## POWER SYSTEMS-2 JNTU previous years question papers

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. (a) Derive an expression for the inductance per phase for a 3-phase overhead transmission line when conductors are unsymmetrical placed but lines are untransposed.

(b) Calculate the inductance and reactance of each phase of a three-phase 50Hz overhead high-tension line (HTL) which has conductors of 2.5cm diameter. The distance between the three-phases are [8+8]

i. 5m between A and B,

ii. 4m between B and C and

iii. 3m between C and A . Assume that the phase conductors are transposed regularly.

2. (a) What is a nominal circuit? Find ABCD constants of nominal-T and nominal circuits of a transmission line.

(b) A balanced 3-phase load of 35 MW is supplied at 110 kV, 50Hz and 0.8 p.f lag by means of a transmission line. The series impedance of a single conductor is (15+j35) ohms and the total phase-neutral admittance is 300 × 10-6 mhos.Use nominal - T method to determine

i. A, B, C, D constants of the line,

ii. Vs

iii. regulation of the line and

iv. efficiency.

3. (a) Derive the equivalent ABCD constants of a transmission line connected in series with an impedances at both ends.

(b) The per-unit-length parameters of a 215kV, 400km, 60Hz, three phase long transmission line are y = j3.2 × 10-6 mhos per km per phase and z = (0.1 + j 0.5) ohm/km. The line supplies a 150 MW load at unity power factor. Determine

i. the voltage regulation

ii. the sending-end power and

iii. the efficiency of transmission. [8+8]

4. (a) What is the outcome of the transient in the transmission lines? Develop the differential equation for the transient in the transmission system.

(b) A 500 kV, 2 µsec, duration rectangular surge passes through a line having surge impedance of 350 and approaches a station at which the concentrated earth capacitance is 3×103 pF. Calculate the maximum value of surge transmitted to the second line. [8+8]

5. (a) What is corona? What are its effects and how are they reduced?

(b) Find the disruptive critical voltage and visual voltage of a 3-phase transmission line operating at 132 kV, the conductors being 1.5 cm in diameter and arranged in delta with spacing of 4 m. The surrounding air is at a temperature of 320C and barometric pressure 74 cm of mercury. The various factors are irregularity factor = 0.85, surface factor for local corona = 0.7, surface factor for decided corona = 0.80, Breakdown strength of air = 21.21 kV/cm, assume fair weather condition. [8+8]

6. (a) Explain why suspension type of insulators are preferred for high voltage overhead lines. Sketch a sectional view of one unit of the suspension type insulator and describe the construction.

(b) An insulator string containing five units has equal voltage across each unit by using disc of different capacitances. If the top unit has a capacitance of C and pin to tower capacitance of all units is 20 percent of the mutual capacitance of top unit. Calculate mutual capacitance of each disc in a string. [8+8]

7. (a) Explain the factors affecting the mechanical design.

(b) An over head line with stranded copper conductor is supported on two poles 200 meters apart having a difference in level of 10 m the conductor diameter is 2 cm and weighs 2.30 kg/m square meter of the projected area and the factor of safety is 4. The maximum tensile strength of the copper is 4220 kg/square meter. [6+10]

8. (a) Describe briefly some commonly used insulating materials for cables.

(b) A 12.5 kV single-core cable has an outside diameter of 8 cm. Determine the radius of the core and the electric field strength that must be withstand by the insulating material in the most economical (optimal-ratio) configuration