Synchronous Genarator

8/2/2019 Synchronous Genarator

1/8

Calculations :

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

OpenCircuitVoltage/V,S

hortCircuitCurrent/A

Field Current/A

Open Circuit Voltage,Short Circuit Current Vs Field

Current

Open Circuit Voltage Short Circuit Current


2/8

For the Thevenins equivalent circuit,

Zs =

= [(ra2+Xs2)]1/2

If ra is small,

Xs =

At field current equals to 0.1A ,

Xs =

= 20

If(A) Vo/c(V) Is/c(A) Xs()

0.1 100 5 20

0.2 172 10 17.2

0.3 217 15 14.470.4 240 19 12.63

0.5 254 24 10.58

0.6 266 29 9.17

0.7 274 33 8.3


3/8

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Xs()

If(A)

Xs Vs If


4/8

Short Circuit Ratio =

SCR =

=

By the graph, Ia= 17 A and Ia = 5 A

SCR =

= 3.4

In p.u. quantities, Xs (sat) =

Xs (sat) =

= 0.294 pu

In ohms,

Xs (sat) = [ Xs (sat)]pu*Xsbase

Xs (sat) = 0.294*

=13.53


5/8

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7 8 9 10

LoadVoltage/V

Load Current/A

Load Voltage Vs Load Current


6/8

Using the phasor diagram;

V2

- (2IXsSin)V + (I2Xs

2E

2) = 0

E = 230V and Xs = 13.53

Current/A Terminal

voltage(Cos=0)

Terminal

voltage(Cos=1)

Terminal

voltage(Cos=0.9)

2.5 196.18 227.5 213.23

3.1 188.06 226.14 208.6

3.7 179.94 224.49 203.72

4.2 173.17 222.87 199.47

4.7 166.41 221.03 195.05

5.2 159.64 218.97 190.44

5.6 154.23 217.16 186.63

6.0 148.82 215.2 182.7

6.4 143.41 213.08 178.656.9 136.64 210.2 173.41

7.2 132.58 208.35 170.17

7.5 128.53 206.4 166.86

7.9 123.11 203.65 162.32

8.2 119.05 201.47 158.83

8.6 113.64 198.4 154.06

8.9 109.58 195.96 150.38


7/8

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10

Term

inalVoltage/V

Load Current/A

Terminal Voltage Vs Load Current at various

Power Factor Angales

Terminal voltage(Cos=0) Terminal voltage(Cos=1) Terminal voltage(Cos=0.9)


8/8

Discussion :

(1)Importance of the SCR with respect to the generator performance

Short Circuit Ratio =

The SCR value of a synchronous generator can reveal information about the machines

performance. When the SCR value is low, in order to maintain a constant terminal voltage for a

given change in load, a larger change in field current is required when compared with a higher

SCR value. Also, low SCR value means the steady state stability limit of the machine is lower.

Therefore, a machine of low SCR is dependent on an excitation system that is able to provide large

changes of field current quickly and reliably. An appropriate excitation system can also increase

steady state stability. Also the physical dimensions and properties are relatively smaller on low

SCR machines, so their cost is also lower. Normally for high speed generators the Short Circuit

Ratio is about 0.5 to 0.6.

(2) Variation of synchronous reactance with field current

The synchronous reactance is defined as follows,

Xs =

As the graph indicates relevant to the observations that were taken for the short-circuit and the

open-circuit characteristic, it can be seen that the ratio

is nearly constant for lower values of the

field current; i.e. before the machine begins saturation, when the open-circuit characteristic curve is

nearly linear. But once the machine begins saturation, the synchronous reactance begins to drop

rapidly.

(3) Synchronous generator has characteristic of a current transformer.

If the rotor of the synchronous generator assumed as primary winding and stator is assumed

as the secondary winding of a current transformer, then the field current is the primary current and

the armature current is the secondary current. In a synchronous generator armature current is so

much higher than field current, so it can be considered as a step up current transformer. In a current

transformer when the secondary is short circuited the secondary current is proportional to the

primary current. Always in asynchronous generator short circuited armature current is proportional

to the field current. So those two characteristics are same in a synchronous generator and a current

transformer. When a load is connected to a CT its primary current depends on the load which is

connected. Also in a synchronous generator the field current depends on the load which is

connected to it.

(4) The variation of terminal voltages for various power factors

When the power factor increases, the graph shifts upwards, this is happened, because more

power is delivered to the load and therefore it will have a relatively high load voltage.

Synchronous Genarator

Documents

Transcript of Synchronous Genarator