NEXT GENERATION BUSHINGS
When dealing with what appears to be proprietary or protected information related to newer technologies, it is hard to separate the company from the technology and provide valuable, leading edge information. In this article, Robert and Eric have been able to walk that fine line between promoting their unique bushing solution and providing leading edge information that every transformer professional will value. Well done Robert and Eric.
A Short Bushing History
To better understand the latest developments in bushing technologies it would certainly be helpful to readers not fully familiar with this segment of the power industry to understand how we got there.
HV AC bushings appeared in the late 19th century with the first AC transformers. Those first bushings were bulk bushings, meaning the insulation relied on the thickness of a solid material placed around the high voltage conductor. As voltages increased, the diameter of those bushings became quickly prohibitive.
About a century ago the capacitance graded insulation structure was introduced. It inserts a succession of capacitive layers within the core insulation, providing a more linear and controlled electric field distribution. A key benefit of this technology was the significant reduction of the bushing’s diameter. It has been the core design concept for HV bushings ever since.
Still, from this same concept, differences between the successive bushing technologies are directly related to the type of materials and fillers used to create the capacitive structure.
The most used over the past century has been the OIP (Oil Insulated Paper) insulation, where capacitive layers are separated by a special paper and immersed in insulating oil.
To try to mitigate the challenge of managing large volumes of oil, from leaks to potentially dramatic failures, the RIP (Resin Impregnated Paper) insulation was introduced in the mid-20th century. This technology substituted a resin for the insulating oil. Leaks were eliminated, but the presence of paper still presented a potential hazard if stored incorrectly or operating in high humidity conditions. But it was considered a “dry type” bushing.
The next generation of bushings then had to get rid of paper to fully deliver the larger benefits expected from a dry type bushing: safety, reliability and maintenance-free operation.
The innovative RIF bushings were introduced in 2003 and became the first entirely paperless dry type bushings in the industry. They pioneered what later became the RIS (Resin Impregnated Synthetics) category.
The innovative RIF bushings were introduced in 2003 and became the first entirely paperless dry type bushings in the industry. To date there are over 22,000 units in service worldwide.
RIF bushings deliver a unique set of features that set them apart; namely, their overall performance for high reliability, the option of integrating monitoring into their structure and the possibility to provide custom designs for replacement needs at standard costs.
RIF Bushing Technology
RIF transformer bushings were introduced to the market in 2003 and to date there are over 22,000 units in service worldwide. These bushings have proven to be ultra-reliable under all types of operating and environmental conditions.
The RIF bushing utilizes a finely graded condenser design and a core insulation that is composed of fiberglass impregnated with epoxy resin wrapped between capacitive screens. The outer insulation for the RIF bushing is silicone rubber sheds that are adhered directly to the condenser core. This ensures there is no gap or opening in the overall bushing structure and eliminates the need for filler fluid or material.
The electrical field is controlled by a finely graded capacitive core which ensures a linear surface potential profile from the conductor to the grounded flange (100% to 0%) which greatly improves its flashover resistance. Additionally, the thermal insulation strength of the resin-impregnated fiberglass is IEC Class B (temperature limit rating of 130°C), which gives the RIF bushing a larger thermal margin than other bushing types. The simpler manufacturing process, which is primarily a wrapping and heat curing process, introduces minimum internal stresses in the capacitive core that can affect the long-term operational life of the bushing. Finally, RIF bushings require no special storage conditions, thereby reducing the handling costs.
In summary the RIF bushing technology has introduced the next level of reliability and safety that operators need for their transformer assets.
The Smart RIF Bushing
RIF bushings can be provided with built-in smart measuring circuitry that continuously monitors the bushing’s core insulation condition. Sealed within the primary core is a built-in signaling capacitance to collect and measure stray capacitive current generated by a damaged condenser screen layer.
RIF bushings deliver a unique set of features that set them apart: their overall performance for high reliability, the option of integrating monitoring into their structure and the possibility to provide custom designs for replacement needs at standard costs.
A self-powered LED sensor collects and processes the signal which is compared to a benchmark voltage. The sensor is factory-calibrated to provide a GREEN LED indication for a normal insulation condition and RED LED pre-alarm indication for a deteriorating insulation condition.
Figure 1. Smart measurement terminal and LED sensor for core insulation condition monitoring
The RIF bushing’s built-in sensing circuitry can also be used to capture high frequency signals generated from partial discharge activity inside the power transformer by installing a PD sensor in the smart measurement terminal. This monitoring can be continuous without any need to shut down the transformer.
If the transformer is equipped with all smart RIF bushings, the location of the PD within the transformer can be accurately determined. The use of the RIF bushing as a PD sensing device dramatically simplifies transformer monitoring while providing high PD monitoring reliability and accuracy with its direct coupling and shielding design (Figures 2 and 3).
This can provide significant added value for critical system transformers.
RIF bushing technology has introduced the next level of reliability and safety that operators need for their transformer assets.
Figure 2. Signal coupling (left) Figure 3. RIF bushing with smart terminal and PD sensor (right )
Custom-designed “Like for Like” Replacement Bushings: The Strength of the RIF Bushing Technology
OIP and RIP type bushings have been the industry standard for transformers for many years and in general have performed satisfactorily. However, as transformer assets age, paper deterioration in these bushings can eventually cause the bushings to fail, some even catastrophically. If you are seeing deteriorating test results (power factor) and sealing systems, it may be time to consider a bushing replacement program. When considering a replacement program, it is important to remember the age of your operating transformer inventory and the standards that the originally supplied bushings were built to, which may be obsolete today. Because of the age of many transformers, the biggest challenge for the program is to be able to get custom-engineered “like for like” replacement bushings at reasonable prices and lead times. Also, to alleviate safety and environmental concerns dry type bushings should be specified for your replacement program.
The very simple manufacturing processes involved in the production of RIF bushings allows for cost effective custom-engineered designs with minimal effect on lead times. Transformer outages can therefore be scheduled without the worry of insecure supply and long delivery times.
The value-added benefits of RIF bushings over other bushing technologies are shown in the table below:
In summary, the RIF bushing technology provides the safety and environmental benefits that customers want to see in transformer bushings. This technology offers the customer an explosion-resistant design, a proven record for reliability under all types of extreme operating conditions and a maintenance-free bushing. It is particularly noteworthy that long term operating experience has shown that the dissipation factor and partial discharge level remain stable over its lifetime.
Further to those performance gains, the optional built-in monitoring features increase significantly the value that RIF bushings can bring to grid operators at a reduced cost when compared to existing solutions.
Last, RIF bushings are available in custom sizes that allow economical replacement of older difficult-to-procure bushings while improving the transformer’s safety and reducing maintenance costs.
Robert Middleton