EE 3183-Laboratory Practice V

SYNCHRONOUS GENERATORSemester 5

Instructed by: Mr. S.P.M. Sudasinghe

Group Members:H.N.T.Wijesekara120716U

L.S.Wijesinghe120720C

W.M.K.M.Wijesooriya120724R

D.T.P.Wijesooriya120726B

V.Y.Wong120737J

Name : H. N. T. WijesekaraIndex No. : 120716 U Group : G-22Field : Electrical EngineeringDate of Perform : 19/06/2015Date of Submission : 03/07/2015

THEORY

Part A.

Open circuit and Short circuit test

Open circuit and Short circuit characteristic are as followsOpen circuit characteristicShort circuit characteristicField CurrentV, IVTIaIaIfIaABC

Eg=Vo/craxaIs/cIf

By the Thevenins theorem for the circuit

For small

Also rated voltage

But

Therefore in per unit quantity

Load testWhen the Inductive load is connected as followsEg=Vo/craxsVo/cIf

For the circuit By neglecting

Part B. Understanding the operation of synchronization

When the synchronous generated output connecting to a system the system and generator should have same frequency, same voltage, same phase angle and phase sequence in each terminalVLrVLbVLyRRYYBBSystem sideRYBRYBGenerator sideLrLyLbAt the instance of switch connecting the system and generator, each phases of the generator should coincide with the corresponding phases of system.

PROCEDUREOpen circuit testConnect the circuit as follows and run the generator in synchronous speed, by varying the output voltage observe the field current.A220VDC

V

Short circuit testConnect the circuit as follows and run the generator in synchronous speed, by varying the short circuit current observe the field current.A220VDC

A

Load testConnect the circuit as follows and run the generator in synchronous speed, by varying the connected load observe the load current and load voltage.Synchronization methodA220VDC

VA

System sideRYBRYBGenerator sideLrLyLb

Connect the circuit as follows and run the generator in synchronous speed.

If the two sets of pharos have the very small relative rotational speed (s) voltage across lamps will rise and fall slowly. We can observe it by the brightness change in the bulbs.At the correct phase position lamps are equally dark during the middle of this period Switch (Circuit Breaker) must be closed to cause no interruptions.

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

2000.24

2200.29

2400.38

2600.51

2700.61

Short circuit characteristics

Short circuit current (A)Field current (A)

00.00

10.01

20.03

30.05

40.08

50.10

60.12

Calculations01) Synchronous reactance, Xs

whereVOC = Open circuit voltage ISC = Short circuit current

Calculation for field current 0.1 A, VO/C= 107 V (from graph) IS/C= 5 A(from graph)XS= 107 / 5 = 21.4

Similarly, calculate synchronous reactance for given field currents.

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

0.10107521.400

0.20177822.125

0.302211415.786

0.402471913.000

0.502602410.833

0.60268289.571

0.70277338.394

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

Field Current (A)Synchronous reactance ()

0.1021.400

0.2022.125

0.3015.786

0.4013.000

0.5010.833

0.609.571

0.708.394

02)

From the name plate data,Rated Voltage= 240 VRated Current= 6.65 Afrom the open and short circuit characteristics,Field current for rated open circuit voltage= 0.367 AField current for rated short circuit current= 0.133 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)

4.0240

4.5236

5.0234

5.2233

5.5231

6.0228

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

Load levelLoad Current (A)Load Voltage (V)

00.00240

10.42240

20.86238

31.28237

41.70236

Single Phase Equivalent Circuit Calculations

Neglecting ra,

Therefore,

Sample Calculation for Terminal Voltage for Resistive LoadsFor Purely Resistive Loads (Cos () =1),E = 240/ V=138.56 V Sin () = 0

When I = 1 A = 137.89 VTherefore total terminal voltage = = 238.83 V

Sample Calculation for Terminal Voltage for Inductive LoadsFor Purely Inductive Loads (Cos =0),E = 240/ V=138.56 V Sin() = -1 (Current lags the voltage for inductive loads)When I = 1 A

124.93 VTherefore total terminal voltage = = 216.39 V

Sample Calculation for Terminal Voltage for Load of Power Factor of 0.9Assume an inductive LoadE = 240/ V=138.56 V Cos () = 0.9Sin () = -0.4359 When I = 1 A

132.08 VTherefore total terminal voltage = = 228.77 V

Theoretical Values for Terminal Voltage vs. Load Current

Load Current (A)Terminal Voltage (V) [Purely Resistive Load]Terminal Voltage (V) [Purely Inductive Load]Terminal Voltage (V) [Load with p.f. =0.9]

1238.83216.39228.77

2237.67192.78215.63

3236.49169.17200.51

4235.31145.57183.27

5234.12121.96163.75

6232.9398.35141.60

7231.7374.75116.33

8230.5251.1487.11

9229.3127.5352.41

10228.093.928.70

11226.87--

12225.64--

Theoretical Values for Terminal Voltage vs. Load Current

Load Current (A)Terminal Voltage (V) [Purely Resistive Load]Terminal Voltage (V) [Purely Inductive Load]Terminal Voltage (V) [Load with p.f. =0.9]

1238.83216.39228.77

2237.67192.78215.63

3236.49169.17200.51

4235.31145.57183.27

5234.12121.96163.75

6232.9398.35141.60

7231.7374.75116.33

8230.5251.1487.11

9229.3127.5352.41

10228.093.928.70

11226.87--

12225.64--

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

The ratio of the field current for rated open circuit Voltage and field current for rated short circuit current give SCR. In addition, SCR is given by inverse of per unit saturated synchronous reactance. Therefore, if we know SCR value we can have an idea about the Xs.The synchronous reactance is higher when the SCR is low. So it becomes comparatively very high value than armature resistance. This results in a significant phase angle (lagging) between the induced Voltage and the armature current.As the value of SCR gets higher and higher, the Synchronous reactance reduces considerably causing a good voltage regulation at the output terminals.In the cases of parallel operation of generators, Synchronous reactance is essential in determining the circulating current within the two generators. As Xs is a function of SCR we can have an idea about the circulating current by knowing the SCR.

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

When we examine the graph of Synchronous Reactance vs. Field Current Graph, It can be seen that the graph can be divided in to two parts a linear region and a non linear region. First, the synchronous reactance linearly reduced and then it started to reduce in non-linear fashion.Linear region is due to the fact that Xs reduces when rotor frequency is reduced. In this region machine has not been saturated. But this Xs is not only the winding reactance it also includes the armature reaction to the field produced. This is high at low values of field current and it reduces afterwards. This is the reason for the non linear behavior

3. Synchronous generator has characteristic of a current transformer. Explain.

Lets take that the rotor of the Synchronous generator represents the primary winding and stator represents the secondary winding. Then the field current is the primary current and the armature current is the secondary current.Now as the armature current is much higher than the field current, this looks like a step up current transformer.

In a current transformer when the secondary is short circuited, that short circuit current is proportional to the primary current.

From the graph of Ia Vs If it is visible that short circuited armature current is proportional to field current. Therefore in that sense also the synchronous generator is acting as a current transformer.

When a load is connected to the secondary of a current transformer, its primary current depends on the load.

In the same way, field current of a synchronous generator is also depend on the load while it is connected to the armature.

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

When there are loads with lagging power factor terminal voltage tends to reduce rapidly, but that reduction is close to being linear.

With resistive loads terminal voltage reduces slower but the non linear manner of the curve is more visible than the inductive loads.

When capacitive loads are used terminal voltage tends to get higher than the original Voltage.

Value of Xs is essential in finding the terminal Voltage.