Three-phase Transformers
Industrial-grade three-phase transformers for heavy-duty manufacturing, commercial facilities and large-scale power distribution.
Robust three-phase power solutions from 10kVA to 2500kVA. Engineered for 50/60Hz systems with voltage ratings from 380V to 11kV, featuring copper or aluminum windings, oil-immersed or dry-type construction, delta/wye configurations, and comprehensive protection systems for industrial machinery, commercial buildings, and power distribution networks.
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Heavy-duty three-phase power solutions for industrial manufacturing, commercial infrastructure and large-scale facilities
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Comprehensive range of three-phase transformers for industrial and commercial power distribution
Three-phase Transformers
Industrial-grade three-phase transformer engineered for heavy-duty power distribution in manufacturing and commercial facilities.
Capacity
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Determine the optimal three-phase transformer capacity for your industrial application
Based on your requirements, we recommend the above three-phase transformer capacity. Contact our engineering team for detailed load analysis, vector group selection, and protection coordination studies.
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Real-world performance in critical applications
Frequently Asked
Questions
Technical answers to common inquiries about three-phase transformers
What is the difference between dry-type and oil-immersed transformers?
How do I select the correct vector group (Dyn11, Yyn0, etc.)?
What is K-factor rating and when is it needed?
What protection devices are required for three-phase transformers?
How do I calculate transformer impedance and why does it matter?
What maintenance is required for three-phase transformers?
Can I operate a transformer at higher altitude or ambient temperature?
What is the typical lead time and what information do you need for a quote?
Technical
Reference
Comprehensive technical reference for three-phase transformers
What Are Three-Phase Transformers?
Three-phase transformers are electrical devices that convert three-phase alternating current (AC) from one voltage level to another using electromagnetic induction. Unlike single-phase transformers designed for residential and light commercial loads, three-phase units serve industrial, commercial, and utility power distribution applications ranging from 10kVA to 2500kVA and beyond. They step voltage up or down (e.g., 10kV to 380V, 480V to 208V) while maintaining the three-phase power system's balanced configuration essential for efficient operation of motors, industrial machinery, and large commercial facilities.
Core Technologies & Construction
Core Design
Three-limb or five-limb core construction using high-grade CRGO (Cold Rolled Grain Oriented) silicon steel laminations, 0.27-0.35mm thickness. Three-limb cores are standard for distribution transformers, five-limb cores provide better zero-sequence impedance for unbalanced loads. Step-lap or mitred joint construction minimizes core losses. No-load losses typically 0.8-1.5 W/kg at 1.7T flux density.
Winding Technology
Concentric cylindrical winding arrangement--LV winding inside, HV winding outside for optimal cooling. Copper conductor (99.95% purity) standard, aluminum available for cost reduction. Continuous disc winding for HV, layer winding for LV. Class F (155°C) or Class H (180°C) insulation systems. Proper radial and axial bracing to withstand short-circuit forces (up to 25× rated current).
Insulation Systems
Dry-type: Vacuum pressure impregnation (VPI) with polyester resin, or cast resin encapsulation for superior moisture resistance and mechanical strength. Nomex paper, aramid fiber, glass tape for layer insulation. Oil-immersed: Kraft paper and pressboard insulation, mineral oil (IEC 60296) for cooling and dielectric strength. BIL (Basic Impulse Level): 75kV for 10kV class, 95kV for 11kV class.
Cooling Methods
Dry-type: AN (Air Natural) for < 1000kVA, AF (Air Forced) with fans for higher ratings. IP00 open construction or IP21-IP44 enclosed. Oil-immersed: ONAN (Oil Natural Air Natural) standard, ONAF (Oil Natural Air Forced) with fans for 25-40% capacity increase. Radiators or corrugated tank for heat dissipation. Hermetically sealed or conservator tank design to prevent moisture ingress.
Technical Specifications
| Parameter | Dry-Type | Oil-Immersed | K-Factor |
|---|---|---|---|
| Capacity Range | 10kVA - 1600kVA | 50kVA - 2500kVA | 15kVA - 750kVA |
| Common Voltages | 10kV/380V, 480V/208V | 10kV/380V, 11kV/400V | 480V/208V, 380V/220V |
| Efficiency | > 98% | > 99% | > 97.5% |
| Impedance | 4-7% | 4-8% | 5-7% |
| Temperature Rise | 80K / 100K | 65K (top oil) | 80K / 115K |
| Insulation Class | F / H | A (105°C oil) | H (180°C) |
| Installation | Indoor, near loads | Outdoor capable | Indoor, IT loads |
| Maintenance | Minimal | Oil testing required | Minimal |
Selection Guidelines
1. Load Analysis & Capacity Determination
Calculate total connected load in kW. Include all motors, lighting, HVAC, process equipment. Convert to kVA: kVA = kW ÷ Power Factor. Industrial average PF = 0.8, commercial = 0.85-0.9. Apply diversity factor (not all loads operate simultaneously): Small facilities 0.7-0.8, Large facilities 0.6-0.7. Add safety margin: 20-30% for future expansion and load growth. For motor loads, use locked rotor current for sizing--starting current can be 5-7× rated current. Example calculation: 500kW connected load, PF = 0.8, diversity = 0.75, safety margin = 1.25. Required capacity = (500 ÷ 0.8) × 0.75 × 1.25 = 586kVA. Select next standard size: 630kVA transformer.
2. Voltage & Vector Group Selection
Primary voltage: Match utility supply (10kV, 11kV, 6.6kV most common). Verify available fault level--ensure transformer impedance limits downstream fault current to acceptable levels. Secondary voltage: 380V/220V (IEC countries), 400V/230V (Europe), 415V/240V (Australia), 480V/277V (North America). Vector group: Dyn11 most common for industrial/commercial distribution--provides neutral for single-phase loads, 30° phase shift blocks triplen harmonics, suitable for unbalanced loads. Yyn0 for utility substations, parallel operation with existing Yyn0 transformers. Dyn5 alternative in some regions. Zigzag (Dzn0) for extreme unbalanced loads. Tap changer: Off-circuit tap changer standard (±2.5%, ±5%). Adjust voltage without load. On-load tap changer (OLTC) for critical applications requiring voltage regulation under load. Adds 15-25% to cost.
3. Type Selection: Dry vs Oil
Choose dry-type when: Indoor installation near occupied areas (offices, hospitals, schools). Fire safety critical--no flammable liquids. Environmental regulations prohibit oil. Capacity < 1600kVA. Minimal maintenance capability. Choose oil-immersed when: Outdoor installation or dedicated transformer room. Capacity > 1000kVA (lower cost per kVA). Heavy industrial loads with high overload requirements. Long service life priority (30-40 years vs 20-25 years). Maintenance staff available for oil testing. Ambient temperature > 40°C (better cooling efficiency). K-factor rated transformer when: Non-linear loads > 30% of total load. Data centers, IT facilities, VFD-heavy applications. THD (Total Harmonic Distortion) > 5%. Specify K-4 (light), K-13 (moderate), or K-20 (heavy) based on harmonic analysis.
4. Protection & Monitoring Requirements
Minimum protection: Primary overcurrent protection (fuses or circuit breaker). Secondary main breaker. Temperature monitoring with alarm and trip. Recommended for > 500kVA: Differential protection (detects internal faults). Restricted earth fault protection. Buchholz relay (oil-immersed). Pressure relief device. Critical applications: Online monitoring: Temperature, load current, partial discharge, oil condition (DGA). Remote alarming via SCADA/BMS integration. Redundant protection systems. Automatic transfer to backup transformer on fault. Surge protection: Lightning arresters on primary side (10kV class II, 12kV MCOV). Consider secondary surge protection for sensitive loads. Ensure protection coordination: Use time-current curves to verify selectivity. Downstream devices must clear faults before upstream protection operates.
Installation Best Practices
- 1. Site Preparation & Foundation: Dry-type: Concrete pad or structural steel support rated for 150% transformer weight. Level within ±2mm/m. Vibration isolation pads for noise-sensitive areas. Oil-immersed: Reinforced concrete foundation with oil containment basin (110% oil volume capacity). Gravel bed for drainage. Foundation must support transformer weight plus oil (total weight typically 1.5-2× nameplate). Clearances: Front 1.5m (maintenance access), Sides 1m, Rear 0.5m, Top 2m (ventilation). Indoor installations require dedicated transformer room with fire-rated walls, ventilation, and emergency lighting.
- 2. Electrical Connections & Terminations: Cable sizing: Primary cables per voltage drop and fault current withstand. Typically 3-5× transformer full-load current for short-circuit rating. Secondary cables sized for load current plus 125% margin. Use copper cables--aluminum requires special terminals and anti-oxidant compound. Termination: Remove cable insulation per manufacturer specifications (typically 15-20mm). Use cable lugs crimped with proper tool--verify crimp force. Apply anti-oxidant compound on aluminum conductors. Torque terminal bolts: M10: 15-20 Nm, M12: 25-35 Nm, M16: 50-70 Nm. Do not over-torque. Phase sequence: Verify correct phase rotation (L1-L2-L3) using phase sequence meter before energization. Incorrect phasing causes motor reverse rotation. Neutral and ground: Bond secondary neutral to ground at transformer only (Dyn11) or at main service entrance (Yyn0)--never both. Use minimum 50mm² copper for ground connection. Ground resistance < 5Ω per IEC 61936.
- 3. Protection System Integration: CT (Current Transformer) installation: Install on primary and secondary sides for metering and protection. Burden rating must match connected relays. Short-circuit secondary terminals before installation--open circuit causes dangerous high voltage. PT (Potential Transformer) / VT (Voltage Transformer): Required for HV metering and protection (> 1kV). Fuse primary side (typically 1A rated). Protection relay configuration: Overcurrent protection: 51 (time overcurrent), 50 (instantaneous). Settings: 125-150% rated current (51), 10-15× rated current (50). Earth fault: 51N (time), 50N (instantaneous). Settings: 20-40% rated current (51N), 2-5× rated current (50N). Differential: 87T. Slope characteristic 15-40%. Set pickup 10-20% rated current. Temperature: 49T. Alarm 90°C (dry) / 85°C (oil), Trip 105°C (dry) / 95°C (oil). Buchholz relay: Gas alarm 50cm³, Trip 150cm³ or sudden pressure. Test all protection functions before commissioning: Inject test currents, verify relay operation, check trip circuit continuity, simulate temperature alarms.
- 4. Commissioning & Testing: Pre-energization checks: Visual inspection--no shipping damage, oil level correct (oil-immersed), all connections tight, no foreign objects. Insulation resistance: Megger test at 2.5kV (LV), 5kV (HV). Readings should be > 1000 MΩ (new), > 100 MΩ (in service). Winding resistance: Measure all three phases--deviation should be < 2% between phases. Turns ratio: Verify within ±0.5% of nameplate rating using turns ratio tester. Polarity and phase relationship: Verify correct vector group using vector group analyzer. Oil tests (oil-immersed): Breakdown voltage > 30kV (IEC 60156), Moisture content < 30 ppm, Acidity < 0.03 mg KOH/g. Energization procedure: Remove all loads from secondary. Close primary breaker, monitor no-load current (should be 0.5-2% rated current). Measure secondary voltages--should be within ±5% nameplate. Listen for abnormal noise. Check temperature rise after 2 hours no-load operation. Apply 25%, 50%, 75%, 100% load in steps, monitoring voltage, current, temperature at each step. Full-load test: Operate at rated load for 4 hours minimum. Verify temperature stabilizes below rated rise. Measure voltage regulation (should be within impedance specification). Record all test data for baseline reference.
- 5. Common Installation Mistakes to Avoid: Undersizing transformer--forgetting diversity factor, future expansion, power factor correction. Results in overheating and premature failure. Poor ventilation--blocking air inlets/outlets causes temperature rise beyond ratings. Dry-type transformers require free air circulation. Incorrect vector group--parallel operation with incompatible vector groups causes circulating currents and overheating. Always verify phase relationship before paralleling. Inadequate protection coordination--upstream and downstream devices not properly coordinated leads to nuisance trips or failure to clear faults. Improper grounding--multiple neutral-ground bonds (Dyn11) or no ground bond (Yyn0) creates safety hazards and ground fault detection issues. Overloading--continuous operation above 80% rated capacity significantly reduces transformer lifespan. Temperature rise accelerates insulation aging exponentially. Ignoring harmonic loads--standard transformer on high harmonic loads overheats due to eddy current losses. Specify K-rated transformer or apply derating factor. Neglecting oil maintenance--oil-immersed transformers require regular oil testing. Degraded oil causes insulation breakdown and internal faults. Poor cable termination--loose connections cause hot spots and arcing. Use proper torque wrench and verify tightness after initial thermal cycle (48 hours). Inadequate fault current analysis--installing transformer with low impedance in high fault level system requires expensive high-interrupt capacity switchgear. Verify available fault current and select appropriate impedance.
Why Choose Wenfei?
Wenfei Electrical specializes in industrial-grade three-phase transformers for manufacturing facilities, commercial buildings, and power distribution networks. Our 10,000m² production facility in Liushi, Wenzhou employs advanced manufacturing technologies including CNC winding machines, vacuum impregnation systems, automated core stacking, and comprehensive testing laboratories. Every transformer undergoes rigorous factory acceptance testing including no-load loss, load loss, impedance voltage, temperature rise, induced overvoltage, applied voltage, partial discharge, and lightning impulse tests per IEC 60076 international standards.
Our engineering team provides complete application support including load analysis, short-circuit calculations, protection coordination studies, and harmonic analysis. We offer custom designs for special requirements: non-standard voltages, vector groups, impedance values, K-factor ratings, seismic specifications, tropical climate designs, and altitude derating. With ISO 9001:2015 quality management, CE compliance, and full type test reports, Wenfei three-phase transformers deliver reliable performance for industrial manufacturing, commercial infrastructure, data centers, hospitals, and power distribution applications worldwide. Our global service network provides installation supervision, commissioning support, maintenance training, and spare parts supply to ensure maximum uptime and long-term reliability.
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Liushi Town, Wenzhou, Zhejiang, China