Why Switchgear Is Vulnerable to PD

Switchgear operates in some of the harshest electrical environments. Compartments are often crowded with bus bars, circuit breakers, disconnect switches, CTs/VTs, and cable terminations—all operating at medium voltages (3.3 kV to 36 kV) or high voltages (up to 245 kV). The confined space and complex geometry create high electric field gradients, particularly at sharp edges, loose connections, and around stress concentration points.

Common PD sources in switchgear include:

• Cable terminations and stress cones: Improper installation or aging causes surface discharge along the termination interface
• Bus bar supports and insulators: Contamination tracking or internal voids in cast epoxy supports
• Breaker chambers: Internal PD in vacuum interrupter operating mechanisms or SF6 gas compartments
• Current and voltage transformer compartments: PD in resin-insulated transformers due to manufacturing voids
• Bushings and feed-throughs: Surface tracking from moisture ingress or pollution buildup

Latin American utilities face additional challenges: high ambient humidity (often exceeding 85% RH in tropical regions), salt spray in coastal substations, and altitude derating effects in Andean countries like Colombia, Ecuador, Peru, and Bolivia. These factors accelerate PD activity and make regular PD testing essential for reliable switchgear operation.

TEV Method for Air-Insulated Switchgear

Transient Earth Voltage (TEV) is the most widely adopted online PD detection method for air-insulated switchgear (AIS). When PD occurs inside a switchgear panel, electromagnetic waves travel through the metal enclosure, inducing a transient voltage on the outer surface. A TEV probe placed against the panel surface measures these pulses in decibels (dB).

Advantages of TEV for switchgear:

• Non-invasive: no need to open compartments or de-energize equipment
• Fast scanning: a typical 10-panel switchboard can be surveyed in 15-20 minutes
• Quantitative results: dB readings correlate with discharge severity (0-10 dB minor, 10-20 dB moderate, >20 dB requires immediate investigation)
• Works on metal-clad and metal-enclosed switchgear (most common types)

The Unitech Tools SC Series PD Detector offers dual TEV + ultrasonic sensors, allowing engineers to cross-validate TEV readings with ultrasonic confirmation. The SC Series measures TEV in the range of 0-60 dB with a resolution of 1 dB and stores up to 500 records for trend analysis.

Ultrasonic Detection for Surface PD

Ultrasonic detection is the complementary method to TEV. While TEV detects internal discharges, ultrasonic sensors pick up the audible and near-ultrasonic signals produced by surface discharge and corona. These sensors can be contact-type (placed on the panel surface to detect structure-borne ultrasound) or airborne (for open-area substations).

Ultrasonic detection excels at locating surface discharge on bushing surfaces, cable termination stress cones, and insulator surfaces—PD sources that may not generate strong TEV signals. The characteristic "frying bacon" sound is unmistakable to trained operators using the HPD200 Handheld PD Detector with its ultrasonic sensor and headphone output.

Acoustic Imaging for Large Switchgear Rooms

For utilities managing extensive switchgear rooms or open-terminal substations, acoustic imaging cameras offer a game-changing advantage. Instead of scanning panel by panel with a contact probe, the engineer can stand in the center of a switchgear room and see all PD sources on a single screen.

The GSW Series Acoustic Camera from Unitech Tools visualizes ultrasonic signals from PD sources in real time, overlaying them onto a visible-light image. With its 136-microphone array and detection range up to 50 meters, the GSW can locate PD sources across an entire substation in minutes. This is particularly valuable for:

• Large indoor switchgear rooms with 40+ panels
• Open-terminal substations where PD sources are spread across a wide area
• Rapid troubleshooting when multiple PD sources are suspected
• Safety: stand at a safe distance (5-10 m) from energized equipment while detecting PD

Method Comparison by Switchgear Type

Switchgear Type	TEV	Ultrasonic	Acoustic Imaging	HFCT	Recommended Product
Metal-clad AIS (indoor)	✔ Best	✔ Good	✔ Good	✖	SC Series
Open-terminal substation	✖	✔ Best	✔ Best	✖	GSW Series
GIS (gas-insulated)	✖	✖	✔ Good	✔ Best	HPD500
Distribution switchgear (11 kV)	✔ Best	✔ Good	✖	✖	HPD200
Cable-connected switchgear	✔ Good	✖	✖	✔ Best	HPD500

Recommended PD Inspection Workflow for Switchgear

Based on field experience with power utilities across Latin America and Africa, Unitech Tools recommends the following multi-stage PD inspection workflow for switchgear:

1. Initial TEV survey: Scan all accessible panels with the SC Series PD Detector. Mark any panels with TEV readings above 10 dB for further investigation. Typical survey time: 15-30 minutes per switchboard.
2. Ultrasonic confirmation: For panels flagged in step 1, use the SC Series ultrasonic sensor or HPD200 to pinpoint the PD source. Listen for the characteristic discharge sound and note the sensor position on the panel.
3. Acoustic imaging scan (optional): For large switchgear rooms or when multiple panels show elevated readings, use the GSW Series acoustic camera to visually map all PD sources simultaneously.
4. HFCT measurement (cable compartments): If the switchgear includes cable termination compartments, clamp an HFCT sensor on the cable grounding conductor to measure PD magnitude in pC.
5. Trend analysis: Repeat the survey at 3-month intervals. Compare dB readings against previous measurements to track PD progression. A consistent increase of 5 dB or more over three successive readings indicates developing insulation deterioration.
6. Scheduled maintenance: When PD levels exceed critical thresholds (TEV > 20 dB, ultrasonic > 15 dB), schedule maintenance at the next planned outage. Use the PD location data to target inspection efforts on specific compartments.

"A leading Chilean mining operation reduced switchgear PD-related failures by 65% in 18 months by implementing a monthly TEV + ultrasonic PD survey program using the SC Series detector. The program paid for itself within 4 months of deployment."

— Unitech Tools Field Report, Minera Escondida, Chile

Unitech Tools Solutions for Switchgear PD

Unitech Tools offers a complete ecosystem of PD detection products designed for switchgear maintenance programs:

• SC Series PD Detector – The workhorse of switchgear PD inspection. Dual TEV + ultrasonic sensors, 0-60 dB range, 500-record storage, and a rugged IP54-rated enclosure for field use. Ideal for monthly switchgear PD surveys.
• HPD200 Handheld PD Detector – Cost-effective ultrasonic PD spot-checker for distribution switchgear. Lightweight (1.2 kg) with 8-hour battery life. Audio output for audible PD detection training.
• HPD500 Advanced PD Analyzer – Multi-method instrument with TEV, ultrasonic, HFCT, and optional UHF. Features phase-resolved PD pattern analysis (PRPD) for detailed diagnostics. Ideal for critical GIS and cable-connected switchgear.
• GSW Series Acoustic Camera – Rapid PD survey tool for large installations. 136 microphones, real-time sound visualization, detection range up to 50 meters. Includes tablet-based control and reporting software.