Two Marks Questions with Answers

Two Marks Questions with Answers

Q.1 Define sag.

Ans. : When a conductor is suspended between two points then it takes the shape of parabola or catenary and sags down.

The difference in levels between the point of support and the lowest point on the conductor is called sag. 

Q.2 What is stringing chart ? What is its use ?

Ans. : The tension at the time of erection of a transmission line is given by a cubic equation hence it is time consuming to solve such equation. Instead of solving such a equation the graph of tension in kg against the temperature in °C and the graph of sag in metres against the temperature in °C is obtained. Such graphs are called the stringing chart.

The stringing chart is very useful to find the tension and the sag at any temperature and the loading conditions when these values at any other temperature and the loading conditions are known to us.

Q.3 What is the effect of wind and ice loading on sag ?

Ans. : Due to wind pressure the conductor is subjected to the additional forces acting horizontally. While due to ice loading the weight of the conductor increases which acts vertically downwards. Both these factors decide the resultant weight of the conductor which decides the sag.

Q.4 What is the economical value of span for 400 kV transmission line ?

AU : May-10

Ans. : The span must be between 200 m to 400 m. For river and ravine crossings exceptionally long spans up to 800 m or so is sufficient.

Q.5 Give two factors which affect the sag in transmission line.

AU : Dec -10, 12,16, May-17

Ans. : The two important atmospheric factors affecting the sag in transmission line are,

i) Ice coating on the conductor which increases weight of the conductor

ii) Wind pressure due to which the conductor gets subjected to the additional forces

Apart from these two factors the span, weight of the conductors and tension in the conductor also affect the value of sag.

Q.6 Give the expression of sag when the supports are at equal level.

Ans. : The sag is given by, S = (w L²)/(8 T) where w = Weight per unit length of conductor, L = Length of span and T = Tension in the conductor

Q.7 Define factor of safety.

Ans. : The ultimate stress which conductor can sustain without fail is called breaking stress. The normal tension in the conductor is called working stress. The ratio of breaking stress to the working stress is called the factor of safety denoted as Sf.

Q.8 What is sag template ? What is its use ? 

Ans. : For normal spans and for standard towers, the sag and the nature of the conductor curve are calculated under expected load conditions and plotted on a thin stiff plastic sheet. Such a graph is called sag template.

The sag template can be used for i) Maintaining safe ground clearance along the entire route of the line ii) Choosing span and height of towers with good accuracy iii) Positioning the towers properly iv) Selecting the towers which carry equal loads  v) Obtaining overall economical layout of the system.

Q.9 Which are the types of vibrations possible in overhead conductors ?

Ans. : The two types of vibrations possible in overhead conductors in vertical plane are, i) Aeoline vibrations having frequency range of 8 to 40 Hz with amplitudes varying from 2 to 5 cm. ii) Galloping or dancing of conductors at the low frequency and high amplitude.

Q.10 Mention the factors affecting the conductor vibrations.

Ans. : 1. Atmospheric conditions such as icing, raining and high winds 2. Span of conductor 3. Tension in the conductor  4. Types of clamps used 5. Conductor configuration 6. Height of the towers 7. Number of dampers

Q.11 Give the minimum ground clearance required for 33 kV, 66 kV, 110 kV and 220 kV lines.

Ans. : A sufficient clearance of a conductor from ground under all loading conditions is required to be maintained for safety. The clearance distance depends on transmission voltage.

For 33 kV, a clearance of 5.18 m is to be provided and for every additional 33 kV or for 33 kV line ground clearance is 5.18 m (17 feet) part thereof, additional 0.3048 m clearance is to be provided.

For 66 kV, 66 kV = 33 kV + (1 × 33)

Ground clearance = 5.18 + 1 × 0.3048 = 5.4848 m (18 feet)

For 110 kV line

110 kV = 33 kV + 2.33 × 33

Ground clearance = 5.18 + (2.33 × 0.3048) = 5.89 m (19.32 feet)

For 220 kV line

220 kV = 33 kV + 5.66 × 33

Ground clearance = 5.18 + 5.66 × 0.3048 = 6.90 m (22.63 feet)

Q.12 What is deviation tower ?

Ans. : The transmission line goes as per available straight paths as far as possible. Due to unavailability of shortest distance straight corridor, transmission line has to deviate from its straight way when obstruction comes. The towers used in such cases to deviate the route of the transmission lines are called deviation towers or angle towers. The design of such towers is such that they can withstand large mechanical stress.

Q.13 What is meant by tower spotting ?

Ans. : The art of locating structures of towers in a right way and selecting their type and height so as to meet all the necessary electrical requirements is called tower spotting. The sag template is used for tower spotting.