Dielectric Oil Testing: Protecting the Workhorses of our Power Grid

Dielectric oil is widely used in fluid-filled high voltage equipment such as transformers, capacitors, pipe-type cables, and circuit breakers for the transmission and distribution of electric power. Of this equipment, transformers belong to some of the most important and cost-intensive in the electric power grid – from generation to transmission and ultimately to the distribution of electric power to the consumer. As a result, the operation of transformers must guarantee a continuous and error-free power supply over a lifetime of decades. In order to better understand the condition of a transformer continuous testing must be done.

As the majority insulation consists of composite liquid-paper, one of the easiest parameters to test is the quality of the dielectric insulating oil. Samples can be taken from the transformer while still energized in the field and then tested on-site or in a laboratory.

What causes transformer ageing?

Over the lifetime of a transformer, degradation processes will occur that negatively affect the quality of the insulation. These include:

• Thermal degradation from hotspots or general overheating will cause the paper to degrade forming dissolved CO and CO₂ gases in the oil, as well as reduce the oil’s capacity to act as a cooling system.
• Electrical degradation can occur from the presence of PD or high-energy arcing. This can in turn cause bond cleavage forming very reactive radicals. Smaller free radicals will react with one another to form dissolved gases, while larger ones will form sludge.
• Chemical degradation. Free radicals will also react with molecular oxygen within the oil to produce water and acids in a process known as auto-oxidation.
• Mechanical degradation in the form of deposits from pumps, fans, and gaskets due to the vibration of the energized transformer.

What tests can be conducted?

The easiest and most common electrical tests to conduct on insulating oil to measure the dielectric properties is breakdown voltage testing, dissipation factor/power factor testing, and specific resistivity testing.

• Breakdown voltage testing measures the ability of the insulating oil to withstand electric stress without failure. A low breakdown value can be a clear indication of contamination such as acids, humidity, or foreign particles (cellulose, sludge, etc.). Typical standards that are used are ASTM D1816, ASTM D877, and IEC 60156.
• Dissipation factor (or power factor) measures the dielectric losses within an insulating liquid. The higher this value, the higher the dielectric losses are, which is a clear indication of contamination. The typical standard to follow is ASTM D924 or IEC 60247.
• Specific resistivity is a measure of the insulating liquid’s electrical insulating properties. As a rule of thumb, the lower the resistance, the higher concentration of conductive contaminants are present. The typical standard to follow is ASTM D1169 or IEC 60247.

Why is it important to test transformer oil?

In 2014 the Infrastructure Security and Energy Restoration Office of Electricity Delivery and Energy Reliability U.S. Department of Energy issued a report titled “Large Power Transformers and the U.S. Electric Grid”. The report refers to the critical role power transformers have on the U.S. power grid:

“Large power transformers have long been a concern for the U.S. Electricity Sector, because the failure of a single unit can cause temporary service interruption and lead to collateral damage, and it could be difficult to quickly replace it. Key industry sources have identified the limited availability of spare LPTs as a potential issue for critical infrastructure resilience in the United States”.

Not only are the transformers difficult to quickly replace due to a very lengthy and costly procurement process, the costs of such transformers are extremely high, as the following image shows. These costs don’t even take testing services, transportation, or the thousands of gallons of insulating oil into account!

A failure in service not only means physical damage of the transformer itself, but also financial damage due to the interruption of service to customers. Furthermore, major environmental damage could occur from oil leaking into the ground or burning uncontrollably. Worst of all, people could get hurt or even die if they were to be in the vicinity of a catastrophic failure. These are all reasons why testing transformer oil on a regular basis is the smart thing to do.

In 2012 SPX Transformers presented data at the Doble Client Conference in which the root causes of transformer failures between 1991 and 2010 was analyzed. This research showed that the third leading cause of failure is due to insulation defect, underscoring why testing dielectric oil quality on a consistent basis is crucial to avoiding major transformer failures.

HVT’s Complete Range of Dielectric Oil Testers:

For years HVT has been offering the best of the best on the market – BAUR oil testers!

BAUR is the pioneer and world market leader in insulating oil testing with over 70 years of experience. All the BAUR instruments offer maximum quality, reliability, precision, and safety. They are also extremely well-known for providing the highest life time of use in this industry. It is this reason why our service department is still calibrating instruments that are over 20 years old.

Watch a video on the BAUR oil testers here.