What is a partial discharge in an insulation system?
A partial discharge (PD) in an insulator system is a localized electrical discharge that occurs when the electric field within the insulation exceeds its dielectric strength in specific areas, without completely bridging the insulating material between conductors. When high voltage is applied to an insulation system, the electric field is generally distributed across the insulating material. If there are weak points in the insulation, such as air voids or contaminants, these areas may experience a much stronger electric field compared to the surrounding insulation. This is because due to low insulation at these locations, the concentration of electric field lines becomes more compared to that of surrounding. As a result, the electric field in these weak areas may exceed the breakdown (dielectric) strength of the insulation medium within the voids, and hence, a partial discharge occurs.
Partial Discharge Phenomenon in GIS
Partial discharge occurs in a gas insulated switchgear system because of dust or metal articles inserted during assembly, or any other impurities present in SF₆ gas or voids or contaminants in solid insulation materials in the gas chambers. This discharge is partial because it does not completely bridge the gap between conductors or from a conductor to the metal enclosure. During the process, not only in GIS but also in all similar electrical systems, two main things happen during partial discharge.
Rapid release of energy: The discharge creates a short, intense burst of current in the defect area.
Electromagnetic wave generation: The current pulse generates a transient electromagnetic wave, radiating across a broad frequency spectrum, with significant energy in the UHF range (300 MHz to 3 GHz).
Propagation of Electromagnetic Waves in a GIS
The metallic enclosure of the GIS acts as a waveguide, which influences the propagation of UHF electromagnetic waves:
- Waveguide effect: The GIS enclosure contains and directs the UHF signals along its length, ensuring minimal attenuation over distances.
- Internal reflections: The smooth internal surface of the metallic GIS structure allows the waves to reflect and propagate efficiently.
- Attenuation: Some attenuation occurs due to losses in the enclosure and gas medium, but the UHF range suffers less from attenuation compared to lower frequencies.
The propagated UHF electromagnetic spectra are sensed by UHF sensors fitted within the GIS.
UHF Sensor Principles
UHF sensors are specially designed to detect the electromagnetic waves generated by PD. UHF sensors mainly use capacitive coupling to detect the transient signals i.e. electromagnetic waves generated by PD. The electromagnetic wave induces a voltage signal in the sensor, which is proportional to the strength of the UHF wave. The captured signal is weak and needs amplification for further analysis. Low-noise amplifiers are used to boost the UHF signal while preserving its characteristics.
The sensor is either:
- External: UHF sensors are mounted on the GIS enclosure, capturing signals through dielectric windows or couplers at strategic locations.
- Internal: Installed inside the GIS during manufacturing to directly sense PD activity.
PD Signal Analysis
The captured UHF signals undergo several steps of processing and analysis.
Filtering
- Noise filtering: External noise or interference is removed by filtering the signals to isolate the UHF band (typically 300 MHz to 3 GHz).
- Focus on PD frequency range: Signal processing targets the specific frequency ranges that are characteristic of PD activity.
Characterization
- Parameters such as amplitude, pulse width, repetition rate, and frequency spectrum are extracted to describe the PD activity.
Localization
- Multiple UHF sensors are used to determine the location of the PD source by:
- Time-of-arrival difference: Measuring the time difference between signals arriving at various sensors.
- Triangulation: Calculating the PD source location using the geometry of the sensor placement.
Advantages of UHF Detection in GIS
- Non-Intrusive: Sensors can be placed externally or can be installed in dedicated dielectric windows on the GIS enclosures, allowing online monitoring without disrupting operation.
- Sensitivity: UHF detection can pick up even low-energy PD events.
- Minimal Noise Interference: The UHF frequency range is less prone to interference from power-frequency noise, corona discharges, or other low-frequency phenomena.
- Wide Coverage: The waveguide effect in GIS enables UHF signals to propagate long distances, allowing effective monitoring with fewer sensors.
Examples of Online Partial Discharge Measurement of GIS
For instance, in Siemens or ABB 220kV GIS systems:
- UHF sensors are installed at cable terminations, busbar joints, and circuit breaker compartments.
- Dielectric windows allow the UHF sensors to pick up signals without compromising the gas-tight integrity of the GIS enclosure.
- Signals are analyzed using advanced digital signal processing techniques to identify and locate PD sources, enabling predictive maintenance.
Conclusion
The detection of electromagnetic waves generated by PD in GIS relies on the waveguide effect of the metallic enclosure, capacitive coupling of UHF sensors, and advanced signal processing. This approach enables early fault detection, helping maintain the reliability and safety of high-voltage systems while reducing the risk of catastrophic failures.