What is online transformer bushing monitoring? Insulation monitoring and local discharge monitoring in detail
Date: May 26, 2026, 2:12:02 p.m.
- Monitoring Objects: Transformer high-voltage bushings—key components that connect the internal windings of a transformer to the external busbars. The insulation condition of the bushings directly affects the safe operation of the transformer, and insulation degradation is the most common failure mode for bushings.
- Monitoring Principles: By collecting the grounding current signal from the end screen of the bushing via a zero-flux current transformer, three key parameters—leakage current, dielectric loss, and capacitance—are extracted. The condition of the bushing insulation is then assessed based on trends in these parameters.
- Dual monitoring dimensions: Insulation monitoring evaluates the overall insulation level of the bushing by measuring changes in dielectric loss and capacitance; partial discharge monitoring detects early-stage insulation defects by capturing partial discharge signals from within the bushing.
- Non-stop installation: The sensor card is mounted on the grounding wire at the end of the bushing and is completely isolated from the high-voltage circuit; installation can be performed without powering down the system.
1. Why is online monitoring of casing necessary?
High-voltage bushings are among the most critical components of a transformer, as they bear the full phase voltage and load current. The insulating structure of a bushing consists of an internal conductive rod, a dielectric core, and an outer porcelain sleeve (or composite insulating jacket). During operation, the dielectric core gradually ages due to electrothermal stress and environmental factors, leading to increased dielectric loss and capacitance drift; in severe cases, this can result in insulation breakdown.
The consequences of bushing failure are extremely serious—a single bushing breakdown can cause a transformer to catch fire or even explode. Traditional bushing inspection relies on periodic preventive testing during power outages, with intervals of several years between tests, leaving changes in insulation condition unmonitored during that time. Online bushing monitoring fills this gap by providing real-time data on bushing insulation status through continuous monitoring.
2. Key Monitoring Parameters
| parameters | hidden meaning | Normal range | Unusual trends |
|---|---|---|---|
| Leakage current | A weak power-frequency current insulated by a sleeve | Tens to hundreds of milliamps (depending on the voltage level) | A continuous increase indicates contamination on the insulation surface or moisture inside |
| Dissipation | Proportion of energy loss in insulating materials under an alternating electric field | Typically less than 0.01 (1%) | An increase indicates internal aging or moisture in the insulation |
| Capacitance | Equivalent capacitance value of the sleeve capacitor core | Close to the factory default value (tens to hundreds of pF) | Significant changes indicate a short circuit or open circuit between capacitance layers |
3. The Difference Between Insulation Monitoring and Partial Discharge Monitoring
3.1 Insulation Monitoring—Assessing Overall Condition
Insulation monitoring focuses on the overall insulation performance of the bushing. By continuously measuring the dielectric loss and capacitance, the degree of insulation aging in the bushing can be assessed. A gradual increase in dielectric loss indicates normal insulation aging, while an accelerated increase in dielectric loss or a sudden shift in capacitance signals a problem with the insulation. Insulation monitoring emphasizes medium- to long-term trends.
3.2 Partial Discharge Monitoring—Identifying Localized Defects
Partial discharge monitoring focuses on detecting whether partial discharges are present inside the bushing. Even when the overall insulation is in good condition, minute air gaps, impurities, or mechanical damage inside the bushing can trigger partial discharges. Partial discharge monitoring captures these early-stage defect signals and focuses on providing short-term fault warnings.
3.3 Integrated Solution
Some bushing monitoring systems integrate both insulation monitoring and partial discharge monitoring functions, providing a comprehensive assessment of bushing condition—from overall to localized—on a single platform. For critical transformers, this integrated solution offers the most comprehensive bushing monitoring coverage.
4. Frequently Asked Questions FAQ
4.1 Q: What is the difference between bushing monitoring and transformer body monitoring?
Answer: Bushing assemblies are components separate from the transformer body, with their own independent insulation structures and failure modes. Chromatography or partial discharge monitoring of transformer oil primarily covers faults within the transformer tank, whereas issues with bushings—such as cracks in porcelain bushings or moisture ingress in capacitive cores—may go completely undetected by the main monitoring system. Therefore, bushings require a dedicated monitoring solution.
4.2 Q: Does an increase in dissipation loss necessarily mean the bushing needs to be replaced?
A: Not necessarily. A gradual increase in dielectric loss is a normal part of the insulation aging process. If the rate of increase remains stable and the absolute value is still within acceptable limits, the equipment can continue to operate. However, if the dielectric loss begins to increase rapidly or shows a significant spike within a short period of time, maintenance or replacement should be scheduled as soon as possible.
4.3 Q: Can the casing monitoring system monitor other equipment as well?
Answer: The technical principle behind bushing monitoring (measuring the grounding current at the end screen to extract insulation parameters) is equally applicable to other capacitive equipment, such as current transformers, voltage transformers, and high-voltage surge arresters. A single system can simultaneously monitor the insulation status of various types of capacitive equipment within a substation.
4.4 Q: Are outdoor conduits significantly affected by environmental factors?
A: It has a significant impact. Rain, contamination, and temperature fluctuations can all affect the surface leakage current and dielectric loss of the bushing. A good monitoring system uses environmental compensation algorithms and trend analysis to distinguish between environmental fluctuations and actual insulation degradation.
4.5 Q: Does installing casing monitoring require any modifications to the casing itself?
A: No. The sensor is installed on the grounding wire of the bushing’s end screen; it does not come into contact with the high-voltage components and does not require any modifications to the bushing itself. However, before installation, you must verify that the grounding method of the bushing’s end screen is suitable for installing the sensor.
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.
Looking for an online monitoring solution for transformer bushings? Contact Innotongda for product information and technical selection support. Service Hotline: 13959168359 (also on WeChat).
