Bushing Failures – A look into Bushing Monitoring and Testing

Qualitrol Bushing Monitor Transformer Bushing Monitoring

Bushings are a critical component (bottlenecks) in electricity transportation. Bushings are used to transfer load currents in and out of grounded metal enclosures, such as transformers, at system voltages and are often exposed to high permanent stresses. But what are High Voltage (HV) bushings in transformers, you may ask? In a transformer, HV bushings are one of the weakest links due to high stress resulting from high voltage.

There are many types of bushings. The below chart explains the basic characteristics of some of those.

Different Types of Bushings

Oil to Oil BushingsTransformers – cable connection (installed between the power transformer tank and cable box)
Oil to SF6 BushingsDirect gas-insulated switchgear connection to the power transformer (installed between power transformer tank and AIS)
Oil to Air BushingsAIS connection to power transformer (installed between power transformer tank and AIS)
Wall BushingsIndoor AIS (installed between indoor AIS and overhead line)
SF6 to Air BushingsSF6 gas-insulated switchgear – overhead line connection (installed between gas-insulated switchgear and overhead line) – with condenser core
High Current BushingsGSU transformer and high current duct from the generator (installed between power transformer tank and GSU duct)
Railway BushingsTrains for traction transformers or wall feed-through bushings (installed between transformer tank and AIS)
Generator BushingsGenerator – high current duct connection (installed between generator and connection to high current duct connection)

What Causes Power Transformer Bushing Failures?

Transformers do not completely fail generally, but the failure of one or more components causes the transformer to fail in different ways. The major causes include winding and/or insulation failure, tap changer failures, and bushing failures. The bushing failures are of very high importance due to the failure mechanism. They generally fail suddenly with fire or explosion, posing a major risk to property and personnel. Combined with the significant portion (17%) of transformer failures occurring due to bushings, the violent failure mechanism warrants an effective and reliable maintenance strategy.

Power Transformer Bushing Testing and Monitoring

Little can be done for maintenance on bushings e.g. checking oil level or SF6 level. Many bushings do not even use oil or SF6. Hence, they need to be replaced periodically to avoid catastrophic failures. To plan the replacements, the bushings can either be tested periodically by taking outages or be monitored online. Online monitoring provides a much more cost-effective and reliable way to make informed decisions on bushing replacements.

Power transformer gas insulated switchgear bushing monitoring

Off-line and Online: Power Transformer Maintenance Practices for Bushings

There are off-line periodic testing practices and online monitoring practices for transformer maintenance. The online continuous monitoring practice has been gaining traction due to high return on investment and effectiveness in not only avoiding failures but also reducing time-based maintenance costs. The off-line period maintenance practice for bushing may include visual inspection, Infrared inspection, Cleaning using different methods, power factor testing (tan delta or tan δ), and capacitance testing.