Digital Partial Discharge Tester: Environmental Factors Affecting PD Measurements and Correction Techniques
Partial discharge activity is highly sensitive to environmental conditions. A digital partial discharge tester that reads 50 pC on a dry day at 20°C may read 300 pC on a humid day at 35°C—for the same asset in the same condition. Without understanding and correcting for environmental factors, trending becomes unreliable, false alarms increase, and genuine defects may be masked. This article examines how temperature, humidity, altitude, contamination, and electromagnetic environment influence PD measurements, and provides practical correction techniques for accurate, repeatable diagnostics.
Why Environmental Factors Matter in PD Testing
PD is a stochastic process governed by electric field strength, available free electrons, and the physical state of the dielectric. Environmental conditions alter all three:
Temperature changes affect insulation permittivity, conductivity, and gas pressure within voids.
Humidity alters surface conductivity, enabling surface discharges that would not occur in dry conditions.
Altitude reduces air density, lowering corona inception voltage in air-insulated equipment.
Contamination (dust, salt, chemicals) creates conductive paths that intensify surface PD.
Electromagnetic environment determines whether the digital partial discharge tester can detect low-level PD above background noise.
Temperature Effects on PD Activity
Temperature influences PD through multiple mechanisms:
| Mechanism | Effect on PD | Typical Magnitude |
|---|---|---|
| Gas pressure in voids | Higher temperature increases gas pressure, raising PD inception voltage (PDIV) and reducing PD magnitude | PD magnitude decrease of 20–40% from 20°C to 80°C |
| Insulation conductivity | Higher temperature increases conductivity, reducing field stress in some geometries | Complex; may increase or decrease PD |
| Thermal expansion (slot discharge) | Winding expands with heat, reducing slot discharge in motors/generators | PD reduction of 50–80% at operating temperature vs. cold |
| Moisture desorption | Higher temperature drives moisture out of insulation, reducing surface PD | Transient effect lasting hours |
| Oil viscosity (transformers) | Higher temperature reduces oil viscosity, improving PD signal transmission | Signal amplitude increase of 10–30% |
Because temperature effects vary by defect type, always record temperature at the time of measurement and compare measurements at similar temperatures. For trending, temperature correction may be applied using empirically derived coefficients, but caution is advised—correction models are asset-specific.
Humidity Effects on PD Activity
Humidity is the most influential environmental factor for surface PD. Effects include:
Surface discharge initiation: Above 60% relative humidity (RH), adsorbed water layers on insulation surfaces reduce surface resistivity, enabling discharge at lower voltages. PD magnitude may increase 3–5× as RH rises from 40% to 90%.
Corona in air: High humidity increases corona inception voltage slightly (due to electron attachment to water molecules) but increases discharge magnitude once initiated.
Internal voids: Humidity has minimal effect on voids sealed within solid insulation—but if the void connects to the surface via a crack, moisture ingress dramatically increases PD.
Condensation: If surface temperature drops below dew point, water droplets form, causing severe local PD. Measurements during condensation conditions should be flagged as potentially misleading.
Correction approach: Record RH with every measurement. For trending, restrict comparisons to measurements within ±10% RH. If humidity correction is essential, use asset-specific correction curves derived from controlled laboratory tests.
Altitude and Air Density Effects
At altitudes above 1,000 meters, reduced air density lowers the breakdown voltage of air gaps:
Corona inception voltage decreases by approximately 1% per 100 meters of altitude gain.
PD magnitude for air-insulated equipment increases correspondingly—up to 30% higher at 2,000 meters compared to sea level.
For equipment designed and tested at sea level but installed at altitude, PD measurements must be interpreted with altitude correction (per IEC 60060-2).
Digital partial discharge testers themselves are unaffected by altitude (internal electronics), but their readings of external PD reflect the reduced air density.
Contamination Effects
Contamination—dust, salt spray, industrial chemicals, or bird droppings—creates conductive or semi-conductive paths on insulation surfaces, leading to:
Increased surface PD magnitude and repetition rate.
Reduced PD inception voltage.
Accelerated tracking and erosion.
Erratic PD patterns that may be misclassified as internal defects.
A digital partial discharge tester with PRPD analysis can distinguish contamination-induced surface discharge (broad phase distribution, humidity-sensitive) from internal voids (stable, symmetrical pattern). However, visual inspection is essential—if contamination is present, clean the asset and retest before interpreting PD results.
Electromagnetic Environment and Background Noise
The electromagnetic environment determines the noise floor and thus the minimum detectable PD. Key factors:
Ambient corona: Nearby energized equipment generates corona that may mask low-level PD. Test during dry conditions when corona is minimal, or use noise gating.
Radio frequency interference (RFI): AM/FM broadcast, TV, cellular, and Wi-Fi signals couple into sensors. A digital partial discharge tester with frequency selection can avoid bands with high RFI.
Power electronics: Variable frequency drives, UPS systems, and switching power supplies generate broadband noise from 10 kHz to 100 MHz. Measurements near such equipment require differential sensing or fiber-optic isolation.
Ground loops: Multiple ground paths create circulating currents that appear as PD-like pulses. Single-point grounding and fiber-optic signal transmission eliminate this.
Environmental Correction Techniques
| Factor | Correction Method | Limitations |
|---|---|---|
| Temperature | Record and group measurements by temperature band (±5°C). Apply asset-specific correction curves if available. Compare only within same band. | Correction curves are asset-specific and rarely available; may introduce errors if applied incorrectly |
| Humidity | Record RH; restrict trending to ±10% RH. For surface PD, apply linear correction if validated: Q_corrected = Q_measured × (1 - k × (RH - 50)/100), where k ≈ 0.5–1.5 | Correction factor varies by contamination level and insulation type |
| Altitude | Apply altitude correction factor per IEC 60060-2: K_alt = e^(m × (H/8150)), where H = altitude in meters, m = 1.0 for air gaps, 0.5–0.8 for insulators | Only applicable for air-insulated equipment; not for solid or liquid insulation |
| Contamination | Clean before testing if visual contamination is present. Compare pre-clean and post-clean PD to isolate contamination effect. | Cleaning may not be possible for energized equipment |
| EMI | Use frequency selection, noise gating, differential sensors, or fiber-optic links. Measure background noise before each test and subtract if using RMS averaging. | Cannot subtract impulsive noise that overlaps PD frequency band |
Best Practices for Environmentally Robust PD Testing
Standardize measurement conditions: Where possible, perform PD tests at the same time of day, same season, and similar ambient conditions each year. This reduces environmental variability.
Record comprehensive metadata: Temperature, humidity, altitude, weather conditions, contamination observations, and load level. A digital partial discharge tester with built-in environmental sensors simplifies this.
Use control measurements: Measure a known-good reference asset under the same conditions. If the reference shows elevated PD, the environment is influencing results—not the test asset.
Trend corrected values: If correction is applied, trend corrected values consistently. Never mix corrected and uncorrected data in the same trend.
Flag extreme conditions: Measurements taken during rain, fog, or after rapid temperature changes should be flagged as "non-standard conditions" and used for information only, not for pass/fail decisions.
Repeat measurements: If PD results are unexpected, repeat the measurement under different conditions (e.g., morning vs. afternoon) to assess environmental influence.
Case Study: Humidity-Induced False Alarm
A utility performed annual PD testing on a 132 kV current transformer in June (35°C, 85% RH) and measured 220 pC. The previous year's measurement (October, 18°C, 45% RH) was 45 pC. The asset was flagged for urgent investigation. However, a review of environmental data showed the measurement conditions were vastly different. A repeat test in October (similar conditions to the original baseline) measured 52 pC—essentially unchanged. The apparent 5× increase was entirely due to humidity and temperature. The utility now schedules all PD testing for the same month each year and records environmental data as mandatory metadata.
Instrument Features for Environmental Compensation
Modern digital partial discharge testers increasingly include features to address environmental factors:
Built-in temperature and humidity sensors: Automatically record environmental conditions with each measurement.
Altitude input: Allows the tester to apply altitude correction automatically.
Humidity correction algorithms: For selected asset types, apply pre-programmed correction curves.
Environmental flagging: Mark measurements as "standard" or "non-standard" conditions based on user-defined thresholds.
Trend views with environmental overlays: Display PD trend alongside temperature and humidity trends to visually identify environmental influence.
Developing an Environmental Correction Procedure
For organizations requiring rigorous PD trending, develop a documented procedure:
Establish baseline: Measure each critical asset under a range of environmental conditions to understand its sensitivity to temperature and humidity.
Derive correction factors: Using regression analysis, determine the relationship between PD magnitude and environmental variables for that specific asset type.
Validate: Test the correction model on independent measurements to ensure it reduces scatter without masking genuine changes.
Document and train: Include correction procedure in SOPs and train all operators on its application.
Review annually: Update correction factors as more data becomes available.
Selecting a Digital Partial Discharge Tester for Environmentally Challenging Sites
For field measurements in diverse climates, prioritize:
Integrated environmental sensors (temperature, humidity, pressure).
Wide operating temperature range (-20°C to +55°C).
IP54 or higher rating for dust and moisture protection.
Automatic altitude correction capability.
Software that records environmental metadata and supports correction algorithms.
Battery performance stable across temperature extremes.
Environmental factors are not noise to be ignored—they are essential variables that must be measured, recorded, and accounted for in any serious PD testing program. A digital partial discharge tester provides the measurement; environmental awareness provides the context. Together, they enable accurate, repeatable, and actionable PD diagnostics regardless of where and when testing occurs. Organizations that master environmental correction achieve higher confidence in their condition assessments, fewer false alarms, and better maintenance decisions.
