Combined Thermal Camera and Partial Discharge Inspection for Substations: A Complete Guide

Why Combine Thermal Imaging and PD Detection?

A common misconception among maintenance teams is that if the thermal scan looks clean, the equipment is healthy. This is not true. Many developing faults — particularly internal partial discharge activity in switchgear, cable terminations, and transformer bushings — produce no detectable temperature rise until they are minutes from failure.

Conversely, a PD detector alone cannot identify overloaded circuits, loose connections, or cooling system blockages that manifest as thermal anomalies. By combining both technologies, an inspection team catches two fundamentally different failure mechanisms in a single pass:

"In our experience, roughly 30% of faults found during combined thermal + PD inspections are invisible to thermal cameras alone. You simply cannot do a thorough substation assessment without both tools." — Senior Engineer, Latin American Power Utility

What Thermal Cameras Reveal in Substations

A handheld thermal camera is the first line of defense for substation electrical inspection. It detects infrared radiation emitted by equipment and converts it into a temperature map, making overheating components immediately visible.

The Hammer II Thermal Camera from Unitech Tools, with 640x480 IR resolution and sensitivity down to 15 mK NETD, is ideal for substation applications. It resolves small thermal anomalies at distance — critical when scanning busbars, disconnect switches, and transformer bushings from a safe perimeter.

The EasIR Series, with its compact form factor and 4+ hour battery life, excels during extended inspection rounds covering dozens of panels and breakers.

Key thermal targets in a substation:

• Disconnect switches: Blade-to-clip contact heating indicates misalignment or pitting
• Circuit breakers (SF6 / oil / vacuum): Top-to-bottom temperature delta on the tank suggests internal arcing or poor contact
• Busbars and joints: Phase-to-phase delta > 3-5 degrees C indicates high resistance connection
• Power transformers: Cooling tube blockages (cold tubes), bushing uneven heating, oil level anomalies
• CTs and PTs: Uneven heating can indicate internal insulation degradation
• Cable terminations: Hot spots at stress cones or lug connections

What PD Detectors Catch That Thermal Cameras Miss

Partial discharge is a localized electrical breakdown of insulation that does not completely bridge the gap between conductors. It produces high-frequency electrical pulses (TEV), ultrasonic emissions, and sometimes light or ozone — but rarely enough heat for a thermal camera to detect in its early stages.

The SC Series PD Detector from Unitech Tools simultaneously measures TEV (transient earth voltage) and ultrasonic emissions, giving inspectors two complementary detection channels. The HPD200 adds HFCT capability for cable PD detection.

PD faults that thermal cameras cannot see:

• Internal PD in MV switchgear: TEV signals from internal arcing inside closed panels — invisible to thermal until the arc flash
• Surface tracking on insulators: Creepage discharge across porcelain or polymer surfaces, detectable via ultrasonic emissions
• Cable void discharge: Internal PD within cable insulation (detected via HFCT) with no external thermal sign
• GIS particulate contamination: Loose particles inside gas-insulated switchgear causing discharge events
• Transformer winding PD: Early-stage PD in oil-paper insulation, detectable via UHF or acoustic PD sensors

A thermal camera can confirm a bad connection is getting worse. A PD detector can warn you that a breaker is about to fail internally — when no external temperature change is measurable. They are complementary, not competing, tools.

The Combined Inspection Workflow

Here is the recommended step-by-step workflow for conducting a combined thermal + PD substation inspection:

1. Pre-inspection data review: Review load data, previous PD levels, and thermal baselines for comparison
2. Thermal scan (wide area): Walk the substation with the Hammer II in auto-range mode. Scan all visible energized components from a safe distance. Note any hotspots > 5 degrees C above ambient or > 3 degrees C phase-to-phase delta
3. PD survey with TEV: Switch to the SC Series PD Detector. Scan each switchgear panel at the TEV sensor contact point (typically on the panel door gasket or vent). Record dBmV levels. Levels above 10 dBmV warrant investigation; above 20 dBmV require immediate action
4. PD survey with ultrasonic: Use the ultrasonic contact probe on panel gaps, gaskets, and vents. Listen for the characteristic buzzing/hissing of surface discharge. Also scan insulators and bushings from a distance with the airborne ultrasonic sensor
5. Correlate findings: Cross-reference hotspots with PD sources — a panel that is both hot and showing PD activity is a high-priority maintenance target
6. Document and report: Capture thermal images and PD spectral data together in one report. Flag priority items for scheduling

Real Application: 138/69 kV Substation Case Study

A regional utility in northeastern Brazil implemented a combined thermal + PD inspection program across 12 transmission substations (138/69 kV) in 2025. Their previous practice was quarterly thermal-only inspections with a spot thermometer for connection temperature checks.

Results after 6 months of combined inspections:

• Total abnormal findings: 47 (up from 19 in the thermal-only period)
• PD-only findings (no thermal signature): 14 (30% of total)
• Thermal-only findings: 21 (45% of total)
• Combined (both thermal + PD active): 12 (25% of total)
• Unplanned outages: reduced by 61% compared to the same period the previous year

The utility now uses the Hammer II for thermal scanning and the HPD500 PD detector (which adds UHF sensor capability for GIS monitoring) as their standard inspection kit for all transmission and distribution substations.

"The combined approach changed our maintenance philosophy. We used to react after a failure. Now we see the full degradation path — from the first PD pulse to the final thermal runaway." — Maintenance Manager, Brazilian Utility

Recommended Equipment for Combined Inspection

For teams implementing a combined thermal + PD inspection program, Unitech Tools recommends the following equipment configuration:

Frequently Asked Questions

Can a thermal camera detect partial discharge?

Only in very advanced stages where PD activity has generated enough heat (typically 10+ watts of discharge energy at a single site). In most cases, PD is detected acoustically or via TEV long before any thermal signature appears. A thermal camera is not a substitute for a PD detector.

Do I need to power down the substation for PD inspection?

No. Both TEV and ultrasonic measurements are performed >online< (equipment energized). This is one of the key advantages of combined thermal + PD inspection: zero downtime required.

How often should combined inspections be performed?

For transmission substations, quarterly combined inspections are recommended. Distribution substations can be inspected every 6 months unless critical PD levels are detected, in which case monthly monitoring is advised.