Transformer bushings are one of the most critical components of a transformer. Today, up to 22% of major failures on high-voltage transformers can be tracked back to bushings. Almost half of these failures result in catastrophic failures such as explosions, fires, or oil spills. The cost of these damages and the lost opportunity to deliver energy could be several hundred times higher than the price of a bushing.
Most modern power transformers will have two sets of bushings during their lifetime because transformers are expected to last 50 years, while bushings have an expected lifetime of 25 years. Experience suggests two points of failure within the lifetime of the bushings:
The first failure point occurs between the ages of 10 and 13 years. This failure can arise from various factors, including design flaws, transformer operation, and potential quality issues, even though they are not aged out in life-cycle management terms.
The second failure point occurs between the ages of 20 and 30 years. While bushings can fail before reaching this age, there are instances of transformers with bushings exceeding 50 years of age. Therefore, age alone cannot be the sole determining factor in bushing failure.
To prevent costly replacements of bushings based solely on age and without considering their actual condition, it is crucial to identify bushing failure mechanisms and causes. Bushing conditions are influenced by various operating conditions of the transformer, including overheating, load variations, frequent transient exposure, and intense pollution. Additionally, designs, types, and manufacturing processes can significantly impact the lifespan of bushings.
Moisture is a bushing’s worst enemy. As a bushing ages, gaskets lose their ability to seal, thereby paving the most common way for moisture to enter the bushing. If moisture is allowed to enter the bushing, it will begin to break down the oil and paper insulation system. When this happens, the remaining concentric capacitors will have increased voltage stress across each capacitor.
As the breakdown goes unchecked and moisture continues to enter the bushing, the rate at which contamination is generated inside the bushing increases. This causes other layers to fail. At some point, there is a catastrophic failure of the bushing. Risk increases dramatically with age and voltage level. Bushings rated greater than 230 kV and older than 25 years fall into the highest risk category. The second highest risk category comprises bushings rated between 100 kV to 230 kV and older than 35 years.
While moisture ingress is the leading cause of bushing failures, the good news is that methods to assess the deterioration of a bushing’s insulation system are available. Performing power factor measurements at multiple test frequencies helps identify compromised insulation. The power factor frequency sweep test involves performing power factor measurements at a series of various test frequencies (e.g., 15 Hz, 30 Hz, 45 Hz, 60 Hz, 150 Hz, 200 Hz, 300 Hz, and 400 Hz). As an oil-and-paper insulation system deteriorates, the frequency sweep trace typically becomes flat, or worse, decreases versus frequency.
Compromised insulation shows a distinct fishhook shape in the low-frequency range of the sweep (i.e., at frequencies below 60 Hz). If this occurs, the insulation system is considered questionable and warrants further investigation or increased testing frequency. One advantage of performing power factor frequency sweep measurements on the C1 insulation system of a bushing is that a bushing mounted on a power transformer has two or three similar-unit bushings that can be tested and compared to each other.
The frequency sweep traces should exhibit reasonably similar shapes when comparing similar-unit bushings. If the shape of the trace of one bushing deviates significantly from the shapes of the other similar-unit bushings, the dissimilar bushing should be investigated further or tested more frequently in the future.
In an effort to get the best indication of excessive moisture in a bushing, dielectric frequency response (DFR) should be considered in the testing process. For more information, access IEEE Std. C57.152, Guide for Diagnostic Field Testing of Liquid-Filled Power Transformers, Regulators, and Reactors, or call intellirent at 888-902-6111.