DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

ELECTRICAL MACHINES LAB REPORT

DATE: 10TH

NOVEMBER 2009

TITLE: PARALLEL DRIVING OF SYNCHRONOUS GENERATOR

OBJECTIVES:

To synchronize a generator to infinite bus bars.

To investigate the behavior of parallel driven generators on the distribution of load.

APPARATUS:

DC motor

Synchronous Generator

Wattmeter

Power Factor Meter

Starter

2 Potential Transformers

2 Current transformers

Bulbs

Voltmeters

Ammeters

THEORY OF EXPERIMENT:

A synchronous machine operates at synchronous speed, that is, at the speed at which the magnetic field created

by the field coils rotates. This machine may either be a generator or a motor. The synchronous speed Ns in

revolutions per minute (rpm) is given as Nf

Ps

120 .

f is Frequency (Hz)

P is the number of poles

When generators are supplying power to the load, they are all connected in parallel. The bus bar connecting

these generators to the load is termed as infinite bus bar. This is because it consists of many other connections

to other loads and alternators. To connect a generator to infinite bus bar is called synchronizing.

SYNCHRONIZING

Synchronizing is the operation of connecting an alternator in parallel with another alternator or with common

bus-bars. In most cases alternators are used in power systems where they are in parallel with other alternators.

The System to which an alternator is connected is most probably already

connected with many other alternators and loads and as such that the

incoming alternator must maintain the existing system specifications in

terms of voltage and frequency. As such, the terminal voltage, speed and

phase voltage of the alternator must match those of the bus-bar.

Failure to fulfill this condition may lead to the destruction of the incoming

alternator or even destabilization of the entire system. To achieve

synchronization, synchronizing bulbs can be used. They are connected

between the alternator and the bus-bar as shown.

Figure: Connection of bulbs, alternator and infinite bus-bar

REACTIVE POWER

If the voltage is greater than the bus bar voltage reactive cross current flows from high voltage side. This current

gives a magnetizing action to the low voltage side (leading power factor to the low voltage generator) and

demagnetizing current to the high voltage side (lagging power factor to the high voltage generator). This action

in effect keeps the voltage constant while the power factor as well as the phase angle changes. The opposite

happens when the field excitation of the generators is reduced.

LOADING THE GENERATOR

After synchronizing the generator to the infinite bus bars, the load connected to it is that seen by all generators

to those bus bars.

PROCEDURE:

1. Parallel driving (lamp sequence method)

The circuit was connected as shown in figure 1. The dc motor was then driven to synchronous speed. The

terminal voltage of the generator is adjusted to bus bars voltage by varying the field current. If the phase angle

between the incoming generator and that of the bus bars is large, the rotation indicated by turning ON and OFF

the lamps is high. When the phases agree bulb B2 turns OFF and the other two have equal brightness. Switch S3

is turned ON immediately. The incoming generator is then running in parallel with infinite bus bars generators.

Note: if the power factor us lagging for under excitation, interchange secondary connections of both current

transformers.

2. Loading

Load the generator from zero to the rated capacity by increasing the dc motor output (by varying the field

current of the dc motor) and the instrument readings were noted each time.

3. Reactive Power

The generator was driven with the rated current then the field current was varied under excited and overexcited

conditions and the instrument readings were noted each time.

RESULTS:

Loading:

Varying power by steps of 3watts.

Power(w) Armature

current,Ia Ig p.f Vg

0 0 1.3 1 410

3 0.5 1.3 -0.95 410

6 1.0 1.3 -0.91 410

9 1.25 1.3 -0.90 410

12 1.65 1.3 -0.89 410

15 2.05 1.3 -0.89 410

18 2.4 1.3 -0.89 410

21 2.7 1.3 -0.89 410

24 3.2 1.3 -0.89 410

Varying generator field current by 0.2A.

Armature current,Ia Power(w)W * 80 Ig p.f Vg

0.9 24 4.2 -0.99 410

1.1 24 3.5 -0.94 410

1.3 25 2.9 -0.73 410

1.5 25 2.8 -0.39 410

1.7 25 3.15 -0.22 410

1.9 24 3.7 -0.30 410

2.1 24 4.6 -0.50 410

0.88

0.93

0.98

1.03

0 0.5 1 1.5 2 2.5 3 3.5

Pf

if (A)

p.f. loading

pf

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2 2.5 3 3.5

Ig (

A)

If (A)

ig vs if for loading

ig

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5

PF

IF (A)

p.f. vs if for reactive power

pf

00.5

11.5

22.5

33.5

44.5

5

0 0.5 1 1.5 2 2.5

Ig (

A)

If (A)

ig vs if for reactive power

ig vs if for reactive power

DISCUSSION:

Synchronizing Indicator commonly used in practice

Lamps can be used in Synchronization but they are not quite accurate, because they depend on a sense of

correct judgment of the operator. To eliminate this element of personal judgment in routine operations of

alternators, the machines are synchronized by a more accurate device called a synchronoscope.

A synchronoscope consists of 3 stationary coils and a rotating iron vane which is attached to a pointer. Out of 3

coils, a pair is connected to one phase of the line and the other to the corresponding machine terminals, potential

transformer being used. The pointer moves to one side or the other from its vertical position depending on

whether the incoming machine is too fast or too slow. For correct speed, the pointer moves vertically up.

Lamps as a synchronizing indicator

If machine 2 has a different speed from that of machine 1, then its frequency will also be different, hence there

is a phase difference between their voltages ( even when they are equal in magnitude). This phase difference

will be continuously changing with changes in frequency. Sometimes the resultant voltage is maximum and

some other times its minimum. Hence the current is alternating maximum and minimum.

Due to this changing current through lamps, a flicker will be produced, the flicker of the lamp being (f2 – f1).

Lamps will glow up and dark out alternatively. Darkness indicates that the two voltages E1 and E2 are in exact

phase opposition relative to the local circuit and hence no resultant current through lamps. Lamps will glow

brightest when the two voltages are in phase with the bus bar voltage because then voltage across them is twice

the voltage of each machine.

Effect of unequal voltages

The phasor diagram is as shown:

If E1 is greater than E2, then their resultant is Er = (E1– E2) and is in phase with E1.

This Er sets up a synchronizing current ISY which is almost 900 behind Er and hence

behind E1 also. This lagging current produces demagnetizing effect on the first

machine, hence E1 is reduced. The other machine runs as a synchronous motor, taking

almost 900 lead current. Hence it’s field current is strengthened due to magnetizing

effect of armature reaction. This tends to increase E2. These two effects act together

and hence lessen the inequalities between the two voltages and tends to establish

stable conditions.

Effect of unequal phase

If E2 falls back by a phase angle α electrical degree. Though E1 = E2 in magnitude but there is a resultant

voltage Er which circulates a synchronizing current. This current ISY sets up a synchronizing torque, which

tends to retard the generating machine with E1 and accelerate the motoring machine with E2.

CONCLUSION

The experiment was a success as we were able to synchronize a generator to infinite bus bar. The objectives met

despite a few errors attributed to;

Human error while taking the readings.

Instruments error thus giving inaccurate values.

REFERENCES

A text book of Electrical Technology By Theraja

http://www.en.wikipedia.org

Electric Machinery – Fitzgerald Kingsley