MCQs on Power Systems

01․ The conductor carries more current on surface as compared to core. This is called as

Accumulation of current on the surface of the conductor is called skin effect. Due to skin effect the effective area of current flowing path is reduced. Where, f = frequency µr = relative permeability σ = conductivity Skin effect depends on 1. Supply frequency 2. Relative permissibility 3. Conductivity σ 4. Size of the conductor

02․ Corona is accompanied by

Corona is the electrical discharge of atmospheric air. If the operating voltage is more than dielectric strength of atmospheric air, air molecules are ionized and start accumulating on the conductor surfaces producing corona. The voltage at which corona is just initiated is called critical disruptive voltage(Vc). At this critical disruptive voltage, corona is not visible but noise can be heard. The voltage at which corona is visible with blue or violet colour is called critical visual voltage(Vv). The effects of corona are 1. Interference increases 2. Power loss increases 3. Charging current increases

03․ If sag in an overhead line increase, tension in the line

Where, l = length of the conductor w = weight of the conductor T = tension Therefore, by increasing of temperature, the length of the conductor will increase and this will increases the sag. Tension is inversely proportional to sag. Sag S ∝ 1/T As sag increases, tension will decrease.

04․ By using guard ring in a transmission line, its string efficiency

Guard ring serves two purposes 1. Equalization of voltage drop across the units 2. Protects the insulator string from flash over whenever on over voltage appears between the tower structure and the power conductor Due to these its string efficiency increases.

05․ Suspension type insulators are used for voltage beyond

Overhead line insulators: 1. Pin insulators used up to 33 kV. 2. Suspension type insulators all voltages 33 kV to 765 kV. 3. Strain insulators are used in river/ road/ railway crossings and dead end end tower to reduce sag. Pin type insulators are used only up to 33 kV. Beyond this voltage more insulation material is required, which increases cost of the insulation. At higher voltage suspension or string insulators are used.

06․ In a transmission line the insulators to be used at the dead end or sharp curves are

Strain insulator is one of the suspension insulator is used to reduce sag in the transmission lines connected between the two towers and it is practically used in railway crossings, road crossings, river crossings and in the dead end towers.

07․ For 66 kV lines the number of insulator discs of each 11 kV used are

Number of insulator discs n = operating voltage per phase/ maximum voltage of each disc For saftey purpose, one extra insulator is required. Therefore the total number of discs are 4+1 = 5.

08․ The corona loss on a particular system at 60 Hz is 1 kW/km per phase. What is the corona loss at 50 Hz in kW/km per phase?

09․ A pin type insulator has 100 kV puncture voltage. If 10 kV flash over voltage occurred in a transmission line, then find the safety factor of insulator?

Safety factor

10․ A string insulator has 4 units. The voltage across bottom most unit is 10 kV and its efficiency is 90%. Find the total operating phase voltage?

Where, n = number of discs