What is the significance of surge impedance

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Reference no: EM131180195 , Length: 4

Question 1.

a) What is the significance of Surge Impedance Loading 2 x 10 in over head transmission Line?

b) What is resistance grounding? Write its merits and demerits.

c) Draw a single line diagram of a typical a.c. power supply.

d) What is the effect of load p.f. on regulation and efficiency of a transmission lines?

e) A 30 MVA, 11 kV generator has a reactance of 0.2 pu referred to its rat ngs as bases. Determine' the per-unit reactance when referred to base of 50,000 kVA and base kV of 33 kV.

f) Discuss in brief about the various types insulators used with over head lines.

g) Prove that the Insulation Resistance vanes inversely with the length of the cable.

h) Write the advantages, disadvantages and methods of improving corona loss.

i) Why is 3-Φ symmetrical fault more severe than a 3-Φ unsymmetrical fault?

j) State Kelvin's Law and give limitations on economic choice of conductor size.

Question 2. a) Derive an expression for the loop inductance per kilometer of a single-phase transmission line without ground.

b) A 100 km double circuit transmission line with 7 strand copper conductors, each strand having 5 mm diameter, has 6 conductors arranged in the form as shown in the given figure.

The ine is transposed at regular intervals. Evaluate the GMD and GMR. Calculate the overall inductance and capacitance per phase of the line.

Question 3. a) Derive the mathennatical expression for sag with the effect of both ice and wind load on transmission line.

b) A transmission lire conductor at a river crossing is supported from two towers at heights of 50m and 80m above water level. The horizontal distance between the towers is 300 meters. If the tension in the conductor is 2,000 kg, find the clearance between the conductor and water level at a point mid-way between the towers. Weight of conductor per meter = 0.844 kg. Assume that the conductor takes the shape of parabolic curve.

Question 4 a) Derive the expression for the capacitarce of an unsymmetrical transposed 3-phase transmission line.

b) Determine the most economical cross section for a 3- Φ transmission line, 1 km long to supply at a constant voltage of 110KV for the following daily load cycle:

6 Hours        20MW     at 0.8 p.f. lagging
12 Hours      5MW       at 0.83 p.f. lagging
6 Hours        6MW       at 0.9 p.f. lagging

The line is used for 365 days annually. The cost per km of line including erection is Rs (9000 + 6000a) where 'a' is the area of X-section of conductor in cm². The annual rate of interest and depreciation is 10% and energy costs 8P per kWh. The resistance per km of each conductor is 0.1765/a.

Question 5. a) A 25 MVA, 13.2 kV alternator with solidly grounded neutral has a sub-transient reactance of 0.25pu. The negative and zero sequence reactances are 0.35 pu and 0.1 pu respectively. A single phase to ground fault occurs at the terminals of this unloaded generator. The fault impedance is j0.1 pu. Compute fault current, terminal voltage under fault condition.

Express all currents and voltages under faulted condition as a percentage of rated values. Express sequence components of currents in the form of percentage of full load current.

b) Determine the regulation and efficiency of transmission for a 3-phase 200 km, 50 Hz transmission line delivering 40 kW at a power factor of 0.75(lagging) and 66 kV to a balanced load. The inductive reactance is 37.5 ohm per phase and capacitance is 0.8851.1F per phase. Assuming the total resistance of the line to be 10 ohm per phase. Use nominal T-method for analysis.

Question 6. a) Compare EHVA.C. and HVDC Transmission System.

b) Determine (i) the Critical Disruptive Voltage, (ii) the Visual Critical Voltage and (iii) The Corona Loss under foul weather condition for a 3-phase line of 160 km long, conductor diameter of 1.036 cm; 2.44 m delta spacing. Air Temperature 26.6°C, corresponding to an approximate barometric pressure of 73.15 cm of mercury, operating voltage 110 kV at 50 Hz, surface irregularity factor 0.85. Assume a value of mv=0.72. lDisruptive voltage under foul weather--0.8 X fair weather value. (Use Peek's Formula).

Question 7. a) A DC Distributor AB is fed at both ends. At feeding point A, the voltage is maintained at 240V and at B, .241V. The total length of the distributor is 300m and loads are tapped off as:

30A at 50m from A

50A at 100m from A

25A at 175m from A

30A at 250m from A

If the resistance per Km of one conductor is 0 5Ω. Calculate the minimum voltage and the point at which it occurs.

b) A simple Power System is shown in the Figure given below. Redraw the System where the per-unit impedances are represented on a common 5,000 VA base and common system base voltage of 250 V.

Question 8. Write short notes on (Any two)
a) Grading of cable
b) Methods of improving string efficiency.
c) Transposition of transmission line
d) Classifying the substations.

Reference no: EM131180195

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len1180195

8/24/2016 3:32:36 AM

Answer given problems including problem no 1 which is required. The figures in the margin indicate marks. You are required to give their solution in their own words as far as practicable and all sections of a problems should be answered at one place only.

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