Case 2024-06-06 — High-Voltage Substation Transformer
Introduction
Continuous online monitoring of a high-voltage substation transformer detected a progressive and accelerating deterioration in the H1 bushing power factor over five months.
The monitored asset is a high-voltage power transformer equipped with high-side bushings operating at approximately 38.4–38.5 kV. ZTZ Services installed its ZVCM-1001M bushing monitoring system on the transformer’s high-voltage side, providing continuous measurement of power factor, capacitance, leakage current, and relative imbalance across all three phases — H1, H2, and H3.
The ZVCM-1001M is FM certified, uses hard-wired sensor connections with redundant safety circuits, and applies advanced algorithms to track parameter trends over time. Data is accessible remotely through the ZTZ online monitoring portal, enabling ZTZ technicians to identify developing conditions well before threshold alarms are triggered.
Monitoring Evidence
Three monitoring data views together provide unambiguous evidence of a single-phase insulation defect developing in the H1 bushing.
Power Factor Trend: January–June 2024
The power factor trend chart for ZVCM-1001M #6134 covers the period from early January through early June 2024.
H2 and H3 power factor remained flat near 0.20% throughout the entire monitoring period, consistent with healthy insulation at nameplate condition. H1 power factor began near 0.25–0.27% in early January and trended progressively upward. By late April, H1 entered the yellow warning zone. From mid-May onward, readings repeatedly breached the red alert threshold, reaching peaks exceeding 1.3% by early June — more than five times the nameplate-equivalent value of the reference phases.
Relative Imbalance Trend: January–June 2024
The relative imbalance trend for the same period corroborates the H1 power factor findings.
Figure 2. ZVCM-1001M
Relative imbalance — a composite index comparing the three phases — rose from near 0–0.35% in January, reached 0.65–0.70% peaks by March, and climbed to 0.75–1.0% through April and May. By early June, the imbalance peaked at approximately 1.5%. The consistent upward trajectory across both the power factor and imbalance metrics confirms that the H1 anomaly is real, sustained, and worsening — not a transient measurement artifact.
Real-Time Dashboard: June 6, 2024
The ZTZ online monitoring portal dashboard, captured at 07:35 on June 6, 2024, confirmed the H1 condition in real time. [G1]
Figure 3. ZVCM-1001M
The dashboard displayed a warning on H1, with power factor at 1.18% shown in red — approximately six times the H2 and H3 values of 0.19%. The H1 capacitance deviation of +0.51% indicated measurable change in the bushing’s dielectric geometry, consistent with advancing insulation degradation. The relative imbalance gauge read 1.19%, with the needle firmly in the red zone. H2 and H3 remained stable, confirming the defect was isolated to H1.
ZTZ Services Technical Expertise
The progressive separation between H1 and the stable H2/H3 reference phases was assessed as a definitive signature of single-bushing insulation degradation, not a measurement artifact or system-wide disturbance.
The diagnostic assessment considered three converging indicators: the rising absolute power factor on H1, the positive capacitance deviation of +0.51% indicating structural change within the bushing condenser core, and the corroborating relative imbalance trend. Together, these pointed to moisture ingress or early partial discharge activity within the H1 bushing — a known and progressive failure mode for oil-impregnated paper condenser bushings.
ZTZ technicians identified the H1 anomaly through continuous trend analysis, assessed the degradation mechanism, and prevented an uncontrolled failure.
