The vital issues within the style and operation of a cable area unit the determination of fall, line losses and potency of transmission. These values area unit greatly influenced by the road constants R, L and C of the cable. as an example, the fall within the line depends upon the values of the higher than 3 line constants. Similarly, the resistance of cable conductors is that the most significant explanation for power loss within the line and determines the transmission potency. during this chapter, we have a tendency to shall develop formulas by that we will calculate voltage regulation, line losses, and potency of transmission lines. These formulas area unit was vital for 2 principal reasons.
Firstly, they supply a chance to grasp the consequences of the parameters of the road on bus voltages and also the flow of power. Secondly, they assist in developing associate degree overall understanding of what’s occurring within the power system.
Important Terms:
While learning the performance of a cable, it’s fascinating to work out its voltage regulation and transmission potency. we tend to shall justify these 2 terms successively. The distinction in voltage at the receiving finish of a cable between conditions of no load and full load is termed voltage regulation and is expressed as a proportion of the receiving finish voltage.
 Voltage regulation:
When a cable is carrying current, there’s a fall within the line thanks to resistance and inductance of the road. The result’s that receiving finish voltage (VR) of the road is mostly but the causation finish voltage (VS). This fall (VS −VR) within the line is expressed as a proportion of receiving finish voltage VR and is termed voltage regulation.

Transmission efficiency:
The power obtained at the receiving finish of a cable is mostly but the causation finish power thanks to losses within the line resistance. The magnitude relation of receiving finish power to the causation finish power of a cable is understood because of the transmission potency of the road i.e.
Classification of OverClassification of Overhead Transmission Lines:
A cable has three constants R, L and C distributed uniformly on the full length of the road. The resistance and inductance kind the series resistivity. The capacitance existing between conductors for a 1phase line or from a conductor to neutral for a 3phase line forms a shunt path throughout the length of the road. Therefore, capacitance effects introduce complications in cable calculations. relying upon the way during which capacitance is taken under consideration, the overhead transmission lines area unit classified as,
 Short transmission lines: once the length of associate degree overhead line is up to regarding 50km and therefore the line voltage is relatively low (< twenty kV), it’s typically thoughtabout as a brief line. because of smaller length and lower voltage, the capacitance effects area unit tiny and, therefore, may be neglected. Therefore, whereas finding out the performance of a brief line, solely resistance and inductance of the road area unit taken into consideration.
 Medium transmission lines: once the length of associate degree overhead line is regarding 50150 klick and therefore the line voltage is moderately high (>20 potential unit < a hundred kV), it’s thoughtabout as a medium line. because of sufficient length and voltage of the road, the capacitance effects area unit taken into consideration. For functions of calculations, the distributed capacitance of the road is split and lumped within the kind of condensers shunted across the road at one or more points.
 Long transmission lines: once the length of associate degree overhead line is quite one hundred fifty klicks and line voltage is extremely high (> a hundred kV), it’s thoughtabout as a protracted line. For the treatment of such a line, the road constants area unit thoughtabout uniformly distributed over the total length of the road and rigorous strategies area unit utilized for the answer.
It may be stressed here that the precise resolution of any line should contemplate the very fact that the constants of the road don’t seem to be lumped however area unit distributed uniformly throughout the length of the road.
Single Phase Short Transmission Lines:
As declared earlier, the results of line capacitance square measure neglected for a brief line. Therefore, whereas finding out the performance of such a line, solely resistance and inductance of the road square measure taken under consideration. The equivalent circuit of one section short line is shown in Fig. 10.1. Here, the whole line resistance and inductance square measure are shown as focused or lumped rather than being distributed. The circuit could be a straightforward a.c. electric circuit.
ThreePhase Shor eePhase Short Transmission Lines:
For reasons related to the economy, the transmission of electrical power is finished by the 3phase system. this method is also thought to be consisting of 3 singlephase units, every wire transmittal common fraction of the whole power. As a matter of convenience, we have a tendency to typically analyze the 3phase system by considering one section solely.
Therefore, the expression for regulation, efficiency, etc. derived for one section line may also be applied to a 3phase system. Since just one section is taken into account, the section values of the 3phase system ought to be taken. Thus, VS and VR square measure the section voltages, whereas R and XL square measure the resistance and inductive electrical phenomenon per section severally.
Medium Transmission Lines:
In short line calculations, the consequences of the road capacitance square measure neglected as a result of such lines have smaller lengths and transmit power at comparatively low voltages (< twenty kV). However, because the length and voltage of the road increase, the capacitance bit by bit becomes of larger importance. Since medium transmission lines have a decent length (50150 km) and typically operate at voltages larger than twenty kilovolts, the consequences of capacitance can’t be neglected.
Therefore, so as to get affordable accuracy in medium line calculations, the load capacitance should be taken into consideration. The capacitance is uniformly distributed over the complete length of the road.
However, so as to form the calculations straightforward, the load capacitance is assumed to be lumped or focused within the style of capacitors shunted across the road at one or additional points. Such a treatment of localizing the road capacitance provides fairly correct results. the foremost normally used ways (known as localized capacitance methods) for the answer of medium transmissions lines square measure :
 finish condenser technique
 Nominal T technique
 Nominal π technique.
Although the on top of ways square measure used for getting the performance calculations of medium lines, they will even be used for brief lines if their line capacitance is given in a very explicit drawback.
Nominal T Method:
In this methodology, the full line capacitance is assumed to be focused at the central purpose of the road and 0.5 the road resistance and electrical phenomenon area unit lumped on its either facet. Therefore, during this arrangement, a full charging current flows over 0.5 the road. one section of the 3phase conductor is shown because it is advantageous to figure in section rather than linetoline values.
Nominal π Method:
In this methodology, the capacitance of every conductor (i.e., the road to neutral) is split into 2 halves; one 0.5 being lumped at the causing finish and therefore the partner at the receiving finish. it’s obvious that capacitance at the causing finish has no result on the road drop. However, its charging current should be added to line current so as to get the overall causing finish current.
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