For transformer maintenance engineers, two diagnostic techniques dominate insulation assessment: dissolved gas analysis (DGA) and partial discharge (PD) testing. Both detect the early stages of insulation deterioration – but they see different things, respond on different timescales and have different blind spots. Understanding when to use each – and when to combine them – is the core of modern transformer asset management.
What DGA sees
DGA measures the gases dissolved in transformer oil – hydrogen, methane, ethane, ethylene, acetylene and carbon oxides. Their ratios indicate the fault type and severity:
- Thermal faults (overheating) produce characteristic hydrocarbon gases
- Electrical faults (arcing, sparking) produce hydrogen and acetylene
- Cellulose degradation (paper involvement) raises carbon oxides
DGA is the established, mandatory screening tool: oil samples are taken periodically, and trends are compared with factory baselines and industry norms.
What PD testing sees
Partial discharge testing measures the electrical and acoustic activity of incipient insulation defects directly:
- HFCT clamps on grounding and neutral leads measure PD current pulses
- Ultrasonic sensors on the tank detect the acoustic emission of internal discharge and help localise it
- UHF sensors detect the electromagnetic signals of PD in the winding and bushing region
PD testing detects the discharge itself, often before gas levels rise above DGA thresholds.
The blind spots of each method
DGA limitations
- Gases accumulate and diffuse slowly – a sudden insulation defect may take weeks to show a clear DGA trend
- Sampling is periodic; a defect developing between samples goes unseen
- DGA indicates a fault family but not its precise location
PD testing limitations
- Interference in the substation environment can mask or mimic PD signals
- Some developing faults (e.g. slow thermal ageing) produce little or no PD until late
- Field PD testing requires trained operators and careful background interpretation
Why they work best together
DGA and PD testing are complementary, not competing:
- Screen with DGA – routine oil sampling catches the overall trend and flags anomalies at fleet scale, cheaply.
- Confirm and locate with PD – when DGA suggests an electrical fault, or as a check between samples, field PD testing confirms activity and locates it on the tank.
- Monitor the gap – for critical units, permanent PD monitoring closes the window between DGA samples, so a fast-developing defect is caught within hours, not months.
Utilities that run both see the complete picture: DGA gives the chemical trend, PD gives the real-time activity and location.
A practical combined programme
- Annual (or per-utility-schedule) DGA sampling on all power transformers.
- Field PD testing when DGA flags an electrical-fault signature, or on units with known risk.
- Permanent PD monitoring on critical transformers, feeding the asset management system.
- Correlate both datasets: PD activity rising with DGA confirms progression; PD alone may catch defects DGA has not yet revealed.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) supplies field PD testing instruments – handheld detectors, HFCT clamps and ultrasonic sensors – and online PD monitoring systems for transformers, supporting utilities and contractors worldwide with OEM/ODM supply and application guidance. Contact the HUWOR sales team for a transformer diagnostic configuration.