Power Transformer Terminations:
Bushings, Cable boxes and Cable sealing end chambers provides the means by which power transformers or reactors are connected to the power supply system without from any high voltage connection to the earthed tank. These devices in the power transformer also prevent the ingress of water into the interior. Connections to the power cables are made in the cable boxes or cable sealing end chambers and to overhead lines by open type bushings. Bushings are also used inside the trunking of metalclad switchgear for connection between power transformers and switchgear terminations.
Bushings in Power Transformers:
The simplest bushings construction used for voltage up to 33kV consists of highly gazed porcelain insulators with a conductor through the centre. The external porcelain section comprises of number of rain sheds to provide creepage and weather protection. The porcelain must be fully vitrified and free from voids when ionization is initiated, thus leading to eventual breakdown of the insulator.
For the voltage exceeding 33kV, oil /air transformer bushings are usually of the oil-filled condenser type. The oil between the core and the porcelain weatherproof shell being contained separately from that in the transformer tank. The core of such bushings consists of metallic tube wound with layers of resin bonded or oil impregnated paper, interleaved with layers of metal foil or paper coated with a conducting material. The capacitance between the foil is designed to give a uniformly graded voltage between the line and earth by variation in the radial spacing of the foils to compensate for their change in length. Oil impregnated cores are enclosed in a two-part shell: the upper part is a weatherproof shedded porcelain, and the lower part is of either plain porcelain or cast resin.
With oil filled bushings for connection to overhead lines, the oil expansion vessel is supported at the top of the extremal porcelain complete with an oil level gauge. The vessel not only allows for expansion of oil but also provides space for higher operating temperatures kept within acceptable limits. It is essential, particularly in the case of oil impregnated paper bushings that they are adequately sealed against ingress of moisture.
As the external connection palm is on the removable top cover, the connection between the top cover and the temperature lead or bus conductor to which the winding is connected must be flexible. It must be capable of withstanding the mechanical vibrations and of carrying the maximum rated current of the transformer, including any requirements for loading beyond name plate rating
Provision is usually made for the accommodation of torodially wound current transformer by a earth band at the oil immersed end of high voltage bushings. The arrangement being such that the bushings can be removed without disturbing the separately mounted current transformers
The original oil impregnated paper type high voltage bushings for connection to SF6 insulated busbar should have pressure gauges and switches to ensure against overpressure of the bushing oil due to ingress of gas from the busbar chambers. These have now been replaced by epoxy-resin impregnated paper (ERIP) types on which such facilities are unnecessary
Low voltage high current bushings have to be used on very large generating transformers for connecting to the generator busbar. Such connections are usually isolated phase ducting. These bushings comprise the main conductor in a porcelain shell. The oil in the transformer is allowed to circulate through the bushings., thus because of the improved cooling, a high current density can be used for bushing conductor compared with that used for sealed oil bushings.
Cable Boxes and Sealing end Chamber:
The design of the cable boxes depends on the voltage and type of the cables used. For many years, solid paper insulated and armoured cables have been used up to 33kV. These have compound filled boxes and plumbed glands, but the modern tendency is to employ cables having extruded plastic insulation which are terminated in air enclosures with compression type glands. It is important to provide a dry situation in such enclosures by providing adequate ventilation and drainage facilities. On elastomeric type cables, fully insulated elbow connectors are available for use at upto 33kV. Such connectors have the advantage of simple assembly and dismantling.
For voltage exceeding 33kV, oil filled or gas pressure type cables are used, which are terminated in sealing ends. For connections to cables up to 275kV, transformers are fitted with oil filled sealing end chambers incorporating disconnecting facilities between the transformer oil/oil bushings and cable sealing ends.