EE1F24- Parallel connection of alternators

Question 1

Why do we operate synchronous generators in parallel?

Answer 1

-Bigger load can be supplied for a given size
-Increased redundancy
-Maintenance less distruptive
-Full load management can increase gen efficiency

Question 2

What are the conditions required for paralleling?

Answer 2

-RMS line voltage of two generators must be equal
-Two generators must have same phase sequence
-Phase angle of the two phases must be equal
-Frequency of the new gen (on coming gen) must be slightly higher than the frequency of system while running

Question 3

Why must RMS line voltage be equal?

Answer 3

Two generators operating in parallel hence the share voltage must be equal to avoid power in-flow

Question 4

Why must generators have same phase sequence?

Answer 4

Same phase sequence avoids counter rotation that manifest in differential relative rotating magnetic field therefore doubling the terminal voltage across low impedance.

Question 5

The phase angle of the two phases must be equal?

Answer 5

If the two sets of three phase rotate synchronously but at a phase angle there will be active power flow.

Question 6

Why must frequency of on coming gen be slightly higher?

Answer 6

If the on coming gen runs at a slightly lower frequency then it will appear as load to the main generator.

Question 7

What is procedure for paralleling generators?

Answer 7

-Field current of on coming generator adjusted until its terminal voltage equal to line voltage of running system
-Phase sequence of on coming generator must be compared to the phase sequence of the running system
-Frequency of on coming generator adjusted to be slightly higher (ideally using synchro scope)

Question 8

What is an infinite bus?

Answer 8

An infitinite bus is a power system so large voltage and frequency do not vary no matter amount of real or reactive power drawn from it.

Question 9

What controls reactive power supplied by generator to the system?

Answer 9

The field current.