Industry: HVDC Power Transmission | Asset: ±800 kV Converter Transformer | Location: China State Grid | Product: DGA-900 LPAS | Result: 72-Hour Early Warning Enabled Planned Outage
Background
A ±800 kV HVDC converter station in China’s State Grid network is one of the world’s highest-capacity power transmission links. HVDC converter transformers are among the most expensive and critical assets in any power grid — a single unit can cost $10-15 million with a replacement lead time exceeding 18 months. These transformers experience unique electrical stresses from AC/DC composite voltages, harmonic currents, and polarity reversals that produce complex fault gas signatures distinct from conventional AC transformers.
The station deployed a PAS DGA DGA-900 Laser Photoacoustic Spectroscopy (LPAS) monitor on its most critical converter transformer in 2023, replacing a previous-generation online GC monitor that required monthly carrier gas replacement.
The Challenge
HVDC converter transformers require comprehensive multi-gas monitoring because their fault signatures are more complex than AC transformers. Harmonic heating can produce CH₄ and C₂H₄ patterns similar to thermal faults. Commutation notches can cause partial discharge unlike any AC PD pattern. Relying on hydrogen-only monitoring would miss crucial diagnostic information — a full 9-gas picture is essential.
The previous online GC monitor provided 9-gas measurement but required monthly maintenance visits to replace carrier gas — an unsustainable operational burden for a remote converter station.
PAS DGA Solution
The DGA-900 LPAS system was commissioned in early 2023. Key configuration details:
- Measurement: 9 fault gases (H₂, CO, CO₂, CH₄, C₂H₆, C₂H₄, C₂H₂, O₂, N₂) + moisture, every 60 minutes
- H₂ sensor: Integrated Pd alloy thin-film sensor (1-2 ppm detection) — because H₂ has no IR signature and cannot be measured by the LPAS optical system
- Communication: IEC 61850 Edition 2 station bus + MODBUS TCP for SCADA + DNP3.0 for the wide-area monitoring system
- Diagnostics: Onboard Duval Triangle 1, Rogers Ratio, and IEC 60599 ratio calculations
- Early warning: 72-hour predictive trending algorithm based on gas rate-of-change analysis
Detection Event
In March 2024 — approximately 14 months after commissioning — the DGA-900’s early warning algorithm flagged an anomaly:
| Date | H₂ (ppm) | CH₄ (ppm) | C₂H₄ (ppm) | C₂H₂ (ppm) | CO (ppm) | Event |
|---|---|---|---|---|---|---|
| Day 0 | 15 | 28 | 12 | 0.0 | 85 | Baseline |
| Day 1 | 22 | 30 | 15 | 0.5 | 88 | C₂H₂ first detected — Early Warning Triggered |
| Day 2 | 38 | 35 | 22 | 1.8 | 92 | C₂H₂ rising rapidly |
| Day 3 | 52 | 40 | 28 | 2.3 | 95 | Alarm: Planned Outage Initiated |
The appearance and rapid rise of acetylene (C₂H₂) — from 0 to 2.3 ppm in 72 hours — was the definitive signature of a developing electrical discharge. C₂H₂ forms only above 700°C, making it the unambiguous marker for arcing.
Duval Triangle 1 diagnosis: %CH₄ = 56.3%, %C₂H₄ = 38.4%, %C₂H₂ = 5.3% → Zone D1: Low-energy discharge
The DGA-900’s 72-hour early warning window gave the operations team time to prepare a planned outage rather than reacting to an emergency trip.
Findings & Resolution
Internal inspection during the planned outage revealed a loosened internal shield connection — a low-energy sparking discharge that had just begun to produce carbonized tracking on adjacent insulation. The connection was cleaned, re-torqued, and the affected insulation was replaced. The transformer was returned to service after 5 days.
Without the early warning, the discharge would have escalated: the carbonized tracking path would have grown until it bridged two phase potentials, resulting in a high-energy phase-to-phase arc (D2 fault) — likely causing winding damage, oil tank rupture risk, and a catastrophic failure requiring complete transformer replacement.
Key Takeaways
- Only multi-gas DGA can detect arcing with certainty. H₂-only monitoring would have shown elevated hydrogen but could not distinguish between partial discharge (benign, monitor over months) and arcing (dangerous, requires immediate action). C₂H₂ is the definitive discriminant.
- Rate-of-change is the most valuable diagnostic metric. The absolute C₂H₂ of 2.3 ppm is below the IEEE typical alarm level of 5 ppm. But the rate of change — 2.3 ppm in 72 hours — was the true alarm signal. Continuous online monitoring is required to capture rate-of-change data; quarterly lab sampling cannot provide it.
- Zero-consumable operation matters operationally, not just financially. The previous GC monitor’s monthly carrier gas requirement meant 2 days of technician travel per month to the remote station. Zero-consumable LPAS eliminated this entirely.