We regularly get questions about transformer maintenance from our customers.
- Do I need to regularly inspect my transformer?
- How often should I perform maintenance?
- Does maintenance look different for liquid-filled vs dry-type transformers?
Overall, transformers are incredibly reliable. Many customers hook up their transformers and leave them for decades without service. While technically possible, this is not recommended. It’s a good idea to develop a routine maintenance plan for your unit. If issues arise, this plan will help you find them before they become major problems.
A good maintenance program is like your yearly visit to the doctor. You may feel healthy, but it's still a good idea. Some problems can be solved long before they become serious issues. If you wait till you’re sick and your symptoms are unbearable, it may be too late. It’s the same way with transformers. Some problems start small, and can easily be dealt with early on. If those problems persist, they may grow into something that ultimately fails the transformer.
Leading Causes of Transformer Failure
According to a study by Hartford Steam Boiler Inspection & Insurance Co., the two leading causes of transformer failure are insulation failure and design / manufacture errors. Insulation failures are usually caused by overloading/overheating, transformer aging, or catastrophic events. Both overheating and aging can be identified with a regular maintenance program. Design / manufacturing errors may be identified with initial testing. A maintenance plan can also catch some less obvious design issues before they cause bigger problems.
In this article, we will provide some routine maintenance tips. We will start with oil-filled transformer maintenance, and then move on to dry-types.
Note: This article is not a replacement for a manufacturer’s instructions. Only use this article as a resource for general transformer maintenance.
Routine Oil-filled Transformer Maintenance
Perform annual inspections of oil-filled units. Some circumstances may require more frequent inspections. You will need to de-energize your transformer for certain sections of your maintenance plan. If you have site wide shut downs, try to schedule this maintenance at that time. Never perform maintenance on a transformer without the proper protective equipment. Ensure you follow all local and national safety standards such as OSHA and NFPA 70E.
After taking the proper safety precautions, check the following list during annual maintenance:
- External visual inspection
- Oil sampling
- Gauge readings
External Inspection
Inspection comes before maintenance. From a safe distance, ensure the unit is operating properly while energized. Listen for loud or strange noises. Unusually loud humming may indicate overloading or overvoltage issues. Review the NEMA TR-1 standard sound levels. Sounds like electrical buzzing, zapping, or the sound of a weed wacker inside the transformer can indicate significant internal arcing. Look for any signs of external tampering, wear, or leaks. Minor weathering and wear is normal. If you do find an oil leak, de-energize the unit and address it. Prolonging repairs on a leak could lead to bigger problems later on.
Internal Inspection
After checking the outside of the unit, inspect the inside of the cabinet. This is especially important if you are energizing a transformer from storage. Before opening the cabinet, de-energize the transformer, and any connected cables or components. With the transformer off, visually inspect the high and low voltage bushings.
Inspect the seal around each gasketed component for leaks. Make sure all valves and devices are operating properly. Finally, make sure the enclosure’s hinges and locking system are free of corrosion and work well. This will help protect your cabinet from the elements, tampering, and other hazards.
Oil Sampling
Oil insulates and cools liquid-filled transformers. It works together with the paper in the windings to provide a transformer’s overall dielectric strength. Over time, both the paper and oil will break down and lose some of their insulating ability (dielectric strength). This breakdown can be witnessed through the generation of certain gasses in the oil. A Dissolved Gas Analysis (DGA) test measures the breakdown of such gasses. It is one of the most reliable ways to check the overall health of your transformer. A DGA can also indicate problems like overloading, catastrophic faults, loose internal connections, and moisture ingress. You should test your transformer’s oil every year by taking an oil sample.
DGA tests will show minimal gassing if the transformer is within its limits. For transformers with a high load, consider performing a DGA test every six months. Otherwise, an annual test is typical.
Gauge Readings
The temperature, pressure, and oil level gauges monitor internal pressure and fluid levels / temperature. Record the unit’s gauge readings. Take careful note of any abnormal values.
Check Liquid Level Gauge
Transformer oil, like all liquids, expands and contracts with temperature changes. Most liquid level gauges use a 25°C reference for standard conditions. It is normal for the gauge to read below the 25°C mark in colder climates. Under warmer temperatures, the gauge may read higher.
Check Pressure Vacuum Gauge
As the oil expands and contracts, the unit's pressure will also fluctuate. With this in mind, compare the pressure gauge with the oil level and temperature gauge readings. The readings between all three should make sense. The ideal pressure reading under normal operation is between 2 and 5 psi.
Oil Temperature Gauge
The liquid temperature gauge displays the temperature of the fluid inside the tank. Ambient outside temperature and transformer loading influence the oil temperature. The red pointer on the temperature gauge is the maximum level indicator. It shows the historical high point of the gauge’s day-to-day movement.
Record what you observe on all three gauges and save the information. Over time you can prepare a trend analysis to better understand your transformer. If your transformer has abnormal gauge readings, consider all the factors at play. Check out our article for more information on how to read a transformer’s gauges. If you find a leak and need help repairing it, reach out to Maddox for repair help.
Routine Dry-type Maintenance
It is a good idea to also inspect dry-type transformers once a year. More inspections may be needed depending on the transformer's age, wear, and location. Dry-type transformers use air instead of oil for cooling.
These units can function for years without ever having a problem. But if a dry-type suddenly goes down, it could mean thousands of dollars in losses. That’s why many companies schedule bi-annual and even quarterly inspections. When performing routine maintenance on dry-type units, consider the following tips:
- Inspect enclosure
- Keep unit clean
- Check temperature monitoring systems
External Inspection
Check the area around your dry-type to ensure there is no external damage to the unit. Also ensure that there aren’t any boxes or other objects resting on the unit or covering up the air vents. Listen for loud or unusual noises. Like with liquid-filled units, unusually loud humming may indicate overloading or overvoltage issues.
You can inspect the external enclosure from a safe distance while your unit is energized. Ensure to de-energize your unit for any internal inspection or maintenance.
Enclosure Cleaning
Due to their open ventilation, dry-types accumulate dust and debris inside their enclosures. Built up dirt provides tracking paths for arc faults. Regularly remove this dust and dirt to keep your transformer running safely. Also make sure the ventilation ports are free of debris, so the unit can cool properly.
This requires regular cleaning. Since air is the cooling medium, restricted air flow can affect a dry-type's performance. If debris blocks air flow, the transformer could overheat. During an inspection, remove accumulated dust around the filter grates. For details on cleaning out the enclosure refer to our 600v Dry-Type manual.
Check Temperature Monitoring System
If your unit has cooling fans, inspect them for signs of wear or corrosion. Make sure the blades are not deformed. The motor should run smoothly without making any strange noises.
Warnings and Recommendations
For most systems, an annual maintenance schedule works well. Transformers serving mission critical applications may require more frequent inspections. Consider the following maintenance tips for both types of transformers.
Don’t Overload
A transformer’s kVA should account for the highest loading situation. If the unit is overloaded, you’ll notice higher temperatures. Continuous overloading results in overheating and eventually insulation failure.
Consult Manufacturer's Manual
Always refer to the manufacturer's instructions for your transformer before consulting other resources. The manufacturer's guide will provide information about your specific transformer. For more tips, consider Maddox’s digital manuals.
Transformer Repair and Replacements
If you identify issues during an inspection or test, address them quickly. If you need your transformer examined or repaired, fill out the form below. We offer a variety of field service and repair options including inspections, tests, and full remanufacturing.
