Terminal equipment

The nondepreciable investments, ie, land and working capital, are often assumed to be constant preoperational costs that are fully recoverable at cost when the project terminates. Equipment salvage is another end-of-life item that can represent a significant fraction of the original fixed capital investment. However, salvage occurs at the end of life, can be difficult to forecast, and is partially offset by dismantling costs. Eor these reasons, a zero salvage assumption is a reasonable approximation ia preliminary analysis. 

To protect terminal equipment or other (weaker) portions of the system, restraints (such as anchors and guides) shall be provided where necessary to control movement or to direct expansion into those portions of the system that are adequate to absorb them. The design, arrangement, and location of restraints shall ensure that expansion-joint movements occur in the directions for which the joint is designed. In addition to the other thermal forces and moments, the effects of friction in other supports of the system shall be considered in the design of such anchors and guides. 

Prospective amplitude, V, (Figure 17.4), to define the insulation endurance of the current-carrying system. Only the first highest peak is of significance for this purpose, which will contain the maximum severity. The subsequent peaks are of moderate magnitudes and of little consequence for the system or the terminal equipment. 

W = energy released iti kW-s or kj V, = prospective crest of the surge in kV Zj = surge impedance of the power system and the terminal equipment in Q... 

Effect of steep-fronted TRVs on the terminal equipment (motor as the basis)... 

The first few turns of the line end coil of a motor or transformer and short lengths of interconnecting cables and overhead lines and their associated terminal equipment, will thus be subject to severe stresses and will be rendered vulnerable to damage by such sleep-fronted transient voltages. 

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. 

The front of the transferred surge will, however, be less steep and dampened than on the primary side due to capacitive dampening. But sometimes this may also exceed the BIL, particularly of the tertiary (if provided) and also the secondary windings of the transformer, as well as the cable and the terminal equipment connected on the lower voltage side. This is especially the case when the primary side voltage is very high compared to the secondary. Protection of the secondary windings, in all probability, will be sufficient for all the cables and terminal equipment connected on the secondary side. 

The terminal equipment connected on the secondary side of the transformer is thus automatically protected as it is subject to much less and attenuated severity of the transferred surges than the secondary windings of the transformer. Nevertheless, the BIL of the interconnec-ting cables and the terminal equipment must be properly coordinated with the BIL of the transformer secondary, particularly for larger installations, say, 50 MVA and above, to be absolutely safe. Example 18,2 will explain the procedure. 

The BIL of the interconnecting cables and the terminal equipment on the secondary must be at least equal to the capacitive and Inductive transferences of the primary surges as determined above. If it is not so, the of the primary arrester must be re-chosen or an arrester also provided on the secondary side. 

As in lEC 60071-1,2a higher insulation level (BIL) will be necessary for all insulators and terminal equipment when the ground fault persists for more than 8 hours per 24 hours or a total of more than 125 hours during a year. 

The more recommended practice is to ground the neutral solidly or through an impedance, commensurate with the requirements of the protective scheme and the fault current limited to a desired level. The terminal equipment and the windings of all the machines may now be designed for a voltage corresponding to the relevant GFF. 

The voltage will now maintain a flat profile from the transmitting end through the receiving end and all the insulators or terminal equipment would be equally stressed. 

The number of thyristors in series, each selected for an impulse voltage of a little less than the impulse voltage with.stand level of the terminal equipment (Table 11.6) can effectively limit the switching overvoltages within desired safe limits. Then connecting them in anti-parallel will mean that the voltage will be forward for either of... 

However, they should remain insulated when terminating with an equipment or a device such as at the ends of generators, GTs, DATs or VTs. It is essential to avoid IPB longitudinal currents through the terminal equipment. Now the bellows necessarily should be of rubber. Figure 31.4(d) shows a rubber bellows but in this small part of the bellows the conductor field will not be nullified and occupy the space affecting the metallic structures, beams and equip-menl/devices in the vicinity. This needs to be taken into account at site and it should be ensured that the nearest structure, beam or equipment is at least 600 mm away from the IPB enclosure. 

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