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In electric power, a bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier such as the case of a transformer or circuit breaker without making electrical contact with it. Bushings are typically made from porcelain, though other insulating materials are also used.
All materials carrying an electric charge generate an electric field. When an energized conductor is near a material at earth potential, it can form very high field strengths, especially where the field lines are forced to curve sharply around the earthed material. The bushing controls the shape and strength of the field and reduces the electrical stresses in the insulating material.
A bushing must be designed to withstand the electrical field strength produced in the insulation, when any earthed material is present. As the strength of the electrical field increases, leakage paths may develop within the insulation. If the energy of the leakage path overcomes the dielectric strength of the insulation, it may puncture the insulation and allow the electrical energy to conduct to the nearest earthed material causing burning and arcing.
A typical bushing design has a conductor, usually of copper or aluminium, occasionally of other conductive material, surrounded by insulation, except for the terminal ends.
In the case of a busbar, the conductor terminals will support the busbar in its location. In the case of a bushing, a fixing device will also be attached to the insulation to hold it in its location. Usually, the fixing point is integral or surrounds the insulation over part of the insulated surface. The insulated material between the fixing point and the conductor is the most highly stressed area.
The design of any electrical bushing must ensure that the electrical strength of the insulated material is able to withstand the penetrating 'electrical energy' passing through the conductor, via any highly stressed areas. It must also be capable of enduring, occasional and exceptional high voltage moments as well as the normal continual service withstand voltage, as it is the voltage that directs and controls the development of leakage paths and not current.
Insulated bushings can be installed either indoor, or outdoor, and the selection of insulation will be determined by the location of the installation and the electrical service duty on the bushing.
For a bushing to work successfully over many years, the insulation must remain effective both in composition and design shape and will be key factors in its survival. Bushings can therefore vary considerably in both material and design style.
The earliest bushing designs use porcelain for both indoor and outdoor applications. Porcelain was originally used due to its properties of being impervious to moisture once sealed by fired glaze, and low manufacturing cost. The main disadvantage with porcelain is that its small value of linear expansion has to be accommodated by using flexible seals and substantial metal fittings, both of which present manufacturing and operational problems.
A basic porcelain bushing is a hollow porcelain shape that fits through a hole in a wall or metal case, allowing a conductor to pass through its center, and connect at both ends to other equipment. Bushings of this type are often made of wet-process fired porcelain, which is then glazed. A semi-conducting glaze may be used to assist in equalizing the electrical potential gradient along the length of the bushing.
The inside of the porcelain bushing is often filled with oil to provide additional insulation and bushings of this construction are widely used up to 36 KV where higher partial discharges are permitted.
Where partial discharge is required to conform to IEC60137, paper and resin insulated conductors are used in conjunction with porcelain, for unheated indoor and outdoor applications.
The use of resin (polymer, polymeric, composite) insulated bushings for high voltage applications is common, although most high-voltage bushings are usually made of resin impregnated paper insulation around the conductor with porcelain or polymer weather sheds, for the outdoor end and occasionally for the indoor end.
Another early form of insulation was paper, however, paper is hygroscopic and absorbs moisture which is detrimental and is disadvantaged by the inflexible linear designs. Cast resin technology has dominated insulated products since the 1960s, due to its flexibility of shape and its higher dielectrical strength.
Typically, paper insulation is later impregnated either with oil (historically), or more commonly today with resin. In the case of resin, the paper is film coated with a phenolic resin to become synthetic resin bonded paper (SRBP), or impregnated after dry winding with epoxy resins, to become resin impregnated paper or epoxy resin impregnated paper (RIP, ERIP).
SRBP insulated bushings are typically used up to voltages around 72.5 kV. However, above 12 kV, there is a need to control the external electrical field and to even out the internal energy storage which marginalises the dielectric strength of paper insulation.
To improve the performance of paper insulated bushings, metallic foils can be inserted during the winding process. These act to stabilize the generated electrical fields, homogenising the internal energy using the effect of capacitance. This feature resulted in the condenser/capacitor bushing.
The condenser bushing is made by inserting very fine layers of metallic foil into the paper during the winding process. The inserted conductive foils produce a capacitive effect which dissipates the electrical energy more evenly throughout the insulated paper and reduces the electric field stress between the energised conductor and any earthed material.
Condenser bushings produce electric stress fields which are significantly less potent around the fixing flange than designs without foils and, when used in conjunction with resin impregnation, produce bushings which can be used at service voltages over one million with great success.
Since the 1965s, resin materials have been used for all types of bushing up to the highest voltages. The flexibility of using a castable form of insulation has replaced paper insulation in many product areas and dominates the existing insulated bushing market.
As with paper insulation, the control of electric stress fields remains important. Resin insulation has greater dielectric strength than paper and requires less stress control at voltages below 25 kV. However, some compact and higher rated switchgear designs, have earthed materials closer to bushings than in the past and these designs may require stress control screens in resin bushings operating as low as 12 kV Fixing points are often integral with the main resin form, and present fewer problems to earthed materials than the metal flanges used on paper bushings.
However, care must be observed in resin insulated bushings designs which use internally cast screens such that the benefit of electrical stress field control is not off set by increasing partial discharge caused by the difficulties of eliminating micro voids in the resin around the screens during the casting process. The need to eliminate voids in resin becomes more sensitive as voltages increases, and it is normal to revert to resin impregnated, foiled paper insulation for bushings rated over 72.5 kV.
Bushings sometimes fail due to partial discharge. This is sometimes due to the slow and progressive degradation of the insulation over many years of energized service; however, it may also be a rapid degeneration which destroys a good bushing in a matter of hours. At present, there is great interest by the electricity supply industry in monitoring the condition of high-voltage bushings. However, some bushings failing early in service are due to failures to control voltage or carry out essential maintenance, while others relate to incipient failure mechanisms built in at manufacture. This view is evidenced by the minority of bushing failures worldwide.
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples are non-metals.
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Faraday's law of induction, discovered in 1831, describes the induced voltage effect in any coil due to a changing magnetic flux encircled by the coil.
In physics, the term dielectric strength has the following meanings:
An electrical cable is an assembly of one or more wires running side by side or bundled, which is used as an electrical conductor, i.e., to carry electric current. One or more electrical cables and their corresponding connectors may be formed into a cable assembly, which is not necessarily suitable for connecting two devices but can be a partial product. Cable assemblies can also take the form of a cable tree or cable harness, used to connect many terminals together.
In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress. While a corona discharge (CD) is usually revealed by a relatively steady glow or brush discharge (BD) in air, partial discharges within solid insulation system are not visible.
A Lichtenberg figure, or Lichtenberg dust figure, is a branching electric discharge that sometimes appears on the surface or in the interior of insulating materials. Lichtenberg figures are often associated with the progressive deterioration of high voltage components and equipment. The study of planar Lichtenberg figures along insulating surfaces and 3D electrical trees within insulating materials often provides engineers with valuable insights for improving the long-term reliability of high-voltage equipment. Lichtenberg figures are now known to occur on or within solids, liquids, and gases during electrical breakdown.
In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material, subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. The voltage at which a given insulating object becomes conductive is called its breakdown voltage and, in addition to its dielectric strength, depends on its size and shape, and the location on the object at which the voltage is applied. Under sufficient electrical potential, electrical breakdown can occur within solids, liquids, or gases. However, the specific breakdown mechanisms are different for each kind of dielectric medium.
Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure.
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
A power cable is an electrical cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. Power cables that are bundled inside thermoplastic sheathing and that are intended to be run inside a building are known as NM-B.
Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled wet transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.
In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of a tree. Treeing of solid high-voltage cable insulation is a common breakdown mechanism and source of electrical faults in underground power cables.
A capacitor is an electronic device that stores electrical energy in an electric field by accumulating electric charges on two closely spaced surfaces that are insulated from each other. It is a passive electronic component with two terminals.
Electronic packaging is the design and production of enclosures for electronic devices ranging from individual semiconductor devices up to complete systems such as a mainframe computer. Packaging of an electronic system must consider protection from mechanical damage, cooling, radio frequency noise emission and electrostatic discharge. Product safety standards may dictate particular features of a consumer product, for example, external case temperature or grounding of exposed metal parts. Prototypes and industrial equipment made in small quantities may use standardized commercially available enclosures such as card cages or prefabricated boxes. Mass-market consumer devices may have highly specialized packaging to increase consumer appeal. Electronic packaging is a major discipline within the field of mechanical engineering.
Magnet wire or enameled wire is a copper or aluminium wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, generators, speakers, hard disk head actuators, electromagnets, electric guitar pickups, and other applications that require tight coils of insulated wire.
Electrical insulation papers are paper types that are used as electrical insulation in many applications due to pure cellulose having outstanding electrical properties. Cellulose is a good insulator and is also polar, having a dielectric constant significantly greater than one. Electrical paper products are classified by their thickness, with tissue considered papers less than 1.5 mils (0.0381 mm) thickness, and board considered more than 20 mils (0.508 mm) thickness.
A high-voltage cable is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a fully rated insulation system that will consist of insulation, semi-con layers, and a metallic shield. This is in contrast to an overhead line, which may include insulation but not fully rated for operating voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (DC) power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent the breakdown of the insulation.
Condition monitoring of transformers in electrical engineering is the process of acquiring and processing data related to various parameters of transformers to determine their state of quality and predict their failure. This is done by observing the deviation of the transformer parameters from their expected values. Transformers are the most critical assets of electrical transmission and distribution systems, and their failures could cause power outages, personal and environmental hazards, and expensive rerouting or purchase of power from other suppliers. Identifying a transformer which is near failure can allow it to be replaced under controlled conditions at a non-critical time and avoid a system failure.
Transformers and Electricals Kerala Limited (TELK) is a public sector undertaking in Kerala incorporated in 1963 under an agreement with Kerala State Industrial Development Corporation (KSIDC) and Hitachi Limited of Japan. The company was formed to design and manufacture extra High Voltage Electrical equipment in India. The first product rolled out from TELK in 1966 is power transformers. In 2009, TELK became a joint venture company of the Government of Kerala and NTPC Limited. The equipment's TELK manufactures includes power transformers, current transformers, voltage transformers, SF6 Circuit Breakers and reactors.
