The operation of connecting an alternator in parallel with another alternator or with common bus-bars is known as synchronizing.
- The voltage between the incoming alternator and the bus-bars can easily be checked by a voltmeter and the phase sequence of the incoming alternator and the bus-bars can also easily be checked by phase sequence indicator.
- The difference between frequency and phase voltage of the incoming alternator and bus-bars can be checked by following two methods-
- By Three Lamp (one dark, two bright) method
- By synchroscope
(i) Three Lamp Method
In this method of synchronizing of the alternator, three lamps L1, L2, and L3 are connected as shown in Fig.
- The lamp L1 is connected between phase R1 and R2 and the other two lamps L2 & L3 are cross-connected to the phase (Y1, B2) & (B1, Y2) respectively.
- When the difference between frequency and phase voltage of incoming alternator and bus-bars is same then the straight connected lamp L1 will be dark and other two lamps (L2 & L3) which is connected cross with the phase will be bright equally.
- At this instant, the synchronization will be perfect and the switch of the incoming alternator can connect it to the bus-bars.
(ii) Synchroscope
- A synchroscope is a measuring/ indicating instrument that is indicated by a revolving pointer.
- The difference between frequency and phase voltage of the incoming alternator and bus-bars can be checked by synchroscope.
The fig shows the synchroscope.
- A synchroscope is a small motor, in this instrument, the supply to the field is fed by the bus-bars through a potential transformer and rotor from the incoming alternator.
- When the alternator is running fast then the frequency of alternator will be more than the frequency of bus-bars then that condition the pointer will move on the clockwise direction.
- When the alternator is running slow then the frequency of alternator will be less than the frequency of bus-bars then that condition the pointer will move on anti-clockwise direction.
- When the frequency of alternator is same as the supply frequency of bus-bars then that condition no torque act on the rotor(pointer) of synchroscope and the instrument pointer points vertically upwards (“12 O’ clock”).
The fig showed the connection of A synchroscope to the alternator and busbars.
MCQs on Parallel Operation and Synchronization of Alternators:
1. Which of the following conditions are necessary to connect an alternator into the bus-bar grid?
A) Same terminal voltage.
B) Same prime mover speed.
C) Same phase sequence.
D) Same frequency.
Explanation: Before connecting an incoming alternator with an already operating alternator, the following conditions must be satisfied:
- Equal voltage: The terminal voltage of the incoming alternator must be equal to the bus-bar voltage.
- Same frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
- Phase sequence: The phase sequence of the three phases of the alternator must be similar to that of the grid or busbars.
- Phase angle: The phase angle between the generated voltage and the voltage of the grid must be zero.
2. If the excitation of one alternator is changed during the parallel operation of two alternators, what happens?
A) It only causes large variation in kW output without any circulating current.
B) It causes circulating current, and kW output remains approximately constant.
C) It does not cause any circulating current and variation in kW output.
B) It causes circulating current, and kW output remains approximately constant.
C) It does not cause any circulating current and variation in kW output.
D) None of these
Explanation: When two alternators run in parallel, changing the excitation of one alternator causes circulating current and keeps the kW output approximately constant.
3. What are the advantages of parallel operation of alternators?
A) Increase in reliability.
B) Increase in efficiency under light load conditions.
C) Continuity supply during maintenance or repair.
D) All of the above.
Explanation: Parallel operation of alternators offers advantages such as increased reliability, improved efficiency under light load conditions, continuity of supply during maintenance or repair, and easy future expansion.
4. Which of the following is a necessary condition for synchronizing an alternator with the grid?
A) Equal terminal voltage.
B) Same frequency.
C) Similar phase sequence.
D) Zero phase angle between generated voltage and grid voltage.
Explanation: Synchronization requires matching the phase angle between the alternator’s generated voltage and the grid voltage to be zero.
5. During parallel operation of alternators, what impact does a variation in voltage or frequency have on the bus bar?
A) Significant impact.
B) Minimal impact.
C) No impact.
D) Cannot be determined.
Explanation: Any variation in voltage or frequency will hardly make a significant impact on the bus bar voltage and frequency during parallel operation of alternators.