Unit4questions
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The document contains 13 problems related to calculating fault currents in synchronous generator and transformer systems. The problems involve determining line currents, voltages and sequence currents for different fault types including line-to-ground, line-line and line-line-ground faults. Sequence networks are formed and fault calculations are performed based on the given positive, negative and zero sequence reactance values of generators, transformers, transmission lines and other system components.
Unit4questions
- 1. 1. A 25 MVA, 11kV generator has a X’’d = 0.2 pu. It’s negative and zero sequence reactances are respectively 0.3 and 0.1 pu. The neutral of the generator is solidly grounded. Determine the subtransient current in the generator and the line-to-line voltages for the subtransient conditions when an LG fault occurs at the generator terminals. Assume that before the occurrence of the fault, the generator is operating at no load at rated voltage. Ignore resistances. 2. Repeat problem 1 for (a) an LL fault and (b) an LLG fault. 3. A synchronous generator is rated 25 MVA, 11kV. It is star-connected with the neutral point solidly grounded. The generator is operating at no load at rated voltage. Its reactances are X’’ = X2 = 0.2 and X0 = 0.08 pu. Calculate the symmetrical subtransient line currents for (a) single line-to-ground fault (b) double line fault (c) double line-to-ground fault and (d) symmetrical three-phase fault. Compare these currents and comment. 4. For the generator in problem 3, calculate the value of reactances to be included in the generator neutral and ground, so that line-to-ground fault current equals the three-phase fault current. What will be the value of the grounding resistance to achieve the same condition? With the reactance value (as calculated here) included between neutral and ground, calculate the double line fault current and also double line-to-ground fault current. 5. Two 25 MVA, 11 kV synchronous generators are connected to a common bus bar which supplies a feeder. The star point of one of the generators is grounded through resistance of 1.0 ohm. While that of the other generator is isolated. A line-to-ground fault occurs at the far end of the feeder. Determine (a) the fault current (b) the voltage to ground of the sound phases of the feeder at the fault point and (c) voltage of the star point of the grounded generator with respect to ground. The impedances to sequence currents of each generator and feeder are given below : Generator (per unit) Feeder (ohms/phase) Positive Sequence j0.2 j0.4 Negative Sequence j0.15 j0.4 Zero Sequence j0.08 j0.4 6. Determine the fault currents in each phase following a double line-to-ground short circuit at the terminals of a star-connected synchronous generator operating initially on an open circuit voltage of 1.0 pu. The positive, negative and zero sequence reactance of the generator are respectively j0.35, j0.5 and j0.20 and its star point is isolated from ground. 7. A three-phase synchronous generator has positive, negative and zero sequence reactances per phase respectively of 1.0, 0.8 and 0.4 ohm. The sequence winding resistances are negligible. The phase sequence of the generator is RYB with a no load voltage of 11 kV between lines. A short circuit occurs between lines Y and B and earth at the generator terminals. Calculate sequence currents in phase R and current in the earth return circuit (a) if the generator neutral is solidly earthed and (b) if the generator neutral is isolated. Use R phase voltage as reference. 8. A generator supplies a group of identical motors as shown in the figure. The motors are rated 600 V, 90% efficiency at full load unity power factor with sum of their output ratings
- 2. being 5 MW. The motors are sharing equally a load of 4 MW at rated voltage 0.8 power factor lagging and 90% efficiency when an LG fault occurs at the low voltage side of the transformer. Specify completely the sequence networks so simulate the fault so as to include the effect of prefault current. The group of motors can be treated as a single equivalent motor. Find the subtransient line currents in all parts of the system with prefault current ignored. 9. A double line-to-ground fault occurs on lines b and c at point F in the system of given figure. Find the subtransient current in phase c of machine 1 assuming prefault currents to be zero. Both machines are 5%. Each three-phase transformer is rated 1200 kVA, 600 V-Δ/3300 V. The reactances of the neutral grounding reactors are 5% on the kVA base of the machines. 10. A synchronous machine 1 generating 1 pu voltage is connected through a Y/Y transformer of reactance 0.1 pu to two transmission lines in parallel. The other ends of the lines are connected through a Y/Y transformer of reactance 0.1 pu to a machine 2 generating 1 pu voltage. For both transformers X1 = X2 = X0. Calculate the current fed into a double line-to- ground fault on the line side terminals of the transformer fed from machine 2. The star point of machine 1 and of the two transformers is solidly grounded. The reactances of the machines and lines referred to a common base are :
- 3. X1 X2 X0 Machine 1 0.35 0.25 0.05 Machine 2 0.30 0.20 0.04 Line (each) 0.40 0.40 0.80 11. The given figure shows a power network with two generators connected in parallel to a transformer feeding a transmission line. The far end of the line is connected to an infinite bus through another transformer. Star point of each transformer. Generator 1 and infinite bus are solidly grounded. The positive, negative and zero sequence reactances of various components in per unit on a common base are : Positive Negative Zero Generator 1 0.15 0.15 0.08 Generator 2 0.25 0.25 ∞(i.e. neutral isolated) Each Transformer 0.15 0.15 0.15 Infinite Bus 0.15 0.15 0.05 Line 0.20 0.20 0.40 (a) Draw the sequence networks of the power system. (b) With both generators and infinite bus operating at 1.0 pu voltage on no load, a line-to- ground fault occurs at one of the terminals of the star-connected winding of the transformer A. Calculate the currents flowing (i) in the fault (ii) through the transformer A. 12. A star connected synchronous generator feeds bus bar 1 of a power system. Bus bar 1 is connected to bus bar 2 through a star/delta transformer in series with a transmission line. The power network connected to bus bar 2 can be equivalently represented by a star- connected generator with equal positive and negative sequences reactances. All star points are solidly connected to ground. The per unit sequence reactance of various components are given below : Positive Negative Zero Generator 0.20 0.15 0.05 Transformer 0.12 0.12 0.12 Transmission Line 0.30 0.30 0.50 Power Network X X 0.10 13. The reactance data for the three-phase system of the given figure is : Generator X1 = X2 = 0.1 pu. X0 = 0.05 pu
- 4. Xg (grounding reactance) = 0.02 pu Transformer X1 = X2 = 0.1 pu Xg (grounding resistance) = 0.04 pu Form the positive, negative and zero sequence bus impedance matrices. For a solid LG fault at bus 1, calculate the fault current and its contributions from the generator and transformer. Answers: 1. –j6.56 kA, ΙVbcΙ = 12.83 kV, ΙVabΙ = 6.61 kV, ΙVcaΙ = 6.61 kV 2. (a) Vab = Vac = 1.8 pu, Ib = Ic = -2√3 pu (b)Vab = Vac = 0.816 pu, ΙIbΙ = ΙIcΙ = 5.69 pu 3. (i)–j6.25 (ii)-4.33 (iii) 6.01 (iv)-j5 pu In order of decreasing magnitude of line currents the faults can be listed as: (a) LG (b) LLG (c) 3-phase (d) LL 4. 0.1936 ohm. 0.581 ohm, -4.33 pu, j5 pu 5. (a) 3.51 pu (b) Vb = 1.19(-159.50) pu, Vc = 168(129.80) pu (c) 0.726 pu 6. Ib = -Ic = -2.881 pu 7. (a) IY = -5.79 + j5.01 kA, IB = 5.79 + j5.01 kA, IG = j10.02 kA (b)I8 = -IY = -6.111 kA, IG = 0 8. Iag = 0, Ibg = -j2.08 pu, Icg = j2.08 pu, Iam = -j3.51 pu, Ibm = -j1.2 pu, Icm = -j1.2 pu 9. 5,266 A 10. j2.0 pu 11. If = -j6.732 pu, Ia(A) = -j4.779 pu, Ib(A) = -j0.255 pu, Ic(A) = -j0.255 pu 12. 0.42 pu, -j9.256 pu 13. –j11.152 pu, -j2.478 pu, -j1.239 pu