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Maintenance of Oil-Filled Transformers – Ultimate Guide
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Oil-filled transformers, also known as oil-immersed and liquid-type transformers, are power transformers housed in oil-filled welded steel tanks with their coils submerged in mineral oil. The transformer generates heat in its coils and iron core when it is in use. This heat is transferred from the insulating oil to the cooling medium, where it is regulated. The oil inside oil-filled transformers acts as the cooling agent to keep the machine temperature under control.

Oil-filled transformers are the most commonly used transformers in the world and require regular maintenance to remain in good working conditions. If a transformer is not maintained through scheduled upkeep, it may cause system-wide operational issues and may result in system failure. It is important to stay ahead of equipment concerns by implementing maintenance procedures proactively rather than reactively or after a fault condition has occurred. Doing so can help address many equipment issues before they occur and achieve a better outcome for the equipment and the system.

This comprehensive guide provides valuable insights and practical tips on effectively maintaining oil-filled transformers through unit inspections and maintenance schedules. Following this guide will help you avoid repair and replacement costs arising from unmaintained transformer units.

How To Perform Routine Maintenance on Oil-Filled Transformers?

Different strategies should be adopted for routine maintenance of oil-filled transformers based on the frequency of the maintenance work. Check out the following sections for valuable guidance on establishing maintenance schedules for daily, weekly, monthly, and yearly tasks.

Daily Procedures

These are the maintenance procedures that must be implemented daily to maintain the working condition of an oil-filled transformer. The daily routine maintenance schedule includes inspecting the transformer temperature, electrical performance, and noise level. These are described below:

Oil and temperature: To perform daily checks on the transformer’s temperature, we must inspect the ambient, oil, and winding temperatures and compare them against rated values. The oil level gauge indicator should vary with temperature changes, and if the oil level gauge is of a magnetic model, the indicator should rotate in the presence of a test magnet. If the indicator does not respond in either case, then the indicator must be replaced.

Electrical Performance: To perform daily checks to evaluate the transformer’s electrical performance, we must inspect the load voltage and current values and compare them against the rated figures. The load settings dictate which type of transformer can be used to feed it. Due to its importance, a transformer is often protected by differential, overcurrent, and ground-current relays that must be monitored and maintained daily. Using the wrong transformer and not maintaining protective relays can cause electrical damage and electrical failure of the equipment, respectively.

Noise level: It is a more prominent performance indicator for dry-type transformers. However, the noise level emitted by an oil-filled transformer’s coils, core, and cooling system might also be used to evaluate its performance. If an oil-filled transformer emits loud noises during operations, then it is important to promptly identify the source of the noise and measure it through the following three techniques:

  • Sound Power Level Method
  • Sound Intensity Method
  • Sound Pressure Level Method
Weekly and Monthly Procedures

Some maintenance procedures must be implemented weekly or monthly to maintain the working condition of an oil-filled transformer. The weekly routine maintenance schedules should include the following tasks:

  • Check the oil level gauge against oil temperature to analyze the oil level inside the oil-filled transformer.
  • Fully inspect the exterior of the transformer visually to look for signs of chipped paint and discolorations.
  • Inspect the Buchholz relay to look for signs of gas collection.
  • Ensure that the gasket radiators and joints are tight with no signs of oil leakage.
  • Inspect the sealing of the vents of all cover-mounted pressure-relief devices.
  • Investigate the operations of all pressure-relief devices.

Some maintenance work must be performed monthly or quarterly unless needed sooner. Such routine maintenance schedules should include the following tasks:

  • Check the dielectric strength of the transformer oil and top off the transformer oil as required after every few months.
  • Check the transformer terminal bushings and cable box for damage and tightness every few months.
  • Check the bushings for cracks every month and during yearly inspections.
  • Inspect and clean the transformer accessories and the auxiliary circuits every six months.
  • Collect transformer oil samples and periodically test them for moisture, acidity, deterioration, discoloration, and other properties to decide whether the oil needs to be filtered or replaced.
  • Perform Dissolved Gas Analysis (DGA) on the oil samples to identify the types of gases dissolved in the oil. This can be done on-site if the testing equipment is available or by sending the samples to a laboratory. The most common gases found in such transformer oil samples include hydrogen, oxygen, methane, ethane, carbon monoxide, acetylene, and ethylene.
Yearly Procedures

A yearly maintenance schedule should include the measures already discussed in the daily, weekly, and monthly schedules and additional tasks for components requiring less frequent maintenance. These tasks may require the assistance of transformer repair and maintenance professionals. The yearly routine maintenance schedules should include the following tasks:

  • Check the pressure-relief valve, cover-mounted pressure-relief device, Schrader valve, and surge relay once every year for damage and operational efficiency.
  • Check the off-circuit selector and temperature indicators annually. Replace any of these components if they are damaged or not working properly.
  • Check the condition of the transformer protective relays and measure their insulation resistances.
  • Clean the marshaling boxes and ensure that their weatherproof seals remain undamaged.
  • Check the condition of the fan and oil-pumping motors and their resistances.
  • Examine the top of the transformer near the pressure-relief tank for leaks every three to five years.
Oil-Filled Transformer Maintenance Checklist

Preparing a maintenance checklist before performing maintenance on the oil-filled transformer can help make the process much easier and ensure that all necessary steps and requirements are fulfilled. A checklist can help in-house and outsourced workers from trusted companies perform their duties. The following steps are some of the basic items to check off the list during the maintenance process to ensure a thorough job:

  • Ensure that the transformer is de-energized and offline before performing any maintenance work.
  • Visually inspect the equipment for any rust, dirt accumulation, chipped paint, or discoloration.
  • Listen to the unit when it is energized for any signs of abnormal vibrations or noise.
  • Clean the transformer and remove all signs of dust, debris, and dirt from it.
  • Check the transformer temperature and oil level indicators.
  • Inspect all cables, bolts, and hardware for tightness. Tighten anything that may be loose.
  • Re-energize the transformer before performing tests and analyses like breakdown voltage testing or infrared temperature analysis.
  • Inspect the transformer for signs of leakages. If found, address the leak immediately. If a transformer leaks often, it might be better to fully replace the transformer unit. Common sources of transformer oil leaks are bad gaskets, improperly welded points, and the presence of rust on the bottom of the unit.
  • Record all events and conditions of the maintenance work. For example, note down the names of the workers, which parts of the transformer they work on, how and when they do their maintenance duties, etc.
The Advantages of Performing Preventive Maintenance on Oil-Filled Transformers

Preventive maintenance has important benefits for all power system machines and combining it with predictive maintenance steps can make the overall upkeep strategy more effective. Keeping a watchful eye on transformers and following predetermined maintenance plans can help avoid unexpected problems and ensure the transformer operates smoothly without any failure or hazardous working conditions. Naturally, there are many advantages to having a proper plan for repairs. Some of these are described below:

Extended Equipment Life

A transformer will have a longer service life when properly maintained, which means that the transformer can be used for longer without needing itself or its expensive parts to be replaced. For example, an effective upkeep strategy can allow a generator step-up (GSU) transformer to last up to 25 years.

Enhanced Worker Safety

The likelihood of hazardous working conditions for a transformer is greatly reduced with sufficient maintenance. A well-maintained transformer with a well-kept protective relay system and additional safety measures is unlikely to explode or cause electrical failures.

Optimized time management

A well-maintained transformer works more efficiently and breaks down less often than an unmaintained transformer, thus introducing less unplanned downtime to system operations. Additionally, an upkeep plan increases employee productivity by giving them a clear strategy on how to perform less complicated maintenance tasks more quickly. The time they save on maintenance work can then be used to perform essential job duties.

Improved Budget Control

Budget management improves directly due to preventive maintenance as it requires workers to plan their tasks far in advance, thus giving them ample time to allocate the limited budget more efficiently. The company finances also become more transparent to all workers, as they must estimate the total maintenance costs for monthly, quarterly, and yearly maintenance work. Fewer transformer issues and less replacement and repair work can save the company a lot of money to be put toward areas of business that need it most.

Bottom line

Before purchasing an oil-filled transformer, any potential buyer should consider the maintenance work needed to keep the device in good working condition. When it comes to maintenance, it is important to stay ahead of equipment concerns by solving them before they become known issues.

That said, you can invest in the Meta Power Solutions (MPS) oil-immersed transformers to lessen the tasks on your transformer maintenance schedules. We not only have a large production capacity but also have the technological solutions to help you prevent power losses, save substantial energy units and energy sources, and reduce operational costs.

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