Transmission and Distribution Objective Questions With Easy Explanation Part-1

1. ……………. are the conductors, which connect the consumer’s
A. Terminals to the distribution
B. Distributors
C. Service mains
D. Feeders

Answer: (C) Service mains

  • A feeder is a conductor which connects the sub-station (or localized generating station) to the area where power is to be distributed. 
  • Generally, no tapings are taken from the feeder so that current in it remains the same throughout. 
  • The main consideration in the design of a feeder is the current carrying capacity.

  • A distributor is a conductor from which tapings are taken for supply to the consumers.
  • The current through a distributor is not constant because tapings are taken at various places along its length. 
  • While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals.

Service mains: 
  • A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
  • Service mains of the consumers may be either connected to the distributors or sub-distributors

2. Distributors designed from the point of view of
A. its current carrying capacity
B. operating voltage
C. voltage drop in it.
D. operating frequency

Answer: C. voltage drop in it.

  • Distributors are the conductors from which numerous tapping's are taken for providing power supply to the consumers. 
  • Therefore the current loading on the distributors varies along the length. 
  • So distributors are designed from the point of view of voltage drop in them.

3. Which of the following are connected by the service mains?
A. Transformer and earth
B. Distributor and relay system
C. Distributor and consumer terminals
D. Distributor and transformer

Answer: C. Distributor and consumer terminals

Service mains: 
  • A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
  • Service mains of the consumers may be either connected to the distributors or sub distributors. 
  • Distributor and consumer terminals are connected by the service mains.

4. The feeder is designed mainly from the point of view of
A.  its current carrying capacity
B. voltage drop in it
C. operating voltage
D.  operating frequency

Answer: A.  its current carrying capacity

  • A feeder is a conductor having constant current density.
  • The size of the feeder designed based on current-carrying capacity.
  • For V ≤ 220 kV, selection of conductor done based on the current-carrying capacity.
  • For V > 220 kV, the selection of conductor is done based on the concept of corona or electric field intensity.
  • In both cases, our concern is about power loss which leads to temperature rise in the conductor.
  • Hence, the selection of the size of the conductor for the feeder is designed based on temperature rise.
  • Size of the distributor designed based on voltage drop.

5. The underground system cannot be operated above
(A) 440 V
(B) 11 kV
(C) 33 kV
(D) 66 kV

Answer: (D) 66 kV

The underground system cannot be operated above 66 kV because of insulation difficulties but overhead system can be designed for operation up to 400 kV or higher even.

6. The usual spans with R.C.C. poles are
(A) 40 – 60 meters
(B) 80 – 100 meters
(C) 80 – 100 meters
(D) 300 – 500 meters

Answer: (C) 80 – 100 meters

Wooden poles:
  • These are made of seasoned wood (sal or chir) and are suitable for lines of the moderate X-sectional area and of relatively shorter spans, say up to 50 meters
  • Such supports are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economic proposition
  • The wooden poles generally tend to rot below the ground level, causing foundation failure
  • In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil
  • They have a comparatively smaller life (20-25 years) and cannot be used for voltages higher than 20 kV
  • They have less mechanical strength and require periodical inspection
Steel Poles:
  • They are used for system voltages up to 33 kV in low and high-voltage distribution systems
  • When compared to wooden poles steel poles have advantages like lightweight, long life, and greater strength
  • These are used for a longer span, i.e., from 50 to 80 m
  • These are costlier than wooden and RCC poles
  • All steel supports should be well-galvanized and have a life of at least 30 years
Concrete poles (RCC Poles):
  • Reinforced concrete poles have become very popular as line supports in recent years.
  • They have greater mechanical strength, longer life, and permit longer spans than steel poles.
  • Moreover, they give a good outlook, require little maintenance, and have good insulating properties.
  • The maximum permissible span for RCC poles is 80 - 100 meters.
  • The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
  • Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation
Steel Towers:
  • These towers are robust in construction
  • They can be used for spans 300 m or above
  • They are used for the transmission of power above 66 kV and are more useful for valleys, railway lines, rivers, etc
  • They are mechanically very strong and have a longer life than steel poles
  • They are capable of withstanding the most severe climatic conditions and cannot be destroyed by forest fires

7. The corona is considerably affected by which of the following?
(A) Size of the conductor
(B) Shape of the conductor
(C) Surface condition of the conductor
(D) All of the above

Answer: (D) All of the above

  • A corona discharge is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. 
  • Spontaneous corona discharges occur naturally in high-voltage systems unless care is taken to limit the electric field strength. It is affected by the size of the conductor, shape of the conductor and surface condition of the conductor.

Effect of supply voltage:
  • If the supply voltage is high corona loss is higher in the lines. In low-voltage transmission lines, the corona is negligible, due to the insufficient electric field to maintain ionization.

The condition of conductor surface:
  • If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. 
  • The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. 
  • Thus, rough line decreases the corona loss in the transmission lines.

Air Density Factor:
  • The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air.
  • Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.

Effect of system voltage:
  • Electric field intensity in the space around the conductors depends on the potential difference between the conductors. 
  • If the potential difference is high, electric field intensity is also very high, and hence corona is also high. Corona loss, increase with the increase in the voltage.

The spacing between conductors:
  • If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor.
  •  If the distance between them is extended beyond certain limits, the dielectric medium between them get decreases and hence the corona loss also reduces.

8. Which of the following are the constants of the transmission lines?
(A) Resistance
(B) Inductance
(C) Capacitance
(D) All of the above.

Answer: (D) All of the above

  • a b c d constants of the transmission line( Primary line constants)
  • The line constants are parameters that describe the characteristics of conductive transmission lines.
The primary line has the following constants
  • R = Resistance per unit length Ω
  • L = Inductance per unit length (Henry) H 
  • C = Capacitance per unit length (Henry) H
  • G = Conductance per unit length ℧
  • All these constants are independent of frequency, therefore, they are called primary constants and these constants are measured by considering both the wires of the transmission lines.
  • R and L elements are in series with the line (because they are properties of the conductor) and C and G are elements shunting the line (because they are properties of the dielectric material between the conductors).
  • G represents leakage current through the dielectric and in most cables is very small.

9. The phenomenon of rising in voltage at the receiving end of the open-circuited or lightly loaded line is called the
(A) See-back effect
(B) Ferranti effect
(C) Raman effect
(D) none of the above.

Answer: (B) Ferranti effect


Ferranti Effect: 
  • At no load (or) at light load, the voltage at the receiving end of the transmission line is more than the sending voltage. 
  • It is known as the Ferranti effect. It is due to the charging current of the line.

How to reduce Ferranti effect:
  • This effect can be controlled by placing the shunt reactors at the receiving end of the lines.
  • A shunt reactor is an inductive current element connected between line and neutral to compensate for the capacitive current from transmission lines.
  • Synchronous phase modifier(SPM) is used as under excitation it works similarly to shunt reactor which absorbs lagging reactive power and delivers leading reactive power.
Important Notes:
See-back effect:
The See-beck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.

Raman Effect:  
Raman effect, change in the wavelength of light that occurs when a light beam is deflected by molecules. When a beam of light traverses a dust-free, transparent sample of a chemical compound, a small fraction of the light emerges in directions other than that of the incident (incoming) beam.

10. The square root of the ratio of line impedance and shunt admittance is called
(A) surge impedance of the line
(B) conductance of the line
(C) regulation of the line
(D) none of the above.

Answer: (A) surge impedance of the line

  • The transmission line generates capacitive reactive volt-amperes in its shunt capacitance and absorbing reactive volt-amperes in its series inductance.  
  • The load at which the inductive and capacitive reactive volt-amperes are equal and opposite, such load is called surge impedance load.
  • It is also called natural load of the transmission line because power is not dissipated in transmission. In surge impedance loading, the voltage and current are in the same phase at all the point of the line. 
  • When the surge impedance of the line has terminated the power delivered by it is called surge impedance loading.
  • Shunt capacitance charges the transmission line when the circuit breaker at the sending end of the line is close. 
  • The series inductance of the line consumes the electrical energy when the sending and receiving end terminals are closed.
  • Surge impedance loading is also defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.
  • Surge impedance loading depends on the voltage of the transmission line. Practically surge impedance loading always less than the maximum loading capacity of the line.
  • If the load is less than the SIL, reactive volt-amperes are generated, and the voltage at the receiving end is greater than the sending end voltage. On the other hand, if the SIL is greater than the load, the voltage at receiving end is smaller because the line absorbs reactive power.
  • If the shunt conductance and resistance are neglected and SIL is equal to the load than the voltage at both the ends will be equal.
  • The square root of the ratio of line impedance and shunt admittance is called surge impedance of the line
  • The shunt admittance of a line consists of the conductance and the capacitive susceptance. The conductance is usually ignored because it is very small compared to the capacitive susceptance. The capacitance of a line is the result of the potential difference between conductors.

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