Synchronous Generator

Name : S.P.M SudasingheIndex No. : 100523GGroup : G - 12Field : EEDate of Performance : 2013/08/28Date of Submission : 2013/09/20

EE 3092Laboratory Practice V

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

OBSERVATIONS

Name: S.P.M Sudasinghe

Index no: 100523G

Group: G – 12

Date: 2013/08/28

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

1. Open Circuit Test

VO/C (V) If (A)20 0.0140 0.0460 0.0580 0.07100 0.10120 0.12140 0.14160 0.17180 0.20200 0.24220 0.30240 0.38260 0.51270 0.592. Short Circuit Test

Is/c (A) If (A)0 01 0.022 0.043 0.054 0.085 0.106 0.12

3. Load Test – Inductive Load

IL (A) VL (V)3.5 2204.0 2164.5 2125.0 2065.5 2026.0 1964. Load Test – Resistive Load

IL (A) VL (V)0.38 2180.76 2161.16 2161.48 2145. Nameplate data of Synchronous machine

Rated Voltage = 240 V

Rated Current = 6.66 A

CALCULATIONS1. Calculations of Synchronous Reactance SynchronousReactance=X S=

V O /C

I S /CFrom open circuit & short circuit characteristics (graph 1),VO/C = 60V and IS/C = 2.5 A when If = 0.05 A∴ XS=

602.5

=25Ω

Similarly XS can be calculated for other values of field currents using the graph 1If (A) Observations from graph 1 Calculated XS (Ω)VO/C (V) IO/C (A)0.05 60 2.5 25.000.10 106 5.0 21.200.15 145 7.5 19.330.20 176 10.0 17.600.25 202 12.5 16.160.30 220 15.0 14.670.35 235 17.5 13.430.40 246 20.0 12.300.45 254 22.5 11.290.50 260 25.0 10.400.55 265 27.5 9.640.60 270 30.0 9.00

2. Calculation of Short Circuit Ratio ShortCircuit Ratio=Field current for rated o /c voltage

Field current for rated s /c currentAs I f ∝ I aSCR= Armature current at rated o/c voltage

rated s /c currentFrom nameplate data of synchronous machine,Rated voltage = 240 VRated current = 6.66 A

From graph 1,Armature current at rated o/c voltage = 18 A∴SCR=¿ 18

6.6 = 2.7

3. Calculation of saturated synchronous reactance In per units,X S (SAT ) , pu=

1SCR

∴ XS ( SAT ) , pu=¿ 12.7

= 0.37 puIn ohms,

X S (SAT )=X S (SAT ), pu×V rated

I rated

∴ XS (SAT )=¿ 0.37 × 2406.66

= 13.33 Ω4. Calculation of Load Voltages

E=V +~I (ra+ jX s)Where,E – No load voltageV – Terminal voltageXs – Synchronous reactance per phasera – resistance per phase

Neglecting resistance,E=V + I ¿

E=(V−I X s sinφ )+ jI X scos φ

∴E2=V 2−2VI X ssin φ+ I2 X s2

No load voltage is set to 220V. By graph 1, field current = If = 0.3 A and Is/c = 15 AHence synchronous reactance can be calculated using graph 1 as,X S (@I f=0.3 A )=

V o / c

I s/ c=220

15=14.67Ω

Assuming synchronous reactance depends only on field current, hence it is constant throughout the load test,X S=14.67ΩSample calculation for 1A Load current

-------------- (1)

a) For purely resistive load (cos φ = 1) cos φ=1

⇒

φ=0From equation 1,V 2−2V ×1×14.67×0+12×14.672=2202

V = ±219.51 V = 219.51Va) For cos φ = 0

cos φ=0⇒

φ=±900For φ=900,V 2−2V ×1×14.67×sin 900+12×14.672=2202

V 2−29.34 V−48184.79=0

V=234.67∨V=−205.33

∴ V= 234.67 VFor φ=-900,V 2−2V ×1×14.67×sin (−900 )+12×14.672=2202

V 2+29.34V−48184.79=0

V=205.33∨V=−234.67

∴ V= 205.33b) For cos φ = 0.9 cos φ=0.9

⇒

φ=±25.840For φ=25.840,V 2−2V ×1×14.67×sin 25.840+12×14.672=2202

V 2−12.79V−¿0

V=225.99∨V=−213.21

∴ V= 225.99 VFor φ=-25.840,V 2−2V ×1×14.67×sin (−25.840 )+12×14.672=2202

V 2+12.79V−48184.79=0

V=213.21∨V=−225.99

∴ V= 213.21VSimilarly terminal voltages can be calculated for other load currents.IL (A) Terminal Voltage (V)cosφ=1 cosφ = 0 cosφ = 0.9Lagging Leading Lagging Leading

(φ = -900) (φ = 900) (φ = -25.840) (φ = 25.840)1 219.51 205.33 234.67 213.21 226.002 218.03 190.66 249.34 205.62 231.204 212.03 161.32 278.68 187.99 239.146 201.62 131.98 308.02 166.88 243.618 186.08 102.64 337.36 141.83 244.1410 163.95 73.30 366.70 112.04 239.9112 131.95 43.96 396.04 75.91 229.36

DISCUSSION

1. Importance of SCR with respect to the generator performance

The ratio of field current required to produce rated voltage on open circuit to field current required to

circulate rated current on short-circuit, while the machine is mechanically driven at synchronous speed

is known as short-circuit ratio (SCR).

A small value of SCR indicates a smaller value of current under short circuit conditions owing to large

value of synchronous reactance. A machine with high value of SCR will have lower value of Xd

leading to

(i) Higher synchronizing power and so higher stability limit

(ii) Good inherent voltage regulation

(iii) Higher short-circuit current.

(iv) Satisfactory parallel operation of machines due to higher synchronizing power.

SCR for low speed generators is 1.0 to 1.5 and for modern turbo-generators is 0.5 to 0.6

2. Variation of synchronous reactance with field current

Synchronous reactance=V o /c

I s / c

It may be seen from graph 2 that the synchronous reactance decrease with the increase in field current.

This is due to magnetic saturation effect. As long as the resultant flux density due to the effect of

armature and field currents, is below the knee point of the saturation curve the flux produced per

ampere of armature current is approximately constant. Therefore the synchronous reactance is

constant. For higher flux densities the flux produced per ampere decreases and consequently the

synchronous reactance decreases.

3. Synchronous generator has characteristic of current transformer

When the secondary winding of a current transformer 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 from graph 1(the short circuit characteristic curve) that the short circuit

armature current is proportional to the field current. Hence, the synchronous generator shows

characteristics of a current transformer.

Synchronous generator’s field current depends on the connected load. Similarly in a current

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

Hence the armature winding of the synchronous generator act as secondary winding of the current

transformer and the field winding of the synchronous generator is similar to the primary winding of a

current transformer.

4. Variation of terminal Voltage with Load current for various power factor loads

For unity power factor, and for lagging power factors Terminal voltage always decrease when the load

current increase. But for leading power factors terminal voltage increases first and begin to fall after

some point when the load current increases. From the graph 5 we can see that load having 90 leading

power factor angle (zero leading power factor) is approximately a straight load characteristic and

always increase with load. Zero lagging power factor load has also a straight load characteristic and it

decrease always.

Normally the terminal voltage falls with the increase in load current. Reason is the increase in voltage

drop through synchronous impedance due to increasing load current.

When the power factor is leading (highly capacitive load) the effect of armature flux is to help the

main flux, hence to generate more emf. This causes the terminal voltage to increase first. But after

some value of load current effect of voltage drop through synchronous impedance becomes higher than

the increase in generated emf. Then terminal voltage starts to decrease after some value of load

current.

When power factor is lagging (highly inductive load) the effect of armature flux is to oppose the main

flux, hence to reduce generate emf. Hence terminal voltage falls rapidly with load current for lagging

power factors. Reduction of voltage with load current for lagging power factors is more than unity and

leading power factors.