase Loads Large industrial heating systems, welding equipment, UV curing lines, and other single-phase machinery above 100kVA sometimes require a dedicated step-down transformer that a standard three-phase distribution unit cannot efficiently serve.
Key Structural Features & Manufacturing Advantages
Fully Sealed Corrugated Tank — Maintenance-Free Oil System Our standard single phase oil immersed transformer uses a hermetically sealed corrugated-wall steel tank, thermally breathing through elastic deformation of the corrugated wall panels rather than through an oil-air interface. This eliminates conservator tank, breather, and oil sampling maintenance entirely. Transformer oil quality is preserved throughout the unit's service life with zero scheduled oil maintenance — critical for pole-mounted units in remote rural areas where maintenance access is infrequent.
CRGO Silicon Steel Core with Step-Lap Lamination The single-limb (or shell-type for larger ratings) core is manufactured from cold-rolled grain-oriented silicon steel (CRGO), the same material as our three-phase product line. Step-lap joint construction at all lamination interfaces minimises flux leakage, reduces no-load (magnetising) losses, and lowers audible noise — important in residential pole-mounted installation environments where transformer hum is a community concern.
Vacuum Pressure Impregnation (VPI) Winding Processing After winding, both HV and LV coils are vacuum-pressure impregnated with insulating varnish before tank assembly. VPI processing eliminates micro air voids from the cellulose paper insulation system, increasing dielectric strength, improving moisture resistance, and extending service life in tropical and high-humidity environments where moisture ingress is a primary insulation aging mechanism.
Patented HV Wire Fixing Device (CNIPA Patent ZL 2020 2 1536485.3) Honway Power's proprietary HV winding fixing device — granted by the China National Intellectual Property Administration — reinforces high-voltage coil-end mechanical stability under fault current impulse forces. This directly reduces the risk of internal winding deformation during distribution system fault events, one of the leading causes of field transformer failures.
Anti-Corrosion Surface Treatment System Tank exterior surface preparation: shot-blasting to Sa2.5 cleanliness level, followed by two-coat anti-corrosion primer (epoxy-based) and polyurethane topcoat. All external fasteners are 304 stainless steel. This surface system provides reliable corrosion protection for outdoor pole-mounted and pad-mounted installations in tropical, coastal, and industrial-atmosphere environments — locations where single-phase distribution transformers are most commonly deployed and where corrosion-accelerated failure is most prevalent.
IEC 60296 Grade I Mineral Oil Fill Transformer mineral oil compliant with IEC 60296, with a dielectric breakdown voltage of ≥70kV at delivery, low moisture content (≤10 ppm water), and inhibited antioxidant additive for extended oil service life. Oil sampling ports are fitted as standard for future dissolved gas analysis (DGA) monitoring if required.
Quality Assurance — Factory Testing Protocol
Every single phase oil immersed transformer shipped by Honway Power passes a mandatory routine test series aligned with IEC 60076-1 Clause 10:
| Test | Standard | Criterion |
|---|---|---|
| Winding resistance (all tap positions) | IEC 60076-1 §10.2 | Within ±0.5% of design value |
| Voltage ratio measurement | IEC 60076-1 §10.3 | Within ±0.5% of rated ratio |
| No-load loss & no-load current | IEC 60076-1 §10.4 | ≤ rated grade (S11/S13) |
| Load loss & short-circuit impedance | IEC 60076-1 §10.5 | Within ±10% of specification |
| Induced voltage (AC dielectric) test | IEC 60076-3 §12.3 | 2× rated voltage, 60s, no breakdown |
| Separate source HV withstand test | IEC 60076-3 §12.2 | Per voltage class, 60s |
| Insulation resistance | Megger | ≥1,000 MΩ at 20°C |
| Oil pressure leakage test | Factory protocol | Zero leakage at 0.05 MPa |
Available Type Tests (on request): Temperature rise (IEC 60076-2), lightning impulse (IEC 60076-3), short-circuit withstand (IEC 60076-5), sound level (IEC 60076-10). Third-party witnessed testing (SGS, Bureau Veritas, TÜV Rheinland) available with advance arrangement.
Certifications & Recognition: ISO 9001 · CE (EU-destined units) · National High-Tech Enterprise (China) · Hebei Province Specialized, Refined, Distinctive & Innovative SME (Cert. No. SKX202508A2050001) · CNIPA Patent ZL 2020 2 1536485.3 · 128+ authorized industry certificates
OEM & Customisation Options
Honway Power accepts full OEM and custom orders for single phase oil immersed transformers with non-standard requirements:
Electrical Customisation Any voltage ratio within production capability; non-standard kVA ratings; custom short-circuit impedance for parallel operation; extended HV tap range; OLTC (on-load tap changer) for under-load voltage regulation; dual-secondary winding (two independent LV outputs).
Mechanical & Installation Customisation Pole-mounting hardware (cross-arm brackets, hanger straps, earthing lugs) for various pole sizes; pad-mounting skid frame; platform substation base; cable sealing boxes for HV and LV XLPE cable entry; compact enclosure for indoor or kiosk-type installation.
Environmental Variants High-altitude derated versions (1,000m–4,500m); extreme cold versions (−40°C ambient); tropical enhanced coating; sealed bushing variants for dust-intensive industrial environments; explosion-proof classification for Zone 1/Zone 2 hazardous areas.
Protection Configuration CSP (internal HV fuse + surge arresters + LV breaker); HV expulsion fuse cutout; separate MOV arrester mounting brackets; Buchholz relay (conservator variants); winding temperature indicator.
OEM / White-Label Custom nameplate, colour, and documentation for distributor and EPC contractor OEM customers. Typical OEM minimum: 5 units; discuss single-unit OEM requirements with our sales team.
Lead Times: 30kVA–315kVA standard catalog: 10–15 working days 400kVA–1,000kVA standard catalog: 15–25 working days Custom / OEM / special environment: 25–45 working days
Request a Factory
Tell us your project requirements and our engineering team will respond within 24 hours with a complete technical proposal and factory-direct pricing for your single phase oil immersed transformer order.
Include in your enquiry for fastest response:
- Rated kVA required
- Primary (HV) voltage and grid system (single-phase or phase-to-neutral connection on three-phase line)
- Secondary (LV) voltage required
- Grid frequency (50Hz or 60Hz)
- Installation type (pole-mounted / pad-mounted / indoor)
- CSP protection required: Yes / No
- Applicable standard (IEC / ANSI / local national standard)
- Quantity and delivery destination
- Special environment conditions (altitude, temperature, atmosphere)
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