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ANALYSIS OF SYNCHRONOUS MACHINECONNECTED TO THE POWER SYSTEM

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INTRODUCTIONSynchronous machines are basically divided into two

types viz. Motor and Generator, but synchronous

machines operating on general power supplynetworks may be divided into five types due to theirspecial features.

These five categories are:

yHydro-Generator

y Turbo-Alternator

y Engine Driven Generators

y Motors

y Compensators

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LITERATURE REVIEWTypes of Synchronous Machiney According to the arrangement of the field and armature

windings, synchronous machines may be classified as rotating-

armature type or rotating-field type.

According to the shape of the field, synchronous machines maybe classified as cylindrical-rotor(non-salient pole)machines and salient-pole machines

Synchronous machines are similar to DC machines in that:they have a field winding and an armature winding; a dc currentis passed through the field winding to produce flux; the fluxfrom the field winding induces a voltage in the armature

winding. However, unlike DC machines: the field winding is onthe rotor and the armature winding is on the stator; thearmature winding is an ac winding.

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METHODOLOGYThe Synchronous generator operating alone

y 1. Decreasing the field resistance increases the field currentin the generator.

y 2. An increase in the field current increases the flux in themachine.

y 3. An increased flux leads to the increase in the internalgenerated voltage.

y4. An increase in the internal generated voltage increasesthe terminal voltage of the generator.

Therefore, the terminal voltage of the generator can bechanged by adjusting the field resistance.

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Conditions required for parallelingy 1. The rms line voltages of the two generators must be

equal.

y 2. The two generators must have the same phasesequence.

y 3. The phase angles of two a phases must be equal.

y 4. The frequency of the oncoming generator must be

slightly higher than the frequency of the runningsystem.

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General procedure for paralleling generators

y 1. Adjust the field current of the oncoming generator to make its terminal voltage

equal to the line voltage of the system (use a voltmeter).y 2. Compare the phase sequences of the oncoming generator and the running

system. This can be done by different ways:

y A. Connect a small induction motor to the terminals of the oncoming generatorand then to the terminals of the running system. If the motor rotates in the samedirection, the phase sequence is the same;

y B. Connect three light bulbs across the open terminals of the switch. As the phasechanges between the two generators, light bulbs get brighter (large phasedifference) or dimmer (small phase difference). If all three bulbs get bright anddark together, both generators have the same phase sequences.

y Ifphase sequences are different, two ofthe conductors on the oncominggenerator must be reversed.

y C. The frequency of the oncoming generator is adjusted to be slightly higher thanthe systems frequency.

y D. Turn on the switch connecting second generatorto the system when phaseangles are equal. The simplest way to determine the moment when twogenerators are in phase is by observing the same three light bulbs. When all threelights go out, the voltage across them is zero and, therefore, machines are inphase.

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Synchronizing Procedure in power

systemBefore a synchronous generator can be synchronized

onto live busbars, the following conditions must besatisfied:

y The voltage of the generator must be equal to that ofthe busbars;

y The frequency of the generator must be equal to thatof the busbars;

y The phase sequence of the generator must be thesame as that of the busbars; and

y At the instant of synchronizing, the voltage phasorsof the generator and the busbars must coincide.

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Synchronous machinesconnected

toinfinite Bus

330kv

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Mac ine

y Their motors can be made to operate at a leadingpower factor and thereby improve the power factor of

an industrial plant from lagging to one that is close tounity.

y It gives constant speed from no load to full load.

y Electro-magnetic power varies linearly with voltage.

y These motors operate at higher efficiency, especially inthe low speed at unity power factor range.

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Disadvantages ofSynchronous

achinesy It cannot be used for variable speed job as there is no

possibility of speed adjustment.

y

It requires d. c. excitation which must be suppliedfrom external source.

y It cannot be started under loaded condition. Itsstarting torque is zero.

y It has a tendency to hunt.

y It may fall out of synchronism and stop when overloaded.

y Collector rings and brushes are required.