What Is Partial Discharge?
Partial discharge (PD) is a localized electrical discharge that partially bridges the insulation between conductors. Unlike a complete breakdown, partial discharge does not immediately cause equipment failure, but it is one of the most reliable early warning signs of insulation degradation in medium-voltage (MV) and high-voltage (HV) electrical equipment. In distribution switchgear, ring main units (RMUs), and cable terminations operating at 10kV and above, PD activity signifies the gradual deterioration of solid, liquid, or gaseous insulation systems.
The physics of partial discharge involves the ionization of small voids, cracks, or impurities within insulating materials. When the electric field strength across a defect exceeds the breakdown strength of the gas trapped inside, a micro-discharge occurs. Each discharge event — lasting mere nanoseconds — erodes the insulation surface, enlarges the void, and accelerates aging. Over months or years, this cumulative damage can lead to complete insulation failure, arc flash incidents, and catastrophic equipment destruction.
Types of Partial Discharge in Switchgear
In distribution switchgear and ring main units, three primary types of partial discharge are commonly observed:
- Internal Discharge: Occurs within voids or delaminations inside solid insulation materials such as epoxy resin bushings, cast-resin transformers, or cable terminations. Internal PD is particularly dangerous because it is invisible from the outside and progresses silently.
- Surface Discharge: Develops along the surface of insulation materials when contamination, moisture, or tracking creates a conductive path. Surface PD is common in polluted industrial environments or coastal installations where salt deposition compromises creepage distances.
- Corona Discharge: Occurs in air around sharp points or edges of high-voltage conductors. While corona discharge produces audible hissing and visible glow in darkness, it contributes to ozone generation and gradual conductor corrosion.
Why Online PD Monitoring Matters
Traditional insulation testing methods — such as insulation resistance measurement, dielectric loss factor (tan δ) testing, and very-low-frequency (VLF) testing — require equipment shutdown and disconnection. These offline tests provide only a snapshot of insulation condition at a single point in time and cannot detect intermittent PD activity that occurs under specific thermal, humidity, or load conditions.
Online partial discharge monitoring continuously tracks PD activity while equipment remains energized. This approach offers several critical advantages:
- Real-time trending: Continuous data collection reveals PD growth patterns, enabling operators to distinguish between stable, benign discharge and rapidly escalating deterioration that demands immediate intervention.
- Condition-based maintenance: Maintenance resources can be allocated based on actual equipment condition rather than fixed calendar intervals, reducing unnecessary outages and labor costs.
- Fault prevention: Early detection of PD onset provides weeks or months of lead time before catastrophic failure, allowing scheduled repairs during planned outages.
- Life extension: By preventing accelerated aging through early intervention, online monitoring can extend the service life of capital-intensive switchgear assets by years.
Detection Technologies: Ultrasonic and TEV Methods
Modern online PD monitoring systems, such as the integrated monitoring solution deployed by Sichuan Yachen Electric, employ dual-mode detection combining ultrasonic (AE) and transient earth voltage (TEV) sensing.
Ultrasonic (AE) Detection
Partial discharge events generate acoustic emissions in the ultrasonic frequency range of 20kHz to 500kHz. Piezoelectric AE sensors mounted on the inner walls of switchgear enclosures detect these high-frequency sound waves. Because ultrasonic signals attenuate rapidly through air gaps and solid barriers, AE detection offers excellent spatial localization — a sensor responds primarily to PD occurring within its immediate vicinity. This makes AE particularly effective for pinpointing the specific compartment or bushing where discharge originates.
Transient Earth Voltage (TEV) Detection
When partial discharge occurs inside metal-clad switchgear, electromagnetic waves propagate along the inner surfaces of the enclosure. These waves induce transient voltage pulses on the grounded metalwork — hence the term transient earth voltage. TEV sensors, typically capacitive couplers placed on the internal metal surfaces, detect these pulses across a frequency range from 3MHz to 100MHz. TEV detection is highly sensitive and can identify PD activity occurring anywhere within a metal-enclosed compartment.
The Integrated Approach
The combination of AE and TEV detection in a single monitoring platform provides complementary information: TEV offers broad sensitivity across the entire switchgear panel, while AE provides precise localization. When both sensor types register simultaneous discharge events, confidence in PD diagnosis is significantly elevated and false alarms from external electromagnetic interference are minimized.
The power grid monitoring product line from Qingdao Britop brings these advanced detection capabilities to distribution networks worldwide. By deploying wireless dual-function sensors with integrated signal filtering, amplification, and analog-to-digital conversion, these systems transform raw sensor data into actionable maintenance intelligence — enabling utilities to move from reactive fault response to proactive asset management.
Related Products from Qingdao Britop
- Integrated Online Monitoring Device for Partial Discharge, Temperature and Leakage Current of Distribution Switchgear Ring Main Unit
- Power Grid Online Monitoring: Partial Discharge, Temperature, and Leakage Current Detection
- Power Grid Monitoring Products
Related Products:
– Filter Plate Selection Guide for Filter Press: Polypropylene, Cast Iron, and Membrane Plate Materials and Configurations
– Hydraulic Hose Skiving: Precision Rubber Removal for Reliable Crimp Connections and Leak-Free Assemblies
– Lightning Monitoring and Overvoltage Protection: Smart Grid Technologies for Transmission Line Resilience
– Cable Sheath Circulating Current in High-Voltage Systems: Fundamentals and Monitoring
Related Products
- Eagle-beak Shear Excavator Attachment – QINGDAO BRITOP QUALITY PRODUCT
- Auger Excavator Attachment – QINGDAO BRITOP QUALITY PRODUCT
- Steel Structure Shear Excavator Attachment – QINGDAO BRITOP QUALITY PRODUCT
- Jaw Crushing Pliers Excavator Attachment – QINGDAO BRITOP QUALITY PRODUCT
- Jaw Crushing Bucket Excavator Attachment – QINGDAO BRITOP QUALITY PRODUCT
