Bushing failures contribute significantly to the failure statistics of power transformers. On average, one out of four transformer failures are due to issues with the bushings. Not only are failures likely to occur but 50% of bushing failures end in fires. Bushing failure can occur for several reasons such as reduction in dielectric strength, thermal integrity or mechanical strength.
What is Online Bushing Monitoring?
Offline tests are performed on a routine schedule – maybe once every few years. Yet many failure mechanisms on bushings can occur in a very short period of time measured in days to weeks. Additionally, some failures are temperature and voltage dependent that are difficult to simulate during offline tests.
Online bushing monitoring overcomes these offline test limitations to increase the performance and reliability of the transformer.
Online monitoring of bushings provides data during all weather conditions, loads, and at rated voltage, with the same sensitivity as an offline measurement. Learn more about online bushing monitoring methods and case studies in this white paper.
50% of bushing failures end in fires!
On Average 25% of transformer failures are bushing related
On Average 1 Out Of Every 200 Transformers Experience A Major Failure Each Year.
What are the failure characteristics of bushings?
OIP versus RIP/RIS Bushings
The integrity of the OIP (Oil Impregnated Paper) bushings is heavily dependent on the health of the gasket system, which is susceptible to damage from high temperatures. Gasket failures allow for water ingress, creating self-sustained breakdown conditions from associate heating that can be detected through online bushing monitoring. Defects in the bushing change the leakage current and phase angle(s), resulting in detectable current imbalance, tan delta (power factor) and capacitance changes. RIP (Resin Impregnated Paper) and RIS (Resin Impregnated Synthetic) bushing constructions are more resistant to water ingress and explosive failure modes, but the propagation of partial discharge associated failures is much faster. The time for a RIP or RIS bushing to fail and cause an unplanned outage is far shorter than the typical periodic maintenance interval making continuous online bushing monitoring an ideal way to prevent failures.
Check out our webcast presentation to learn more about the failure characteristics of OIP, RIP or RIS bushings.
How Do Offline Bushing Tests Compare to Online Monitoring?
Limitation of Offline Testing
Offline bushing tests and online monitoring are both important to evaluate the health of your bushings. However, there are times when the data from offline tests and online monitors may not match. This can be due to several reasons. Typically, offline testing is performed at near ambient temperature with a test voltage of 10kV. This is not a true representation of the bushing’s normal operating conditions or stresses. Failure modes are easier to detect online while the transformer is energized at operating voltage and temperature. It is common for online monitoring to detect incipient faults that are not detected by offline tests. Further, the time for a fault to develop is typically less than the time between offline test cycles. Online systems are measuring the performance of the bushing at full operating voltage and temperature. Since the “test” parameters are different, the results may be different.
To learn more about why the data may not match, check out our webcast presentation on Online Monitoring vs Offline Testing.