Transformer bushings are very important devices that make it possible to pass electric conductors through a grounded transformer tank without compromising insulation. Incorrect choice of a bushing can cause partial discharge, oil leakage, aging, or failure of a transformer.
This article explains the essential technical points that engineers and procurement personnel must evaluate when selecting an appropriate transformer bushing for their intended use.
1. Rated Voltage and Insulation Level
The rated voltage of the bushing is the first and foremost aspect to consider.
A transformer bushing has to be selected not only based on the system operating voltage but also based on its requirements for insulation coordination, which include:
- Power Frequency Withstand Voltage
- Lightning impulse withstand voltage
- Switching impulse voltage (for higher voltage classes)
These parameters help to prevent flashover or internal insulation failure because bushings must withstand normal operating conditions as well as transient overvoltages produced by lightning or switching. Underrated bushings increase the danger of flashover or internal insulation failure.
2. Rated current and thermal performance
Bushings must be able to handle safely the current rating of the transformer without an excessive rise in temperature. Factors that must be considered in the calculation of the current rating include:
- Continuous operating current
- Short-time current withstanding ability
- Ambient Temperature and Cooling Requirements
Overheating causes the aging of the insulation. The selection of the size of the conductors, the type of contacts, and the heat dissipation properties are critical, particularly for current-carrying transformers.
3. Porcelain vs. Composite Bushings
Insulation material selection is also a very important consideration.
Porcelain bushings are in widespread use owing to the following reasons:
- Excellent Mechanical Strength
- Long service life
- Performance in high voltage applications
- Protection against UV and Aging
They are especially recommended in substations, transformer stations, and in cases where mechanical strength and reliability are most important.
Compared with the composite insulators, composite bushings are lighter. Yet, for weight-sensitive applications such as seismic areas, composite bushings may be preferred. However, issues of aging, surface corona, and maintenance may have to be weighed.
Bushing types vary based on conditions.
In indoor systems, matters like the system size and the required height may be more important.
In outdoor mounting, bushings need to resist:
- Pollution and contamination
- Humidity, rainfall
- Fluctuations in
- UV exposure
In heavily polluted or coastal regions, greater creepage distances and optimized shed profiles may be required to resist surface flashover.
Transformer bushings may be made to accommodate various mounting arrangements, such as:
- Top-mounted
- Side-mounted
- Wall & turret-mounted
Engineers must also consider compatibility with the transformer tank design, the sealing system, or oil or resin insulation interfaces. Incompatible assembly or sealing can cause oil damage or water ingress, both of which can affect insulation performance adversely.
Standards promote consistency, safety, and interchangeability. Some of the commonly used standards include:
- IEC standards for international power equipment use
- EN / DIN standards for European markets
- Technical specifications or requirements, utility-specific
When choosing a bushing, it is important to ensure that it has met the necessary standard of design, testing, and performance as required by the relevant standard for the intended use.
Apart from detailing the product features, experience and quality control expertise of the manufacturer play a significant role. The ideal supplier should be able to provide:
- Stable Ceramic Formula and Firing Procedures
- Ceramic devices
- Strict Dimension and Electrical Testing
- Homogeneous glaze quality and finish
- Traceability and quality documentation
Transformer bushings are long-life devices, and the effect ofTransformer bushingreliability has a direct effect onTransformer availabilityMaintainability.
It is essential to strike a balance between the various requirements when selecting a suitable transformer bushing. Carefully chosen bushing not only aids in safe operation but also helps in extending the life of a transformer. The involvement of a skilled manufacturer, which is experienced and knowledgeable about the system requirements and practical working conditions, plays a crucial part in ensuring the best performance within any power system installation.
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