Alternator

3.1 Alternator

An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Any AC electrical generator can be called an alternator an alternator also called synchronous Generator.

3.2   Voltage regulation:

Steady state condition:

The generator, when driven at rated speed and operating with normal excitation control system, which includes an automatic voltage regulator (AVR), will maintain the voltage under steady state conditions within + 1.0% of rated voltage for all loads between no-load and rated load at rated power factor.

Transient condition:

      The generator, when driven at rated speed and giving the rated voltage on no-load under the control of excitation and voltage regaling system, is switched on to a symmetrical load which would absorb 100% of rated current (100% impedance) at rated voltage at a power factor between 0.4 and zero (O) lagging, the initial voltage drop will be limited to 30% of rated voltage and the voltage will recover to at least 94% of rated voltage in less than 1.0 second

Deviation factor of a wave:

The deviation factor of the line –to –line terminal voltage on open-circuit at rated speed and voltage will not exceed 10%

Dielectric strength:

Values of the test voltage will be of in accordance with JEC-144

3.3 Accessories of Alternator

1. Stator coil

2. Field coil

3. AC exciter

4. Silicon rectifier

5. Coupling

6. Bearing

7. Fan

8. Damper coil

1. Stator coil: Supports revolving magnetic field, generates voltage and causes load                   current to flow.

2. Field coil: Generates revolving magnetic field (main magnetic flux)

3. AC exciter: Generates power for main magnetic flux

 4. Silicon rectifier: Converts AC current generated by the AC exciter to DC.

5. Coupling: Connects with the prime mover to transmit power.

6. Bearing: Supports revolving parts for stable revolution

7. Fan: Installed in the revolving area to feed in cooling air

The synchronous generator most commonly used, the revolving –armature type AC exciter is installed on the shaft used for DC excitation of the field coil. The output is converted to DC with the silicon rectifier for supply to the coil.

No. of Revs. And Poles

The relation between revolution speed and frequency in generators is represented by the following formula

3.4 Three Phase Electromotive Force

  Fig: 3.2 Three-phase electromotive force with wave form

3.5  Protection of Alternator

Some of important faults, which may occur on an alternator:

Failure of prime-mover

Failure of field

Over current

Over speed

Over voltage

Unbalanced loading

Stator winding faults

Failure of prime-mover:

 When into to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the supply system. This monitoring conditions is known as “ inverted running”

Engine driven alternators, when running inverted, draw a considerable amount of power from the supply system and it is a usual practice to provide protection against motoring in order to avoid damage due to possible mechanical seizure. This is achieved by applying reverse power relay to the alternators, which isolate the latter during their motoring action. it is essential that the reverse power relays have time-delay in operation in order to prevent inadvertent tripping during system disturbance caused by faulty synchronizing and phase swinging.

  Failure of field

The changes of field failure of alternator are undoubtedly very rare. Even if it does occur, no immediate damage will be caused by permitting the alternator to run without a field for a short-period. It is sufficient to rely on the control room attendant to disconnect the faulty alternator manually from the system bus-bars. Therefore, it is a universal practice not to provide automatic protection against this contingency.

 Over current

It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Over current protection for alternators is considered unnecessary because of the following reasons.

The modern tendency is to design alternator with very high values of internal impedance so that they will stand a complete short –circuit at their terminals for sufficient time without serious overheating. On the occurrence of an overload, the alternators can be disconnected manually.

The disadvantage of using overload protection for alternators is that such a protection might disconnect the alternator from the power plant bus on account of some momentary trouble outside the plan and, therefore, interfere with the continuity of electric service.

Over speed

The chief cause of over speed is the sudden loss of all or the major part of load on the alternator. Modern alternator is usually provided with mechanical centrifugal devices mounted on their driving shafts to trip the main value of the prime-mover when a dangerous over speed occurs.

Over voltage

The field excitation system of modern alternators is so designed that over voltage conditions at normal running speeds cannot occur. However, over voltage in an alternator occurs when speed of the prime-mover increase due to sudden loss of the alternator load.

Unbalanced loading

Unbalance loading means that there are different phase currents in the alternator. Unbalanced loading arises from faults to earth or faults between phases on the circuit external to alternator. The unbalanced currents, if allowed to persist, may either severely burn the mechanical fixing of the rotor core or damage the field winding. 

Stator winding faults

These faults occur mainly due to the insulation failure of the stator windings .the main types of stator winding faults, in order of importance are:

fault between phase and ground

fault between phases

inter-turn fault involving turns of the phase winding

The stator winding faults are the most dangerous and likely to cause considerable damage to expensive machinery. Therefore, automatic protection is absolutely necessary to clear such faults in the quickest possible time in order to minimize the extent of damage. For protection of alternators against such faults, differential method of protection (also knows as merz-prize system) is most commonly employed due to its greater sensitivity and reliability.  This system of protection is discussed in the following section.

3.6 Common Electrical Equipments

Generator control panel-1

Generator control panel-2

HT Switch Gear

LT Switch Gear

Synchronizing panel

Auto load sharing panel

Auxiliary panel

Out going Feeder panel

Bus Tie Panel

Rectifier Panel

3.7  Generator Control Panel Mounted on the inside

Common Bus-bar

AVR

CT

PT

CB

Relay

Timer

Fuse

Metering & measuring Instruments

Connecting Cables

Control switch for governor motor.

Control switch for synchronizing mode.

Bell for alarm (BL).

Tumbler switches (TS).

Anti –compensating heater (SH)

1-AC voltmeter (V)

1-Voltmeter change-over switch (VS)

1-AC ammeter (A)

1-Ammeter change-over switch (AS)

1-Frequency meter (F)

1-Power factor meter (PF)

1-Indicating wattmeter (W)

Signal lamp .red and green globe (RL, GL)

Control switch for engine, start-stop

Control switch for ACB

Control switch for governor motor

Push button switch for auto synchronizing mode

Bell for alarm (BL)

Under voltage, Over voltage, Reverse power relay