Maximize the Service Life of Your Transformer

Executive summary

To help you achieve reliable and maximum uninterrupted service life of your transformer, Hartford Steam Boiler offers comments and recommendations. Much of this information is part of a collection of loss prevention and maintenance documents that is available on our web site ( for a variety of equipment types. Our advice is intended to complement the equipment manufacturers’ recommendations — not replace them.

The guidelines for transformers includes a substation overview and information about:

  • Installation
  • Oil Testing
  • Gauges
  • Connections
  • Cooling
  • Safety
  • Cost

The full article

Maximize the Service Life of Your Transformer


Every year, Hartford Steam Boiler investigates numerous transformer failures. The primary reasons for these failures are poor or nonexistent preventive maintenance programs, or improper electrical loading. To help you achieve reliable and maximum uninterrupted service life of your transformer we offer the following comments and recommendations. This includes information that is part of a collection of loss prevention and maintenance documents for a variety of equipment types. The documents are available online at our web site, From our home page, click on Quick Links and then EquipmentCare for a list of available topics. You can also go directly to the Equipment Care site at

These guidelines are intended to provide the transformer owner with consistent and effective directions for maintaining transformers, and where appropriate, the surrounding substation. In order to ensure that maintenance is conducted efficiently and with sufficient scope, the approach taken should be risk based, rather than object oriented. What this means is that it may not always be necessary or desirable to perform maintenance on every object in every substation. One also cannot just maintain the transformer, and ignore all the other miscellaneous appurtenances and apparatus that surround the transformer.

Substation Overview

The equipment usually found in utility substations consists of one or more three-phase transformers; or one or more banks of three single-phase transformers. In large investor-owned utilities, the transformer may be 100,000 kVA or larger. In cooperative and municipal utilities a three-phase transformer is typically in the range of 1,500 kVA to 50,000 kVA. When single-phase units are installed it is generally as a bank of three with a size range of 333 kVA to 3,333 kVA. Sometimes a forth unit is installed as a spare.

In addition to the transformers, there will be various electrical apparatus in the substation, such as circuit breakers, mechanical or motor operated disconnect switches and fused disconnects. Other common apparatus that may be found is metering equipment, control relays, capacitor banks, reactors, surge arresters and voltage regulators.


  • The installation and operation of your transformer should be checked to ensure that you are keeping the electrical loading of the unit within the design capability.
  • The transformer must be compatible with the location in which it is installed (for excample, an air cooled unit in a dry clean environment).
  • The transformer installation should be free from external hazards such as trees and weeds that could cause potential shorting of the unit.

Oil Testing

The single most important area of preventive maintenance for liquid filled transformers should be annual sampling of the unit’s insulating fluid. A sample of the fluid should be taken for screen testing and gas-in-oil analysis. These tests may indicate if internal problems within the unit exist and what if any corrective action should be taken.


Most liquid-filled transformers come with a number of gauges. The top oil temperature and winding temperature gauge give an indication of temperature inside. Both of these are usually equipped with a pointer that indicates the highest level reached. Oil level gauges are used to determine the level of oil in the main tank. Pressure gauges indicate the gas pressure in the tank above the oil. All gauges should be checked monthly, and calibrated on a three-year basis.


On all transformers, the bushings and insulators must be kept clean and in good repair, with broken porcelain and brittle gaskets replaced as needed. All electrical connections must be tight because loose or high resistance connections can cause short circuits or single phasing which will cause winding damage.


Whether the unit is air or liquid cooled, it is important that adequate, unrestricted outside air is allowed to circulate freely. Dust and dirt must be removed from cooling vents and windings on all air cooled units. The radiators on liquid filled units should be leak-free and clean.


As with any maintenance process, safety is of foremost importance. When service is performed on a transformer, it is important to note that all electrical safety precautions are followed. Energized transformers represent significant shock hazards. All personnel working on your unit must be properly trained and qualified. When in doubt, put safety first.


The cost of unscheduled and unwanted breakdowns of your transformer can be more than you might expect. Even when boiler insurance is available, the deductible alone may still account for a considerable out-of-pocket expense. Having insurance may be of little comfort when your transformer is down and you are left in the dark. In addition, if you have tenants or are trying to run a business, you don’t need the added headaches of irate customers because your system is down from minor maintenance oversights.

[Our advice is intended to complement the equipment manufacturers’ recommendations — not replace them. If you have doubts about any particular procedure, contact your equipment service representative.] 

Disclaimer statement:

All recommendations are general guidelines and are not intended to be exhaustive or complete, nor are they designed to replace information or instructions from the manufacturer of your equipment. Contact your equipment service representative or manufacturer with specific questions.

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