Introduction
The economic implications of switchgear failures are substantial. A single unplanned outage at a 10kV distribution substation can affect thousands of commercial and residential customers, incurring regulatory penalties and reputational damage. For industrial facilities, downtime costs can range from tens of thousands to millions of dollars per hour depending on the process criticality. This makes partial discharge monitoring not merely a technical nicety but a financial and operational imperative.
Britop Electric, in partnership with Sichuan Yachen Electric, offers a comprehensive suite of online partial discharge monitoring solutions under the SCYC product family. At the heart of this offering is the Integrated Online Monitoring Device for Temperature of Partial Discharge and Leakage Current of Distribution Switchgear Ring Main Unit, a device that combines PD detection, temperature sensing, and leakage current measurement into a single, field-deployable platform designed for 10kV and above distribution networks.
The Physics of Partial Discharge
To understand why PD monitoring is essential, one must first understand the physical mechanisms that produce partial discharges. PD occurs when the local electric field strength exceeds the dielectric strength of a portion of the insulation system, while the overall insulation path remains intact. This localized breakdown is almost always associated with imperfections in the insulation structure.
Common PD Sources in Switchgear
Internal Voids and Cavities: During the manufacturing process of epoxy resin insulation components — bushings, post insulators, CT/VT bodies — microscopic air bubbles can become trapped within the solid dielectric. These voids have a lower permittivity than the surrounding material, causing the electric field to concentrate across them. When the voltage across a void exceeds the Paschen minimum for the gas within, a discharge occurs.
Surface Tracking and Contamination: In environments with high humidity, salt spray, or industrial pollutants, conductive contamination layers can form on insulation surfaces. Under the influence of the electric field, leakage currents flow along these paths, causing localized heating, carbonization, and eventual tracking — the formation of permanent conductive paths that progressively degrade the creepage distance.
Floating Electrode Discharges: Metallic components that become electrically isolated — due to loose bolts, broken earth bonds, or corrosion — act as capacitive voltage dividers. The floating potential can rise to values sufficient to cause sparking to adjacent grounded or energized parts, producing strong, repetitive PD pulses.
Corona Discharge: In air-insulated switchgear (AIS), sharp points or edges on high-voltage conductors create regions of extremely high electric field gradient. When the gradient exceeds the dielectric strength of air (approximately 3kV/mm at standard temperature and pressure), corona discharge occurs. While corona may initially appear benign, the associated ozone production and nitric acid formation accelerate insulation aging.
Particle-Induced Discharge: Free metallic particles within gas-insulated switchgear (GIS) can levitate under the influence of the electric field, bouncing between conductors and creating intermittent discharges. Each bounce produces a characteristic PD signature that varies with particle size, shape, and trajectory.
Detection Methodologies: TEV and Ultrasonic
Modern PD detection in switchgear relies predominantly on two complementary techniques: Transient Earth Voltage (TEV) sensing and ultrasonic (acoustic) emission detection. Each method has distinct strengths, and their combined use provides comprehensive coverage of PD activity.
Transient Earth Voltage (TEV) Detection
When a partial discharge occurs within the insulation structure of a switchgear panel, the high-frequency (typically 3-100 MHz) current pulse propagates along the metallic enclosure. At any discontinuity in the enclosure — joints, gaskets, viewing windows — a transient voltage appears on the outer surface relative to earth. This is the TEV signal.
TEV sensors are capacitive coupling devices placed on the external surface of the switchgear panel. They detect the magnitude (typically in dBmV or dBμV) and pulse repetition rate of the TEV signal. Key advantages include:
- Non-intrusive installation: sensors are externally mounted, requiring no shutdown or panel opening
- Wide bandwidth: capable of distinguishing PD from external noise sources
- Quantitative trending: TEV amplitude correlates with PD severity over time
Britop’s SCYC integrated monitoring device employs dual TEV channels with sophisticated noise discrimination algorithms, enabling reliable PD detection even in electrically noisy substation environments.
Ultrasonic (Acoustic Emission) Detection
The rapid expansion and contraction of the discharge channel creates a pressure wave that propagates through the surrounding medium. In air-insulated equipment, this acoustic emission is in the ultrasonic range (typically 20-100 kHz), well above the audible spectrum and above most mechanical noise sources.
Ultrasonic PD detection offers several unique benefits:
- Immunity to electromagnetic interference: ultrasonic sensors are completely unaffected by the strong electric and magnetic fields present in substations, making them ideal for EMC-challenged environments
- Source localization capability: by measuring the time-of-flight difference between multiple sensors, or by observing signal amplitude variation with sensor position, the physical location of the PD source can be triangulated
- Surface discharge sensitivity: ultrasonic methods excel at detecting surface tracking and corona, which may produce relatively weak TEV signals
The SCYC device incorporates a contact ultrasonic sensor (AE type) designed to operate in the 20-100 kHz band, with a resonant frequency centered at 40 kHz for optimal sensitivity to common switchgear PD signatures.
Combined TEV + Ultrasonic Approach
No single detection method is universally optimal. Internal PD in epoxy insulation may produce strong TEV signals but weak acoustic emissions, while surface corona may show the opposite pattern. By deploying both TEV and ultrasonic sensors simultaneously and correlating their outputs, the SCYC monitoring device achieves:
- High detection probability: capturing PD events regardless of their physical nature
- PD type classification: TEV-dominant signals suggest internal insulation defects, while ultrasonic-dominant signals indicate surface or corona activity
- False positive rejection: requiring coincidence between TEV and ultrasonic channels eliminates spurious triggers from external radio-frequency interference or mechanical vibration
The SCYC Integrated Online Monitoring Device: Architecture and Capabilities
Britop Electric’s flagship SCYC Integrated Online Monitoring Device for Temperature of Partial Discharge and Leakage Current represents a significant advancement in distribution network asset management. Designed specifically for 10kV and above switchgear and ring main units (RMUs), it combines three critical monitoring functions in a single, compact unit.
Technical Specifications
| Parameter | Specification |
|---|---|
| ———– | ————— |
| PD Detection Methods | TEV (Transient Earth Voltage) + Ultrasonic (AE) |
| TEV Frequency Range | 3-100 MHz |
| Ultrasonic Frequency Range | 20-100 kHz, centered at 40 kHz |
| Temperature Sensors | Up to 6 wireless passive sensors per unit |
| Temperature Range | -40°C to +80°C (ambient); -40°C to +135°C (sensor nodes) |
| Temperature Accuracy | ±1°C |
| Leakage Current Channels | Up to 3 channels |
| Leakage Current Range | 0.1 mA to 10 A |
| Communication | RS485 (Modbus RTU), 4G/LTE, NB-IoT, optional fiber optic |
| Enclosure Rating | IP65 |
| Operating Temperature | -40°C to +80°C |
| Power Supply | AC 85-264V or DC 110-370V; optional CT self-powered |
| Edge Computing | On-board DSP for real-time PD pattern analysis |
Wireless Passive Temperature Sensing
One of the most innovative features of the SCYC device is its passive wireless temperature sensing capability. Traditional temperature monitoring in switchgear requires either wired sensors (impractical for high-voltage environments) or battery-powered wireless sensors (requiring periodic replacement). The SCYC system instead employs electric field space energy harvesting.
Each temperature sensor node is embedded within an insulating plug installed in the cable compartment or busbar compartment of the ring main unit. The sensor contains no battery; instead, it harvests energy from the ambient electric field present within the energized enclosure. This approach offers three transformative benefits:
- Lifetime maintenance-free operation: no battery replacement cycles, no sensor recalibration
- Intrinsic safety: the absence of metallic connections between the sensor and the monitoring unit eliminates any risk of compromising the insulation coordination
- Compact form factor: sensor nodes are small enough to be integrated into standard insulating plugs without affecting equipment ratings
The sensors communicate using 2.4 GHz wireless transmission with a range sufficient to cover the entire RMU enclosure. Each SCYC unit can manage up to six wireless temperature sensors simultaneously, providing comprehensive thermal mapping of critical connection points.
Edge Computing and Data Processing
Raw sensor data is only valuable if it can be transformed into actionable information. The SCYC device incorporates an ARM-based digital signal processor (DSP) that performs real-time analysis at the edge:
- PD pulse counting and amplitude trending: the DSP maintains running statistics on PD pulse rate (pulses per second or per minute) and peak amplitude, generating trend curves that reveal progressive insulation degradation
- Phase-resolved partial discharge (PRPD) pattern generation: by correlating PD events with the power frequency voltage waveform, the device constructs PRPD patterns that are characteristic of specific defect types — internal voids produce symmetric patterns centered on the zero crossings, while surface discharges show asymmetric patterns concentrated near voltage peaks
- Temperature anomaly detection: the DSP compares each sensor’s temperature against configurable thresholds and against the average of all sensors, flagging hot spots that may indicate loose connections, oxidation, or overload conditions
- Leakage current analysis: for cable terminations and surge arresters, leakage current magnitude and harmonic content are monitored; a rising trend in resistive leakage current is a precursor to insulation failure
All data is timestamped with 1 ms resolution using the device’s internal real-time clock, synchronized via NTP over the 4G/LTE or Ethernet connection.
Communication and Integration
Processed monitoring data is transmitted to the central SCADA or asset management system via multiple communication options:
- RS485 (Modbus RTU): for local integration with substation RTUs and PLCs
- 4G/LTE cellular: for remote sites without wired network infrastructure
- NB-IoT: for urban distribution sites where cellular coverage is reliable and low power consumption is beneficial
- Fiber optic Ethernet: for EMC-critical substations where copper cabling is undesirable
Deployment Best Practices
Sensor Placement Strategy
Effective PD monitoring begins with strategic sensor placement. For a typical 10kV ring main unit with three or four functional units (two incoming, one or two outgoing/transformer feeders), the following configuration is recommended:
- TEV sensors: one sensor per functional unit panel, placed on the front surface at approximately mid-height, avoiding panel edges and viewing windows that may produce spurious signals
- Ultrasonic sensors: one sensor mounted internally within each cable compartment, where PD from cable terminations is most likely to occur. For externally mounted sensors, the area around bushing plates and cable entry points should be scanned
- Temperature sensors: one sensor per cable termination (three per cable compartment for three-phase systems), and additional sensors on busbar joints if accessible
Threshold Configuration
Alarm thresholds should be established based on a baseline survey conducted after installation but before the equipment is placed under significant load. A typical configuration approach:
- Commissioning baseline: record TEV amplitude, ultrasonic amplitude, temperature, and leakage current for 24 hours under normal operating conditions
- Statistical thresholding: set the “alert” threshold at the 95th percentile of baseline values and the “alarm” threshold at the 99th percentile plus a safety margin
- Trend-based alarming: in addition to static thresholds, enable trend-based alarming that triggers when any monitored parameter shows a statistically significant increasing or accelerating trend over configurable time windows (e.g., 7 days, 30 days)
Maintenance Integration
Online monitoring data should be integrated into the utility’s computerized maintenance management system (CMMS) to close the loop from detection to action. When a PD alert is generated:
- The CMMS automatically creates a work order with the device ID, monitored parameter, current value, and trend data attached
- Maintenance planners review the work order and schedule an inspection, potentially using portable PD measurement equipment (handheld TEV/ultrasonic instruments) for detailed diagnosis
- If the inspection confirms active PD, the asset is flagged for repair or replacement during the next scheduled outage window, avoiding an emergency response
This condition-based maintenance approach, enabled by continuous online monitoring, has been shown to reduce switchgear maintenance costs by 30-50% compared to time-based preventive maintenance, while simultaneously improving reliability by catching defects before they become failures.
The Broader Britop Electric Monitoring Ecosystem
The SCYC Integrated Online Monitoring Device is one component of Britop Electric’s comprehensive Smart Power Grid product portfolio. Complementary devices include:
- SCYC-HLJC2304: Integrated Online Monitoring for High-Voltage Cable Sheath Circulating Current and Partial Discharge, designed for 35kV to 220kV cable systems with IP68 stainless steel construction and CT induction self-powering above 25A load current
- DTE2100: Online Monitoring Device for Transformer Core and Clamp Grounding Current, providing ±1% or ±1mA accuracy for early detection of multi-point grounding faults
- DT801: Distributed Online Monitoring System for Zinc Oxide Surge Arresters in 110kV+ substations, measuring full current, resistive current, bus voltage, harmonics, and environmental parameters
- SCYC-CW30: Passive Wireless Temperature Monitoring System specifically designed for ring main unit cable compartments, using the same field-harvesting wireless sensor technology
These devices share a common communication architecture and can be integrated into a unified monitoring platform, providing substation-wide visibility of equipment health.
Conclusion
Partial discharge in MV and HV switchgear is a progressive degradation mechanism that, left undetected, inevitably leads to catastrophic failure. Modern online monitoring solutions, combining TEV and ultrasonic detection with wireless temperature sensing and leakage current measurement, provide the continuous visibility necessary to transition from reactive to predictive maintenance strategies.
Britop Electric’s SCYC Integrated Online Monitoring Device exemplifies this approach. By deploying edge-computing-capable monitoring hardware that performs real-time PD analysis at the device level, utilities and industrial facility operators can detect insulation defects months or years before they would otherwise become apparent, scheduling repairs during planned outages and avoiding the costs and risks of unplanned downtime.
The return on investment for online PD monitoring is compelling: a single avoided switchgear failure at a critical distribution node can pay for the monitoring system many times over, while the intangible benefits of improved safety, regulatory compliance, and customer satisfaction further strengthen the business case.
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