Transformers and Oil Testing Necessity

The transformer is a key element of the power grid as it ensures efficient electricity distribution with minimal energy loss. To make this possible, transformers first increase the voltage of power station generators before transporting electricity and then bring the voltage of power transmission lines down to a level acceptable for consumers. Obviously, a transformer failure may negatively affect the electricity grid system, result in accidents, and lead to considerable financial damages. In this article, I will focus on one of the weakest elements in oil-filled transformer installations: insulating liquid. One the one hand, it is prone to contamination and subsequent breakdowns, and yet, on the other hand, can be easily tested and replaced if necessary.

Oil dielectric: basic facts

Insulating liquid is a unique dielectric material that easily fills any volume and makes for effective prevention of partial discharges due to electrical stress. Another advantage of insulating dielectric is that it removes excessive heat generated in a device. The downside is the fact that insulating oil tends to lose its dielectric properties over time. If these developments go unnoticed by the operating personnel, the electric installation will malfunction frequently, with negative consequences for the whole power system. Thus, it is critical to conduct oil dielectric testing and evaluate the insulating dielectric condition on a regular basis.

The quality of insulating oil is mainly defined by its resistivity, the ability to oppose an electrical current, and dielectric strength, the ability to avoid an arc discharge caused by a strong current bridging the conductive parts of a circuit. In order to ensure that the insulating liquid being used is of proper quality, a range of testing methods can be applied. The information on these is easily accessible on the web, so I will only discuss the most reliable and time-proven technique: dielectric breakdown voltage testing.

OLT oil tester

Oil dielectric testing

The dielectric breakdown voltage test, typically performed with an oil dielectric tester, allows the user to find out whether the oil dielectric is contaminated and therefore unable to withstand electrical stress. During the test, a sample of insulating liquid is placed within a vessel with two electrodes inside and is then subjected to high voltage stress. Voltage is increased until the liquid dielectric breaks down; the registered breakdown voltage is compared against that specified by the applicable standard. The standard also stipulates a range of other test parameters, including the gap between electrodes, electrode shape (mushroom, plane, sphere, or a combination of these), electrode size, the number of test runs, etc. Having studied the test results along with the standard requirements, the user can decide whether the insulating liquid has passed the test or needs to be replaced.

Oil breakdown testing: general guidelinesToday’s insulating oil testers have numerous benefits. They produce reliable and replicable test results, are safe for the user, are portable, and provide easy PC connectivity. To make the most of what oil breakdown test sets can offer, it is important to bear in mind a few operational principles:

• Set the scene properly. The testing area should be clear of hazardous objects and of objects that obstruct movement;
• New oil should always be tested before it is used. Following that, the oil condition should be checked in the course of regular maintenance on at least a yearly basis;
• Since the oil breakdown test results should be valid and representative of the whole oil volume, the oil sample should not get contaminated. It means that everything the oil sample will come in contact with, beginning with the oil flush system of the device and ending with the oil sample bottle, should be absolutely clean. When filling the bottle, make sure no air bubbles are created in the oil by tilting the bottle so that the oil runs down its side or using a tube to pour oil onto the bottom of the bottle;
• If the oil contains hazardous components, take necessary precaution measures required for safe handling;
• The transformer must be under positive pressure; otherwise, it could go out of order due to air influence.

Also, do keep in mind that insulating liquids tend to get contaminated as a result of the aging processes; in the case of aged transformers, the need to check the oil dielectric quality becomes even more pressing.

To sum up, in a world where we rely on the electrical installations to such an enormous extent, we cannot afford to just wait for a breakdown to occur; the consequences of such negligence are often devastating and result in substantial financial losses. It is then in the immediate interest of those operating electrical devices to conduct timely preventive maintenance and make sure all the elements of their equipment are in perfect working order.

The video below shows an example of automatic insulating liquid breakdown testing using an oil testing kit.

Alexei Tiatiushkin

Marketing manager

KharkovEnergoPribor Ltd.

marketing@keppowertesting.uk

http://www.kep.ua