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Home / Download Center / Electrical Engineering Books and Technical Guides / Schneider Electric MV/LV Tehnical Guides and Studies / Distribution transformer guide: Installation, commissioning, operation and maintenance hints

Transformer service

This guide deals with the distribution transformer construction (core, windings, cooling, tank & cover, conservator, pressure relief device, Buchholz relay, silica gel breather, winding temperature indicator, etc. ), transport & packing & despatch, installation procedure, fittings & accessories, commissioning, operation and maintenance.

Distribution transformer: Installation, commissioning, operation and maintenance hints
Distribution transformer: Installation, commissioning, operation and maintenance hints

1. Transformer location

The transformer should be kept in a well-ventilated place, free from excessive dust, corrosive fumes etc. Adequate ventilation is necessary for tank and radiators so that they can dissipate heat. There should be a clear space of about 1.25 m on all sides of the transformers if it is enclosed in a room.

2. Transformer foundation

Foundation should be firm, horizontal and dry. where rollers are fitted, suitable rails should be provided.

3. Provisions for oil draining

Necessary provisions for oil draining; in the event of a fire, should be made by way of Oil Soak Pits. Fire separation walls should also be provided when necessary.

4. Assembly of dismantled components

Various components dismantled for transportation should be duly assembled. Following are the torque values in Nm for various sizes of fasteners. (nuts & bolts)

Table 1 – Torque Table Reference Chart (Nm)

Bolt Size/GradeBushing Mounting on TankCopper ConnectionsRadiator Valve, Gunmetal Valve
SS4.8 & 5.6 Grade8.8HDG

4.1 Main tank

Keep the main tank in its permanent position of operation. Lock the rollers to prevent any accidental movement on rails. Draw an oil sample from the bottom of the tank and test it for Breakdown-Voltage (BDV).

4.2 Bushings

Clean the bushings and check that there are no hair cracks or other damages. Test IR value of each bushing with a 500V Megger. It should be 100 M ohms or greater. Note details of Bushings in the “Commissioning Report.” Mount all the bushings, ensure that the test cap is fully tightened, thus positively grounding the same.

Adjust the Arcing Horn Gaps in accordance with the Insulation Co-ordination.

4.3 Conservator & MOG

If the M.O.G. is provided with a locking lever, it should be removed. Mount the conservator. When there is an OLTC, its conservator is sometimes provided separately or by making a partitioned compartment in the main conservator. OLTC conservator, if separate, should also be mounted.

Mount the conservator as shown in G.A. Drawing. Very often the small conservator for the on-load tap changer is attached to the main conservator.

Usually, the main conservator is provided with Magnetic Oil Level Gauge having low oil level alarm contacts and the OLTC conservator is provided with an oil sight window. Magnetic oil level gauge shall be mounted after attaching the float to the gauge. Check and confirm the free movement of the float arm and proper locking of float to the float arm. Mount all valves as shown in the GA drawing on the conservator.

Mount the connecting pipe with Buchholz relay from the main tank to the conservator. Ensure direction of Buchholz Relay by the direction of arrow on Buchholz Relay towards conservator.

Mount breather connecting pipes and silica gel breathers for the main tank and OLTC conservators.

4.3.1 Method of mounting flexi separator (air cell) inside the conservator

Flexi separator (air cell) inside the conservator
Flexi separator (air cell) inside the conservator


4.3.2 Procedure – I (oil filling without vacuum)

  1. Set up the air cell inside the conservator. Care should be taken to see that the hooks on the air cell are properly engaged in the brackets provided inside the conservator.
    Inflate the air cell at a pressure as shown in the instruction plate (DO NOT APPLY EXCESS PRESSURE AS IT MAY DAMAGE THE AIR CELL) through the breather connection pipe. Follow the instructions given in the Instruction Plate fixed on the transformer.
  2. Ensure that there is no leakage.
  3. The conservator with Air Cell is pressure tested and despatched from the factory at a slightly positive pressure. Confirm that there is no oil leakage.
  4. Fix three numbers air release valves on the conservator.
  5. Keep air release valves open. Fix air filling adapter on breather pipe and inflate the air cell at an air pressure indicated on the INSTRUCTION PLATE affixed on the transformer and hold air pressure.
  6. Open the air release valves and start oil filling from the bottom filter valve of the transformer.
  7. Observe the air release valves and as soon as oil starts overflowing, close the air release valves one by one. Stop oil filling when all air release valves are closed.
  8. Remove the air filling adapter.
  9. Continue oil filling and observe the Magnetic Oil Level Gauge (MOLG)
  10. Stop the filling when the needle of MOLG shows the level corresponding to the ambient temperature at the time of filling.
  11. Fix silica gel breather.
  1. Do not open any of the air release valves after the completion of oil filling. If the air release valve is opened, air will enter and the oil level will drop.
  2. The plain oil level gauge on the end cover of the conservator should indicate the full oil level always.
  3. If air enters the conservator, it can be seen by a fall in the oil level in the plain oil level gauge.
  4. The plain oil level gauge should be monitored on regular basis.

4.3.3 Filling procedure under vacuum in conservator

The conservator, with its separator, being set up and plugged in above the transformer, is connected to its oil filling reserve by a pipe in its lower part. Proceed as follows:

  1. Create a vacuum in the separator.
  2. With the same source of vacuum, create a vacuum in the conservator.
  3. Open the oil filling valve of the transformer. Because of the vacuum in the conservator, the oil level rises automatically.
  4. Stop the oil filling once the required volume in the conservator is attained.
  5. While maintaining the conservator under vacuum, allow Dry Air or Nitrogen gas to enter into the internal part of the separator. Then, the separator inflates by itself and takes all the free space due to the fact that the conservator was not completely full. In particular, during the operation, the oil is going to rise to the top of the conservator.
  6. Inflate the separator at a maximum as shown in the INSTRUCTION PLATE.
  7. Check on the vent-holes, and confirm that there is no more air in the conservator or, if necessary, adjust the level.

4.4 Buchholz relay

Buchholz Relay floats are tied to prevent transit damage. They should be released. Also, If ‘Test’ lever is provided, it should be in the working position.

Suggested reading – Troubleshooting Buchholz relay and recommended actions

Troubleshooting Buchholz relay and recommended actions when it operates

4.5 Breathers

  1. If OLTC is provided, it may have its own separate breather.
  2. Check that the colour of Silica Gel in the main breather is Blue.
  3. Remove the rubber cap closing the breather pipe and it the breather.
  4. Fill oil in the oil cup and remove the seal which closes the breather opening.
  5. Similarly, mount the OLTC breather.

4.6 Radiators

  1. Radiators are to be assembled only one at a time. Oil required to fill in the radiators is sent separately in drums. Test oil sample from each drum for BDV. Ensure it is more than the minimum specified in I.S. 1866 for the new Transformer.
  2. Fill the conservator full using an oil filter machine, if available.
  3. Clean one radiator externally. Remove blanking plates and clean the gaskets and radiator flanges. If gaskets are damaged, replace using spare gaskets.
  4. If blanking plates are not in position and it is suspected that foreign material might have entered the radiators, clean them internally by flushing fresh and clean transformer oil.
  5. Oil might seep through the tank side radiator valves and be retained by the blanking plates. This oil should be collected in a clean container at the time of removing the top and bottom blanking plates.
  6. Bring the radiator flanges close to the flanges on the tank. Ensure that the tank gasket is in position. Fit by means of bolts, nuts, spring washers etc.
  7. Open the bottom radiator valve using the operating handle. Gradually unscrew the air release plug on the top of the radiator, until air starts escaping.
  8. The air release plug should not be removed from the engaging threads as it would be difficult to control oil coming out of it. When oil comes out steadily from the air release plug and no more air comes through, close the air release plug.
  9. Open the top radiator valve. The oil level in the conservator would now have fallen. Check that there is no oil leakage from the radiator itself and the gasket joints.
  10. Restore the oil level and assemble the next radiator in the same manner.

Suggested reading – Learn how to prepare for transformer acceptance tests and final inspections

Learn how to prepare for transformer acceptance tests and final inspections

4.7 Marshalling box

The marshalling box should be fitted in Position. Pockets of O.T.I. & W.T.I., should be filled with transformer oil. Bulbs of O.T.I. & W.T.I. should be fitted in position in their respective pockets.

Set the ‘Alarm’ & Tip’ contacts of O.T.I. & W.T.I., at required values. O.T.I, setting of A=85°C & T=95°C W.T.I., setting of A=95°C & T=105°C are considered good. They can be worked out to suit local conditions.

4.8 Cable box

When Cable Boxes are provided they should be mounted and cable terminations performed. Oil-filled cable boxes should be duly filled with oil. In the case of “Bus-Duct” connections, the transformer is provided with a flange to receive the bus duct.

Due to connection and fitting should be done to over-head line by a terminal connector.

5. Air release

Release air by slowly unscrewing Air Release Plugs provided at Bushings, Bushing Pockets, Buchholz Relay, Main Tank Cover, Oil Surge Relay etc. From plain porcelain bushing of 11, 22 & 33 kV, air can be released by loosening the nut pressing the rubber gasket and pressing the metal part down.

6. Oil sampling and filtering

Oil should not be filled in the transformer directly from the drums or transport container as there is a chance of precipitated water or sediments may also be pumped into the transformer along with the oil.

Oil from the drums or transport container should be filled into the oil storage tank and oil parameters should be tested before it is filled into the transformer.

Title:Distribution transformer: Installation, commissioning, operation and maintenance hints – Schneider Electric
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Distribution transformer: Installation, commissioning, operation and maintenance hints
Distribution transformer: Installation, commissioning, operation and maintenance hints

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  1. Faridoon Ahmed Khan
    Nov 11, 2021


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