35kV Oil-Immersed Power Transformer: The Backbone of Medium-High Voltage Transmission
35kV Oil-Immersed Power Transformer: The Backbone of Medium-High Voltage Transmission
Wide capacity range (50–31,500 kVA), mature technology, and low lifecycle cost — the workhorse for regional substations, industrial distribution, and renewable-energy step-up.
1. Introduction
Within the voltage hierarchy of power systems, 35kV occupies a pivotal, bridging position — linking the 110kV-and-above high-voltage transmission network upstream and radiating to the 10kV-and-below distribution system downstream. The 35kV oil-immersed power transformer has become the workhorse equipment for regional substations, industrial distribution, and renewable-energy step-up applications.
XIAOPAI Electric manufactures a full range of 35kV oil-immersed power transformers, covering energy-saving series such as S11 and S13. All products are certified to ISO 9001 and CE, and are exported to Southeast Asia, Africa, South America, and other markets. We also provide customized solutions including OLTC/DETC, various vector groups, and anti-corrosion / high-altitude reinforcement.
2. Technical Principles
Double-Winding Structure
The 35kV oil-immersed transformer adopts the classic double-winding electromagnetic design. The high-voltage (HV) winding uses a 35kV-class insulation structure, while the low-voltage (LV) winding is configured for 10.5 kV, 6.3 kV, or 0.4 kV output according to the user's grid requirements.
Core: Built from high-quality CRGO silicon steel, using a 45° fully mitered step-lap joint stacking process that effectively reduces no-load loss and operating noise.
Windings: Made of high-purity electrolytic copper conductor, wrapped in multi-layer kraft paper insulation and treated by VPI to ensure insulation strength and thermal class (Class A / Class E).
Tap-Changing Methods
Switches tapping positions while energized — ideal for grids with frequent voltage fluctuation. Range ±4 × 2.5% (9 steps). Standard with German MR or domestic Changzheng tap changers.
Requires manual tapping change after de-energizing. Simple, economical — suited to stable-voltage grids. Range ±5%.
Insulation and Cooling
The insulating oil meets GB 2536 / IEC 60296 standards (No. 25 or No. 45 mineral insulating oil), offering good dielectric strength and heat dissipation. At the 35kV level, main insulation distances and longitudinal creepage are designed strictly in accordance with IEC 60076.
Cooling is primarily ONAN (Oil Natural Air Natural), relying on natural convection driven by oil temperature differences. Units of 31,500 kVA and above can opt for ONAF (Oil Natural Air Forced), boosting heat dissipation by approximately 30%–40%.
3. Product Features
Standard 50–31,500 kVA, with non-standard capacities on request — from a small 35kV factory incoming line to a medium regional substation.
S11 / S13 / SBH15 amorphous options cut no-load loss up to 75% versus S9.
| Series | No-load loss vs. S9 | Application |
|---|---|---|
| S11 | 30%–40% lower | General industrial & distribution; cost-effective choice |
| S13 | 45%–55% lower (Hi-B silicon steel) | High-efficiency demand; lifecycle-cost priority |
| SBH15 amorphous alloy | ~75% lower | Green-energy benchmark; government-subsidized projects |
Standard Protection Devices
Gas Relay (Buchholz Relay): Monitors internal fault gas accumulation, issuing alarm or trip signals.
Pressure Relief Valve: Explosion-proof protection, actuating at 70 kPa.
Oil Level Gauge: Real-time oil level display with high/low limit alarms.
Silica Gel Breather: Keeps insulating oil dry and extends insulation life.
Winding Temperature Indicator: Dual monitoring of oil and winding temperature.
Optional BWDK-series intelligent temperature controller supports RS485 remote communication and SCADA system integration.
Flexible Customization Options
| Option | Available |
|---|---|
| Tap changer | OLTC (on-load) / DETC (de-energized) |
| Vector group | Yyn0 / Dyn11 / Yd11, etc. |
| Voltage combination | 35/10.5 kV / 38.5/6.3 kV / 40.5/0.4 kV, etc. |
| Corrosion class | Standard outdoor / C5-M high corrosion (coastal / chemical plant) |
| High-altitude reinforcement | 2000 m / 3000 m / 4000 m reinforced insulation |
4. Application Scenarios
The 35kV/10kV step-down transformer is the backbone of urban and county-level distribution grids. A 10,000 kVA station serves 2–3 industrial parks or 3–5 residential communities.
In petrochemical, steel & metallurgy, and cement industries, the 35kV main step-down substation is the power hub — often with OLTC and anti-corrosion tanks.
PV/wind projects use 35kV step-up transformers to raise 0.4–0.69 kV to 35kV via collector lines. Fully sealed structure with reinforced anti-corrosion coating.
5. Selection Guide
Capacity: Total actual load × coincidence factor (typically 0.7–0.85), plus a 10%–15% growth margin.
Voltage level: HV (35 / 38.5 / 40.5 kV) / LV (10.5 / 6.3 / 0.4 kV).
Tap changer: Volatile grid or impact loads → OLTC; stable grid and limited budget → DETC.
Installation conditions: Outdoor or indoor, ambient temperature range, altitude, presence of corrosive gases.
Efficiency class: Choose S11 or S13 based on local rules and electricity pricing.
| Selection parameter | Recommended value |
|---|---|
| Capacity | Actual load × 1.1–1.15 |
| HV (kV) | 35 / 38.5 / 40.5 |
| LV (kV) | 10.5 / 6.3 / 0.4 |
| Vector group | Yyn0 (standard) / Dyn11 (anti-harmonic) |
| Impedance voltage (%) | 7%–8% (standard) / 8%–10% (high impedance) |
| Tap changer | OLTC / DETC |
| Cooling | ONAN / ONAF |
6. Conclusion
With its mature design, reliable quality, and flexible configuration, the 35kV oil-immersed power transformer has long held a core position in medium-high voltage transmission and distribution. As efficiency standards rise and the renewable-energy sector grows rapidly, this classic equipment is evolving toward greater energy efficiency and intelligence.
XIAOPAI Electric brings years of 35kV transformer manufacturing experience, delivering a full product range from 50 kVA to 31,500 kVA, supporting on-load/de-energized tap changing, various vector groups, and special-environment customization. All products pass routine and type tests to ensure delivered quality.
Need a quotation or technical proposal?
Contact our technical sales team for tailored 35kV transformer solutions.
[email protected]Keywords: 35kV Oil-Immersed Transformer, OLTC, DETC, Double-winding, ONAN Cooling, Medium-High Voltage Distribution, Renewable Energy Step-up, Energy Efficiency S11-S13, XIAOPAI Electric
Related Guides: Oil-Immersed Power Transformer · S11-M Oil-Immersed Transformer · S13-M Hermetically Sealed Transformer
