Maintenance of the Transformers

Electrical transformers are an expensive and necessary component in electrical system. Therefore, the most effective method to ensure that the equipment is operating at its full potential and has a longer lifespan is to do routine maintenance checks on it.

In its most basic form, there are two different kinds of transformer maintenance. The first is something that is carried out on a regular schedule, while the second is that is done only in conditions of extreme need. Maintenance checks that are performed on a regular basis reduce the probability that emergency maintenance will be required, which in turn reduces the amount of time spent offline.

Process for Maintenance of Transformer

This involves a wide range of maintenance tasks, including

• Overhauls,
• Prevention and removal of malfunctions,
• Changeover management, and
• Development of specialised tools and equipment.

In addition, the structure includes resources for maintenance, such as

• Tools and equipment,
• Spare parts,
• Procedures and paperwork,
• Maintenance tasks, and so on.

It is possible to condense the Maintenance Process into four stages:

• Planning stage,
• Organisation stage,
• Execution stage, and
• Recording stage.

In addition, there is a feedback step that provides for optimisation.

A power transformer has to be subjected to a wide variety of different kinds of preventative maintenance inspections.

The following are some of the most important tasks pertaining to transformer maintenance that need to be carried out:

Monthly Inspections of the Transformers for Maintenance

• On a monthly basis, the oil level in oil cap has to be checked to ensure that it does not go below a predetermined limit and cause any harm that may be caused by this.
• It is essential that to keep the breathing pores clean in silica gel breather  in order to maintain correct breathing action at all times.
• Be certain that the oil in your electrical transformer’s oil filler bushing is topped up to the appropriate level if the transformer has such a feature.

Daily Inspection and Testing of the Transformers for Maintenance

The following are the preventative maintenance checks that should be performed every day on the transformer:

• Magnetic Oil Gauge (MOG) readings of both the main tank & the conservator tank’s oil levels.
• Always be sure to keep the oil level at the proper level in MOG by maintaining it.
• In the condition that the silica gel becomes pink, it should be replaced.
• If there is any sign of leakage, seal it.

Annual Inspections of the Transformers for Maintenance

• Annual inspections are required for the air fans, oil pumps, and any other components that are used in the process of cooling down a transformer & control circuit.
• Always use a gentle cotton towel to wipe out the bushings of the electrical transformer once a year. This is a required maintenance task.
• On a yearly basis, a thorough inspection of the oil condition of the (On-Load Tap Changer) OLTC is recommended. To do this, get an oil sample from the drain valve and have it analysed for both its dielectric strength (BDV) and its moisture content (PPM). It is necessary to change the oil if the BDV value is determined to be low while the PPM value is found to be high.
• Make it a yearly routine to give the inside of each of the marshalling boxes a thorough cleaning. Check that the space heaters and the lighting heaters are operating in good condition. At least annually, each and every one of the terminals (contact) connections of the control and relay wire has to have their screws tightened.
• It is necessary to use the appropriate cleaning solution to clean each of the control switches, alarms, and relays, as well as the circuits that connect them, the Remote Tap Changer (RTC) Control Panel, and the Relay and Control Panel.
• Check each of the chambers to see whether they have the needed amount of oil in the Winding Temperature (WT) Indicator & Oil Temperature (OT) Indicator, and if not add more oil to the chambers as necessary.
• On an annual basis, the Buchholz relay & Press Release Device relays should both be tested to ensure that they are operating correctly.
• Remember to measure the resistive value of transformer connected to the earth connection, and the rizer need to be measured once a year using an earth resistance meter.
• It is recommended that a DGA (Dissolve Gas Analysis) of Transformer Oil, be carried out on an annual basis for 132 KV transformers, once every 2 years for transformers with voltages lower than 132 KV, and at regular intervals of 2 years for transformers with voltages more than 132 KV.

Half-Annual Inspections of the Transformers for Maintenance

Every six months, the electrical transformer has to be inspected to determine its IFT, flash point, DDA, sludge content,  dielectric strength, acidity, water content, and how resistant it is to transformer oil. Additionally, it is mandatory to have its dielectric strength measured.

Benefits of Transformer Maintenance

• Enhanced assurance of safety
• Problems are identified before they become a danger to the organisation.
• Improved efficiency of operation of the apparatus; decreased costs
• Maintenance on repairs may be planned in an efficient manner.

Procedure for Maintenance

1). Precautions for Safety

Work on the transformer should never be performed until it is removed from all external electrical circuits & all windings are securely earthed.

While operating, keep bright lights and flames away from the transformer.

2). Oil

Transformer oil decreases or becomes contaminated during storage and service. As a result, frequent maintenance is essential to keep it in good condition, and it may ultimately need to be replaced with fresh oil. IS:1866 and 335 describe the causes of oil degradation and guidelines for different examinations. A few pointers are provided below for your convenience.

The oil level must be checked on a regular basis, and any oil leaks should be examined.

To minimise potential problems caused by low oil levels, any leaks should be fixed as quickly as feasible.

Oil samples should be obtained on a regular basis and tested for BDV in accordance with IS-335; the BDV of the oil shouldn’t fall less than 60KVrms. On the filled sample, this test must be repeated six times. The average of the six outcomes should be utilised.

It should be noted that for big transformers, dielectric strength does not provide the accurate indicator of the oil’s quality. In this case, additional tests should be performed.

It is suggested that the oil in big transformers be monitored for acidity. If the acidity is fast growing, the transformer cover should be removed to inspect the inside of the tank, as well as the core and windings. If there is sludge or corrosion in the oil, it should be treated (or) disposed.

When the transformer is opened or the IR readings are low, it must be filtered twice before being sealed.  The transformer is then cooled to ambient temperature, and the oil level is decreased to normal fill level.

3).Bushings

Clean the porcelain bushings and look for cracks.  Small chips may be tolerated, but major damage will need the purchase of new porcelain from the maker. It is suggested that some stock be retained.

4). Connections

All connections must be secure. If they seem darkened, unscrew the connections and use emery paper to polish down to bright metal.

5). Oil Level Gauge and Conservator

Conservators are designed such that the bottom half functions as a sump, collecting any contaminants that enter the conservator.  For draining and sampling, a valve is installed at the conservator’s lowest position. Every two or three years, the  conservator inside should be cleaned with oil.

The oil level indicator needs to be maintained clean at all times. A translucent material allows to see the oil level. In the event of a break, an urgent replacement is required.  The oil gauge should be examined and cleaned when the conservator is removed for cleaning.

6). Breather

Silica gel breathers are equipped with a sight glass to ensure that the crystals colour is seen. As the crystals absorbs moisture, the tint shifts from blue to pink.  When the crystals become saturated with moisture, they turn pink and must be reactivated by heating.  Oil must be accessible in the oil cell.

7).Gaskets

Gaskets might shrink during service. As a result, all bolts used in gasketed joints must be checked for tightness. To eliminate unequal pressure, the bolts must be tightened equally around the joints. Gaskets that are leaking should be changed as soon as possible.

8). Buchholz Relay

Routine operation & mechanical inspection tests are recommended to be performed once a year and twice a year, respectively.  If a gas has been found to be gathering and emitting an alarm during operation, the gas must be examined and investigated to determine the cause of the malfunction.  It is possible that the gas accumulating is merely air at times. The oil might be releasing some absorbed air as a result of the temperature shift. When no hover is used during oil filling, the absorbed air is expelled in the early phases. A chemical study of gas may identify internal defects to a large degree. The Buchholz relay will also emit an alert / trip if the oil level falls below the Buchholz level.

9). Explosion Vent

The diaphragm installed at the exposed portion of the vent needs to be examined on a regular basis and replaced if broken. Failure to replace the diaphragm soon may cause moisture to enter and taint the oil.  If the diaphragm is broken due to a transformer failure, an examination must be performed to ascertain the type and origin of the damage.

10). Temperature Indicators

The amount of oil in the pockets containing thermometer bulbs needs to be checked and replenished during the annual maintenance inspection. If the capillary tube has become loose, it should be tightened anew.  Dial glasses must be maintained clean and, if damaged, should be replaced as quickly as possible to avoid instrument damage.

11). Physical Inspection

The transformer tank cap and other elements should be examined on a regular basis for oil leaks, paint flaking, and corrosion.  Rusted areas should be thoroughly cleaned and repainted. Oil leaks should be addressed quickly, and clamping bolts on the gasketed joints must be tightened as needed, and broken gaskets replaced. Welding again should be used to eliminate leaks via welded seams (this ought to be performed without any of oil in the transformer tank or radiators).

Why it is important to maintain a transformer?

The fundamental objective of the maintenance performed on the transformer is to make sure that all of its components, both internal and exterior, as well as its accessories, are in satisfactory condition, are operating at optimal levels, and can do so in a secure manner. Keeping historical records of the state of the transformer is also an essential function, and this should be done regularly.