The Complete Guide to Partial Discharge Monitoring in MV/HV Switchgear

Introduction: The Silent Threat Inside Switchgear

Partial discharge (PD) is one of the most insidious threats to medium-voltage (MV) and high-voltage (HV) switchgear. Unlike a catastrophic flashover that announces itself with dramatic consequences, partial discharge progresses silently—degrading insulation materials over months or years until a seemingly sudden failure occurs. For utilities, industrial facilities, and renewable energy operators relying on continuous power distribution, an unplanned switchgear failure can mean millions in downtime, equipment damage, and safety risks.

Modern integrated online monitoring devices have transformed how engineers approach PD detection. Rather than relying on periodic manual inspections—which can miss intermittent discharge events—these systems provide continuous, real-time surveillance of switchgear health. This guide explores the complete landscape of partial discharge monitoring, from fundamental physics to advanced deployment strategies.

Understanding Partial Discharge: The Physics of Insulation Breakdown

Partial discharge occurs when the electric field strength in a localized region of insulation exceeds the material’s dielectric withstand capability, causing a small electrical breakdown that does not completely bridge the gap between conductors. The key word is “partial”—the discharge is confined to a portion of the insulation, leaving other sections intact.

Common causes of PD in switchgear include:

• Manufacturing defects: Voids, air gaps, or impurities introduced during cable jointing or component fabrication create weak points where electric field concentrations form.
• Aging and thermal stress: Repeated thermal cycling causes insulation materials to crack, delaminate, or develop microscopic fissures.
• Moisture ingress: Humidity penetration into cable terminations and bushings accelerates insulation degradation and creates conductive paths.
• Mechanical damage: Vibration, improper installation, or external impact can compromise insulation integrity.
• Surface contamination: Dust, salt deposits, and chemical pollutants create tracking paths on insulator surfaces.

PD activity is typically classified into four fundamental types: internal discharges (within voids or cavities), surface discharges (along insulation surfaces), corona discharges (from sharp conductor points into air), and treeing (branch-like degradation channels). Each type produces distinct signal signatures that modern detection systems can identify and classify.

Detection Technologies: AE, TEV, and Beyond

Modern PD monitoring employs multiple complementary detection technologies, each with specific advantages for different switchgear configurations and discharge types.

Acoustic Emission (AE) Detection

AE sensors detect the ultrasonic pressure waves generated by partial discharge events. When a discharge occurs within a gas-filled void, it creates a small explosion-like event that propagates mechanical vibrations through the switchgear structure. AE sensors—typically piezoelectric transducers operating in the 20-200 kHz range—capture these vibrations and convert them to electrical signals for analysis.

AE detection offers several advantages: it is immune to electromagnetic interference (EMI), provides excellent spatial localization capability (especially when using multiple sensors for triangulation), and can detect discharges in inaccessible internal cavities. Its primary limitation is attenuation through solid materials and interfaces, which restricts range and requires careful sensor placement.

Transient Earth Voltage (TEV) Detection

TEV sensors detect the electromagnetic transients that propagate along the inner surfaces of metal-clad switchgear when PD occurs. These transients escape through joints, gaskets, and apertures in the metal enclosure, creating measurable voltage pulses on the external surface. TEV sensors—capacitive probes placed on the switchgear exterior—capture these pulses without requiring direct access to energized conductors.

TEV detection is particularly valuable for metal-enclosed switchgear where internal access is impractical. It enables non-intrusive, online monitoring without requiring outages or modifications to existing equipment. However, TEV signals can be affected by external electromagnetic noise, requiring sophisticated filtering algorithms to distinguish genuine PD from interference.

The AE+TEV Synergy

The most advanced monitoring systems, such as the integrated online monitoring device for switchgear, combine AE and TEV detection into a unified platform. This dual-technology approach provides complementary data streams: AE excels at detecting internal cavity discharges and surface tracking, while TEV is sensitive to corona and discharges near conductor surfaces. Cross-correlation between the two sensor types dramatically reduces false positives—a critical requirement for systems that may trigger automated alarms or protection actions.

Beyond PD: The Multi-Parameter Approach

While PD detection is essential, leading-edge monitoring systems go further by integrating additional parameters into a single platform. The Sichuan Yachen integrated monitoring solution exemplifies this philosophy, combining PD detection with:

• Wireless temperature sensing: Passive sensors using electric field energy harvesting monitor busbar connections, cable joints, and circuit breaker contacts. Temperature rise is often the earliest indicator of a developing fault, preceding detectable PD by weeks or months.
• Leakage current monitoring: Shield wire leakage current measurement detects insulation deterioration in cable systems and identifies tracking paths before they develop into full discharge channels.
• Edge computing analytics: On-board processing enables autonomous fault detection and classification even without network connectivity. The device can independently assess severity levels and trigger local alarms while storing historical data for trend analysis.

This integrated approach reflects a fundamental shift from reactive maintenance—fixing equipment after it fails—to predictive maintenance, where degradation trends are identified and addressed before failure occurs. For operators managing hundreds or thousands of switchgear units across distributed sites, this capability translates directly to reduced outage frequency, lower maintenance costs, and extended asset life.

Deployment Architecture and Communication

A modern PD monitoring deployment typically follows a layered architecture:

Sensor Layer: AE sensors (contact piezoelectric), TEV sensors (capacitive probes), temperature sensors (wireless, self-powered), and current sensors (split-core CTs) distributed across monitored equipment. The wireless temperature sensors used in solutions like the SCYC-CW30 and SCYC-PWTM2304 eliminate the need for battery replacement and complex wiring, dramatically simplifying installation in retrofit scenarios.

Data Acquisition Layer: Local monitoring units (LMUs) that digitize sensor signals, perform initial signal processing, and transmit data via RS485, Ethernet, or wireless (LoRa/4G) to centralized platforms. Modern LMUs incorporate edge computing processors that can execute PD classification algorithms locally, reducing bandwidth requirements and enabling autonomous operation.

Platform Layer: Cloud-based or on-premises software that aggregates data from multiple sites, performs advanced analytics (including machine learning-based pattern recognition), generates alarms, and provides visualization dashboards for operations and maintenance teams.

EMC Resilience: Designed for the Substation Environment

• Electrostatic discharge: Level 4 (15 kV air discharge)
• Electrical fast transient/burst: Level 4 (4 kV on power lines)
• Surge immunity: Level 4 (4 kV line-to-line, 6 kV line-to-ground)
• Power frequency magnetic field: Level 5 (100 A/m continuous)
• Pulse magnetic field: Level 5
• Damped oscillatory magnetic field: Level 5

This EMC robustness, combined with an operating temperature range of -40C to +80C and IP65 enclosure protection, ensures reliable operation in the harshest environments—from desert solar farms to arctic wind installations.

Application Scenarios

Integrated PD monitoring systems are deployed across a wide range of applications:

Utility Distribution Substations: Monitoring of 10kV-35kV switchgear in urban and rural distribution networks, where unplanned outages affect thousands of customers.

Renewable Energy: Wind farm and solar PV collection substations present unique challenges—remote locations, wide temperature swings, and high harmonic content from power electronics all accelerate insulation aging. Continuous monitoring is essential for these critical assets.

Industrial Facilities: Petrochemical plants, steel mills, and semiconductor fabs where process continuity is paramount and a single switchgear failure can halt production lines costing millions per hour.

Data Centers: Mission-critical facilities where even momentary power interruptions are unacceptable, and where predictive maintenance enables planned switchgear servicing without risking uptime.

Transportation Infrastructure: Railway traction power substations, airport terminal power distribution, and port facilities where reliability directly impacts public safety and economic activity.

Conclusion: The Case for Continuous Monitoring

The economics of partial discharge monitoring are compelling. Studies by CIGRE and IEEE have demonstrated that condition-based maintenance programs incorporating online PD monitoring can reduce switchgear failure rates by 50-70% compared to time-based maintenance alone. For a typical utility operating 500 switchgear units, this translates to 5-10 avoided failures annually—each potentially saving hundreds of thousands of dollars in direct repair costs and outage-related penalties.

The technology has matured to the point where integrated multi-parameter monitoring—combining AE, TEV, temperature, and leakage current into a single ruggedized platform—is cost-effective for deployment across an entire fleet, not just on the highest-criticality assets. As power grids evolve to accommodate distributed generation, bidirectional power flows, and increasing load densities, the stress on switchgear insulation will only increase. Continuous online monitoring is no longer a luxury; it is an essential component of modern asset management strategy.

Related Products from Qingdao Britop:

• Integrated Online Monitoring Device – PD + Temperature + Leakage Current for Switchgear/RMU
• SCYC-CW30 – Passive Wireless Temperature Monitoring for Ring Main Units
• SCYC-PWTM2304 – Universal Passive Wireless Temperature Monitoring Device
• SCYC-HLJC2304 – HV Cable Sheath Circulating Current & PD Monitoring
• DTE2100 – Transformer Core Grounding Current Online Monitoring

Related Products:

• Power Grid Monitoring Products

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