High-voltage bushings are and will be with us for a long time
An impregnation of RIP bushings
Reliability has always been one of the core demands of the energy market. One such example of reliability can be seen in the exceptional lifetime of power transformers, which often function for 50 years or more. This core demand is also fundamental for high-voltage bushings – critical components of all electrical networks, as their chief role is to prevent flashovers to ground.
Although many in the electric utility industry still regard the bushing as nothing more than a hollow piece of porcelain housing a conductor, the task it performs is quite extraordinary, involving advanced technologies in manufacturing and design with a lifetime exceeding the requirements of its applications.
Sophisticated calculation and design tools, improved material and production technology and broad expertise are a result of over 100 years of experience in bushings, developed and manufactured at ABB in Sweden. As the world’s largest supplier of bushings, ABB’s flagship plant in Ludvika, Sweden, is equipped with state-of-the-art production lines that meet the increased demands associated with increasing voltage levels and reliability needs.
A bushing serves to insulate conductors that are carrying high-voltage current through a grounded enclosure. To accomplish such task safely without a flashover is a challenge, as the dimensions of the bushing are very small compared with the dimensions of the equipment it is connecting. Controlling the stress (ie, electrical voltage, thermal current and mechanical stress) to get the dimensions right is of utmost importance in terms of bushing performance in the field during its lifetime.
HIGH-VOLT AGE CONDENSER BUSHINGS
Condenser bushings facilitate electric stress control through the insertion of cylindrically applied floating equaliser screens made of aluminium. The condenser core in which the screens are located decreases the field gradient, and distributes the field along the length of the insulator, distributing it radially and axially in the condenser core. The screens are located coaxially, to ensure the optimal balance between external flashover and internal puncture strength (ie, the electrical withstand of the condenser core).
For years, condenser bushings have maintained the same basic design. Special paper envelopes the conductor, with metallic electrodes strategically placed inside the wrapping. These control the electric field of the bushing. The cylinder is impregnated with transformer-grade mineral oil or epoxy resin to further increase the electrical withstand; beyond that, it’s possible with only dry paper. The bushing is, therefore, an enclosed apparatus fully separated from its application environment.
Oil-impregnated paper (OIP) and resin-impregnated paper (RIP) are the two main technologies in highvoltage condenser bushings. RIP technology is making a valuable contribution to better overall performance figures. The somewhat differently designed condenser core is vacuum impregnated by a curable epoxy resin to form a solid unit free from oil.
Both systems are designed and manufactured for long lifetimes and trouble-free performance, ensuring low partial discharge readings at well above the nominal voltage and ample margins for thermal runaways and overheating.
The outer insulation can be either ceramic or polymeric. Ceramic insulators have a long history, and will be used for many years to come. However, it is likely that their role will diminish in the foreseeable future as the industry seeks improved insulator performance in order to reduce overall costs and improve safety, seismic withstand and pollution performance, as well as to reduce insulator weights.
Controlling the electrical voltage, thermal current and mechanical stress to get the bushing dimensions right is of utmost importance.
To meet as many requirements as possible, bushings are configurable and are produced for system voltages up to 1100kV AC and 800kV DC – even higher for test purposes. The largest bushing developed and manufactured at the Ludvika plant is an 1800kV AC transformer bushing with a length of 15 metres. The limitation for the plant is not the voltage levels or bushing sizes, but rather the market need, where 1100kV AC and 800kV DC are the highest voltages being used today.
HISTORICAL REVIEW OF BUSHINGS
At the start of the 20th century, bushings were dry-insulated, made of Bakelite (resincoated) paper and aluminium foil, with a flange glued to the condenser core and an insulator made of porcelain. These were suitable for voltages up to 190kV.
Voltages increased to 220kV in the 1930s, and the conductive layers were changed to graphite. The graphite (ie, the semiconductive layer) concept is still used today by some manufacturers. The space between the condenser core and outer insulation is filled with oil and is open to the transformer.
In the 1960s, oil impregnated bushings became the predominant technology – even today they have a market share of more than 80%.
In the 1940s, voltages rose to 400kV and condenser cores impregnated with oil and placed inside an insulating envelope of oil and porcelain had been introduced.
But the early bushings had a high partial discharge and dissipation factor, which increased with rising system voltage. While sufficient for lower voltages, they left little margin for increasing applications. As voltages rose to 765kV in the early 1960s, this old technology was replaced with OIP systems. ABB thus supplied the first 705kV bushings to Hydro Quebec in Canada.
At that time, oil-impregnated bushings became the predominant technology (this is still true today with a present market share of more than 80%). OIP bushings continued to be developed while resin-bounded bushing production was discontinued. Dry bushings, often referred to as RIP bushings, were introduced and have a growing market share.
In the 1970s, development of HVDC bushings for 600kV began, and test installations requiring bushings for 1800kV AC were also under way. Even though both systems were oil -insulated, they were significantly different in design.
A transformer from the early part of last century
RIP bushing development, which began in the 1960s at ABB’s Swiss sister plant, resulted in the production of 420kV bushings in 1989 and 525kV bushings in 1996. The reintroduction of dry technology is the direct result of powerful modern calculation tools as well as progress in material and production technology.
Recent (2006) HVDC transformer and wall bushings for 800kV DC were developed and verified through extensive short- and long-term testing.
The material properties change with time under DC stress and, therefore, long-term testing enables the verification of the DC design before putting a new product into service. Based on experience in design, production processes and the latest achievements in calculation tools development, complex mechanisms like ion migration and time-dependent charge distributions have been taken into consideration. The next step may well be 1000kV DC as a logical continuation of the achieved knowledge during the development of the 800kV DC bushings.
In 2007, ABB designed and delivered AC bushings of up to 1100kV to China, and the long-established level for AC networks was increased to 1000kV AC from 800kV. This relied on the knowledge from the 1800kV designs of the 1970s.
Developing bushings for increasingly higher voltages and complex applications requires a great amount of experience and knowledge, as well as testing facilities far above the rated voltages – all of which are available at ABB today.
TECHNICAL CHALLENGES
Inherent in such complex devices are, of course, technical challenges. The highest voltage for equipment is generally limited by physical dimensions (ie, longer distances have higher electrical withstand). Also at high-voltage levels another complication arises – namely the dielectric heating of the insulation. While the dielectric losses in a properly processed bushing can be neglected at low voltages, they become substantial at highvoltage levels.
A schematic view of condenser core
Insulation material has its lowest dissipation factor at approximately 60 degrees Celsius, and then increases with temperature. The heat has to be dissipated through the insulation and bushing surface. Thus, for each bushing, a limit exists that if exceeded will result in insufficient heat dissipation and consequently an uncontrolled temperature increase. This phenomenon, known as “thermal runaway”, will eventually result in a breakdown.
The dissipation limit is called “thermal stability”. Bushings naturally require full thermal stability at their highest voltage level while considering both the ohmic losses originating from the load current as well as the capacitive losses described earlier. The maximum allowed currents for a specific bushing and the dielectric losses coming from the voltage must, therefore, be judged both together and separately. For a properly designed bushing, the dielectric losses and the rated current are not critical, and low-loss requirements do not contribute to the service life.
RIP technology represents a much larger challenge than OIP technology. This is because oil impregnation under vacuum is a relatively straightforward, and a forgiving process in the sense that oil fills out all parts of the bushing and remains in a liquid phase throughout its entire life. Void-free products must be developed and manufactured with special attention, using the latest design tools.
To fully simulate the very specific manufacturing process, where exothermic reaction, thermo-chemical shrinkage and air circulations take place, it is necessary to include relevant mathematical equations in the theoretical models. Simulationbased modelling provides for cause-and-effect analysis, without which imperfections such as incorrect curing propagation, high temperature gradients, local overheating, high strain and stress (cracks) and shrinkage can occur. Besides numerical simulations of the manufacturing process, significant attention is placed on proper material selection, tailoring RIP materials for manufacturing optimisation while taking into account field performance.
An 800kV DC converter transformer, circa 2008
Both OIP and RIP bushings are complex products which, compared with the products available on a global scale, require high investments in equipment as well as research and development. The complexity rises heavily with increased voltages and currents. But these challenges push ABB to create optimised, state-of-theart bushings for its customers.
QUALITY ASSURANCE
Bushing production processes have seen decades of continuous development, resulting in extremely high yields. This does not, of course, mean that the end of the road has been reached.
ABB is at the cutting-edge in optimising processes to reach higher degrees of reliability and quality. This is evident in the most critical manufacturing steps, such as in the winding of the condenser cores where stateof- the-art machines control the winding and equaliser screen insertion, or during drying and impregnation where the process is controlled and monitored by computers and highly experienced teams working with machines.
Another area that has seen excellent advancement over the years is statistic process control and the automatic checking of process limit values with the implementation of new technology.
A global reporting network is in place, requiring all ABB units to report all major events within 24 hours. This provides a good base for event classifications. Crossfunctional meetings are held to review and prioritise this information and implement the subsequent corrective or preventive actions in design and production.
ABB’s role in bushing production and development is strong, with sites in Brazil, China, India, Russia, South Africa, Switzerland, the US and, particularly, Sweden.
The article is reprinted from ABB Review, 3/2009.
