Transmission Line

Question 1

Conductors

Answer 1

Conductors inside a cable must be insulated to the ground

Question 2

Overhead Lines

Answer 2

• Cheaper Construction & cable cost
• Advantage of air cooling and insulation of line
  -Vulnerable to strong winds and severe weather conditions
• Negative Visual impact
• Inductive in nature

Question 3

Underground Cables

Answer 3

• Expensive pipe work & cable cost
• Expensive insulation required at high voltage
• Immune to severe weather
• Environment and aesthetic advantage
• Less frequent maintenance work
• Capacitive nature

Question 4

Resistance

Answer 4

• Heat
• Transmission Line losses (I^2R)

Question 5

Conductance

Answer 5

• Leakage current from insulation
• Types and no of insulators
• Electric field strength strong = electrically ionised = Corona effect

Question 6

Inductance

Answer 6

Ampere’s Law (Magnetic field)

Question 7

Capacitance

Answer 7

Gauss’ law (Electric Field)

Question 8

Impedance matrix (short line)

Answer 8

A = 1,B = Z, C = 0, D = 1

Ignore shunt value, series Y only

Question 9

Impedance matrix (medium line)

Answer 9

A = (YZ/2 +1)
B = Z
C= (YZ/4 +1)Y
D = (YZ/2 +1)

Question 10

Receiving End Voltage at No Load

Answer 10

Vr,nl = Vs,fl /A

Question 11

Voltage Regulation

Answer 11

• Variation of load voltage with different loading conditions
• 10% tolerance between no load and full load
• Measures degree of change in voltage at specific PF
  % Regulation = (Vr,nl - Vr,fl) / Vr,fl * 100%

For short line: Vs,fl = Vr,nl as A =1

Question 12

Transmission Efficiency

Answer 12

n = Re(Vr x Ir)/ Re (Vs x Is) * 100%

Question 13

Inductor and capacitor are energy storage elements in an electric circuit

Answer 13

True

Question 14

Loss in a transmission line (i.e. heat) is mainly due to the line reactance

Answer 14

False, resistance provides transmission line losses (heat)

Question 15

Conductance of the transmission line model captures the di-electric effect

Answer 15

True

Question 16

A short transmission line model does not account for shunt admittance

Answer 16

True. Only accounts for series impedance.

A = 1
B = Z
C = 0
D = 1

Question 17

Receiving-end voltage of a transmission line is the same as the sending-end voltage at full-load condition for the medium transmission line model.

Answer 17

False. Only for no load condition where Vr,NL = Vs,FL /A and A=1

Question 18

The di-electric effect within the conductors is modelled as admittance

Answer 18

True

Question 19

Underground cable and overhead line have similar inductance per km for same voltage and power ratings

Answer 19

False. Underground Cable has lesser inductance and more capacitance.