Asset Health Indexing: Where Bushings Fit in Transformer Evaluation

Power transformers rarely develop serious issues without warning; instead, they degrade quietly through signals that often go unnoticed until a maintenance window is missed or a component reaches a critical level.

Most transformer failures are accompanied by measurable deterioration in insulation, oil chemistry, or mechanical contacts. The problem isn't that these signals don't exist, it is that traditional maintenance schedules aren't designed to catch them early enough, and the diagnostic frameworks used to evaluate transformer condition don't always treat every component with equal diligence.

That's where transformer health indexing changes the conversation. Instead of treating an asset as either serviceable or failed, it assigns a condition score based on multiple diagnostic inputs, weighing each subsystem's contribution to the overall risk picture. Bushings have a specific and often underestimated role in this process.

What a Transformer Health Index Actually Measures

A transformer health index (THI) is a composite score derived from the condition of each of the major subsystems combined to give an overall risk rating for determining maintenance, refurbishment, and replacement requirements. No single test result defines it, and no single component determines the outcome in isolation.

These indicators can include dissolved gas analysis (DGA) results, insulation condition, tap changer wear, oil quality, cooling system performance, and bushing diagnostic data. The weights are based on the risk of operational problems and the severity of their consequences, with the scores added together to create an index that places the transformer's condition on a spectrum.

This is a very useful method because it reveals uncovered risk, as a transformer that may look visually well may be poor on DGA or insulation testing. More importantly, an asset that seems to be in good condition overall can have high risk if one subsystem is not included or not assessed. This exclusion occurs more frequently than desired, especially for bushings.

The Problem with How Bushings are Typically Evaluated

Bushings are a critical component of a transformer, providing electrical insulation while giving safe passage to electricity between internal windings of the tank and external connections. Monitoring their condition helps support reliable transformer performance over time.

Oil-Impregnated Paper (OIP) Bushings are one of the oldest technologies used in transmission and distribution infrastructure around the world and suffer a predictable deterioration. Partial discharge activity within the condenser core, oxidation of the oil, and moisture ingress are progressive conditions that take long duration of time to develop. If adequate tests are applied at required frequency, they may be detected through trends before reaching critical levels.

Most bushing issues are caused by the gap between what can be detected and what is actually detected. Visual inspections and periodic oil sampling offer valuable information, but incorporating dedicated bushing condition assessment helps create a more comprehensive asset health profile.

What Bushing Diagnostics Actually Measure

A credible analysis of the bushing’s conditions and performance depends on three primary parameters, each of which captures a different aspect of insulation integrity.

Tan delta (dissipation factor)

Tan delta measures dielectric loss within the bushing insulation. A healthy bushing maintains a low, stable tan delta reading, and a rising value indicates insulation deterioration.

Capacitance measurement

Capacitance measurement works alongside tan delta to identify structural changes in the condenser core. Each bushing has a designed capacitance value based on its construction, and a deviation beyond acceptable limits. This is often the earliest measurable sign of a developing fault within the bushing.

Partial discharge (PD) measurement

Partial discharge detects minute electrical discharges occurring within voids or weak points in the insulation. Sustained discharge activity indicates insulation aging that can lead to thermal or electrical breakdown over time.

Dissolved Gas Analysis

For OIP bushings specifically, oil sampling from the bushing's own oil reservoir adds another diagnostic layer. Dissolved gases in bushing oil, analyzed independently from main tank oil, can isolate localized degradation that would otherwise be masked in bulk DGA results from the transformer itself.

How Bushing Data Shapes the Overall Asset Score

The influence of bushings on transformer asset management decisions is often disproportionate to the weight they are given in health scoring models, and that imbalance has real consequences for how accurately risk is assessed.

Consider a transformer that scores well on oil quality and winding insulation but has bushings with elevated tan delta readings trending upward over three consecutive test cycles. On paper, the overall health index may still appear acceptable. In practice, bushing condition is a key aspect of transformer evaluation, as advanced insulation deterioration can have a substantial impact on transformer operation even when other components remain in good condition.

This is what can be called the "risk multiplication problem," as a weak subsystem doesn't reduce the overall score proportionally. It creates a failure mode that can bypass all other positive indicators entirely. Sound transformer diagnostics recognize this and weigh bushing health accordingly, particularly for high-voltage assets above 110 kV, where bushing failures carry the most severe grid consequences.

Integrating bushing data comprehensively, including trend analysis across multiple test intervals rather than depending on single-point readings, consistently produces health index scores that more accurately reflect operational risk.

Frequently Asked Questions

Q1. How is a transformer health index score actually calculated?

Scores are calculated by weighing diagnostic results from each major subsystem, including oil, insulation, tap changer, cooling, and bushings, then aggregating them into a single risk rating.

Q2. Does bushing insulation type affect how frequently diagnostic testing should be scheduled?

Yes. Different insulation materials age at different rates depending on operating temperature, load cycling, and environmental exposure, so testing frequency should reflect the specific bushing construction and its service conditions.

Q3. Can bushings be replaced without replacing the entire transformer?

Yes. Bushings are replaceable components, and interchangeable formats exist that are dimensionally compatible with equipment from multiple original manufacturers.

Q4. How does online bushing monitoring differ from what happens during a scheduled offline test?

Online monitoring tracks parameters like capacitance and tan delta continuously between test intervals, while offline testing provides a more comprehensive diagnostic snapshot when the asset is taken out of service.

Conclusion

Asset health indexing works when the input data is complete and every major subsystem is weighted according to its actual contribution to operational risk. Bushings are not peripheral components in this process, they are active contributors to transformer condition and leaving them underrepresented in the health index produces scores that look credible but don't reflect reality accurately enough to act on.

The focus is not on whether bushing condition belongs in the health index but on whether it is being assessed accurately and consistently in overall transformer evaluation.

At Yash Highvoltage®, bushings are manufactured and tested in accordance with international standards, helping utilities and OEMs maintain reliable transformer performance while supporting established condition monitoring and maintenance practices.

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