What is Partial Discharge?

Partial discharge is a localized electrical discharge that only partially bridges the insulation between conductors. Unlike a complete breakdown (arc flash), PD does not immediately cause equipment failure — but it progressively erodes insulation over time, eventually leading to complete insulation failure.

PD occurs in voids, cracks, or contaminants within solid insulation; at interfaces between different dielectric materials; and in gas-filled cavities within insulating materials. Each discharge event releases energy in the form of heat, light, sound, and electromagnetic radiation — all of which can be detected with the right instruments.

Key fact: According to IEEE studies, partial discharge is responsible for approximately 40% of all high-voltage equipment failures in power utilities. Early detection can extend equipment life by 10–20 years.

Types of Partial Discharge

Understanding the type of PD present helps determine the severity and appropriate remediation strategy. Modern acoustic imaging cameras like the GSW Series can automatically classify PD into four main types:

PD Type	Location	Severity	Acoustic Signature
Tip Discharge	Sharp conductor edges, protrusions	Medium	Irregular, high-frequency bursts
Suspension Discharge	Floating metal parts, loose connections	High	Periodic, synchronized with power frequency
Surface Discharge	Contaminated insulator surfaces	Medium-High	Continuous, broadband noise
Particle Discharge	Free metallic particles in GIS	Very High	Random, impact-like pulses

Why Early Detection Matters

The economic case for PD detection is compelling. A single transformer failure at a major substation can cost between $500,000 and $5 million in equipment replacement and lost revenue. A routine PD inspection program using acoustic imaging cameras costs a fraction of this — and can identify developing faults years before failure.

Beyond economics, early PD detection enables:

• Planned maintenance instead of emergency repairs, reducing labor costs by 30–50%
• Extended equipment life through timely intervention before irreversible insulation damage
• Improved safety by preventing arc flash incidents and equipment explosions
• Regulatory compliance with IEC 60270 and IEC 62478 standards

Detection Methods Compared

Several technologies are used for PD detection, each with different capabilities, limitations, and ideal applications:

Method	Detects	Range	Through Metal?	Best For
Acoustic Imaging (Airborne)	Surface, tip, suspension PD	0.3–130m	No	Open-air equipment, overhead lines
TEV (Transient Earth Voltage)	Internal PD in metal-enclosed switchgear	Contact	Yes	MV switchgear, cable joints
Thermal Imaging	Heat from severe PD	0.5–50m	No	Confirming PD location, hotspot detection
HFCT (High-Frequency CT)	PD in cables and cable joints	Contact	Yes	Cable systems
UHF Sensors	Internal PD in GIS	Contact/Near	Yes	GIS switchgear

Acoustic Imaging Cameras: The Modern Standard

Acoustic imaging cameras represent the most significant advancement in PD detection technology in the past decade. Unlike traditional single-probe ultrasonic detectors, acoustic cameras use a large array of microphones (typically 64–136 microphones) to create a visual "sound map" overlaid on a real-time camera image.

The GSW Series from Unitech Tools features 136 digital silicon microphones arranged in a circular array, detecting frequencies from 2kHz to 100kHz. This allows it to:

• Visualize the exact location of PD sources in real time
• Detect PD at distances up to 130 meters — safely away from energized equipment
• Simultaneously capture thermal images to confirm hotspots
• Automatically classify PD type (tip, suspension, surface, particle)
• Display PRPD (Phase-Resolved Partial Discharge) spectra for severity assessment

The key advantage over traditional methods is speed: an experienced inspector can survey an entire 110kV substation in 2–3 hours using an acoustic imaging camera, compared to 1–2 days with contact-based methods.

TEV (Transient Earth Voltage) Sensors

TEV detection is the preferred method for inspecting metal-enclosed switchgear (MV switchboards, ring main units, and cable joints). When PD occurs inside a metal enclosure, it generates high-frequency electromagnetic pulses that travel along the enclosure surface and can be detected by a TEV sensor placed on the metal panel.

The SC Series combines TEV detection (3–100MHz) with airborne acoustic (40kHz) sensing and thermal imaging in a single handheld instrument. This triple-function approach allows inspectors to:

• Detect PD inside closed switchgear without opening panels
• Correlate TEV readings with acoustic and thermal data for confident diagnosis
• Generate comprehensive inspection reports with a single instrument

Thermal Cameras for PD Detection

While thermal cameras cannot directly detect PD in its early stages (the heat generated is too small), they play a crucial role in confirming PD locations identified by acoustic or TEV methods. Severe PD activity generates enough heat to be visible as a hotspot on a thermal image.

High-resolution thermal cameras like the PT II Series (up to 1280×1024 IR resolution) are particularly effective for:

• Confirming PD locations identified by acoustic imaging
• Detecting resistive heating from loose connections and overloaded conductors
• Monitoring transformer bushings and cable terminations
• Providing visual evidence for maintenance reports

Best Practices for Substation PD Inspection

To maximize the effectiveness of your PD inspection program, follow these industry best practices:

1. Establish a baseline: Conduct an initial survey when equipment is known to be in good condition. Future surveys can then be compared against this baseline to identify developing faults.
2. Inspect under load: PD activity increases with voltage and load. Inspect during peak load periods for the most accurate results.
3. Use multiple detection methods: Combine acoustic imaging (for open equipment) with TEV (for enclosed switchgear) for comprehensive coverage.
4. Document everything: Record GPS coordinates, equipment ID, measurement date, and acoustic/thermal images for trend analysis.
5. Follow IEC 62478: This standard provides guidance on non-conventional PD measurement methods, including acoustic and electromagnetic techniques.
6. Inspect regularly: For critical equipment (transformers, GIS, cable joints), quarterly inspections are recommended. For less critical equipment, annual inspections are typically sufficient.

Recommended Equipment from Unitech Tools

Based on the inspection scenarios described above, Unitech Tools recommends the following equipment for a comprehensive substation PD inspection program:

Application	Recommended Product	Key Feature
Open-air PD detection (transformers, overhead lines)	GSW Series	136-MIC acoustic imaging, 130m range
MV switchgear inspection	SC Series	AA + TEV dual detection
Precision thermal analysis	PT II Series	Up to 1280×1024 IR resolution
24/7 continuous monitoring	WY30 Series	Solar-powered, 4G cloud transmission