Year 2011
4. (a) Starting from fundamentals, derive the equations for sending end voltage and sending end current for a long transmission line. Use the distributed parameter form of representation of the long line. Obtain the A, B, C, D parameters of the line. 18
(b) A synchronous generator is rated 20 MVA, 13.8 kV. It has X1 = 0.25 p.u., X2 = 0.35 p.u. and X0 = 0.10 p.u. The neutral is solidly grounded. The machine is on no load and is operating at rated voltage when a D-L-G fault occurs at its terminals. Find the subtransient current in all the phases and the fault (all in amps) and the line-to-line voltages in all the phases (in kV). 20
(c) Sketch typical swing curves for a synchronous machine (i) showing that the machine is stable after a disturbance (ii) showing that the machine is unstable after a disturbance. 2
5. (a) A ring feeder with five sections and fed at one point is to be protected using Directional over current (DOC) Relays and Over current (OC) Relays with suitable time grading. Explain the working of the scheme. Show the location of D.O.C’s and O.C’s and their time of operation. Assume a time grading of 5 ms between the relays. The fastest relay needs 5 ms for it to operate. 15
(b) (i) A surge of 100 kV is incident on a Line having a surge impedance of 400 ohms, It meets a cable having a surge impedance of 40 ohms, Derive expression for the transmitted voltage and reflected voltage and compute their values. 7
(ii) Explain the practical importance of this situation. 3
(c) A 3-phase double circuit line is arranged as shown below :
The conductors are transposed. The radius of each conductor is 0.75 cm. Phase sequence is ABC. Find the Inductance per phase per km.
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