EE 3092 Laboratory Practice V

SYNCHRONOUS GENERATOR

Instructed by: Mr. Kariyawasam K. K. M. S.

Name: Gunawardena L.H.P.N.Index No: 100170UGroup: 04Date of Performance: 14 / 08 / 2013Date of Submission: 11 / 09 / 2013

OBSERVATIONS

Name: Gunawardena L.H.P.N.Index No:100170UGroup: 04Date: 14 / 08 / 2013Instructed by: Mr. Kariyawasam K. K. M. S. Practical: Synchronous Generator

A).1).Open Circuit testOpen Circuit Voltage (V)Field Current (A)

200.01

400.04

600.05

800.08

1000.10

1200.12

1400.14

1600.17

1800.20

2000.24

2200.30

2400.39

2600.52

2700.60

2).Short Circuit testShort Circuit Current (A)Field Current (A)

00.00

10.01

20.03

30.05

40.07

50.10

60.12

3).Inductive Load testLoad Current (A)Load Voltage (V)

3.5206

4.0202

4.5196

5.0190

5.5182

6.0176

4).Resistive Load testStepLoad Current (A)Load Voltage (V)

10.40216

20.78216

31.16214

41.52214

C).SynchronizationName plate dataSynchronous GeneratorDC motorTypeT825

Phase3

Frequency50 Hz

Voltage240 V

Current6.65 A

VA2750 VA

Cos 0.8

Tmin1500

TypeT825

V220 V

A15 A

CV3.75

Tmin1500

Open Circuit Characteristics

Open Circuit Voltage (V)Field Current (A)

200.01

400.04

600.05

800.08

1000.10

1200.12

1400.14

1600.17

1800.20

2000.24

2200.30

2400.39

2600.52

2700.60

Short Circuit Characteristics

Short Circuit Current (A)Field Current (A)

00.00

10.01

20.03

30.05

40.07

50.10

60.12

GRAPHS

CALCULATIONS

From the thevenins equivalent circuit,

If ra is small,

whereVO/C - Open circuit voltageIS/C - Short circuit current

Therefore synchronous reactance (XS) can be determined by using the plotted characteristics curves for a given field current.

Sample calculation for field current 0.1 A,VO/C= 104 V(from graph) IS/C= 5 A(from graph)XS= 104 / 5= 20.8

Similarly, we can calculate synchronous reactance (XS) for given field currents and results are listed below.

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

0.101040520.80

0.201761017.60

0.302201514.67

0.402452012.25

0.502592410.79

0.60269299.28

0.70282348.29

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

Field Current (A)Synchronous reactance ()

0.1020.80

0.2017.60

0.3014.67

0.4012.25

0.5010.79

0.609.28

0.708.29

Short Circuit Ratio is defined as,

From the name plate data of synchronous generator,Rated Voltage= 240 VRated Current= 6.65 ATherefore, from the open and short circuit characteristics curves,Field current for rated open circuit voltage= 0.37 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)

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

Load Current (A)Load Voltage (V)

3.5206

4.0202

4.5196

5.0190

5.5182

6.0176

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

Load Current (A)Load Voltage (V)

0.40216

0.78216

1.16214

1.52214

ErVrraXSVI

For the circuit,E=V + I (ra + j XS)E=V + j I XSNeglecting raE=V + j XS I (Cos + j Sin ) E=V + j XS I Cos - XS I Sin E=(V - XS I Sin ) + j XS I Cos

E 2=(V - XS I Sin ) 2 + (XS I Cos ) 2E 2=V 2 2VXS I Sin + XS 2 I 2

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

Sample calculation for load current 1 A,E = 240 VXS = 13.33 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 = 253.33V = 226.67

When Cos = 1 = 0 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 239.63When 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 = 245.51V = 233.89

Theoretical Terminal VoltagesLoad Current (A)Terminal Voltage (V)

= 900 = - 900 = 00 = 25.840 = -25.840

1253.33226.67239.63245.51233.89

2266.66213.34238.51250.42227.18

4293.32186.68234.00258.39211.91

6319.98160.02226.28263.81194.09

8346.64133.36215.01266.45173.49

10373.30106.70199.58265.96149.76

12399.9680.04178.92261.75122.31

Terminal Voltage vs. Load Current for Different Loads

Load Current (A)Terminal Voltage (V)

= 900 = - 900 = 00 = 25.840 = -25.840

1253.33226.67239.63245.51233.89

2266.66213.34238.51250.42227.18

4293.32186.68234.00258.39211.91

6319.98160.02226.28263.81194.09

8346.64133.36215.01266.45173.49

10373.30106.70199.58265.96149.76

12399.9680.04178.92261.75122.31

DISCUSSION

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

The Short Circuit Ratio (SCR) is the ratio of the field current that required for generate rated armature voltage at open circuit to the field current that required for produce rated current at short circuit when the machine is driven at synchronous speed. And SCR is given by the reciprocal of per unit synchronous reactance.

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,

When we observe the short circuit and the open circuit characteristic curves, we can see that the ratio of (VO/C / IS/C) is nearly constant for lower values of the field current as before the machine begins saturation, when the open-circuit characteristic curve is nearly linear. The synchronous reactance must remain constant.

But in higher value of the field current, the ratio of (VO/C / IS/C) is not constant due to open circuit non-linearization. The synchronous reactance begins to drop rapidly. This is because of the fact that 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.

If we take the rotor winding of a synchronous generator to be the primary winding of a transformer and the field winding to be the secondary, then the field current to be the primary current, and the armature current to be the secondary current and a synchronous generator act as a step-up current transformer since the armature current is much higher than the field current.

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. General variation of terminal voltage vs. load current for different loads is shown below. But this is not as same to obtained curves.

The terminal voltage decrease when the load current increases, but for leading power factor, the load characteristic curve rise at the beginning and then decreased. Each curve is nearly straight at the beginning but tends to bow with the load current increases.

The maximum current can be obtained when the generator terminals are short circuited. The steady short circuit current is not much greater than full load rated current in modern synchronous generator. This is purely arranged to prevent excessive current in the event of a short circuit.