Testing the oil in load tap changers provides valuable information concerning the operation of the unit. Monitoring conditions between preventative maintenance inspections is a critical step in preventing expensive maintenance problems and even unplanned outages. In this first article, we are going to talk about how mineral insulating oil ages in the LTC. This aging mechanism is considerably different from that of oil in other types of electrical equipment. Understanding the aging process indicates some of the challenges of maintaining LTCs, but it also presents some opportunities for improving the effectiveness of the overall maintenance program.
The interior of an arc-in-oil LTC is a very energetic environment. There is usually relatively high dissolved oxygen content in the oil. There are frequent discharges (arcing) as taps change while under load. Conditions are harsher compared to those inside other types of electrical equipment. In this environment, oil does oxidize at a more rapid rate than it does in a milder environment such as that inside a sealed and properly maintained transformer. However, unlike oil in transformers, there is a second aging mechanism for oil in LTCs that affects operation of the device, usually long before oxidation proceeds far enough to cause any serious degradation of the oil’s performance.
As oil ages in a load tap changer, it polymerizes and forms a resinous, varnish-like film over the contacts and mechanism. Viewed with the naked eye, this smooth coating is generally very dark and is even frequently mistaken as a carbon deposit or as evidence of coking. The picture below shows deposits of filming compounds on the contacts of an LTC. The smooth appearance is unlike the typical pebbly surface of a coke deposit after it forms in an electrical device. Notice also those surfaces of the contacts that have been wiped clean and show up as bright areas of relatively clean conductor.
This film is both mechanically and electrically resistive. As filming becomes more advanced, it can adversely affect the efficient operation of the device. Quenching of the arc by the oil becomes less effective if the contacts have a heavy layer of film on them, so the arc is sustained longer on the contact surface. This may cause hot spot overheating of contacts and result in premature wear. In extreme cases, overheating of the contacts may lead to contact failure.
Filming also occurs over the mechanism of the load tap changer, and this will provide additional resistance to mechanical movement within the device. As a result, the LTC needs to work harder in order to change. It takes longer for tap changes to be accomplished, which further extends the arcing on the contacts. This additional work results in a general increase in the operating temperature within the device as the mechanism heats up because of the mechanical resistance. Overheating can become more severe in cases where filming of the mechanism is more extensive. Delays in completing tap changes may cause resistors in the LTC to also experience overheating. In extreme cases, the operation of the mechanism may be compromised to the point where there is an increased risk of failure of the device due to binding.
Overheating of the contacts and of the mechanism may also lead to coking. Coking is very destructive within an LTC as the hard deposits inhibit proper operation and may lead to contact failure. Also, particles of coke are very erosive to the contact surfaces and may cause premature wear.
In fact, particles from any source, including those caused by the normal wiping of the filmed surfaces during operation of the device, may be incorporated into the film as it forms. These particles are usually very abrasive, and generally add to the mechanical resistance to proper operation of the mechanism. In the next installments of this series, we are going to discuss the oil tests we recommend for LTCs and the benefits that LTC owners may expect to receive from a properly designed and executed testing program for LTCs. We will start next week with a discussion of the conditions that may cause film formation to be accelerated in an LTC.
