The Ultimate Guide: On the Maintenance of Electrical Power Transformers

Maintenance Inspection Tasks

Although there are many ways to test for power transformer condition, the majority of power transformer problems can be detected by visual inspection. Maintenance engineers are well aware of this. This best practice guide lists the inspections that can be performed on a power transformer in and out of service.

Infrared Thermography

Infrared thermography provides a visual representation of invisible infrared radiation emitted from objects by their thermal conditions. It is not possible to contact the object directly with the image. This can be used to identify a defect in transformer bushings, hot spot on surge arresters, blocked cooling system, and circulating currents that affect localized overheating of tank walls.

Infrared thermography is sometimes used to detect temperature rises in compartment types OLTCs.

These problems can be easily detected on OLTC designs, as the OLTC compartment is usually cooler than the main tank. Any IR scan showing the contrary should prompt an investigation.

Main Tank and Conservator

The transformer active element (core, coil assembly and insulating fluid) is contained in the tank. A compartment for the on-load tap changer can also be included in tank condition assessment. Its mechanical characteristics are the most important determinant of the tank's integrity. The tank should be able to hold pressure and not leak.

The condition of the following factors will determine the integrity of the tank:

* The gasket containing surfaces

* The tap changer door gaskets for the manhole, hand hole and manhole

* and tank integrity.

Oil leakage is a sign of a problem. Inability to seal a tank properly can make it unsuitable for current installations. Leakage prevention systems that use specialty sealants or unique hardware with gasket provisions may prove to be effective.

Deformation signs should be checked on the tank. Tank deformation can be caused by extreme pressure, electrical arc, poor foundation support, frequent removal of spare transformers, or from improper installation.

You can check the integrity of a bladder by inserting a swab stick fitted with a cotton cloth at the end through the bladder access port on the top of your conservator tank. Then, gently wipe the bladder's interior. If the cotton swab is saturated with oil, it indicates that the bladder has a leak and must be replaced.

These transducers can detect rupture:

* A capacitive transducer which detects oil contact

* To detect air, a gas collection relay is mounted at the top of the conservator.

Regularly check the oil level of the conservator system to ensure it is at the correct level. Also, make sure there isn't an oil leak in the transformer. The oil level gauge has a 25degC marking that can be used to adjust the correct oil level.

Conservator tanks weren't designed to withstand vacuum in the past. These conservator tanks were not designed to withstand full vacuum. The valve between the main and conservator tanks must be shut down when a vacuum is applied. Modern conservator tanks can be used for full vacuum, so the valve should not be closed. The transformer manual or manufacturer should always be consulted in all cases.

It is important to check the desiccant used in the bladder-type conservator system on a regular basis and replace it when it has reached its limit of moisture removal. A system like this with a non-functioning deiccant (especially in humid environments) can cause moisture to enter the tank or prematurely age the bladder.

These actions can be done on the main tank:

* Inspect for oil leakages and reseal if necessary

* Inspect for paint damage or corrosion and repair if necessary

* Inspect all compartments for oil levels

* Refill or replace the main conservator bladder and air dryer if necessary

* If an air freezer is installed, check it.

* Verify the grounding of neutral terminals and main tank

* Inspect the electrical insulation at the tank base if it is fitted

Before putting the valves back in service, make sure you check their status

If silica is used to dry silica gel, the silica color will change. This indicates that silica is saturated with water. The air that passes through the breather will stay humid. If silica gel has changed in color by more than 50%, it should be replaced.

The color change must be seen starting from the fresh-air input side (where the air enters the system on its way to the conservator). If silica gel color changes on the conservator side, or if silica gel color remains unchanged for several months, this is an indication that there is air leakage in connection piping, allowing air to bypass your breather.

Cooling System

Oil pumps

Regular visual inspections of transformer oil cooling circuit components must be done. Usually, this should not exceed every 12 months. To ensure proper operation, oil pumps should be manually energized. Any unusual noises, such as grinding, rubbing, scraping or oil flow cavitation, should be taken note and investigated. Flow gauges must indicate flow with no fluctuation. Oil leakage must be ruled out in all areas, including the piping and valves.

To ensure the integrity of bearings, pumps equipped with a bearing condition monitor need to have their readings taken every year. In-service failure is more common for pumps with ball bearings. It is recommended that pumps with ball bearings be replaced by pumps with sleeve-bearings.

Radiators and fans

To ensure proper operation, fans should be manually energized. Similar to oil pumps, any fan motor noises or fan imbalance shudders should be noticed, investigated, and rectified. For the safety of staff, fan blade guards should be maintained. Debris should be spotted and removed from the airflow.

To ensure heat transfer and oil flow, it is important to periodically take infrared images of radiators. Oil leakage must be prevented from all areas, including the piping and valves.

Pipe work flange joints and valve stems are common oil leakage points.

Air Coolers for Oil-Air Forced

Many generator step-uptransformers, mobile transformers, as well as some substation transformers use forced oil-air coolers. To provide the required cooling to dissipate transformer losses, these coolers require full airflow from the fans and full oil flow from the pumps.

These coolers will cease to function if the pumps and fans are not working. The environmental conditions in which the transformer is used can affect how the coolers age over time. These coolers may experience premature death if they are exposed to any one or more of these conditions.

Transformer Cabinets

This component needs special attention as most accessories are connected to the Transformer Cabinet. These inspection tasks should be performed:

* Verify power supply, backup feed, and signaling

* Verify the functionality of anti-condensation heaters

* Clean all contact points and tighten screws on current-carrying components

* If installed, check the service hour counter

Buchholz Relay Operation: Recommended Actions When It Operates

This section discusses the causes of Buchholz relay operation. It also recommends what to do after a Buchholz surge trip, or gas collection alarm is received. This section covers both operational situations as well as situations that may arise during commissioning new transformers.

Buchholz relays mounted on the transformer main tanks have two components, which are described below.

Gas Collection Element

This element captures gas that escapes from the transformer, and closes the contact when it reaches a set limit. If the limit is exceeded, any gas produced will be released from the relay and into the conservator. No further operations will be performed. It is important that the transformer was designed so that gas bubbles do not collect in its internal cavities.

When a certain amount of gas is collected, the gas collection element works. It does not give any indication of the gas production rate.