Insulation Aging and Deterioration

The most common PD cause in medium voltage switchgear is simply age. As switchgear insulation systems — typically epoxy resin, cross-linked polyethylene (XLPE), or EPDM rubber — age over decades of service, their dielectric properties gradually degrade. Thermal cycling, electrical stress, and environmental exposure all contribute to the slow breakdown of polymer chains and the formation of micro-cracks within the insulation material.

In a 15-year study of MV switchgear failures published by CIGRE, insulation aging was identified as the primary PD source in approximately 35% of all cases. The key indicators include:

• Internal void discharge — Small cavities within aged insulation produce symmetrical PRPD patterns centered around voltage zero crossings. These voids gradually expand over time as each discharge event erodes the cavity walls.
• Electrical treeing — In cable terminations and bus bar insulation, aging leads to the formation of branching discharge channels (electrical trees) that propagate through the insulation until complete dielectric breakdown occurs.
• Reduced partial discharge inception voltage (PDIV) — As insulation ages, the voltage at which PD begins decreases. A PDIV below 75% of the rated phase-to-ground voltage indicates advanced aging.

The Unitech Tools SC Series PD Detector can measure PDIV across switchgear compartments during planned outages, providing a quantitative benchmark for insulation aging. Trending PDIV values year over year reveals the rate of degradation and helps schedule replacement before failure.

Moisture Ingress and Condensation

In tropical and coastal environments — common across Latin America — moisture ingress is the second most frequent PD cause in MV switchgear. High humidity, temperature swings, and inadequate sealing allow water vapor to enter switchgear enclosures. When the internal temperature drops below the dew point, condensation forms on insulation surfaces, dramatically reducing surface resistivity and enabling discharge activity.

Moisture-driven PD has several telltale characteristics:

• Surface discharge — Condensation on bushing surfaces and insulator skirts creates conductive paths for leakage current, resulting in surface discharge that shows as asymmetrical PRPD patterns concentrated on the rising and falling edges of the AC waveform (45-90 degrees and 225-270 degrees).
• Seasonal variation — PD levels in moisture-affected switchgear increase significantly during rainy seasons and early morning hours when condensation is highest. This pattern is a strong diagnostic clue for moisture-related PD.
• Intermittent activity — Unlike aging-related PD which tends to be stable, moisture-driven PD can appear and disappear with changing humidity conditions, making it difficult to detect with single-point measurements.

"In a Colombian sugar mill's 13.8 kV switchgear, condensation-driven PD was causing an average of 3 unplanned outages per year. After installing space heaters and passive dehumidifiers, PD levels dropped by 78% within 3 months, and no further switchgear failures occurred over a 2-year follow-up period."

— Unitech Tools Case Study: MV Switchgear in Tropical Industrial Environments

Surface Contamination and Tracking

Dust, salt spray, coal dust, chemical residues — contaminants accumulate on switchgear insulation surfaces over time, especially in industrial environments. When combined with moisture, these contaminants form a conductive layer that redistributes the electric field and promotes tracking (carbonized discharge paths) across insulator surfaces.

The contamination threat varies by industry:

• Cement and mining plants — Conductive mineral dust settles on bus supports and bushings, creating surface leakage paths that can sustain continuous PD at operating voltage
• Petrochemical facilities — Hydrocarbon vapors mixed with moisture produce acidic surface films that accelerate tracking
• Coastal substations — Salt spray (common near Colombian and Peruvian coastal cities) creates highly conductive contamination layers on all exposed insulation surfaces

Surface tracking is one of the most dangerous PD mechanisms because it self-sustains and accelerates. Each discharge carbonizes a microscopic path on the insulation surface, which becomes more conductive and attracts the next discharge. Within months, surface tracking can lead to flashover and catastrophic switchgear failure.

The HPD500 Advanced PD Analyzer with its combination of TEV and ultrasonic sensors excels at identifying surface tracking. The simultaneous TEV-ultrasonic measurement provides high confidence in surface discharge diagnosis — TEV detects the electromagnetic pulse while the ultrasonic microphone picks up the audible surface discharge sound.

Voids and Delamination in Cast Epoxy Insulation

MV switchgear relies extensively on cast epoxy resin components — bus supports, voltage transformer insulators, bushing flanges, and barrier panels. Manufacturing defects during the casting process can leave microscopic voids or delamination layers within the epoxy. These voids have a much lower dielectric strength than solid epoxy, making them the preferred location for PD inception.

Void-type PD exhibits a very distinctive PRPD fingerprint:

• Symmetrical "rabbit ear" pattern — Due to the symmetrical nature of void discharges, the PRPD pattern shows two symmetric clusters of pulses centered around the voltage zero crossings (0 and 180 degrees)
• Stable magnitude — Unlike surface discharge, void discharge magnitude remains relatively constant once the inception voltage is exceeded
• Low apparent charge but high repetition rate — Individual void discharges may be only 20-100 pC, but they can recur thousands of times per second, causing cumulative damage

PD Source	PRPD Pattern	Typical Magnitude	Phase Position	Progression Speed
Internal void (epoxy)	Symmetrical rabbit ears	20-200 pC	0-30 / 180-210 deg	Slow (years)
Surface tracking	Asymmetrical, spreading	50-500 pC	45-90 / 225-270 deg	Moderate (months)
Corona (air-insulated)	Sharp, negative half-cycle	10-100 pC	250-290 deg	Stable to slow
Moisture-induced surface	Variable, humidity-dependent	10-300 pC	45-90 / 225-270 deg	Intermittent
Floating electrode	Very large, stable pulses	500-5000 pC	Variable, consistent	Moderate-fast
Electrical tree (XLPE)	PRPS shows burst pattern	5-100 pC	Variable	Fast (days-weeks)

Identifying void-type PD early is critical because voids gradually expand — each discharge erodes the cavity wall, increasing the void volume and lowering the PD inception voltage further. What starts as a harmless 20 pC void can grow into a full insulation puncture over 2-5 years.

Floating Electrodes and Loose Connections

Loose metallic components within the switchgear — such as ungrounded shield covers, floating bolt washers, or detached corona shields — create floating electrode conditions that produce some of the most dangerous PD types. Floating electrodes are characterized by extremely high discharge magnitudes (often 1000-5000 pC) because the full voltage across the insulation gap appears across the floating element until it discharges.

Key characteristics of floating electrode PD:

• Very high magnitude — Individual discharge events frequently exceed 1000 pC, making them easy to detect even with basic PD instruments
• Stable, reproducible pattern — The PRPD pattern shows two very clear, narrow pulse clusters at specific phase positions
• Risk of secondary damage — While the floating electrode itself may not be immediately critical, the high-energy discharges can damage adjacent healthy insulation, potentially triggering a cascade failure

Loose primary connections — bolted bus bar joints, cable lug connections, and switch contacts — can also produce PD. However, loose connections more commonly generate partial discharge activity through micro-sparking at the contact interface rather than through insulation-based PD. The distinction matters because loose connections require mechanical tightening rather than insulation replacement.

How to Identify Each PD Cause

Identifying the root cause of PD in MV switchgear requires a systematic approach using the right detection tools. Here is a practical workflow used by experienced PD engineers:

1. Broadband survey — Use the SC Series PD Detector (TEV + ultrasonic) to scan all accessible switchgear panels. Note the TEV level in dB and the ultrasonic level in dB. Panels reading above 10 dB TEV or 8 dB ultrasonic warrant closer investigation.
2. PRPD pattern analysis — Switch to PRPD display mode (available on the SC Series and fully featured on the HPD500). Record the PRPD phase-resolved pattern for each suspicious panel. Compare against the reference patterns in the table above to classify the PD type.
3. Multi-sensor verification — Use both TEV and ultrasonic simultaneously. If both sensors show correlated PD activity (TEV pulse coincident with ultrasonic burst), the measurement is confirmed as internal PD and not external noise interference.
4. Magnitude assessment — Classify the severity:
   • TEV < 10 dB / US < 8 dB: Normal background, continue monitoring
   • TEV 10-20 dB / US 8-15 dB: Moderate PD, schedule detailed investigation
   • TEV 20-30 dB / US 15-25 dB: High PD, plan intervention within 3 months
   • TEV > 30 dB / US > 25 dB: Critical PD, intervene immediately
5. Trending — Record PD levels and PRPD patterns in a database. Repeat measurements at 3-month intervals. A rising trend in TEV magnitude or a pattern that shifts in phase position indicates progression that requires early intervention.

"Don't treat all PD readings the same. A TEV reading of 18 dB from surface tracking on a contaminated bushing is far less urgent than a TEV reading of 15 dB from a floating electrode that could flash over at any moment. The PRPD pattern tells you the story — the magnitude alone doesn't."

— Unitech Tools PD Testing Best Practices Guide

Read more about how acoustic cameras detect partial discharge as an alternative method for large-area substation screening.