Download - Syn Gen Transient Analysis
-
8/2/2019 Syn Gen Transient Analysis
1/17
THE OBJECTIVES OF THIS EXPERIMENT
I. To conduct a sudden short circuit test and to determine machine parameters using currentoscillogram.
II. To study field current variation following a sudden short circuit.III. To study starter voltage variation following a sudden open circuit.IV. To conduct a slip test to determine machine parameters in the steady state.
APPARATUS
Synchronous machineD.C. machine set
Digital Oscilloscope with current probe
Current shunt (0.1, 0.1)
DC Voltmeter (300V)
DC Ammeter (1A)
AC Voltmeter (300V)
AC Ammeter (10A)
Rheostar (490/2A, 1040/1A)
Tachometer
On/Off Switch
Three phase variac
THEORY
a) Armature current on sudden short circuitA synchronous generator, if subjected to a sudden three phase short circuit while it is on load, set up a
short circuit current that can be given, for phase A as
( )
(
) (
)
(
)
Where,
VsLine to neutral rms voltage before the short circuit
0displaced angle relative to the positive peak of the voltage waveform of phase A at the instant of
short circuit. This is indicated in fig. 1.
-
8/2/2019 Syn Gen Transient Analysis
2/17
Xd, Xd, XdDirect axis steady state, transient and subtransient reactance.
XqQuadrature axis subtransient reactance.
Td, TdDirect axis transient and subtransient short circuit time constants.
TaArmature time constant
The first three terms of the current expression represent steady state, transient and subtransient current
components; the fourth term DC-offset component and the last term double frequency component. The
overall variation of the current waveform has the shape shown in fig. 2.
The peak to peak variation of this current waveform wherein the Dc-offset component has no effect,
and the double frequency component negligible (because X is nearly equal to X for mostmachines) is given by;
(
) (
)
The peak variation (also called envelop variation) is therefore given by,
(
) (
)
The parameters contained in the Ia,pk expression are obtained using the appropriate current plots as
shown in following steps.
-
8/2/2019 Syn Gen Transient Analysis
3/17
STEP 1
Obtain the half the magnitude of the peak to peak value of the short circuit current waveform and draw
the following graph.
By using the graph steady state and subtransient reactance components can be obtained in the
following way.
STEP 2
By using the component obtained from graph in step 1, draw the following graph.
By using the graph transient reactance and direct axis transient short circuit time constants can be
obtained in the following way.
-
8/2/2019 Syn Gen Transient Analysis
4/17
STEP 3
By using the component obtained from graph in STEP 2, draw the following graph.
By using the graph direct axis subtransient short circuit time constants can be obtained in the following
way.
The open circuit transient and subtransient time constant can also be obtained now as,
STEP 4
The armature time constant Ta can be obtained by taking a plot of envelop mean with time.
Where armature time constant Ta=H.
b) Field current variation following a sudden three phase short circuit at the armature isgiven by,
[ ( )
]
-
8/2/2019 Syn Gen Transient Analysis
5/17
Where If0 is the field current before the short circuit and Tkd the direct-axis damper time constant; other
parameters are as defined before.
Fig. 6 shows variation of field current with time which indicates a sudden jump in the current
envelops. The fundamental frequency current term appears only if the machine has dampers.
c) Armature voltage on sudden open circuit from a steady short circuit.If a synchronous generator is suddenly open circuited from a steady short circuit, then its terminalvoltage build up is given, per phase A, by
The three terms in the expression represent the steady state, transient and subtransient components
respectively, is the position of the rotor q-axis relative to the axis at phase A at the instant of open
circuit, or alternatively the displacement from the positive peak of phase A induced voltage waveform.
-
8/2/2019 Syn Gen Transient Analysis
6/17
-
8/2/2019 Syn Gen Transient Analysis
7/17
-
8/2/2019 Syn Gen Transient Analysis
8/17
Procedure:
(a) obtaining of short circuit armature current oscillogram
The circuit was connected as shown in fig. 12. Three phase breaker (S) was kept open and the
synchronous generator filed switch opened.field Voltage Vf was kept at Zero.
2. DC supply was switched on and started the DC motor using three point Starter.
3. The field rheostat was slowly increased until synchronous generator establishes the speed of
1500rpm.
4. Then switch S was closed and increased excitation current slowly until rated current is established in
the generator and then switch S was opened.
5. After then without changing field current, switch S was closed suddenly and caught the phase
current across the 0.1 current shunt, on the phase oscilloscope.A few numbers of attempts were made,
fully asymmetric and symmetric oscillogram was obtained. Also following readings were taken down.
Pre-short circuit line voltage V
Steady short circuit current = A
Qenerator speed = rpm
Number of generator pole pairs
-
8/2/2019 Syn Gen Transient Analysis
9/17
(b) Obtaining of field current oscillogram
With the circuit condition as in part (a), The field current transient was caught , following a sudden
closure of switch S, across the 0. 1 current shunt in the field current on the oscilloscope. Also the
following information was taken down.
Steady state field current A
(c) obtaining of open circuit armature voltage waveform,
With the circuit conditions as in part (a) and steady state current is flowing into the short circuit,
switch S was opened suddenly and caught the voltage transient on the oscilloscope. The steady
line voltage was taken down after the opening the circuit.
(d) slip test
The circuit was connected as shown in fig. 13 and, with switch S opened, the generator was ran at
about 25 rpm below synchronous speed using the dc motor. Then the three phase variac output was set
until phase current maximum reached about rated current. Phase current waveform was obtained,
covering a number of cyclic fluctuations, the generator speed was trimmed (using dc motor field) if
necessary.
Also, the maximum and minimum value of fluctuating ammeter and voltmeter reading was taken down
in the armature circuit.
Minimum phase current A
Maximum phase current = A
Minimum line voltage = V
Maximum line voltage V
Generator speed pm
-
8/2/2019 Syn Gen Transient Analysis
10/17
Step 1
T (ms) I peak (A)
0 96
20 78
40 68
60 64
80 62
100 60
120 58
140 56
160 55
180 54
200 52
220 50240 50
260 49
280 49
300 48
320 48
340 47
360 47
380 47
400 47
420 47
440 46
460 46
480 46
500 46
-
8/2/2019 Syn Gen Transient Analysis
11/17
0
20
40
60
80
100
120
0 100 200 300 400 500 600
Ia.pk (A) Vs Time (ms)
Ia.pk
46
96
T(ms)
Ia (A)
-
8/2/2019 Syn Gen Transient Analysis
12/17
Step 2
T (ms) X (A)
0 50
20 32
40 22
60 18
80 16
100 14
120 12
140 10
160 9
180 8
200 6
220 4
240 4
260 3
280 3
300 2
320 2
340 1
360 1
380 1
400 1
420 1
440 0460 0
480 0
500 0
-
8/2/2019 Syn Gen Transient Analysis
13/17
-10
0
10
20
30
40
50
60
0 100 200 300 400 500 600
X (A) Vs T (ms)
X (A)
T (ms)
155
8.83
2
4
-
8/2/2019 Syn Gen Transient Analysis
14/17
Step 3
T (ms) y (A)
0 26
5 22.25
10 17.5
15 13.25
20 10
25 7.2
30 5
35 3.2
40 1.8
45 0.7
50 0
-5
0
5
10
15
20
25
30
0 10 20 30 40 50 60
y (A) Vs T (ms)
23
8.5
27
y
(A)
T
(ms)
-
8/2/2019 Syn Gen Transient Analysis
15/17
Step 4
T (ms)
Ia (Peak-
Peak)
0 192
20 156
40 13660 128
80 124
100 120
120 116
140 112
160 110
180 108
200 104
220 100
240 100
260 98
280 98
300 96
320 96
340 94
360 94
380 94
400 94
420 94440 92
460 92
480 92
500 92
-
8/2/2019 Syn Gen Transient Analysis
16/17
0
50
100
150
200
250
0 200 400 600 800 1000
Ia (peak-peak) Vs Time
Ia (peak-peak)
710
51.5
140
T (ms)
Ia (P-P) A
-
8/2/2019 Syn Gen Transient Analysis
17/17
CALCULATION :
(a) Step 1
A= 96 B= 46
Xd= = 0.98
Xd= = 0.47
Step 2
C= 24 D= 155
Xd=
= 0.644
Td= 155 ms
Step 3
F= 27
Td= 27 ms
Tdo= 155 x = 235.87 ms
Tdo= 27 x = 36.99 ms
Step 4
H= 710
Ta= 710 ms
(b) Since it is assumed no dampers, &
If= 2.5+2.5 [ -](c) Assuming 0 is zero