The detection of a transformer winding condition that might lead to a fault and subsequent outage, and being able to take corrective action (e.g. re-clamping or insulation repair) prior to failure, can save a transformer rewind, estimated to cost in the region of $1-million for a large power transformer.

Frequency Response Analysis (FRA) is becoming an increasingly popular technique used to externally monitor and assess the condition and mechanical integrity of transformer windings for short-circuits, open-circuits, deformation, winding insulation breakdown and loss of clamping pressure. The FRA technique can help maintenance personnel identify suspect transformers, enabling them to take those transformers out of service

FRAMIT
FRAMIT is a portable, self-contained and easy to use instrument that had been developed specifically for performing FRA tests on power transformers. Results from the FRAMIT instrument show that they are identical to those obtained from the Sweep Frequency FRA test instrument. If a transformer is suspected of internal damage, the FRAMIT test results can assist in deciding whether or not to take the transformer out of service for further investigation.

Causes of Transformer Deformation
Power transformers are usually very reliable, but when faults occur, the transformer can be affected catastrophically. Transformers fail in service each year. Most of these failures are caused by transformer winding faults and through faults generated by lightning and switching surges. As a transformer experiences a fault, it may suffer mechanical shock that gradually displaces and distorts the windings. In the process of winding movement, the insulation between the turns can be abraded, causing a short circuit and damage to the windings. Mechanical vibrations, initiated by short circuit forces, may cause the windings to loose their clamping pressure, eventually leading to collapse of the windings. The other cause of winding movement may be extensive vibration during transformer transportation. As the windings experience vibration, they may slacken and subsequently become unable to withstand mechanical forces exerted during faults. Ageing also contributes to winding looseness. In addition, harmonics generated under normal operating conditions may cause winding and core vibration.

Short circuit faults are potentially very destructive because if the clamping pressure is not capable of restraining the forces involved, substantial permanent winding deformation or even collapse can occur almost instantaneously, often accompanied by shorted turns. A common cause of failure is a close-up phase to earth fault resulting from a lightning strike.

It is expected that a transformer will experience and survive a number of short circuits during its service life, but sooner or later one such event will cause slight winding movement, and the ability of the transformer to survive short circuits in future will then be severely reduced. As the transformer ages, its components deteriorate and the likelihood of a failure increases.

Performing a new test
The test-leads are connected across a specific phase winding of the transformer. FRAMIT injects a voltage impulse into this phase winding and calculates its frequency response. This frequency response result is then displayed within a Logarithmic/Frequency graph on the laptop computer. This test is repeated for each phase winding of the transformer (6 tests for a 3-phase transformer). This allows each winding of the transformer to be independently inspected and evaluated. The frequency response of all the tested phases together is called the 'fingerprint' of the transformer. This fingerprint is unique to every transformer, and remains unchanged for as long as the transformer's winding structure remains unchanged.

Implementation of FRA Test Technology
The international use of FRA test technology is growing steadily. Many companies in the USA are successfully implementing FRA testing using the FRAMIT instrument. These companies have authorized that FRAMIT tests be performed on all their new transformers before they are purchased from the manufacturer, after they are transported to site, and once they have been commissioned. With future routine tests being performed on a regular basis, this decision is enhancing their transformer preventative maintenance programs.

Before a transformer is loaded onto a truck for delivery to the customer, a FRA test is performed at the manufacturers premises. Once the transformer has been delivered, it is tested again. If the two fingerprints match well, it means that there has been no movement of the winding structure during transport and loading. If a transformer suffers a high through fault condition, it can be tested to determine the extent of the distortion of the winding structure. The test results will indicate what further action (if any) needs to be taken. This can save a lot of time, as well as transformers. Transformers can also be tested periodically, to determine the cumulative effects of high through current faults.

Conclusion
International FRA research has proven that it is possible to detect a variety of different internal transformer conditions. As a preventative maintenance tool, FRAMIT is the ideal complement to the traditional tests of ratio, partial discharge and dissolved gas analysis. Combined with on-going international research into quantifying specific waveform patterns with corresponding winding faults the system is proving invaluable in providing base data on currently healthy transformers. In a world when energy is power, FRAMIT is making a difference, taking electro-mechanical engineering into the future.