The losses occurring in a cable dielectric include leakage loss and hysteresis loss.
Leakage Losses:- Since insulating materials are not truly perfect insulators there is always some current flow across the material. However, the conductivity is so low that this current can be neglected. The current flow through insulation is called leakage current. It occurs in both AC and DC voltages
Hysteresis Loss:- The additional dielectric loss under AC is caused by the hysteresis involved in the process of dielectric polarization.
The increase in the power loss due to dielectric hysteresis is much greater than leakage loss. The dielectric hysteresis loss cannot be measured separately. The total dielectric loss, consisting of dielectric hysteresis loss and the power loss due to leakage current flowing through the insulation resistance, can be measured by means of the Schering bridge.
Ques.22. The breakdown voltage of a cable depends on
Presence of moisture
Working temperature
Time of application of the voltage
All of the above
Answer.4. All of the above
Electric breakdown occurs when the applied voltage can no longer be maintained across the material in a stable fashion without the excessive flow of current and physical disruption. Failure of material, due to the application of voltage stress, is called voltage breakdown; the voltage gradient at failure is called the dielectric strength, or electric strength, and is usually expressed as kV/mm.
The breakdown voltage is related to the waveform, temperature, frequency, pressure time and boost speed of the applied voltage.
Ques.23. In congested areas where excavation is expensive and inconvenient ________ of laying of Underground cable is often adopted.
Overhead Cables
Draw in system
Direct Laying
Solid system
Answer.2. Draw in system
Draw-in System
In this method, a conduit or duct of glazed stone or cast iron or concrete is laid in the ground with manholes at suitable positions along the cable route.
This method of cable laying is suitable for congested areas where excavation is expensive and inconvenient, for once the conduits have been laid, repairs or alterations can be made without open undergrounding the ground. This method is generally used for short-length cable routes such as in workshops, road crossings where frequent digging is costlier or impossible.
Ques.24. The minimum dielectric stress in a cable is at
Conductor surface
Lead sheath
Answer.4. Lead sheath
Lead sheath:- For protecting the insulation material from moisture, gases, and any other harmful liquids in the soil, a metallic sheath is provided over the insulation. It also protects the insulation from mechanical damage. The metallic sheath is usually of lead or leads alloy. Recently, aluminum is also being used as a metallic sheath because it has greater mechanical strength, low weight, and cost when compared to the lead sheath. It has minimum dielectric stress.
Ques.25. Capacitance grading of cable implies
Grading according to the capacitance of cables per km length
Use of dielectrics of different permeabilities
Cables using single dielectric in different concentrations
Capacitance required to be introduced at different lengths to counter the effect of inductance
Answer.2. Use of dielectrics of different permeabilities
The process of achieving uniformity in the dielectric stress by using layers of different dielectrics 1s known as capacitance grading.
In capacitance grading, the homogeneous dielectric is replaced by a composite dielectric. The composite dielectric consists of various layers of different dielectrics in such a manner that the relative permittivity of any layer is inversely proportional to its distance from the center. Under such conditions, the value of potential gradient at any point in the dielectric is constant and is independent of its distance from the center.
In other words, the dielectric stress in the cable is the same everywhere and the grading is an ideal one. However, ideal grading requires the use of an infinite number of the dielectric which is an impossible task. In practice, two or three dielectrics are used in the decreasing order of permittivity. The dielectric of the highest permittivity being used near the core.
Ques.26. Low tension cables are generally used up to
200 V
500 V
700 V
1000 V
Answer.4. 1000 V
Depending on the operating voltage, the cables are classified as follows:
L.T. (Low tension) cable – up to 1 kV
H.T (High tension) cable- up to 11kV
S.T. (Super tension)cable- from 22 kV to 33 kV
E.H.T (Extra high tension) cable- from 33 kV to 66 kV
Extra super voltage cable- beyond 132 kV
Ques.27. A certain cable has insulation of relative permittivity 4. If the insulation is replaced by one of relative permittivity 2, the capacitance of the cable will become
One half
Double
Four times
None of the above
Answer.1. One half
Capacitance C = εA/d
Also, ε = ε0 εr
C = ε0 εrA/d
C ∝ εr
Where, εr = Relative permittivity
ε0 = Absolute permittivity
A = Cross-sectional Area
d = distance between the two layers or plates
Given in question
εr1 = 4 , εr2 = 2
C2/C1 = εr2/εr1
C2/C1 = 2/4
C2/C1 = 1/4
C2 = C1/4
So if the relative permeability is half then the capacitance will also be one-half.
Ques.28. In a cable, the maximum stress under operating conditions is at
Insulation layer
Armour
Sheath
Conductor surface
Answer.4. Conductor surface
The dielectric stress of a conductor is given as
E = V/rln(b/a) volts/meter
Where
a = inner conductor
b = outer conductor
r = radius
The dielectric stress at any point is seen inversely proportional to r i.e E = 1//r
The dielectric stress will be maximum when r is at the minimum value i.e when r = a or at the conductor surface.
Emax = E = V/aln(b/a) volts/meter
Ques.29. The breakdown of insulation of the cable can be avoided economically by the use of
Inter-sheaths
Insulating materials with different dielectric constants
Both (A) and (B)
None of the above
Answer.3. Both (A) and (B)
It has been observed that the voltage gradient is maximum at the surface of the conductor and minimum at the inner surface of the sheath (i.e., the stress decreases from conductor surface to sheath). This causes a breakdown in the insulation. For avoiding this breakdown, it is advisable to have a more uniform stress distribution throughout the dielectric. The process of achieving uniform distribution in dielectric stress is called the grading of cables. There are two methods to achieve it, they are:
The application of layers of different dielectric materials called “Capacitance grading”.
Providing metallic intersheath between successive layers of the same dielectric materials and maintaining the appropriate potential level at the intersheath is called “Intersheath grading”.
Ques.30. If a cable of homogeneous insulation has maximum stress of 10 kV/mm, then the dielectric strength of insulation should be
5 kV/mm
10 kV/mm
15 kV/mm
30 kV/mm
Answer.2. 10 kV/mm
If the homogeneous condition is satisfied, the value of dielectric stress at any point will be constant and independent of the distance from the center and the grading will be an ideal one. Since the dielectric is homogeneous, the maximum stress in each layer is the same i.e 10 kV/mm.
