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.


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.


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.


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,

[ ( )

]


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.


6/17


7/17


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


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


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


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)


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


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


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)


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


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


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

Syn Gen Transient Analysis

Documents

Transcript of Syn Gen Transient Analysis