Maintenance Tips for Power Transformers

Maintenance is a routine and recurring activity of keeping a particular equipment or facility at its normal operating condition so that it can deliver its expected performance or service without causing any loss of time on account of accidental damage or breakdown. In other words, maintenance can be described as measures adopted to ensure that the equipment is kept in a fully serviceable and reliable condition. In the following sections we look at some of the key maintenance considerations for power transformers.

Insulating Oil: Common Issues & Maintenance Tips

Oil forms part of the main insulation of most transformers, however it tends to deteriorate in service owing to factors such as operating temperature, atmospheric conditions (applicable to unsealed, non-conservator-type transformers) and presence of moisture or fibers.

Deterioration of the insulating oil is accelerated by prolonged high operating temperature leading to the development of acidity and sludging, which in turn have a detrimental effect on the solid insulation.

Poor ventilation in a transformer chamber results in condensation inside the transformer, which in the same as aforementioned, is liable to promote acidity and sludging.

The electrical strength of the oil is considerably reduced by included moisture or fibers and specifically by a combination of the two.

Deterioration can be greatly minimized or even arrested by paying attention to operating conditions and by routine precautions. Samples of the oil should, therefore be taken from the transformer at regular intervals and the characteristics checked.

Routine maintenance of transformers includes the testing of oil for moisture, contaminants, and possibly also for dissolved gas content.

In general, with large transformers, little problem is experienced with acidity due to the fact that there is lower operating temperatures in these particular types of transformers. More sophisticated oil preservation equipment is specified for 400 kV transformers.

In small transformers there is a tendency for the development of acidity however with modern oil [provided sensible precautions are taken] no serious inconvenience should be experienced. Discharge under oil may result in the flash point of the oil being reduced, however after a relatively short period of time it may recover. Similar reduction in flash point can happen as a result of abnormal local heating such as may be experienced during the development of an incipient fault such as a core fault involving circulating currents within the core itself due to a breakdown of interlamination resistance or failure of core bolt insulation.

Insulation: Measurement & Maintenance Tips

The measurement of insulation resistance is the standard method of checking the state of insulation of a transformer.  However, it should be noted that a transformer with relatively ‘wet’ insulation may have a high insulation resistance when the measurement is made with the transformer cold, but the value may drop rapidly as maximum operating temperatures are approached. A hot insulation resistance reading below 1 MΩ per 1000 V rating of the tested windings is generally indicative that drying out is necessary.

When suitable equipment is available, measurement of dielectric loss angle gives a reliable check on the state of the internal solid insulation. As the value of the loss angle will depend on the transformer design, a reference value taken on the actual transformer during works test is required for comparison before any useful assessment can be made.

A useful technique mainly used in the factory for checking the state of dryness in solid insulation is the ‘dispersion’ test or ‘recovery voltage’ test, based on the fact that in dry insulation the distribution of elements is such that individual shunt paths of time constant greater than 3 ms are effectively absent and that the presence of moisture introduces time constant within the range of 3-300 ms. This method of test involves the application of a 300 ms pulse followed by a 3 ms short-circuiting pulse. Any shunt paths having time constants greater than 3 ms retain their charge, which is measured as a voltage by a suitable measuring device, the measurement indicating the moisture content.

The windings of a transformer should be inspected at long-term intervals. Any slackness due to insulation shrinkage or to the falling out of the packing can be fixed either by the adjustable coil clamping screws or by packing out the winding. The operation is particularly advisable in the case of transformers subject to heavy load surges, for example furnace transformers. Large transformers are normally designed with preshrunk windings, so that slackening in service is almost entirely done away with.

On-load Tap Changing Equipment Maintenance

A key factor from operational point of view is the period of time that can be allowed to elapse before attention to the switch contacts becomes necessary. If possible, this period should be such that it can be co-ordinated with other outages of plant especially for generator transformers where outage of the unit means the non-availability of generating plant. Traditionally it was a routine practice to carry out maintenance after every 10000 operations somewhat corresponding to a 12-month period between generator overhauls however the modern high-speed resistor tap changer requires diverter switch maintenance only after 100000 or more operations, and tap changer maintenance is no longer a limiting factor from the operational point of view.

Temperature Monitoring of Power Transformers

A correctly specified and loaded transformer should not develop excessive temperatures in operation. Oil and winding temperature is monitored in all with the exception of small say less than 200 kVA distribution transformers.

Apart from the feature to monitor the temperature (useful during controlled overloading), an important feature of the winding temperature indicator is to initiate automatic switch-on and switch-off of cooling fans and oil circulation pumps according to the transformer loading conditions.

Breathers

Breathers are placed in the vent pipes of conservators to dry the air entering the conservator as the volume of oil contracts during transformer cooling.

Traditional breathers use the moisture absorbing properties of the silica gel crystals. These crystals need replacement when they become saturated with moisture. Replacement is indicated by a change in color of the crystals from blue to pink

Related articles:

• Common Causes of Failures of Power Transformers
• Features and Applications of Instrument Transformers
• Transformers
• Testing Power and Distribution Transformers
• What is the difference between a Power transformer and a Distribution Transformer?