What is transformer partial discharge online monitoring? Partial discharge principle and detection method details

• Definition of localizationPartial discharge is a discharge phenomenon that occurs in a localized area of the transformer insulation system, but not throughout the entire insulation medium. It is the early sign of insulation degradation, if left unchecked will continue to develop until the insulation breaks down!
• Online monitoring methodTransformer partial discharge online monitoring system through the installation of the transformer body on the sensor, 24 hours a day to capture the local discharge generated ultrasonic, high-frequency current and ultra-high-frequency electromagnetic wave signals, to achieve non-intrusive real-time monitoring.
• Multi-sensor fusionA single sensor can only capture partial frequency band of localized emission signal, modern localized emission monitoring system generally adopts ultrasonic + high-frequency current + ultra-high-frequency three-in-one sensor combination, covering a wider detection frequency band.
• diagnostic mapping: The system converts PD signals into 3D PRPD (Phase Resolved Partial Discharge Profile) and PRPS (Phase Resolved Pulse Sequence Profile), and identifies the type and severity of the discharge through the features of the profiles.
• Non-stop installationNon-intrusive installation of sensors - ultrasonic sensors are mounted on the outer wall of the transformer tank, and high frequency current sensors are mounted on the grounding wire, eliminating the need for power outages throughout the process.

1. Mechanisms of localized discharges

The insulation system of a transformer consists of a variety of materials such as insulating oil, insulating paper and insulating cardboard. In the manufacturing process may remain tiny air gaps or impurities, in the operation of the insulating material will also gradually aging defects. When the electric field strength in these defective areas exceeds the local breakdown field strength, partial discharge occurs.

Although the energy of partial discharges is much smaller than that of complete breakdown, each discharge causes small irreversible damage to the insulation material. Accumulated over a long period of time, these small damages gradually expand and eventually lead to complete failure of the insulation system. This process can last from months to years, leaving plenty of early warning window for online monitoring.

2. Three core testing methods

3. Workflow for localized release monitoring

3.1 Signal Acquisition

The sensor continuously receives the local discharge signal generated inside the transformer. The system supports multi-channel simultaneous acquisition (typically 4 or 6 channels), and the high-speed acquisition card digitizes the signal at a high sampling rate to ensure that the complete PD pulse waveform is captured.

3.2 Signal Separation and Filtering

There is a large amount of electromagnetic interference in the substation environment - cell phone signals, walkie-talkies, transients from switching operations, etc. may generate responses on the sensors. The system separates the real local discharge signal from the environmental interference through digital filtering and pattern recognition algorithms, which is one of the core technical difficulties of the local line monitoring system.

3.3 Graph generation and diagnosis

The separated PD signals are counted in three dimensions: phase, amplitude and repetition rate to generate a three-dimensional PRPD map. Different types of discharges (internal discharges, surface discharges, corona discharges, suspended discharges) have different distributions of features on the PRPD map, and the diagnostic algorithm determines the type of discharge accordingly.

3.4 Trend tracking and alerts

The system continuously tracks the trend changes in discharge amplitude, discharge frequency and discharge type, and automatically pushes alerts to notify operation and maintenance personnel when the indicators exceed the set thresholds or when the trend is abnormally accelerated.

4. Value of localized monitoring

Partial discharges are the earliest detectable signal of a transformer insulation failure. Whereas oil chromatography sees the chemical products of a fault, and temperature monitoring sees the thermal effects of a fault, local discharge monitoring sees the discharge event itself directly - it is the earliest signal on the timeline.

For transformers where insulation problems are a major risk (e.g., older transformers, transformers subjected to frequent overvoltage shocks), local discharge monitoring provides the most sensitive early warning. Discharges can be quite severe by the time acetylene is detected by oil chromatography, and local discharge monitoring can capture signals at the budding stage of discharges.

5. Frequently Asked Questions FAQ

5.1 Q: Which is more important, local discharge monitoring or oil chromatography monitoring?

A: The two cover different fault dimensions and are not interchangeable. Local Discharge looks directly at the discharge event, while Oil Chromatography looks at the gas products generated by the fault. For discharge faults caused by insulation defects, local discharge is found earlier; for overheating faults, oil chromatography is more effective. The ideal solution is to use both together.

5.2 Q. Is there a high rate of false alarms in local-radio monitoring?

A: Early local discharge monitoring systems do have the problem of high false alarm rate, mainly due to the complex electromagnetic environment and many interference signals at the site. With the development of digital filtering and pattern recognition technology, the interference suppression capability of modern systems has been greatly improved. Multi-sensor cross-validation also helps to reduce the false alarm rate.

5.3 Q. Must all three tests be used at the same time?

A: Not necessarily. The two-sensor combination of ultrasonic + high-frequency current has been able to meet the needs of most scenarios. The UHF method has stronger anti-interference capability but is more complex to install, and is usually used for critical transformers or sites with particularly complex electromagnetic environments. The sensor combination can be selected according to the actual needs of the transformer and site conditions.

5.4 Q: Does the installation of a local discharge sensor require a transformer outage?

A: Ultrasonic sensors and high-frequency current sensors can be installed without power failure. Ultrasonic sensor directly mounted on the tank wall, high-frequency current sensor can be mounted on the grounding wire. Special high-frequency built-in sensors need to be pre-installed in the manufacture of the transformer or installed during power outage maintenance.

5.5 Q: How do you look at a local discharge map? How do you determine which discharge is present?

A: The horizontal axis of the PRPD diagram is the phase of the industrial frequency, the vertical axis is the discharge amplitude, and the color represents the frequency of the discharge. Internal discharges usually appear in the first and third quadrants of the phase, discharges along the surface are more widely distributed, and corona discharges are concentrated near the voltage peak. The diagnostic software automatically labels the type of discharge, and O&M personnel can also make empirical judgments through the morphology of the plots.

6. Recommendations for selection

6.1 220kV and above main transformer is recommended to configure ultrasonic + high frequency current + UHF three-in-one program.

6.2 Ultrasonic single sensor solutions are available for general distribution transformers, which are less costly and easiest to install.

6.3 Choose a system with multi-sensor fusion diagnostic capabilities rather than simply displaying multiple sensor data side by side.

Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements. The technical data and cases involved in this article are from public information and engineering practice, if updated without notice.

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