Overhead lines

All the tibove surges and even the transference of a lightning surge from an overhead line through inter-... 

In Table 11.6 the values of colurrm 2 relate to normal operating conditions. Abnormal conditions may prevail when the machine is exposed to overhead lines, the... 

When the system is not directly connected to an overhead line, such as when connected through a transformer. 

It is equal to the phase impedance of the overhead lines or cables. For low current systems, LT or HT, this impedanee is nearly equal to the resistanee of the circuit, as Due very low A l, the impedance... 

A power system is connected to a number of power supply machines that determine the fault level of that. system (e.g. generators and transformers). The impedances of all such equipment and the impedances of the interconnecting cables and overhead lines etc. are the parameters that limit the fault level of the system. For ease of calculation, when determining the fault level of such a system it is essential to consider any one major component as the base and convert the relevant parameters of the other equipment to that base, for a quicker calculation, to establish the required fault level. Below we provide a few common formulae for the calculation of faults on a p.u. basis. For more details refer to a textbook in the references. 

Similarly, consider an overhead line having the following parameters ... 

Above we have analysed ihe case of an induclion moior during a swiiching sequence for systems of 2.4 kV and above. The phenomenon of voltage surges in iransformers, capacitors, inierconneciing cables or overhead lines elc. is no differem, as Ihe circuit conditions and sequence of switching will remain the same for all. 

Interrupting a cable or an overhead line when extremely underloaded, such as when operating at a near no load. 

Interrupting the equipment such as an induction motor, a transformer, a cable or an overhead line on a fault, such as on a short-circuit or a ground fault. 

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. 

When equipment is exposed to direct lightning strikes Equipment connected directly to an overhead line, or even through a transformer, will fall into this category. Select the highest value of BIL and even then a surge protection will become necessary for critical installations. 

The time of travel from the point the lightning strikes on the overhead lines to the arrester, considering a line length of approximately 420 km and speed of propagation of the surge as 0.3 km/,us,... 

Additioiiiil noise from inductive equipment such as motors, generators md triinsformers and even overhead lines. 

Figure 24.14 Phasor position of sending-end and receiving-end voltages in an overhead line...

Resistance of primary distribution overhead lines, section B-B at the operating temperature,... 

T) Leakage reactance of transmitter-end transformer. Resistance of overhead lines of section B-B. 

If the regenerator temperature eontinues to rise, additional quantities of steam will be added in the form of water spray by either manual or automatie means—first, through one set of injeetion nozzles in the overhead line, then through another set elsewhere. It may beeome neeessary to add steam to the eyelones in order to maintain 1,300°F to the expander. Spray nozzles in the dilute phase might also be used if the temperature eontinues to rise. 

For big vapor lines and condensers (frequent in vacuum systems) always insulate the line, condenser, and top of column. Rain or sudden cold fronts will change column control otherwise. It is possible to have more surface in the overhead line than in the condenser. 

---