MV-515 · Phase 4 Advanced

Condition Monitoring & Failure Forensics

Plot your transformer gas results on the Duval Triangle, judge an infrared hot spot the way a thermographer does, watch a partial discharge trend build over a year, then work through four MV failure investigations to find the root cause.

In short

Dissolved gas analysis (DGA) reads the gases that faults produce in transformer oil. The Duval Triangle 1 uses CH₄, C₂H₄ and C₂H₂ to classify partial discharge, thermal faults and arcing.
Acetylene (C₂H₂) is the warning gas for arcing. Even small, rising amounts need attention.
For thermography of similar components under similar load, a temperature difference of 1 to 3 K suggests a possible problem, 4 to 15 K a probable one, and over 15 K a major one (NETA guidance).
Rising partial discharge on a cable termination or joint usually comes before failure, often by months, which gives time to plan a repair.
Many MV failures trace back to installation workmanship, moisture and overheating, not to the equipment design.

By Eng. Mannan Maqsood, Electrical EngineerPart of the MV Hub

Duval Triangle 1 (DGA) 2D

Enter gas levels in ppm or pick a preset. The point moves on the triangle and the zone gives the likely fault type.

CH4 methane (ppm)

C2H4 ethylene (ppm)

C2H2 acetylene (ppm)

H2 hydrogen (ppm)

CO (ppm)

CO2 (ppm)

Thermography Hot Spot Assessment 2D

A simulated infrared image of three cable terminations. Set the hot spot and the load at the time of the survey. Readings taken at low load hide problems, so the temperature rise is corrected to full load.

Healthy phase (°C)

Hot phase (°C)

Load at survey (%)

Correction exponent

4D Partial Discharge Trend 2D 12 months

An online PD monitor on a cable termination. Play 12 months and watch the trend and the phase-resolved (PRPD) pattern change as the defect grows.

Failure Investigations Interactive

Each case gives the symptoms. Look at the evidence, then choose the most likely root cause.

Why monitor MV equipment condition?

Most MV equipment fails slowly before it fails suddenly. Oil gases build up, joints heat up, and partial discharge erodes insulation over months. Condition monitoring catches these early signs so that repairs can be planned during an outage instead of after a fault. The key is trending: one result tells you little, but a rising trend tells you a lot.

Reading DGA results

Hydrogen with little else suggests partial discharge.
Methane and ethane point to lower-temperature overheating. Ethylene rises with hotter faults.
Acetylene means arcing or very high temperatures. Treat any rise as urgent.
CO and CO₂ come from paper insulation. A CO₂/CO ratio below about 3 suggests the paper itself is being overheated.

ExpertUse the Duval triangle only when gas levels are above typical values or rising, otherwise normal background gases can land in a fault zone. IEC 60599 and IEEE C57.104 give typical concentration and rate-of-increase values. Duval pentagons and triangles 4 and 5 help separate overlapping thermal faults.

Thermography good practice

Survey at high load, with the covers removed only where safe and permitted, and use an emissivity setting suited to the surface. Compare similar components (the three phases of the same termination) rather than relying on absolute temperature. Record load, ambient and distance with each image.

Regional notes

GCC

High ambient temperatures and dust make thermography and connection checks especially important before summer peaks. Coastal humidity and salt drive surface tracking on terminations.

Australia and New Zealand

Distributors run condition-based maintenance programmes with online PD and DGA for zone substation assets, and seismic events can loosen connections, so post-event inspections are common in New Zealand.

Asia

Large urban cable networks use online PD monitoring and periodic DGA, with tropical humidity a common contributor to termination failures.

Test yourself

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6 questions. Pick an answer to see the explanation. Practise about 50 more MV-515 questions in the MV Question Bank.

Frequently asked questions

What is the Duval Triangle?

A graphical method for interpreting dissolved gases in transformer oil, using the relative percentages of methane, ethylene and acetylene to classify faults as partial discharge, thermal faults (T1, T2, T3) or electrical discharges (D1, D2), with a mixed zone DT.

What does acetylene in transformer oil mean?

Acetylene forms at very high energy, such as arcing or temperatures above about 700 to 1,000 degC. Any new or rising acetylene should be investigated quickly.

How is a thermography reading corrected for load?

A common approach scales the temperature rise by the square of the ratio of full load to survey load: delta T at full load = delta T measured x (100 / load %)^2. Some guides use an exponent of about 1.6 to 2.

What is a PRPD pattern?

A phase-resolved partial discharge plot shows PD pulses against the phase angle of the AC voltage. Internal voids, surface tracking and corona each give typical shapes, which helps identify the defect type.

Related MEPVirtual labs

MV-504 Distribution Transformer Lab MV-502 MV Cables & Accessories Lab MV-509 MV Testing & Commissioning Lab E-108 Meter Lab

Standards referenced

IEC 60599 DGA interpretation
IEEE C57.104 Gas interpretation
ANSI/NETA MTS Thermographic survey
IEC 60270 Partial discharge measurement