Transformers

Oil-cooled Transformer with Oil Pillow Equipment

The Oil-cooled Transformer with Oil Pillow Equipment is designed for modern power distribution systems that require stable performance, enhanced cooling, and long service life. By integrating an oil pillow (oil conservator) into the system, this equipment ensures reliable oil expansion, superior insulation, and protection against leakage and pressure fluctuations.

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An oil-cooled transformer with oil pillow is a type of oil immersed transformer that includes an oil expansion device mounted on top of the tank. This pillow, often called an oil conservator, allows the transformer to adjust to temperature variations, ensuring that insulating oil maintains optimal pressure and volume.

This design not only enhances transformer safety but also reduces maintenance risks. The result is a sealed oil cooled transformer capable of delivering reliable power even under heavy or fluctuating loads.

Core Features

  • Oil pillow integration – provides automatic oil level adjustment and expansion.
  • Advanced cooling system – oil immersion ensures efficient thermal regulation.
  • Durable tank design – resistant to pressure changes and environmental stress.
  • Flexible voltage ratings – suitable for medium and high-voltage distribution.
  • Improved insulation – oil acts as both coolant and dielectric barrier.
  • Reliable operation – reduces downtime in industrial and utility networks.

Key Advantages

  • Stable oil management – prevents oil overflow or vacuum inside the transformer.
  • Enhanced efficiency – supports high load conditions without overheating.
  • Extended lifespan – oil conservator reduces stress on seals and windings.
  • Low maintenance costs – easy monitoring through oil level indicators.
  • Adaptable – compatible with outdoor and indoor applications.
  • Safety assurance – reduces the chance of internal faults caused by oil imbalance.
Technical Parameters of 35kV Class S11 Series Oil-immersed Distribution Transformer
Rated
Capacity
(kVA)		 Voltage Combination
(kV)		 Connection
Group
Designation		 Losses
(kW)		 No-Load
Current
(%)		 Short-Circuit
Impedance
(%)		 Weight(Kg) OverallDimensions
(mm)
L(Length)
×B(Width)
×H(Height)		 Gauge
Longitudinal/
Transverse
High
Voltage		 Low
Voltage		 No-Load   Load
(75℃)		 Weight of
Core  and
Windings		 Oil
Weight		 Total
Weight
  30 35
36.5
38.5		 0.4 Yyno
Dyn11		 0.14 0.89/0.84 1.7 6.5 165 230 580 1320×550×1550  550/550
  50 0.17 1.21/1.15 1.6 230 265 730 1360×700×1680
  80 0.22 1.80/1.71 1.7 310 310 870 1395×730×1730
  100 0.23 2.01/1.92 1.4 370 340 1000 1440×760×1810
  125 0.27 2.38/2.26 1.4 440 380 1120 1450×800×1940  660/660
  160 0.29 2.82/2.69 1.3 540 415 1360 1470×800×2910
  200 0.34 3.33/3.16 1.2 635 455 1540 1500×800×2050
  250 0.41 3.95/3.75 1.1 725 490 1745 1530×830×2090
  315 0.49 4.76/4.53 1.1 825 525 1910 1550×870×2150
  400 0.58 5.75/5.47 1.0 980 595 2220 1600×940×2200  820/820
  500 0.69 6.92/6.58 1.0 7210 680 2(340 1750×1000×2280
  630 0.83   7.88 0.9 1110 800 3175 2130×950×2400
  800 0.98   9.41 0.8 1625 850 3595 2210×1075×2450
1000 1.15   11.54 0.8 7955 965 4260 2250×1090×2630
1250 1.41   13.94 0.7 2250 1130 4835 2210×1740×2690 1070/1070
1600 1.70   16.67 0.6 2890 1260 5995 2250×2145×2700
2000 2.09   20.43 0.5 3070 1330 6445 2320×2170×2750
2500 2.52   21.85 0.5 3555 1390 7080 2340×2180×2510
Note:
1.The high voltage can provide a voltage regulation range of±5%or±2×2.5%.(The provided outline dimensions are for model selection reference only,and the
final dimensions shall be subject to the product outline drawing.)
2.The load loss values above the diagonal in the table are applicable to the Dyn11 connection group,and the load loss values below the diagonal are applicable
to the Yyn0 connection group.

Applications

The oil expansion tank transformer is widely used across industries:

  • Utility substations – ensuring grid stability.
  • Heavy industries – supporting continuous operations in steel, mining, and chemical plants.
  • Renewable energy – wind and solar farms requiring stable transformers.
  • Commercial complexes – hospitals, data centers, and shopping malls.
  • Transport infrastructure – railway power systems and metro networks.
Technical Parameters of 35kV Class S9 Series Oil-immersed Power Transformer
Rated
Capacity
(KVA)		 Voltage Combination
(kV)		 Connecfion
Group
Designaion		 Losses
(kW)		 No-Load
Current
(%)		 Short-Circut
Impedance
(%)		 Weight(Kg) Overall Dimensions
(mm)
L(Length)
×B(Width)
×H(Height)		 Gauge
Longitudinal/
Transverse
High
Voltage		 High-voltage
tapping
range(%)		 Low
Voltage		 No-Load Load
(75℃)		 Weight of
Core and
Windings		 Oil
Weight		 Total
Weight
  200 35 ±5 3.15
6.3
10.5		 Yd11 0.43 3.30 1.55 6.5 610 365 1310 1240×1270×1680 660/660
  250 0.52 3.96 1.4 670 370 1405 1290×1280×1680
  315 0.61 4.77 1.4 825 440 1685 1300×1350×1840
  400 0.75 5.76 1.3 920 530 1910 2020×940×1870 820/820
  500 0.87 6.93 1.25 1110 610 2235 2150×1000×1920
  630 1.04 8.28 1.10 1215 630 2420 2260×1060×1920
  800 1.23 9.90 1.00 1540 750 3050 2330×1080×2010
1000 1.44 12.15 1.00 1795 835 3440 2390×1100×2060
1250 1.76 14.67 0.90 1960 900 3860 2450×1210×2090
1600 2.12 17.55 0.80 2275 1025 4540 2475×1230×2300
2000 2.72 19.35 0.70 2560 1050 4960 2380×2010×2300
2500 3.20 20.70 0.60 3390 1245 5730 2460×2150×2400 1070/1070
3150 35~
38.5		 3.80 24.30 0.56 7.0 3785 1500 7600 2550×2220×2420
4000 4.52 28.80 0.56 4690 1790 8500 2670×2390×2510
5000 5.40 33.03 0.48 5570 2015 9790 2870×2450×2750
6300 6.56 36.90 0.48 7.5 7380 2460 12620 3100×2580×2950 1475/1475
8000 35~
38.5		 ±2×2.5 3.15
3.3
6.3
6.6
10.5
11		 Ynd11 9.00 40.50 0.42 8870 2650 14100 3250×2680×3150
10000 10.88 47.70 0.42 10020 2930 16500 3320×2720×3230
12500 12.60 56.70 0.40 8.0 12880 3710 19780 3410×2950×3410
16000 15.20 69.30 0.40 16120 4280 23950 3520×3180×3570
20000 18.00 83.70 0.40 18580 5230 29600 3730×3560×3990
25000 21.28 99.00 0.32 22970 6370 25350 4110×4120×4220 2040/2040
31500 25.28 118.80 0.32 27600 7740 41900 4760×4570×4390

Installation & Maintenance

Installation steps:

  1. Place the transformer on a level, vibration-free foundation.
  2. Securely connect oil pillow equipment and check all seals.
  3. Fill the oil conservator to the recommended level.
  4. Test grounding, insulation, and cooling systems before operation.

Maintenance tips:

  • Regularly monitor oil level through the pillow indicator.
  • Conduct oil testing for dielectric strength every 12–18 months.
  • Inspect gaskets and bushings for leaks or pressure issues.
  • Schedule infrared scans to detect thermal irregularities.

How to Choose the Right Oil-Cooled Transformer with Oil Pillow

When selecting, consider:

  • Capacity & rating – match load demand and grid voltage.
  • Type of oil conservator – sealed vs. breathing oil pillow designs.
  • Cooling class – ONAN (natural cooling) or ONAF (forced cooling).
  • Efficiency standards – high-efficiency models reduce power losses.
  • Application environment – outdoor installations may need extra sealing.
  • Manufacturer certification – ensure compliance with IEC/ANSI standards.

FAQ

What is the purpose of the oil pillow in a transformer?

It acts as an expansion device, maintaining stable oil levels during temperature changes.

How does an oil pillow improve transformer safety?

It prevents vacuum conditions and oil leakage, reducing the risk of electrical faults.

Can this equipment be used in outdoor environments?

Yes, with protective sealing, it operates efficiently in outdoor substations and plants.

How often should the oil level in the pillow be checked?

Monthly inspections are recommended, especially in high-load systems.

Is an oil pillow transformer more durable than sealed designs?

Yes, because it reduces internal pressure and extends component lifespan.

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