ELEMENTS OF POWER SYSTEM

UTTARAKHAND TECH. UNIVERSITY

utu previous year question papers

B TECH (sem iv),2012

SUB:- elements of power system

time : 3hr]

Total marks :100

SECTION A

Q1:- Attempt any four of the following :

1. Compare overhead transmission system with underground transmission system with underground  transmission system on atleast 10 major grounds.
2. Differentiate the following:(a) Skin effect and proximity effect.
   (b) Circuit breaker and isolator.
3. What is the % saving in feeder copper if the line voltage in a 2-wire dc system be raised from 220 volts for the same power transmitted over same distance and having the same power loss?
4. State Kelvin's law for obtaining the size of the conductor for transmission. Discuss its limitations.
5. Describe ACSR conductor used in transmission line.
6. The cost of 3-Ф overhead line having cross-section area 'a' Cm² is Rs. (500+2,000a) per kilometer. Calculate the most economical current density for the most economical current density for the conductor if the rate of interest and depreciation is 12% per annum and the cost of energy wasted is 5 paise per kWh. The resistivity of conductor may be taken as (1.7/a) ohm/km. Take load factor of 12%.

SECTION B

Q2:- Attempt any four of the following :

1. Describe 'Ferranti effect' in long transmission line. How can it be compensated?
2. A 3-phase line, 3 km long delivers, 3000 kW at a power factor of 0.8 lagging to load. If the voltage at the load end efficiency of transmission. The resistance and reactance per km of each conductor are 0.4 ohm and 0.8 ohm respectively.
3. Define the generalized A,B,C,D constants of a transmission line and determine their values for a long transmission line. Hence show A=D and AD-BC =1
4. Calculate the loop inductance per km of a single phase transmission line comparing of two parallel conductors one meter apart and 1.25 cm in diameter. Also calculate the reactance of the transmission line. Frequency is 50 Hz.
5. Explain surge impedance loading with respect to an overhead transmission line. Show that 
6. Derive an expression to calculate the capacitance of a double circuit line. All the conductors are of equal diameter and are spaced hexagonally.

SECTION C

Q3:- Attempt any two of the following :

1. (a) Explain the phenomenon of corona. Also discuss its advantage and disadvantages.
   (b) A certain 3- loss of 53 kW at 106 kV and a loss of 98 kW at 110.9 kV. What is the disruptive critical voltage between lines? What is the corona loss at 113 kV?
2. What are the different methods use to improve the string efficiency. Discuss each of them with the help of neat sketch.
3. Define string efficiency. A string of six insulator units has a self-capacitance. Find the voltage distribution across various units across the string efficiency.

SECTION D

Q4:- Attempt any two of the following :

1. Derive expression for a sag and tension in a power conductor strung between two supports at equal heights taking into account the wind and ice loading also.
2. Describe the following in detail:
   (a) Vibration dampers
   (b) Sag template
3. A transmission line conductor has a diameter of 19.5 mm and weighs 0.85 kg/m the span is 275 meters. The wind pressure is 39 kg/m² of the projected area with ice coating of 13 mm the ultimate strength of the conductor is 10,000 kg. Calculate the maximum sag if the factor of safety is 2 and ice weighs 910 kg/m³.

Q5:- Attempt any two of the following :

1. Derive an expression for the insulation resistance of a single core cable. Calculate insulation resistance for a 5 km length of a 1-core cable. Specific resistance of insulation (impregnated paper) is ohm-cm, insulation thickness is 1 cm and radius of conductor is 2.5 cm.
2. What do you mean by grading of cables? Discuss different methods of cable grading.
3. A single core cable 1.0 km long has a core diameter under sheath 2.0 cm. The relative permittivity of the insulating material is 3.5. The power factor on open circuit is 0.05 and the supply voltage is 11 kV , 50Hz. Determine i) the capacitance of the cable ii) the charging current per conductor iii) the dielectric loss and iv) the equivalent insulation resistance.

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