Chapter 12 - Paralleling & Load Transfer.

Question 1

Explain 2 Reasons for Paralleling AC Generators?

Answer 1

1. Transfer load from 1 Generator to another without interrupting power to the ship.
2. Allow 2 Generators to share the ship’s Electrical Load for increased reliability and security of the supply.

Question 2

List the 4 conditions that must be met before Generators are paralleled?

Answer 2

1. Both Generators must have the same Terminal Voltage. (If Termanal Voltage is not Equal, this will cause very high currents because Voltage Difference will be applied across the Very Low Impedance of the Circuit Breaker).
2. Oncoming generator’a frequency must be slightly higher than the running generator. (A slight freqencey difference ensures a small amount of load is picked up by the Oncoming Generator if it was the other way around the Runnung Generator would supply power to the oncomming generator this is called motoring.)
3. Both generators must have the same phase rotation (ie R.Y.B).(If Phase rotations are not the same then it would be impossible for the phases of two generators two sync)
4. Both generators must be in phase when the circuit breaker is shut. (if not the phases will be out of sync causing a voltage difference which in turn causes high current to flow)

Question 3

Explain the procedure for paralleling 2 AC Generators using a Synchroscope including the reason for each action?

Answer 3

The procedure for paralleling 2 AC Generators using a Synchroscope.

• Adjust the frequency until the Synchroscope is rotating slowly clockwise. (This ensures that the oncoming generator’s frequency is slightly higher than the running generator.)
• Close the breaker for the Oncoming Generator when the Synchroscope read roughly 5 to 12. (Breaker is operated at 5 to 12 instead of on 12 due to a time delay from operating and the braker actually Closing.)
• When the Supply Breaker is closed the Oncoming Generator pick up a small amount of the load.(it pick up a small amount of load due to the frequency difference)
• The next step is to transfer some, or all of the load to the oncoming generator from the running generator.

Question 4

Describe the operation of a Synchroscope?

Answer 4

• The synchroscope is a meter with an iron rotor that is free to rotate.
• The stator coils of the meter are connected to one phase of the bus-bars.
• The iron rotor is connected to the corresponding phase of the Oncoming Generator.
• When the coils are energised the pointer rotates at a speed equal to the frequency difference between the oncoming and the running generators.

Question 5

How do you transfer load from one generator to another once paralleled?

Answer 5

• When the oncoming generator’s circuit breaker is Closed, the oncoming generator picks up a small amount of true load due to the frequency difference.
• The next step is to transfer some or all the load from the running generator to the on coming generator.

Question 6

How do you transfer True Power?

Answer 6

To transfer True Power with to Paralleled Generators, we need to increase the prime mover speed of one compared to the other.

Question 7

How do you transfer Reactive Power?

Answer 7

To transfer Reactive Power with to paralleled Generators, we need to increase excitation current on one and decrease on the other.