Question

(a) State the necessary conditions required prior to the synchronizing of electrical alternators.

(b) Describe the type of cumulative damage that may be caused when alternators are incorrectly Synchronized.

(c) Explain how the damage referred to in (b) can be avoided/reduced.

(d) For two alternators operating in parallel the consequences of:

(i) Reduced torque from the prime mover of one machine.

(ii) Reduced excitation on one machine.

Answer 1

Part (a)

Necessary Conditions Required Before Synchronizing Alternators:

Voltage: Voltage should be equal to or slightly higher than the busbar voltage. This is checked using a voltmeter.
Frequency: The frequency should be equal to or slightly higher than the busbar frequency. In practice, the frequency of the incoming alternator is kept slightly higher so that when load is applied, it matches the busbar frequency. The synchroscope should move clockwise at a slow speed.
Phase Angle: There should be no phase angle between the incoming and running generator. The synchroscope pointer should be at 12 o'clock, indicating a zero or acceptable phase angle difference between the incoming alternator and the busbar.

Part (b)

Cumulative Damage from Incorrect Synchronization:

Mechanical Surge Torque: A significant surge of torque is exerted on the rotor. This can cause damage to the rotor shaft (twisting, keyway damage), coupling (breakage), and stator windings (deformation). The stator core might also shift relative to its frame.
Electrical Surge: A surge of current and power circulates through the system. This greatly strains the entire system, potentially leading to overheating and component failure. The sudden inrush of current could lead to circuit breakers tripping to protect the system.

Part (c)

Avoiding/Reducing Damage from Incorrect Synchronization:

Automatic Synchronization: Systems with automatic synchronization pre-program the correct voltage, frequency, and phase angle, greatly reducing the chances of errors.
Manual Synchronization with Synchroscope: With manual synchronisation, a synchroscope carefully compares the incoming alternator's frequency and phase angle to the busbar's. Adjust the incoming alternator’s voltage to match the busbar. When the synchroscope pointer moves slowly clockwise and approaches the 12 o'clock position, close the alternator breaker to ensure proper synchronisation.

Part (d)

Consequences of operating two alternators in parallel:

(i) Reduced torque from the prime mover of one machine:

If one alternator's prime mover (the engine driving the alternator) experiences reduced torque, that alternator will begin to reduce its load contribution to the busbar. The other alternator will compensate for the reduced output, taking on the additional load. If the torque continues to decrease on the first alternator, it will eventually draw power from the busbar, acting as a motor rather than a generator. This will trip a reverse power relay, shutting down the affected alternator for protection.

(ii) Reduced excitation on one machine:

If the excitation of one alternator is reduced, its generated voltage decreases. This creates a circulating current between the alternators, almost 90 degrees out of phase, due to the inductive nature of alternator windings. The other alternator carries both its original load current and the circulating current, leading to an increased current and a more lagging power factor. The affected alternator will have a reduced current and a less lagging power factor. Both will continue to share the load (kW) despite operating at different currents and power factors. The reduced excitation can lead to instability and, in some cases, result in the alternator becoming overloaded and tripping offline.

✓ Verified Model Answer (Text Solution)

Necessary Conditions Required Before Synchronizing Alternators:

Cumulative Damage from Incorrect Synchronization:

Avoiding/Reducing Damage from Incorrect Synchronization:

Consequences of operating two alternators in parallel: