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USN

Time: 3 hrs.

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10EE61

Max. Marks:100

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Sixth Semester B.E. Degree Examination, June/July 2014Power System Analysis and Stability

Note: Answer FIVEfull questions, selectingat least TII/O questions from each part.

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Draw the impedanceper unit impedances

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PART _ Afor ttre power system shown in Fig.Ql(a) and mark on

base of 50 MVA, l3 kV in the circuit ofnces calculated on a Dase oI )u MVA, lJ KV m the cfcult oI ger

Power systemcomponent

Rating

Generator G1 25 MVA, 13 kV, X:: 0.15 PU

Generator Gz 35 MVA, 22 kV , X; : 0. 12 PU

Transformer T 30 MVA, 220Y il3.84 kV X: l0%Transforrner T' 40 MVA, 220120 kV X: 12%Transformer T:l Bank of lQ Transformers, each rated

10 MVA, t27lt\ kV X :8%Load, Ln 3+i1 QLoad, Ls 4+i2 OTransmission L ne Ttr i60 oTransmission L ne TLu i90 o

Fig.Ql(a) (12 Marks)b. Write in matrix form, the node equations necess ary to solve for the voltages of numbered

buses as shown in Fig.Ql(b). All the impedances are marked in per unit. The emf s shownare Eu : 1.5/.0o , Eu : 1.51-36.87 o and E. : 1.510". (08 Marks)

Fie.Q1(b)

2 a. Explain in detail the transients on a transmission line. (08 Marks)b. For the radial network shown in Fig.Q2(a), a 3 phase fault occurs at F. Determine the fault

cur:rent and line voltage at 11 kV bus under fault condition. Select a base of 100 MVA,11 kV on generator side. (12 Marks)

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3a.

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Prove that a balanced set of 3-phase voltages will have only positive sequence componentsof voltages only. (06 Marks)A delta connected balanced resistive load is connected across an unbalanced 3-phase supplyas shown in'fig.Q3(b). With currents in lines A and B specified, find the symmetricalcomponents of line currents. Also, find the symmetrical components of delta currents(phase currents). (14 Marks)

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Fig.Q3(b)4 a. Draw the zero sequence equivalent circuit for the following conditions of transformer:

i) ox1t , "iii) o;rro "

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v) o-th- " (10 Marks)b. Draw the positive, negative and zero sequence networks for the power system shown in

Fig.Qa@). Choose a base of 50 MVA,220 kV in the 50 f) transmission lines. Mark all thereactances in p.u. The ratings of the generators and transformers are

Gen Gr : 25 MVA, I I kV, X" :20o/o

Gen Gz : 25 MVA, 1l kV, X" :20o/o

Three phase transformers (each): 20 MVA, llYl220Y kV, X : l5ohThe negative sequence reactance of each slmchronous machine is equal to its subtransientreactance. The zero sequence reactance of each machine is 8%. Assume that zero sequencereactance of lines are 250Yo of their positive reactances. (10 Marks)

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108E61

PART _ B5 a. Draw the interconnected sequence networks for the following cases:

(r) L-G fault through fault impedance zr(ii) L-L fault through fault imped&ncr- 21

(iii) L-L-G fault through fault impedance ziclearly indicating positive, negative and zero sequence impedance, symmetrical componen[sof voltages and currents. Also, write the expressions for fault current in the above threecases. (12 Marks)

b. A three phase generator with line-to-line voltages of 400 V is subjected to L-L-G fault. IfZ;j2 Q, Z2:i0.5 f) and Z0:j0.25 f). Determine the symmetrical components of currentsand fault current. (0S Marks)

Derive an expression for fault current in case of line-to-ground fault on an unloadedgenerator. (10 Marks)Draw the sequence networks for the system shown in Fig.Q6(b). Determine the fault cunentif a line-to-line fault occurs at point F. The p.u. reactances all referred to the same base areas follows. Both the generators tins 1.0

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Fig.Q6(b)

7 a. A turbo generator, 6 pole, 50 Hzof capacity 80 MW working at 0.810 MJ/MVA.i) Calculate the energy stored in the rotor at synchronous speed.

(10 Marks)

p.f. has an inertia of

(08 Marks)

ii) Find rotor acceleration if the mechanical input is suddenly raised to 75 MW for anelectrical load of 60 MW.

iii) Supposing the above acceleration is maintained for a duration of 6 cycles, calculate thechange in torque angle and rotor speed at the end of6 cycles. (10 Marks)

Explain the equal area criterion when there is sudden loss of one of the parallel lines.(t0 \larks)

A load free alternator supplies 50 MW to an infinite bus, the steady state stability being100 MW, determine if the alternator will remain stable if the input to alternator is abruptlyincreased by 40 MW.Write short notes on any three of the following :

i) Analysis if three phase induction motor with unbalanced voltageii) Methods of improving steady state stabilityiii) Impedance and reactance diagramiv) Swing curvev) Selection of circuit breakers.

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Component Xe Xr XzGr 0.05 0.30 0.20Gi 0.03 4.25 0. 1s

Line-l 0.70 0.30 0.30Line-2 0.70 0.30 0.30Tr a.t2 0.r2 0.r2Tt 0. 10 0.10 0.10

(12 Marks)



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