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Home / Technical Articles / The most common accessories you can find on oil filled transformer

Transformer accessories

Every oil filled transformer has a variety of accessories for different purposes. Here, I’ll try to name the most common of them, together with picture and description. Note that pictures can be different depending on manufacturer.

The most common accessories you can find on oil filled transformer
The most common accessories you can find on oil filled transformer (photo credit:

Also, you will notice an absence of transformer protection relay. I have written about it few times about a year ago (read here and here). Oil conservator is considered as a standard part of transformer, so not an accessory.

If you find anything missing in this list below, feel free to sharpen the pencil and leave the comment ;)

Ok, here’s the list of the ten accessories I’m talking about:

  1. Liquid level gauge
  2. Liquid temperature gauge
  3. Pressure-vacuum gauge
  4. Pressure relief device
  5. Winding temperature gauge
  6. Transformer cooling fans
  7. High voltage bushings
  8. Low voltage bushings
  9. De-energized tap-changer
  10. Insulating liquid

1. Liquid level gauge

Liquid Level Gauge
Figure 1 – Liquid Level Gauge

A liquid level indicator is provided to aid in the systematic inspection of the transformer under load. It consists of a float-arm inside the tank, an indicating pointer and a magnetic coupling between the two across a liquid-tight separation.

The gauge may be furnished with SPDT (Single Pole Double Throw) alarm contacts to give a remote annunciation of low liquid level. For contact wiring and terminal points, see the accessory connection diagram furnished with the transformer.

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2. Liquid Temperature gauge

Liquid Temperature gauge
Figure 2 – Liquid Temperature gauge

The temperature gauge is furnished to indicate the top liquid temperature in the tank in degrees Centigrade. The temperature-sensitive element is mounted in a leak-proof well, permitting removal of the thermometer without lowering the oil level.

The device is furnished with an additional red pointer to show the highest temperature attained since the last reset.To reset the maximum indicator, turn the knob in the center of the dial. The thermometer can be furnished with two SPDT contacts for a high temperature alarm, for energizing a fan circuit or for a low temperature alarm.

For wiring and contact settings, refer to the schematic furnished with the delivered transformer .

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3. Pressure-Vacuum gauge

Pressure-Vacuum gauge
Figure 3 – Pressure-Vacuum gauge

The pressure-vacuum gauge indicates whether the gas space in the tank is under positive or negative pressure. The pressure will vary depending on the transformer temperature. If the transformer is de-energized or operating under light load in low ambients, the pressure may be negative .

Note: If the indicator reads zero and does not change under any load condition, the transformer should be checked for a possible leak in the seal. If sufficient air has been absorbed by the liquid during shipment or storage, the transformer may operate indefinitely in the vacuum range, depending upon the loading conditions .

This, in itself, is not cause for concern, provided the pressure vacuum gauge does not remain on zero for any length of time an indication of a leak. The unit may be equipped with pressure vacuum switches with two SPDT contacts for remote alarm on positive and negative pressure.

For wiring and contact ratings, refer to the schematic furnished with the delivered transformer .

When required, the pressure gauge is furnished with a pressure regulator that will automatically regulate the tank pressure between 7.0 psig positive and 3.0 psig negative. The pressure regulator is fitted with a valve and fitting to take gas samples.

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4. Pressure Relief Device

Pressure Relief Device
Figure 4 – Pressure Relief Device

All substation transformers are furnished with a:

  1. Mechanical pressure relief valve (PRV), or
  2. Pressure relief device (PRD).

The cover-mounted PRD consists of a selfresetting, spring-loaded diaphragm and a mechanical operation indicator. Should the tank pressure increase above that for which the device is set, the gas pressure will lift the diaphragm and let the gas escape quickly. Immediately after the pressure returns to normal, the diaphragm will reset and reseal the transformer.

A mechanical indicator will protrude vertically. This must be reset manually to indicate subsequent operations.

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5. Winding Temperature gauge

Winding Temperature gauge
Gigure 5 – Winding Temperature gauge

Transformers may be furnished with a winding temperature gauge as optional equipment. A temperature sensitive stem is mounted in a leakproof well, permitting removal of both the instrument and stem without lowering the liquid level.

The well is heated by both the surrounding liquid and a heater element which is energized from a current transformer mounted inside the tank to simulate the hot spot winding temperature gradient.

The combination of the two temperatures is indicated on the gauge. An additional red pointer is furnished to show the highest temperature attained since the last reset.

The maximum indicator is resettable by means of a pushbutton through the bottom of the dial bezel .

The gauge has three separate SPDT switches for fan control and alarm circuits. For wiring the contact settings, refer to the schematic furnished with the delivered transformer .

The equipment is calibrated to indicate the hottest spot of the transformer windings. All contacts are factory set to operate at the temperatures shown in the connection diagram. If readjustment of the contacts is desired, consult the manufacturer for detailed instruction.

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6. Transformer Cooling Fans

Transformer Cooling Fans
Figure 6 – Transformer Cooling Fans

In order to increase the transformer load without overheating the windings, a set of fans can be furnished as an optional item .Fan control consists of a contact on either the liquid temperature gauge or the winding temperature gauge (when furnished), and “Manual-Auto” control switch .

For continuous run, the switch is turned to the “Manual” position. In the “Auto” position, the fans are controlled by the contacts on the temperature gauge. For contact and temperature settings, refer to the schematic furnished with the delivered transformer.

Refer to wiring schematic for control equipment supplied.

Warning! Fan guards are furnished and installed for your protection. Do not remove fan guards or probe into the fan with long objects. Doing so can result in severe personal injury and equipment damage.

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7. High Voltage Bushings

High Voltage Bushings
Figure 7 – High Voltage Bushings

High voltage leads for ratings 2 .4 kV and up are normally brought through the tank end wall using a porcelain bushing. To prevent excessive mechanical loading of the bushing, only flexible connections should be made to the bushing terminal.

The bushing should never be used as a structural member to support other current-carrying parts.

Care must be taken in handling the bushing to avoid cracking the porcelain or damaging its surface.

Should it become necessary to replace a bushing or its gasket, proceed as follows:

  1. Vent the tank to the atmosphere until pressure is zero.
  2. Lower the liquid level to a point below the bushing level.
  3. Remove the nuts and washers used to clamp the flange of the bushing.
  4. Pull the bushing outward as far as necessary to replace the gasket and/or to unfasten the cable connection at bushing inner end.

Only in rare circumstances will there be insufficient slack in the cable leads to facilitate bushing replacement through the wall .The alternative means of gaining access to the connections is through the manhole.

When reinstalling the bushing, install a new gasket in the gasket recess on the underside of the flange to insure that the gasket is properly seated in the groove. A flat washer and lock washer should be placed between the mounting nut and the flange. After the nuts are finger tight, each one should be tightened to a torque of 60±5 inch pounds .

After completion, pressure test the transformer. When condenser-type bushings are used, supplementary leaflets forming a part of the complete instruction book will be provided.

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8. Low Voltage Bushings

Low Voltage Bushings
Figure 8 – Low Voltage Bushings

Low voltage leads for ratings in the 1 .2 kV Class are normally brought through the tank wall using an indoor bushing. This is a cast resin or porcelain bushing.

The low voltage bushing should not be used as a structural member.

As a rule, only flexible connections should be made to any bushing. Avoid rigid connections between the bushing and other bus supports to eliminate thermal expansion forces to the bushing.

If a bushing is damaged and leaking transformer coolant, contact your factory representative for proper repair procedures.

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9. De-energized Tap-Changer

De-energized Tap-Changer
Figure 9 – De-energized Tap-Changer

Warning: Do not operate the tap-changer while the transformer is energized! Doing so can result in severe personal injury and equipment damage.

The tap-changer provides a means of changing the voltage ratio of a de-energized transformer without breaking the transformer seal. It is operated by means of a rotatable handle located on the side of the transformer. The tap-changer is normally provided with five or seven positions, as indicated on the tap-changer dial plate and transformer instruction nameplate.

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10. Insulating liquid

The insulating liquid in substation transformers is either conventional transformer oil, R-Temp™ fluid or Envirotemp™ FR3™ fluid.When makeup liquid is required, use only approved fluid of the same type that is in the transformer.

It is important to check the proper liquid level in the transformer at all times by periodically observing the liquid level gauge.

In addition, the dielectric strength of the insulating liquid must be maintained at a high value. It is recommended that a sample be taken of the liquid and tested within one week after energization, and annually thereafter.

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Reference: Substation Transformer Installation, Operation, and Maintenance Instructions and Parts Replacement Information – Cooper Industries

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. adedayo olajide
    Mar 07, 2020

    love your good work mr edward,fantastic analysis with excellent breaking down of accessories

  2. Phil
    Feb 09, 2019

    As a Supplier of the Qualitrol Brand happy to see these images used, an alternative to FR3 a Natural Ester which has many benefits is Midel’s En1204 they also do a Synthetic Ester, this product has superior cooling properties to Mineral Oil or Silicone Fluids, and has no fire risk.

  3. Sebastian Müller
    Apr 23, 2018

    Thank you for your overview. My company produces many of this transformer accessories. There are some more interesting products such as Buchholz relay or transformer protection device for hermetically sealed transformers. Depending on transformer there are many different accessories to be used.

  4. vinton
    Apr 24, 2015

    oil filled high voltage bushing what is the purpose of the oil inside the bushing.

  5. Sandip kumar pandit
    Feb 03, 2015

    Please tell me about electric traction basics

  6. karam mohamed sayed
    Nov 01, 2014


  7. Jim Stead
    Oct 06, 2014

    A pressure vacuum gauge on a transformer with a conservator generally reads zero (if it is near the top of the transformer, slightly positive if it is located lower). On a transformer with a conservator, a changing pressure indicates that the conservator breather is blocked, or the valve connecting the transformer to the conservator is closed.

  8. Adebayo Olusola
    Jun 03, 2014

    Thanks for your great works Mr Edvard. Pls put up topics on Practical design LV and MV switchgears (Panels). Ifthey are here already, pls guide me to them.

  9. engr shahid
    May 18, 2014

    Hydron meter is also a common accessory of power transformers now a days.

  10. Navdeep
    May 17, 2014

    Dear you forget about conservator tank, Silica gel breather

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