VIE's Health Metrics Explained: A Plain-Language Guide to WHr, WHa, Impact, Oil, and Thermal
A health metric is only useful if the person reading it knows what to do next. VIE produces six categories of metrics. Each one detects a specific type of mechanical, electrical, or thermal change in the transformer. Each one has a directional logic: what a low value means, what a rising value means, and what that rise should trigger.
This article covers all six. For the framework behind leading versus coincident versus lagging indicators, see [Leading, Coincident, and Lagging Indicators: What the Economics Analogy Gets Right].
Radial Winding Health Metric (WHr)
What it detects: Mechanical stress on the transformer windings in the radial direction, outward from the core.
What it maps to: Winding looseness, early deformation, high tensile stress, the beginning of buckling, and spiraling of the conductor stack.
What a rising value means: The winding is experiencing increased radial mechanical force. Something has changed in the winding structure or in the forces acting on it. The winding has not necessarily failed, but the conditions that precede radial failure are present and trending.
Rule of thumb: High WHr points toward winding looseness or deformation.
What it triggers: A rising WHr value calls for a targeted MEGGER test or insulation resistance measurement. VIE's Radial metric shows strong correlation with insulation resistance across modern transformers, independent of transformer age. Traditional insulation resistance thresholds (1 megohm per kilovolt of rated voltage) are a pass/fail gate. They do not show how quickly insulation is deteriorating. A rising WHr, tracked continuously, can detect that rate of change long before a point-in-time IR test would flag a problem.
Axial Winding Health Metric (WHa)
What it detects: Mechanical stress on the winding stack in the vertical (axial) direction.
What it maps to: Core lamination loss, microbending of conductors, tilting of the winding column, axial bending, telescoping, and collapse of winding end support structures.
What a rising value means: The core or winding structure is under increasing axial stress. This can reflect lamination loosening, mechanical settling after a through-fault event, or progressive structural degradation over the transformer's service life.
Rule of thumb: High WHa points toward high stress on core lamination and structure.
What it triggers: A rising WHa in isolation calls for the same confirmatory path as WHr: insulation resistance measurement and correlation with Dissolved Gas Analysis (DGA) trends. When WHa and WHr are both elevated at the same time, the combined reading changes the diagnostic picture.
WHr and WHa Together: Reading the Combined Signal
When both the Radial and Axial Winding Health Metrics are elevated simultaneously, the interpretation is more specific than either metric alone would suggest.
High compressive force on the winding in both the radial and axial directions at the same time indicates a structural condition with meaningful buckling risk. This is not two separate problems developing in parallel. It is a single mechanical condition expressing itself across both axes.
A combined elevation of WHr and WHa is a stronger signal than either individual value. It warrants a higher-urgency response: prioritize Sweep Frequency Response Analysis (SFRA) testing to assess mechanical displacement, correlate with any available DGA history, and consider accelerating the inspection timeline.
Impact Metric (NHv, NHa)
What it detects: The magnitude and character of mechanical impacts on the transformer structure, in both vertical and horizontal/axial directions.
What it maps to: Physical shocks to the core or winding assembly, significant mechanical events, and structural compromise that is already underway.
What a rising or elevated value means: Unlike the Winding Health Metrics, the Impact Metric is a coincident indicator. It does not predict. It reports on what is happening now. A high or rising Impact Metric means the transformer structure is under significant mechanical stress at this moment. Core or winding structural integrity is likely already compromised.
What it triggers: An elevated Impact Metric calls for immediate action, not a scheduled follow-up. Validate with available DGA trends and electrical test data. Risk of failure is highest when the Impact Metric is elevated at the same time that oil or electrical tests are also trending negatively.
One important distinction: A single transient spike in the Impact Metric may reflect an external event (a nearby fault on the network, a physical shock during transportation or nearby construction) rather than a developing internal fault. Trend matters. A sustained elevation is more diagnostic than an isolated event.
Oil Health Metrics (V2P and S2P)
What they detect: The condition of the transformer's insulating oil, measured continuously through changes in the oil's ability to transmit vibration pressure waves.
What they map to: The four primary oil degradation modes VIE tracks as leading indicators: oxidation and hydrolysis, sludging, contamination, and fluid integrity loss.
How they work: As oil degrades, dissolved gases and breakdown products change the oil's bulk modulus, its resistance to compression. This changes how pressure waves travel through it. VIE detects that change in the vibration signal arriving at the tank wall. No sample extraction is required. No site visit is needed.
What a rising value means: Oil quality is deteriorating. The rate and pattern of the rise help characterize the likely degradation mode. A slow, sustained rise over weeks or months points toward oxidation or gradual contamination. A more abrupt change correlates with a thermal event or contamination incident.
What it triggers: Rising V2P or S2P values call for an oil quality lab test covering moisture content, interfacial tension, dielectric strength, acid number, power factor, and total dissolved combustible gas. For units over 20 years old or with prior thermal history, add furan analysis to assess paper insulation condition.
One important note: VIE's oil metrics can extend the interval between required oil lab tests when values are stable and trending normally. They do not replace the annual lab DGA. That test remains a required safety net regardless of what VIE's continuous monitoring shows.
Thermal Metrics (Excess Heat Flux)
What they detect: Thermal abnormalities across the height of the transformer tank, based on the difference between measured surface temperature and the temperature the thermal model predicts given current load, ambient conditions, and sensor position.
What they map to: Localized hotspots from insulation stress or winding issues, and systemic cooling problems from oil contamination, sludging, or cooling equipment degradation.
Why sensor height matters: VIE places sensors at multiple heights on the tank body. That vertical distribution is what separates two very different problems that can produce similar alarm conditions.
Excess heat flux concentrated at the top of the tank points toward thermal stress in the upper winding region, consistent with overloading, insulation degradation, or oil quality problems that affect heat dissipation. This scenario calls for increased monitoring frequency and DGA correlation.
Excess heat flux appearing at lower sensor heights, or a heat flux profile that does not match the expected top-weighted thermal gradient, points toward a cooling obstruction. Blocked radiator fins, contaminated oil reducing flow, or a failed cooling pump all produce this pattern. Addressing the cooling issue directly may prevent further thermal damage and, in some cases, may prevent failure entirely.
What it triggers: Any sustained excess heat flux trend calls for DGA correlation. If the thermal trend and oil quality metrics are both rising, treat the combination as a priority flag. If only the thermal metric is elevated and oil metrics are stable, inspect the cooling system before escalating to electrical testing.