# Objective Questions on Electrical Transmission

1. The concept of an electrically short, medium and long line is primarily based on

L=length of the line. If L less than equal to 100km, L is short line. If L less than equal to 250km, L is medium line. If L is greater than 250km then L is long line.

2. The main consideration for higher & higher operating voltage of transmission is to

always increase power transfer capability for higher opering voltage of transmission.because power also depent on voltage.

3. For transmission line which of following relations is correct?

The condition for a reciprocal transmission is AD − BC = 1.

4. Which statement is false for long transmission line?

For long transmission line if the whole line length is divided into n sections, each section will have line constants 1/n th of these for the whole line.

5. In nominal T method,

In this method, the whole line capacitance is assumed to be concentrated at the middle point of the line and half the line resistance and reactance are lumped on its either side. Therefore in this arrangement full charging electric current flows over half the line.

6. Which combination is true for short lines?

Vs =A VR +B IR -------(1)
Is =CVR +D IR --------(2)
For short transmission line, Is = IR --------(3)
Vs = VR + IR Z--------(4)
Now, comparing equation (1) and (2) with (3) and (4) we have,
A=1, B=Z ,C=0 ,D =1

7. In a DC transmission line,

When a sinusoidal voltage is applied through a purely inductive or capacitive circuit, the electric current flowing is determined by the reactance of the circuit. In DC lines the electric current does not alternate (oscillate) like it does for an AC line. That is why there is no effects due to inductive and capacitive reactances.

8. A 220 kV,20 km long,3-phase transmission line has the following A, B, C, D constants where A = D = 0.96, C = 0.037, B = 25. Its charging electric current will be

IS=CVR+DIR
Here, VR = 220/√3
Let, IR=0, ∴ IS=0.05X220/√3=11/√3A

9. For a 500 Hz frequency excitation, a 50 km long power line will be modelled as

A line longer than λ/20 may be modelled as a long line. For 500 Hz, λ=(3X108)/500=600km
Now, λ/20=600/20=30, therefore a line of 50 km legth should be modelled as long line.

10.  list-1 list-2 Type of transmission line type of distance relay preferred a. Short line 1. Ohm relay b. Medium line 2. Reactance relay c. Long line 3. Mho relay

For short distance transmission line reactance relay is preferred, for medium transmission line ohm relay is preferred, and for long transmission mho or admittance relay is preferred.

11. High voltage DC transmission (HVDC) transmission is mainly used for

HVDC systems are economical for long distance bulk power transmission by overhead lines, due to reduced power & cable costs..

12. The capacitance effects of short transmission line is

The length of an overhead short transmission line is upto about 50 km and the line voltage is comparatively low (< 20 kV), it is usually considered as a short transmission line. Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected.

13. Up to what length of transmission line the capacitance can be neglected?

For short transmission line (upto 75km) capacitance C can be neglected. In medium and long transmission line cannot be neglected.

14. Ferranti effect is seen in which transmission line?

When long transmission line having high capacity is unloaded / lightly loaded it is sometimes observed that the receiving end voltage becomes more than the sending end voltage and this is observed in long transmission line.

15. For a 3Φ transmission line transmission efficiency of a line decreases with

For a given amount of power to be transmitted, the receiving end voltage, the load current I is inversely proportional to the load p.f. cosφR.
Since P = V I cosφR.
∴ With the decrease in load p.f. the load current is increased, hence the line losses are increased. This leads to the conclusion that transmission efficiency of a line decreases with decrease in p.f.

16. For a medium transmission line which one is true?

In a medium transmission line.
IS =CVR +DIR-----------------(1)
and VS = V1 + ISZ/2
where V1= VR + IR Z/2
Now electric current flowing through the line,
IC = IS - IR
= V1Y = Y (VR+ IR Z /2)
or IS = Y VR+ IR(1 + YZ/2)---------------(2)
Comparing equation (1) & (2), D = 1 + Y Z/2

17. Which combination is true for any transmission line?

For any transmission line AD − BC = 1
Now by trial method, in option A, AD − BC = 52X0.385 − 25X0.8 = 1

18. In nominal π method, find the sending end voltage. Given, R = 27.5 Ω, X = 97.4 Ω, Y = 7.38 × 10-4 ∠90° mho, VR = 76.2∠0°, IR = 109.3× 10-3∠-36.9°.

Z = R + jX = 101.2∠74.2o
For nominal π method VS = (1+1/2 YZ)VR+ZIR
= 82.5∠5.2o

19. If in a short transmission line, resistance and reactance are found to be equal and regulation appears to be zero, then the load will be

The voltage regulation becomes zero for a loading power factor load whose power factor angle θ is given by tan-1X/R where X and R are respectively the reactance and the resistance of the short line. As the resistance and reactance to be equal, θ = 45o and therefore the p.f. of the load shuld be cos45o = 0.707(leading)

20. A long transmission line has the following auxiliary constants: A = D = 0.98∠2°, B = 28∠69°, C = 0.00282∠53°. Two similar transformers, each of impedance 12∠80° Ω are interposed at the two ends of the above transmission line. Determine the values of Ao to represent the line with the transformer.

Ao = A + CZse
= 0.98∠2o + 0.00282∠53o X 12∠80o
= 0.9563 + j0.059 = 0.958∠3.53o