Overhead conductor systems

Impedance and Admittance Formulation of an Overhead Conductor System... 

Summarizing the earlier sections, the impedance and the admittance of an overhead conductor system are given in the following form ... 

Chapter 3 examines the transients in a cable system. A cable system is, in general, more complicated than an overhead line system, because one phase of the cable is composed of two conductors called a metallic core and a metallic sheath. The former carries a current and the latter behaves as an electromagnetic shield against the core current. Another reason why a cable system is complicated is that most long cables are cross-bounded, that is, the metallic sheaths on phases a, b, and c in one cable section are connected to that of phases b, c, and a in the next section. Each section is called a minor section and the length of each normally ranges from some 100 m to 1 km. Three minor sections comprise one major section. The sheath impedances of three phases thus become nearly equal to each other. As a result, a transient on a cable system is quite different from that on an overhead line system. 

It should be noted that the impedance and admittance in this equation become a matrix when a conductor system is composed of multiconductors. Remember that a single-phase cable is, in general, a multiconductor system because the cable consists of a core and a metallic sheath or a screen. In an overhead conductor, no conductor internal admittance y exists, except a... 

Pollaczek s General Formula for Overhead, Underground, and Overhead/Underground Conductor Systems... 

Pollaczek derived a general formula that can deal with earth-return impedances of overhead conductors, underground cables, and multiconductor systems composed of overhead and underground conductors in the following form [7,13] ... 

For instance, when lightning of, say, a nominal discharge current of 10 kA strikes a 400 kV (r.m.s.) overhead line, having a surge impedance of 350 Q, then two parallel waves will be produced each of amplitude 10 x 350/2 or 1750 kV which may be more than the impulse withstand level of the system and cause a flashover between the conductors and the ground, besides damaging the line insulators and the terminal equipment (Table 13.2). It is therefore imperative that the system is protected against such eventualities. 

Spray-aluminised coatings are used for exhaust valves in automobile engines, exhaust and silencer systems (double and triple life), tyre moulds, gas ducting, heat-treatment pots, furnace ladles, carburising boxes and fans handling hot gases. Similar applications utilise Calorised and hot-dip aluminised coatings. Hot-dip aluminised steel wire has been used in steel-cored aluminium conductors for overhead transmission lines. For some... 

Noise protection is needed to protect against RFI/EMI problems caused by mobile and stationary radio, television, and handheld walkie-talkies radio-controlled overhead cranes radar induction heating systems static discharge high-speed power-switching elements high-AC current conductors large solenoids and relays transformers AC and DC motors welders and even... 

A three-necked, round-bottomed flask equipped with a thermometer, reflux conductor, N, inlet and an overhead mechanical stirrer was charged sequentially with H O (5 mL), 50% NaOH (2 mL. 39 mmol), and adenine (2 g, 15 mmol). After the adenine had dissolved, a solution of benzylating reagent (16 mmol) and phase-transfer catalyst (5 mol%) in organic solvent (30-40 mL) was added. The system was stirred under reflux and then cooled to rt. The precipitated solids were filtered, washed with lIjO (2x10 mL), and dried in vacuo to give the crude bcnzylated adenine mixtures. For separation see ref 82. 

For third party safety the concern is mainly coming from the potential accidental touching of live electrical system parts, e.g. the conductor wires of overhead lines. 

Electricity is taken from the National Grid by appropriately located substations which eventually transform the voltage down to 11 kV at a local substation. At the local substation the neutral conductor is formed for single-phase domestic supplies and three-phase supplies to shops, offices and garages. These supplies are usually underground radial supplies from the local substation, but in rural areas we still see transformers and overhead lines suspended on wooden poles. Figures 3.11 and 3.12 give an overview of the system from power-station to consumer. 

Service-entrance conductors, overhead system. The service conductors between the terminals of the service equipment and a point usually outside the building, clear of building walls, where joined by tap or splice to the service drop. 

Where work has to be done near a system where there may be danger, the responsible person has to provide for the safety of the workers. For example, painters, who may not be electrically knowledgeable, may have to decorate an overhead crane track where there are bare live trolley wires. The painters must be warned about the hazard, and measures must be taken to avoid the danger. This might involve the provision of track stops to limit crane movement, and the painters working in a segregated area where the live conductors are screened to prevent direct contact. Where systems are... 

As an example, in TN-C-S supplies to domestic premises a break in the combined neutral/earth (CNE) conductor of an overhead service line would cause the metalwork in the premises to become live at or about the supply voltage if any apparatus, such as a dishwasher, were to be connected and switched on. To prevent this type of hazard, the integrity of the CNE conductor, throughout the system, has to be maintained, so all joints have to be properly made and reliable, and fuses and solid state devices are prohibited in these conductors. 

Many accidents have happened, and continue to happen, when somebody working on or near a system that has been made dead experiences an electrical injury when the system is unintentionally switched on. This could be, for example, an electrician working on a system, or painters working in the vicinity of an overhead travelling crane s uninsulated power conductors. The aim of Regulation 13 is that measures should be taken to prevent this happening. 

Regulation 542-01-02 refers to TN-S systems where the supply company runs a protective conductor from the distribution transformer earth into the consumer s premises and makes it available for connection to the consumer s main earth terminal. This protective conductor may consist of the metallic sheath/armour of the underground service cable or an earthing conductor connected to the protective conductor of an overhead line. 

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