Bushing Failures: Monitoring & Testing Guide

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? 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 list below explains the basic characteristics of some of those.

Different Types of Bushings

• Oil-to-Oil Bushings Transformers – cable connection (installed between the power transformer tank and cable box)
• Oil to SF6 Bushings Direct gas-insulated switchgear connection to the power transformer (installed between the power transformer tank and AIS)
• Oil to Air Bushings AIS connection to power transformer (installed between the power transformer tank and the AIS)
• Wall Bushings Indoor AIS (installed between the indoor AIS and the overhead line)
• SF6 to Air Bushings SF6 gas-insulated switchgear – overhead line connection (installed between gas-insulated switchgear and overhead line) – with condenser core
• High Current Bushings, GSU transformer, and high current duct from the generator(installed between the power transformer tank and the GSU duct)
• Railway Bushings Trains for traction transformers or wall feed-through bushings (installed between transformer tank and AIS)
• Generator Bushings Generator – high current duct connection (installed between the generator and the connection to the high current duct connection)

What Causes Power Transformer Bushing Failures?

Transformers do not generally completely fail, 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.

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

There are offline 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.