Partial Discharge Monitoring in Power Distribution: Principles and Technologies

Partial discharge (PD) is one of the most critical early-warning indicators of insulation degradation in medium and high-voltage electrical equipment. In power distribution networks, where switchgear and ring main units (RMUs) operate continuously under electrical stress, undetected PD activity can progress from minor insulation defects to catastrophic failures within months. This article provides a comprehensive technical examination of PD monitoring principles, detection technologies, and their role in modern power distribution asset management.

Understanding Partial Discharge in Distribution Equipment

Partial discharge refers to localized electrical discharges that partially bridge the insulation between conductors. Unlike a complete breakdown, PD only affects a portion of the insulation system but progressively deteriorates dielectric materials. In distribution switchgear operating at 10kV and above, common PD sources include voids in solid insulation, surface tracking across contaminated insulators, floating electrode discharges, and corona discharge from sharp conductor edges.

The physics of PD involves electron avalanche processes within gas-filled voids or along insulation surfaces. When the local electric field strength exceeds the dielectric strength of the void (typically 3 kV/mm for air at atmospheric pressure), an ionization event occurs. This generates characteristic signals: electromagnetic emissions in the UHF range (300 MHz – 3 GHz), acoustic emissions in the ultrasonic range (20 kHz – 300 kHz), transient earth voltage (TEV) pulses on grounded metal surfaces, and high-frequency current pulses in the ground connection.

TEV Detection: Principles and Applications

Transient Earth Voltage (TEV) detection is the most widely deployed PD monitoring technique for metal-clad switchgear. When PD occurs inside a switchgear compartment, the electromagnetic wave propagates within the metal enclosure. At joints, gaskets, and openings in the metal cladding, a transient voltage pulse appears on the external grounded surface — this is the TEV signal.

TEV sensors are capacitive coupling devices placed on the external surface of switchgear panels. They detect voltage pulses typically in the mV to V range with rise times of 1-5 ns. Modern TEV-based monitoring systems, such as BRITOP’s Integrated Online Monitoring Device, combine TEV sensors with advanced signal processing to measure PD magnitude (in dBmV), pulse count, and phase-resolved patterns.

The key advantage of TEV monitoring is its non-intrusive nature — sensors can be installed on energized switchgear without requiring an outage. However, TEV measurements are influenced by sensor placement and background noise. Modern systems employ time-domain and frequency-domain analysis to distinguish genuine PD signals from external interference.

Ultrasonic Detection for Air-Insulated Equipment

Ultrasonic PD detection captures the acoustic emissions generated by discharge events. When ionization occurs in air (such as corona discharge), the rapid heating and expansion of the ionized channel produces pressure waves in the ultrasonic frequency range, typically 20 kHz to 100 kHz. These acoustic signals are detectable using piezoelectric sensors with resonant frequencies around 40 kHz.

Ultrasonic monitoring is particularly effective for surface PD and corona discharge in air-insulated switchgear. Unlike TEV, ultrasonic signals do not propagate through metal barriers, making them immune to inter-compartment crosstalk but also limiting sensor placement to within the same air volume as the PD source. The comprehensive PD monitoring solutions from BRITOP integrate both TEV and ultrasonic sensors, providing complementary detection capabilities.

UHF Method for GIS and Shielded Equipment

Ultra-High Frequency (UHF) PD detection operates in the 300 MHz to 3 GHz range, capturing the electromagnetic transients radiated by PD current pulses. UHF sensors are typically internal disc-type antennas or external window-type couplers installed in gas-insulated switchgear (GIS) or oil-filled equipment.

The UHF method offers excellent signal-to-noise ratio because the frequency range is far above typical corona and switching noise. It also enables PD source localization through time-of-flight measurements between multiple sensors. For distribution-level applications, UHF monitoring is gaining adoption in compact RMU designs where internal UHF sensors can be factory-installed during manufacturing.

Integrated Multi-Parameter Monitoring

The most advanced PD monitoring systems now integrate multiple sensor technologies with environmental parameter monitoring. BRITOP’s switchgear monitoring solution combines PD detection (TEV + ultrasonic) with wireless temperature monitoring and leakage current measurement in a single platform. This multi-parameter approach provides a holistic view of equipment health:

• PD activity indicates insulation condition
• Temperature anomalies reveal contact resistance issues or overload conditions
• Leakage current trends reflect insulation surface contamination

The integration is powered by edge computing, where local processors perform real-time analysis and only transmit actionable alerts to the central monitoring system. This distributed architecture, also employed in BRITOP’s cable monitoring systems, reduces communication bandwidth requirements and enables millisecond-level response to critical events.

EMC Immunity: Operating in High-Interference Environments

From Periodic Testing to Continuous Online Monitoring

Traditional PD testing relies on periodic handheld instruments deployed during scheduled maintenance outages. While valuable, this approach has fundamental limitations: PD activity is often intermittent and may not manifest during the short test window; trending and degradation rate analysis are impossible with sparse data points; and outage-based testing itself incurs operational costs.

Continuous online monitoring addresses these limitations by providing 24/7 surveillance, enabling degradation trend analysis, and supporting predictive maintenance strategies. The cost-benefit analysis increasingly favors permanent monitoring for critical distribution assets, particularly in renewable energy integration points where equipment is subjected to more frequent thermal cycling and voltage fluctuations.

Related Products from BRITOP

• Integrated Online Monitoring Device for Partial Discharge, Temperature and Leakage Current
• SCYC-CW30 Passive Wireless Temperature Monitoring System for RMU
• SCYC-HLJC2304 Integrated Online Monitoring for HV Cable Sheath and PD
• DT801 Intelligent Monitoring for Substation Surge Arresters
• Electromagnetic Pulse Protection Device (EPPD)

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