Ques 31. The spacing between phase conductors of a 220 kV line is approximately equal to
2 m
3 m
6 m
10 m
Answer.3. 6 m
Explanation:
According to the Electrical safety rule, the spacing between the phase conductor of 220 kV is 6m.
Ques 32. The minimum clearance between the ground and a 220 kV line is about
1 m
7 m
10 m
5 m
Answer.2. 7 m
Explanation:
According to the Electrical safety rule, the minimum clearance between the ground and a 220 kV line is about 7m.
Ques 33. In a D.C. 3-wire distribution system, balancer fields are cross-connected in order to
Boost the generated voltage
Equalize voltages on the positive and negative outers
Balance loads on both sides of the neutral
Make both machines run as unloaded motors
Answer.2. Equalize voltages on the positive and negative outers
Explanation:
The purpose of the Balancer is to redress the voltage on the positive and negative outer.
Let us suppose P and Q are the armatures of 2 DC machines that are connected in series across the outers and the field being cross-connected.
Out of balance load makes the DC machine works automatically as a motor or generator in order to compensate for voltage drop or rise.
The balancer connected to lightly load side always work as the motor while that connected to a heavily loaded side act as the generator.
The energy is always transferred from the lightly loaded side to the heavily loaded side.
Ques 34. In a D.C. 3-wire distributor using balancers and having unequal loads on the two sides
Balancer connected to lightly- loaded side runs as a motor
Balancer connected to heavily- loaded side runs as a motor
Both balancers run as motors
Both balancers run as generators
Answer.1. Balancer connected to lightly- loaded side runs as a motor Explanation:
Check the above question i.e question number 33.
Ques 35. A uniformly-loaded D.C. distributor is fed at both ends with equal voltages. As compared to a similar distributor fed at one end only, the drop at the middle point is
One-half
One-third
One-fourth
Twice
Answer.3. One-fourth Explanation:
When distributed is fed at one end only, the voltage drop at the middle point is =IR/2.
When distributed is fed at both ends with equal voltages, the voltage drop at the middle point is = IR/8.
Where I is the current fed to the distributor R is the total resistance of the distributor.
Therefore, a uniformly loaded DC distributor is fed at both ends with equal voltages the drop at the middle point is one fourth as compared to a similar distributor fed at one end only.
Ques 36. As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only
31.25 percent of copper
66.7 percent of copper
33.3 percent of copper
125 percent of copper
Answer.3. 33.3 percent of copper
As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only 33.3 percent of copper.
For the same conductor length, the same amount of power, same losses and same maximum voltage to earth, 3 wire DC system requires minimum conductor area
For transmitting the same amount of power at the same voltage, a three-phase transmission line requires less conductor material than a single-phase line; The three-phase transmission system is so cheaper
For a given amount of power transmitted through a system, the three-phase system requires conductors with a smaller cross-sectional area; This means a saving of copper and thus the original installation costs are less
Important Point:
Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.
System
Same maximum voltage to earth
Same maximum voltage between conductors
DC system: Two-wire
1
1
DC: Two-wire mid-point earthed
0.25
1
DC: 3 wire
0.3125 = 5 / 16
1.25
Single-phase: 2 wire
2/cos2ϕ
2/cos2ϕ
Single-phase: 2 wire mid-point earthed
0.5/cos2ϕ
2/cos2ϕ
Single-phase: 3 wire
0.625/cos2ϕ
2.5/cos2ϕ
2-phase: 4 wire
0.5/cos2ϕ
2/cos2ϕ
2-phase: 3 wire
1.457/cos2ϕ
2.914/cos2ϕ
3 phase, 3 wire
0.5/cos2ϕ
1.5/cos2ϕ
3 phase, 4 wire
0.583/cos2ϕ
1.75/cos2ϕ
Ques 37. For an overhead line, the surge impedance is taken as
20-30 ohms
400 – 600 ohms
70—80 ohms
100—200 ohms
Answer.2. 400 – 600 ohms
Explanation:
Surge Impedance
In power system, the characteristic impedance is sometimes referred to as surge impedance.
It is defined as the √Z/Y where Z = series impedance of line Y = shunt admittance of the line
For a lossless line, the surge impedance reduces to √(L/C) a pure resistance.
Its value is normally 400 to 600 ohms for an overhead line while for underground cable its value is typically between 40 to 60 ohms.
Ques 38. The presence of ozone due to corona is harmful because it
Corrodes the material
Transfer energy to the ground
Gives odor
Any of the above
Answer.1. Corrodes the material
Explanation:
Corona discharge is the ionization of air molecules due to high electrical potentials. The ionization process creates a plasma which we call corona discharge.
The high-energy ions that create this effect can rapidly deteriorate power systems equipment.
Corona discharge typically occurs at (non-smooth) surfaces because at these points the electric field is the highest.
Ques 39. The power transmitted will be maximum when
Corona losses are minimum
Receiving end voltage is high
Reactance is high
Sending end voltage is high
Answer.4. Sending end voltage is high
Explanation:
In case if the sending end voltage is not more than the receiving end voltage then the current start flowing in reverse direction i.e from receiving end to sending end this phenomenon is called as the Ferranti effect.
This occurs when the line is energized, but there is a very light load or the load is disconnected.
Ques 40. A 3-phase 4 wire system is commonly used on
Primary transmission
Secondary transmission
Primary distribution
Secondary distribution
Answer.4. Secondary distribution
Explanation:
At the local distribution centers, there are step-down distribution transformers.
The voltage level of 6.6 kV and 11 kV are further step-down to 400 V for an industrial region and 240 V for a domestic or household region.
This uses 3 phase 4 wire system and the voltage between any two lines is 400 Volt and the voltage between any of the three-line and neutral is 230 V.
