35KV Mian Power Transformer
Rated Power: Up to 120,000 kVA
Voltage Levels: 33kV
Vector Group: YNd11 / YNyn0d11
Cooling Method: ONAF (Forced Air Cooling)
Notes: The SF20 series 33kV power transformer is customizable in rated power, voltage combination, vector group, cooling method, impedance voltage, tap range (off-circuit or on-load), and protection accessories to meet specific transmission and substation project requirements.
Zhejiang Tianrun Electric Co., Ltd. is one of the leading manufacturers and suppliers of 35kv mian power transformer in China. We are committed to providing high quality customized products with competitive price. We warmly welcome you to buy bulk advanced 35kv mian power transformer from our factory. Also, quotation is available.
Product Overview
The SF20 series power transformer is an oil-immersed transformer designed specifically for large-capacity power transmission and distribution applications. Building upon the foundation of conventional oil-immersed transformers, it is equipped with a forced air cooling system (ONAF - Oil Natural Air Forced), delivering a qualitative leap in heat dissipation efficiency.
This series is ideally suited for power systems, industrial and mining enterprises, substations, and other critical applications that demand high-capacity transformers.
Core Differences vs. Ordinary Oil-Immersed Transformers
|
Aspect |
Ordinary Oil-Immersed |
SF20 Series |
|
Cooling Method |
Oil Natural Air Natural (ONAN) |
Oil Natural Air Forced (ONAF) |
|
Heat Dissipation |
Natural radiation via tank walls only |
Active forced-air cooling system |
|
Capacity Range |
Small to medium capacity |
Medium to large capacity |
|
Overload Capability |
Limited; temperature rise restricts |
Significantly enhanced; forced air rapidly removes heat |
|
Temperature Protection |
Basic protection |
Intelligent temperature control + multi-level protection |
|
Service Life |
Standard |
Extended due to lower winding temperatures |
Core Advantages
1. Forced Air Cooling - Double The Dissipation Efficiency
The SF20 is standard-equipped with multiple low-noise axial fans. When load increases or oil temperature reaches the preset threshold, the fans automatically activate, creating forced convection that rapidly removes heat from the transformer, ensuring windings and core operate safely within temperature limits.
• Heat dissipation capacity increased by 30%–50% compared to conventional oil-immersed transformers.
• Intelligent temperature control system monitors top-oil temperature in real time, automatically starts/stops fans - balancing energy efficiency and cooling performance.
• Redundant fan design: single fan failure does not affect system operation.
2. Higher Capacity, Wider Applicability
Thanks to the forced air cooling system, the SF20 series achieves significantly higher capacity ratings:
|
Rated Capacity (kVA) |
Cooling Mode |
Application Scenario |
|
10,000 – 30,000 |
ONAN (Natural Cooling) |
Normal load conditions |
|
30,000 – 63,000 |
ONAF (Forced Air Cooling) |
High load, continuous full-load operation |
|
≥ 63,000 |
OFAF (Forced Oil Forced Air) |
Large substations, power plants |
Example: At 50,000 kVA, the SF20 in ONAF mode delivers approximately 40% more output capacity than an ONAN transformer of the same physical size.
3. Enhanced Overload Protection
Transformers inevitably encounter short-term peak loads during operation. The SF20's forced air cooling system provides substantial overload margin:
• Normal overload: 120% rated load continuously with fans at full speed.
• Emergency overload: 150% rated load for short durations, providing dispatch response time.
• Intelligent temperature control adjusts fan speed automatically, ensuring temperature rise stays within allowable limits during overload.
4. Extended Service Life, Reduced Maintenance Costs
• Every 6°C reduction in winding temperature doubles insulation life (Arrhenius aging law).
• Forced air cooling maintains uniform winding temperature distribution, eliminating localized hot spots.
• Sealed oil system + high-quality insulation materials - moisture-proof and oxidation-resistant.
• Reduces unplanned downtime, lowering total lifecycle cost.
Technical Specifications
|
Parameter |
SF20 Series |
|
Rated Capacity Range |
10,000 – 120,000 kVA |
|
Voltage Levels |
35 kV / 66 kV / 110 kV / 220 kV |
|
Vector Group |
YNd11 / YNyn0d11 (customizable) |
|
Cooling Method |
ONAN / ONAF (Forced Air Cooling) |
|
Voltage Regulation |
Off-circuit tap changer / On-load tap changer |
|
Insulation Class |
A (oil-immersed) |
|
Temperature Rise Limit |
Top oil ≤ 55 K (Class F insulation) |
|
Noise Level |
≤ 65 dB(A) at 1 m |
|
Service Life |
≥ 30 years |
|
Applicable Standards |
GB/T 1094 (equivalent to IEC 60076) |
Structural Features
Forced Air Cooling System
• High-quality axial fans - stable operation, easy maintenance.
• Stainless steel protective grilles prevent foreign object ingress.
• Low-noise design, matching the transformer's 65 dB noise limit.
Tank & Heat Dissipation
• Corrugated tank or panel-type radiator structure for maximum heat exchange surface.
• Reinforced rib design for high mechanical strength.
• Superior sealing performance - no oil leakage during operation.
Protection System
• Buchholz relay: gas accumulation alarm, sudden pressure trip.
• Pressure relief valve: prevents tank overpressure.
• Oil level gauge: real-time oil level monitoring.
• Temperature controller: PT100 RTD temperature sensing, automatic fan start/stop.
Application Scenarios
- Urban substations: high load density, compact space - requires high volumetric power density.
- Industrial parks: fluctuating loads - SF20's overload capacity handles production peak demand.
- Renewable energy integration: solar/wind output varies - forced air cooling copes with frequent load changes.
- Large industrial enterprises: metallurgy, chemicals, mining - high energy consumption industries.
- Power plant step-up substations: generator output step-up, stable large-capacity transmission.
Selection Guide
|
Operating Conditions |
Recommended Option |
Reason |
|
Annual load rate > 70%, continuous full-load |
SF20 ONAF |
Forced air ensures adequate cooling |
|
Highly variable load, frequent peaks |
SF20 ONAF + intelligent temp control |
Auto-matches cooling to demand |
|
Noise-sensitive locations |
SF20 + low-noise fans + enclosure |
Meets environmental requirements |
|
Limited budget, low load rate |
Standard oil-immersed + future ONAF provision |
Upgrade-ready for future needs |
After-Sales Service
- Warranty: 24 months from factory shipment or 18 months from commissioning (whichever comes first).
- Technical support: 7×24 hotline, on-site arrival within 48 hours.
- Spare parts: original factory parts, fast dispatch.
- Remote monitoring (optional): intelligent online monitoring system for real-time tracking of winding temperature, oil temperature, partial discharge, and other key parameters.
Proccess of Transformer
1-Wire‑wound winding of transfomer
2-Foil‑wound winding of transfomer
3-Vacuum casting of transformer
4-Epoxy resin casting of transfomer
5-Coil assembling over core
6-HV porcelain bushings of oil immersed transformer
7-Lead insulation taping and welding of transfomer
8-Vacuum oil filling of transformer
9-Tank sealing of transformer
Testing of Transformer
1-Measurement of winding resistance testing of transformer
2-Measurement of voltage ratio and check of phase displacement testing of transformer
3-Separate‑source AC withstand voltage testing of transformer
4-Measurement of no‑load loss and no‑load current of transformer
5-Measurement of short‑circuit impedance and load loss of transformer
6-No load loss and no load current testing of transformer
7-Temperature‑rise test of transformer
8-Partial‑discharge measurement of transformer
9-Winding ressitant and voltage ratio testing of transformer
FAQ
Q1: What is the difference between ONAN and ONAF cooling?
A: ONAN (Oil Natural / Air Natural) relies solely on natural oil circulation and air convection - no fans, lower noise, no auxiliary power. ONAF (Oil Natural / Air Forced) adds cooling fans to the radiators, increasing capacity by 20–30% when fans are running. ONAF is preferred for high-capacity units or sites with limited space; ONAN is preferred for low-noise, low-maintenance installations.
Q2: What do YNd11 and YNyn0d11 mean?
A: These are vector groups describing winding connection and phase shift. YNd11 - high-voltage winding star-connected with neutral brought out, low-voltage delta-connected, 30° phase shift; the most common configuration for two-winding power transformers. YNyn0d11 - a three-winding configuration with HV star (neutral out), LV star (neutral out), and a tertiary delta winding, used when a third voltage level or harmonic suppression is required.
Q3: What applications are the SF20 series designed for?
A: SF20 is a power transformer series for transmission and large substation applications - including regional grid substations, power plant step-up/step-down, heavy industrial facilities, and renewable energy collection stations. It handles voltage levels from 35 kV up to 220 kV and capacities up to 120 MVA.
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