Transformer bushing online monitoring FAQ: from principle to operation and maintenance one-stop answer

• Need for casing monitoring: the casing is the transformer high-voltage side of the road to connect with the outside, its insulation state is directly related to the operation of the transformer safety, casing failure can lead to serious consequences!
• Core value of online monitoringFilling the time blindness between regular preventive tests, real-time control of casing insulation status, and timely detection of early deterioration trends.
• Dielectric loss and capacitanceThese two parameters are the core indicators for evaluating the insulation condition of the casing, with increased dielectric loss representing aging or moisture, and capacitance drift representing capacitive interlayer anomalies.

1. Basic cognitive categories

1.1 Q: What is the difference between on-line casing monitoring and outage pre-testing?

A: Outage pre-testing is done every few years and provides comprehensive and accurate data but with long lead times. Online monitoring runs uninterruptedly every day and provides continuous trend data to detect changes in insulation status between two pre-tests. The two complement each other - pre-testing for accurate assessment and online for daily monitoring.

1.2 Q. What problems can be detected by casing monitoring?

A: It can detect common defects such as moisture in casing insulation, aging of capacitor cores, increased surface leakage due to cracks in the porcelain sleeve, and internal partial discharges. Judge the overall health status of casing insulation by the trend change of dielectric loss and capacitance.

1.3 Q. Do all transformers need to be fitted with bushing monitoring?

A: Full coverage is not required. Priority is given to deployment on 220kV and above main transformers, older bushings that have been in operation for more than 15 years, and bushings with a history of insulation anomalies.

2. Installation and utilization classes

2.1 Q. Where are the sensors for casing monitoring installed?

A: Installed on the grounding lead of the casing end screen. The casing end screen is the lead terminal for the outermost pole plate of the capacitor core, which is grounded through the ground wire during normal operation. The sensor is clamped to this ground wire and measures the weak current flowing through it.

2.2 Q. Can different casing types be fitted?

A: Oil-paper capacitive casing, adhesive paper capacitive casing and other casing with capacitive core structure can be installed. Pure porcelain casing cannot use the scheme based on end-screen current measurement because it has no capacitor core and end-screen structure.

2.3 Q. What do I need to pay attention to during installation?

A: Core precautions: confirm the casing end screen grounding method before installation; the installation process shall not disconnect the end screen grounding wire (open circuit will make the end screen potential rise); the sensor card mounted tightly to avoid loosening; terminals to do rust-proof treatment.

3. Data interpretation category

3.1 Q. Is the dielectric loss value significantly affected by ambient temperature?

A: It has an effect. Dielectric loss usually increases slightly when the temperature rises, which is a normal physical phenomenon. When judging, you should pay attention to the value of dielectric loss after temperature rise compensation, or compare the historical data under the same temperature conditions, and cannot directly equate the increase in dielectric loss during the high temperature period in summer with insulation deterioration.

3.2 Q: Do I need to worry about a sudden change in electrical capacity of a few pF?

A: It needs attention but does not necessarily mean failure. Small changes in capacitance can be temperature effects or measurement errors. A sudden, large drop in capacitance could mean a short circuit between the capacitor layers - one or more layers of the capacitor core are shorted out by breakdown, which is a sign of a serious failure.

3.3 Q. Can the monitoring data for the three cannulas be compared?

A: Yes. The same transformer three-phase bushing type and operating conditions are the same, under normal circumstances, three-phase data should be basically the same. If the data of one of the bushings deviates significantly from the other two phases, the focus should be on the bushings.

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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