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5. What do you mean by lightning?
Lightning phenomenon is a peak discharge in which charge accumulated in the clouds
discharges into a neighbouring cloud or to the ground. The electrode separation,
(i.e),cloud-to-cloud or cloud-to-ground is very large, perhaps 100m or more. The
mechanism of charge formation in the clouds and their discharge isb quite a complicatedand uncertain process
6. Give the factors contributing to good line design.
In order to reduce the hazard that lightning poses to power system, certain factor that
determine the line performance must be understood.
First we try to keep the incidence of stokes to the system to a minimum.
The objective of good line design is to reduce the number of outages caused by
lightning.
First we try to keep the incidence of stokes to the system to a minimum.
Then we try to minimize the effects of those strokes that do terminate on the
system. Lightning problems can be eliminated if all transmission was through tunnels at
least 20ft under the ground.
Tall towers are more vulnerable than low goal post-like structures. In order to
prevent the lightning, some adequate clearances must be provided.
High ground impedance or tower footing resistance is to be avoided. High surge
impedance in ground wires, tower structures are to be avoided.
2. What are the characteristics of lightning strokes
3. What are the protective devices used to protect power system equipments against lightning
4. What do you mean by current chopping?
When interrupting low inductive currents such as magnetizing currents of the
transformer, shunt reactor, the rapid deionization of the contact space and blast effect
may cause the current to be interrupted before the natural current zero. This
phenomenon of interruption of the current before its natural zero is called current
chopping.
5. What are the methods of capacitive switching?
Opening of single capacitor bank
Closing of one capacitor bank against another
10. What are the applications of EMTP?The EMTP is a comprehensive computer program designed to solve electrical transient
problems in lumpy circuits, distributed circuits. This program is capable of solving
steady-state circuit problems. Transient analysis can be carried out in circuits with any
arbitrary configuration of lumped parameters (R, L and C).
Explain the mechanism of lightning discharge
1 Explain in detail about the switching suges in a power system.
13. a) Explain in detail how the charges are formed in the clouds
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b) Derive the mathematical model for lighting and also give its interaction with
power system.
Derivation 8 marks
Explanation8 marks
During the charge formation process, the cloud may be considered to be a nonconductor.Hence, various potentials may be assumed at different parts of the cloud. If the charging
process is continued, it is probable that the gradient at certain parts of the charged region exceeds the
breakdown strength of the air or moist air in the cloud. Hence, local breakdown takes place within the
cloud. This local discharge may finally lead to a situation wherein a large reservoir of charges
involving a considerable mass of cloud hangs over the ground, with the air between the cloud and the
ground as a dielectric.
When a streamer discharge occurs to ground by first a leader stroke followed by main strokes with
considerable current flowing. The lightning stroke maybe thought to be a current source of value
10with source impedance Zo discharging to earth. If the stroke strikes an object of impedance Z, thevoltage built across it may be taken as.
The source impedance of the lightning channels are not known exactly, but it is estimated to be
about 1000 to 3000.The objects of interest to electrical engineers, namely, transmission line have surge impedances
less than 500 . (over head lines 300 to 500 , ground wires 100 to 150 , towers 10 to 50 ,etc).Therefore, the value Z/Zo will usually be less than 0.1 and hence can be neglected.
What is short line and kilometric fault and also explain the voltage transientsop.closing and reclosing lines.
Short line and Kilometric fault. 8 MARKS
The circuit breakers undergo less difficulty in interrupting current to a fault located close
to their terminals that the current to a similar fault located away from the terminals.
When the fault is located beyond the terminals, the current can be easily interrupted due
to the added impedance of the line.
This added impedance not only limits the current but also supports some of the system
voltage. The generated voltage is divided on either side of the breaker in proportion to the
impedance of the source and the line.
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The fault current I can be represented by a ramp with a slope
Voltage transients on closing and reclosing times.8 MARKSCase 1: When a switch is closed
When a switch is closed immediately prior to the circuit being completed a certain voltage exists
across the switch contacts.
At the moment the contacts made by pre restriking discharge, this voltage disappears. If the
instantaneous voltage is V, a fraction VL with momentarily appear across the line.
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This voltage will travel down the line as a wave and be reflected from remote end
returning to the source.
To reduce the closing voltage transients the resistance is pre inserted, the pre insertion
resistors share the voltage.
Case 2:Reclosing lines
In an utility system a breaker is reclosed as rapidly as possible, after interrupted a fault.
This allows service to be restored quickly if fault is of a transient nature. It improves the
overall stability of the system.