Synchronous Generator

Name : P.D.M. Chandrasekara Index No : 110072F Group : 03 Submission Date : 2014/07/18 Instructed By : Mr. L.P.M.I. Sampath

OBSERVATION SHEETName : P.D.M. ChandrasekaraIndex No : 110072FField : EEGroup : G-03Date : 2014/07/04Instructed By : Mr. L.P.M.I. Sampath

01) Open Circuit Test Open circuit voltage (V)Field current (A)

200.01

400.03

600.05

800.07

1000.10

1200.12

1400.14

1600.17

1800.20

2000.25

2200.30

2400.39

2600.52

2700.60

02) Short Circuit Test

Short circuit current (A)Field current (A)

00.00

10.01

20.04

30.07

40.10

50.13

60.15

03) Load Test Inductive Load TestLoad voltage (V)Load current (A)

105.6

205.2

304.9

404.5

504.1

603.8

703.4

803.0

902.5

1002.1

1061.9

Resistive Load TestLoadLoad voltage (V)Load current (A)

Load 01200.00

Load 11190.22

Load 21190.44

Load 31180.64

Load 41170.84

04) SynchronisationName plate dataSynchronous GeneratorDC motorPhase3

Frequency50 Hz

Voltage240 V

Current6.6 A

Speed1500rpm

V220 V

A15 A

Speed1500rpm

Open circuit characteristics

Open circuit voltage (V)Field current (A)

200.01

400.03

600.05

800.07

1000.10

1200.12

1400.14

1600.17

1800.20

2000.25

2200.30

2400.39

2600.52

2700.60

Short circuit characteristics

Short circuit current (A)Field current (A)

00.00

10.01

20.04

30.07

40.10

50.13

60.15

Calculations01) Synchronous reactance, Xs

whereVOC - Open circuit voltage ISC - Short circuit current

Calculation for field current 0.1 A, VO/C= 116 V (from graph) IS/C= 4 A(from graph)XS= 120 / 4 = 29

Similarly, calculate synchronous reactance for given field currents .

Field Current(A)Open circuit voltage(V)Short circuit current(A)Synchronous reactance()

0.10116429

0.20178822.25

0.302201218.33

0.402481516.53

0.502651913.94

0.602752212.5

0.702842610.92

02. Synchronous Reactance (XS) vs. Field Current (If)

Field Current (A)Synchronous reactance ()

0.1029

0.2022.25

0.3018.33

0.4016.53

0.5013.94

0.6012.5

0.7010.92

02)

From the name plate data,Rated Voltage= 240 VRated Current= 6.6 Afrom the open and short circuit characteristics,Field current for rated open circuit voltage= 0.375 AField current for rated short circuit current= 0.14 A

Saturated synchronous reactance (XS(sat)) in per unit,

Saturated synchronous reactance (XS (sat)) in ohms, (Using open and short circuit characteristics curves)

03. Load Voltage vs. Load Current for Inductive Load (Cos = 0)

Load Current (A)Load Voltage (V)

5.610

5.220

4.930

4.540

4.150

3.860

3.470

3.080

2.590

2.1100

1.9106

Load Voltage vs. Load Current for Resistive Load (Cos = 1)

Load Current (A)Load Voltage (V)

Load 00.00120

Load 10.22119

Load 20.44119

Load 30.64118

Load 40.84117

03. V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0

calculation for load current 1 A E = 240 V XS = 17.14 WhenCos = 0 = 900 = - 900 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 257.14V = 222.86

When Cos = 1 = 0 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 239.39

When Cos = 0.9

= 25.840 = - 25.840 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 246.97V = 232.03

Load Current (A)Terminal Voltages (V)

1222.86232.03239.39246.97257.14

2205.72223.07237.54252.95274.28

4171.44202.05230261.81308.56

6137.16176.61216.85266.26342.84

8102.88146.97196.97265.61377.12

1068.6109.15167.99258.56411.4

1234.3263.10123.68242.34445.68

Terminal Voltage Vs Load Current

Load Current (A)Terminal Voltages (V)

1222.86232.03239.39246.97257.14

2205.72223.07237.54252.95274.28

4171.44202.05230261.81308.56

6137.16176.61216.85266.26342.84

8102.88146.97196.97265.61377.12

1068.6109.15167.99258.56411.4

1234.3263.10123.68242.34445.68

Discussion1. Importance of the SCR with respect to the generator performance.

The Short Circuit Ratio (SCR) is defined as the ratio between the field current that required for generate rated armature voltage at open circuit and the field current that required for produce rated current at short circuit when the machine is driven at synchronous speed. And this parameter can also be defined as the inverse of the saturated synchronous reactance in pu.

The SCR value of a synchronous generator can represent information about the generator performance. Low value of SCR indicates a low value of current under short circuit conditions owing to large value of synchronous reactance.

When the SCR is higher, stability limit increased and also voltage regulation is improved.

2. Comment on the variation of synchronous reactance with field current.

The synchronous reactance is defined to be as follows,

By observing the short circuit and the open circuit characteristics curves, in open circuit curve we can see that the ratio of (VO/C / IS/C) is nearly constant for lower values of the field current. So the synchronous reactance must remain constant. But in higher values of the field current, the ratio of (VO/C / IS/C) is not constant due to open circuit non-linearization. Then the synchronous reactance drops rapidly. This is because of the rate of increase of open circuit characteristic is reducing faster and at the saturated region the slow growth of flux affects to the armature reaction and the self-inductance of the armature coils. 3. Synchronous generator has characteristic of a current transformer. Explain.

In a current transformer, when the secondary winding is short circuited, that short circuit current is proportional to the primary current. Similarly, when the armature is short circuited in a synchronous generator, we can observe that the armature current is proportional to the field current by the short circuit characteristic curve. Hence, the synchronous generator shows characteristics of a current transformer. A synchronous generators field current depends on the

connected load. Similarly in a current transformer, the primary current depends on the load connected to the secondary winding.

4. Comment on the variation of the terminal voltage with load current for various power factor loads.

We already obtained some terminal voltage vs. load current curves for different power factors. When speed and exciting current constant, the terminal voltage of a synchronous generator changes with the different load.

When the power factor of the load is unity, the reduction in the terminal voltage is small. But when the load is inductive (lagging power factor), the stator mmf opposes the rotor mmf resulting in a reduction in the generated emf which consequently reduces the terminal voltage. The reduction in the terminal is more when the power factor is low. On the other hand, when the load across the alternator is capacitive (leading power factor), the magnetic effect of the armature reaction causes the terminal voltage to increase with an increase in load.