There are many PD detection methods to detecting partial discharge. This article will introduce the partial discharge detection methods in detail.
Electrical Measurement
This is the most widely used and most standardized test method. When partial discharge occurs, a high-frequency short-pulse current signal is released in the insulating medium and propagates along the cable, grounding system or electrical loop. The pulse signal is collected through a high-frequency current transformer (HFCT) or a coupling capacitor, and then the detection instrument analyzes the parameters such as amplitude, phase, waveform and discharge number.
Features:
- High sensitivity
- Signal amplitude is intuitive and easy to quantify
- Suitable for long-term online monitoring and offline detection
- Susceptible to on-site electromagnetic interference, requiring good grounding and filtering
Applications: Transformers, cables, GIS, switchgear, etc.
Ultrasonic/Acoustic
Partial discharge is accompanied by local gas discharge or material collapse, which will generate ultrasonic signals (generally in the range of 20kHz-300kHz). A piezoelectric ultrasonic sensor is attached to the device housing or insulator surface to capture the discharge sound waves, and then the signal is amplified, filtered, and analyzed to determine whether PD exists.
Features:
- Non-contact, online detection
- Suitable for closed equipment such as GIS, transformers, ring network cabinets, etc.
- Strong anti-interference ability to complex electromagnetic environment
- Need to locate the sound source and eliminate the interference of environmental noise
Applications: Transformers, GIS, switchgear, bus ducts, disconnectors.
Transient Earth Voltage Method (TEV)
When partial discharge occurs in metal enclosure equipment, it will induce high-frequency transient voltage signals in the shell or ground loop. TEV sensors can capture discharge transient pulses by contacting or sticking to metal shells without destroying the device structure, and then analyze the discharge intensity and phase.
Features:
- Moderate sensitivity
- No need to shut down the equipment or open the cover
- Easy to deploy, suitable for quick inspection
- Only applicable to discharge detection inside metal-enclosed equipment housing
Applications: Medium and high voltage switchgear, bus duct, substation transformers, etc.
Ultra-high frequency method (UHF)
Partial discharge will release ultra-high frequency electromagnetic waves of 300MHz~3GHz. The UHF method uses an ultra-high frequency antenna or probe to capture the signal and analyze the discharge activity through a spectrum analyzer or partial discharge monitor. Because UHF signals attenuate slowly and have strong anti-interference capabilities, they are suitable for environments with strong electromagnetic interference.
Features:
- Strong anti-interference and low false alarm rate
- Suitable for GIS, sealed equipment
- Support multi-channel, remote online monitoring
- Sensitive to housing material and mounting point location
Applications: GIS, transformer tanks, metal enclosed busbars.
High Frequency Current Transformer Method (HFCT)
The HFCT method is suitable for detecting high-frequency current pulses generated by partial discharge. The open-type HFCT transformer can be clamped on the equipment grounding wire or cable shielding layer to monitor the high-frequency signal in the grounding path or cable shielding layer in real time to determine whether there is discharge inside the equipment.
Features:
- Easy installation, no need to cut off power
- High sensitivity, obvious signal pulse characteristics
- Suitable for grounding systems such as cables, transformers, GIS, etc.
Applications: High voltage cables, transformer bushings, GIS partial discharge monitoring.
Coupling Capacitor
Often used in high-voltage test sites or long-term online monitoring. The PD signal is coupled to the detection system by connecting a coupling capacitor in parallel on the high-voltage side. This method can effectively separate the power frequency voltage and the PD high-frequency signal with high detection accuracy.
Features:
- High sensitivity and stable signal extraction
- Suitable for large equipment field testing and fixed monitoring
- The installation environment is demanding and the cost is relatively high
Applications: Long-term monitoring of high-voltage transformers, generators, and GIS.
Dissolved Gas Analysis (DGA)
Mainly used for oil-immersed transformers. When partial discharge occurs, it will cause the gas in the oil to decompose and produce H₂, CH₄, C₂H₂, C₂H₄, etc. By taking oil samples regularly or using online monitoring equipment, the changes in trace gas concentration in the oil can be analyzed to determine the type and development trend of partial discharge.
Features:
- Suitable for long-term degradation monitoring of oil-immersed equipment
- Can be combined with partial discharge signal to verify accuracy
- Slow response, suitable for trend analysis
Application: Large transformers, on-load switches.
Conclusion
Various partial discharge detection methods have their own advantages and disadvantages depending on the detection signal medium, transmission characteristics and site environment. The electrical method and HFCT have high sensitivity, the ultrasonic method is suitable for complex environments, the TEV method is suitable for metal-enclosed equipment, the UHF method performs well in high electromagnetic interference environments, the coupling capacitor method is suitable for high-voltage test occasions, and the DGA method is suitable for long-term monitoring of oil-immersed equipment.
