Condenser bushings are devices which allow high voltage conductor to pass through grounded walls of power transformer. Condenser bushings should meet the electrical, thermal and mechanical requirements of the application which is taken into consideration. Insulation against internal breakdown and external flashover must be provided and bushings should operate even if exposed to periodic over voltages and contaminated conditions to avoid severe catastrophic failure of substation as shown in figure 1.
The basic construction of condenser bushings has remained the same from 15 kV to 800 kV. Majority of bushings installed on power transformer have oil impregnated condenser core with porcelain serving as external insulator.
In condenser bushings, there are three main insulation systems used around the world which are as follows:
• Resin Bonded Paper (RBP) Bushings
• Oil Impregnated Paper (OIP) Bushings
• Resin Impregnated Paper (RIP) Bushings
Previously, dry insulation without oil and resin bonded bushings were used. Now, oil impregnated bushings are used throughout the world with resin impregnated bushings being the future market needs. These are described briefly in following sections.
Resin Bonded Paper Bushings
These bushings were started in 1940s from 15 kV till 72.5 kV. Resin coated kraft paper was used as insulation paper; hence called resin bonded bushings. The winding machine would itself provide the heat to activate the bond for the resin while the insulation paper was wound with aluminium foil inserts on the aluminium conductor. These inserts served as voltage grading layers for the condenser bushings. After the machining process, bushings were lathe, oven cured and then dipped into varnish. The lower side did not have a porcelain insulator. It was only provided for the upper air end portion of condenser bushings.
These bushings had several concerns and hence they were only manufactured till 1980s. If these bushings were not properly stored then moisture contamination would take place in the lower part which was not covered with external insulator as shown in the figure 2. Over the years, the varnish layer would delaminate and the transformer oil would seep into the insulation paper. Uneven redistribution of electrical stresses would take place and as all the layers were not affected by the same; this would ultimately lead to change in the capacitance of bushing.
Inspite of these concerns in the bushing, these still found a ready market due to competitive price. These bushings were only used for lower voltages as the risk of thermal instability and the dielectric losses of insulation paper would lead to thermal runway.
Oil Impregnated Paper Bushings
This type of insulation philosophy has become a common technology in the substation transformer bushings. At least three out of every four bushings installed around the world have an oil impregnated paper design and the percentage would increase in the countries like China and USA. Figure 3 shows us the schematic diagram of OIP condenser bushings used over the years.
Keeping current advancement in power system components, this long existing technology is fact of no small achievement. Due to the variety of small refinements by the manufacturers, there has always been a strong preference for OIP design by different power utilities. Hence, it has always been an attractive propaganda with ever changing market needs and demand.
The manufacturers always wanted to standardise the high creepage distance in the OIP design. Their goal was to achieve the same bushing design to be used across variety of bushings installed which in turn will reduce the different styles end users hold. There has been continuous effort by the suppliers over a decade or so which has in fact streamlined their further orders and inventorying of porcelain which ultimately leads to reduction in cost and production lead time.
Manufacturers over the years have also carried out design changes to make slimmer profiles by the reducing the diameters of porcelain housings which will also make them lighter. These changes have led to easy handling of bushings during installation and maintenance practices. Reduction in weight due to smaller dimensions is directly proportional to the reduction in purchase price. There will also be reduction in volume of oil which will progressively reduce the risk of oil leaks and fires among the end user’s assets.
Some manufacturers have made additional improvements like better sealing against the leaks; designs suitable for vertical as well as horizontal mounting; ease of visual monitoring of the oil level of condenser bushings; ease for interchangeability between power transformer bushings and other switchgear applications like circuit breakers; better contact between the top terminal and the incoming conductor even if the contact thread out over a period of time. Refinements have also been developed which gives these bushings to change quickly without hampering the operation of the affected device.
One question is still unanswered in the interest of thousands of condenser bushings installed all around the world – How intelligent our bushing designs should be? Especially, in internal monitoring of oil levels and other such crucial parameters. The bushing industry feels that there should be further development in the areas of monitoring systems of bushings where there has been a growing interest in recent years.
Given the fact of such continue fine-tuning in the OIP bushing technology, one wonders whether there is anything left to enhance the functionality or performance of the asset. We can say that this style of bushing today has reached the stage of maturity with very little room for any development. So, finally, one can fairly say that OIP bushings ‘Once built like battleships’, everything possible has been done to make them neater and as small as possible.
Resin Impregnated Paper Bushings
Compared to the previous RBP bushings, significant changes have taken place to introduce a new and successful technology where insulation paper will be impregnated with epoxy resin which will give us dry and void-free bushings. Same like the OIP bushing manufacturers, the suppliers of RIP bushings have also carried out some refinements to seek the heavy cost that utilities will be paying to them. Most important development is the use of silicone rubber as external insulator in the condenser bushings which can be seen in figure 4. Some say that it is the ultimate design evolution one has seen in the bushing history to increase performance levels.
There will be no doubt that in the future; market acceptance of silicone rubber insulators with RIP cores will be greater mainly due to safety and environment concerns. European countries like Germany, Austria and Switzerland have already started to incorporate RIP bushings in their fleet. The motivation behind such transition has factors such as reduced risk to human life, better performance which is invulnerable to pollution, faster lead time in production process, reduction in weight and easier handling. There are many examples of porcelain housing catching flashover under wetting which will not be happening in the case of silicone rubber. However, overpowering the OIP bushings in terms of cost will always remain the biggest challenge for the RIP bushing especially, below the 245 kV level where the business volume is focused. A comparison between the OIP bushings and the RIP bushings is shown in Table 1.
Higher the voltage level, higher will be the volume of material; which will ultimately lead to internal stresses and formation of cracks. Great care has to be taken further in the curing cycle of RIP bushings especially as we rise to higher and higher voltages in the future. Among the more noteworthy developments has been the molding of fibre-reinforced plastic tube directly on to the epoxy core. The direct molding eliminates the use of dielectric materials which fills in the area between the epoxy tube and RIP core. The main advantage of doing so is the potential savings assured up to 5 percent of total cost of the bushing due to direct mold instead of stand-alone cores.
With these large in-house processes which requires a large investments, there are other technical challenges too at higher voltage levels related to it. Problems related to interfacing occur if the chemical bond between the core and silicone is improper. Initially, these bushings might have high dissipation factor as silicone material is directly molded on to the RIP core. Experts say that vapor must have been absorbed by the resin body before the energisation.
Whatever will be the improvements in the future of RIP bushings, engineers will be always striving for better grading of conductive layers with smaller diameters. To increase the robustness of production processes, there will be a need of greater process control which will give us more consistent product which can be controlled and repeated easily every time.
Finally, in the end, the customer selection cannot only be decided by just comparing OIP and RIP bushings. Given the known fact that huge efforts will go in commercial considerations in the future, suppliers of condenser bushings who provide the most features irrespective of the advance technology will be in great success. Features such as interchangeability between related applications, seismic and high altitude capabilities, higher creepage distance and more cantilever strength are more likely to be added in near future.
If not in the first glance but bushing technology has come a far way which looks quite impressive. They look very much like the old designs from the past but upon closer examination, one can say that there have been fine improvements and subtle refinements considering the scale, cost, performance and functionality of the power asset. The ongoing efforts to reduce the cost, lead time of production processes and standardize the bushing to reduce the need of end user to stock up many different styles will always be serving as driving forces in the bushing industry. As commercial considerations will be playing a huge impact on the purchase decisions in the future, the bushing suppliers who offer the most features irrespective of the price will be in great success. Moreover, the engineers have to seek more design optimizations wherever possible due to the internal competition in OIP and RIP styles. Finally, we conclude that, RIP bushings which are a 21st century product might also last the 22nd century in future.
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