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Unformatted text preview: Page 1 0f 6 NORTH CAROLINA STATE UNIVERSITY
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING Lecturer: John W. Gajda, P.E. ECE451 Test 3 Fall 2010
NAME » '53 5 W7 .' Problem No. 1 (possible 25 pts.)
Problem No. 2 (possible 25 pts.)
Problem No. 3 (possible 25 pts.)
Problem No. 4 (possible 25 pts.)
TOTAL (possible 100 pts.) NOTE: THIS IS A STRICTLY CLOSED BOOK EXAM. ALL WORK MUST BE
SHOWN. DO ALL FOUR PROBLEMS. Honor pledge: I certify that I have neither given nor received unauthorized aid while taking this test. Signature: Problem Page 2 of 6 No. 1 (25 points) A circuit breaker is closed in to a faulted circuit.
 The source is 60 Hz, 22.86 kV linetoline RMS nominal voltage, infinite bus  The circuit beyond the breaker is faulted one mile from the substation with a three
phase bolted fault.
 That mile of line has a resistance of 1.0 ohms, and a reactance of 6.0 ohms.
fl. Parts A & B (15’ points): What is the (a) the angle in the cycle (alpha) and (b) the
instantaneous value of the voltage (linetoneutral) which will produce the maximum DC / \ _ .J " W.
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cycles after the breaker closing. ‘_ _ , . 7— s . . ‘ f __ r ‘I J /.
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Part D (2 points): Wthical tripping time for a modern vacuumbottle circuit breaker? Circle one: (1 cycle I w/ 15 cycles 1 30 cycles) Page 3 of 6 Problem No. 2 (25 points) A 60 Hz generator is rated 2.8 MVA, 480 V (linetoline), with X”d = 0.25 per unit. It is
controlling its bus voltage to 1.0 per unit, and it is sewing a 2.0 MW load running at 0.9
lagging power factor. if the generator suddenly undergoes a three phase bolted fault at its
terminals, find the magnitude of the initial symmetrical rms current in the generator. First
find the answer in perunit, then calculate the answer in real amperes, using the machine ratings for your perunit bases. ,uJ )
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x. Page 3 of 6 Problem No. 3 (25 points) Page 4 of 6 The following currents are measured at a feeder circuit breaker monitoring system: A phase = 150 amps, ang 90 deg
B phase = 160 amps, ang 320 deg
C phase = 170 amps, ang 220 deg Assume this is an ABC sequence system. Part A (20 points): Calculate the positive, negative, a
and the zero sequence quantities for B phase and for
each worth 4 points). f i r r
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a , 1 r. Part B (3 points): This circuit breaker serves the only nd zero sequence quantities for A phase,
C phase (you will have five answers, feeder out of a substation with a transformer that has a wyegrounded secondary. How many amps will the feeder breaker
ground relay see? (don't worry about CT ratio here...assume a CT ratio of 1:1, so the relay
sees ) Another way to ask this question: What is the number of amps that will be flowing in the transformer secondary neutral? l 1 Part C (2 points): In generators, what type of sequence current is not produced by the generator but can have dama in heating effe_cts for the generator rotor? Circle one: (positive sequence current [\negative sequence—[email protected] zero sequence current) Page 4 of6 Page 5 of 6 Problem No. 4 (25 points) An isolated generator (not connected to the grid) is rated 419 kVA, 480V, and has X”d = 25%,
X2 = 20%, and X0 = 5%. It has a wye~grounded I wyegrounded stepup transformer with an
impedance of X = 10%. For each question below, assume a bolted fault at the transformer’s
high voltage terminals, and assume that all faults are subtransient. Part A (4 points): Find the magnitude of a three phase fault in amperes perunit. X247 , Xe
, .t \ \ ' ‘1 Part B (7 points): Assume the generator is wyegrounded. Draw the appropriate sequence
network diagram used to find the magnitude of a single linetoground fault {with all impedances labeled), and calculate the perunit value of the singlelinetoground fault in
amperes perunit. Page 5 of 6 Page 6 of 6 Part C (7 points): Assume the generator is grounded with a 1 ohm neutral reactor. Draw the
appropriate sequence network diagram used to find the magnitude of a single line~toground
fault (with all impedances labeled), and calculate the per—unit value of the singlelineto ground fault in amperes perunit. .f I g (a an? ’ ,3 ,T’i" iii ‘ '
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f‘ _ r; ' I Part C (7 points): Assume the generator is ungrounded. Draw the appropriate sequence
network diagram used to find the magnitude of a single linetoground fault (with all
impedances labeled), and calculate the perunit value of the singlelinetoground fault in amperes peraunit. ._< ,‘J ‘I
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 Fall '11
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