Question

With reference to the paralleling of alternators:

Outline the principles of synchronisation;

Explain how KW power and KVAr are shared;

for two machines operating in parallel explain the consequences of EACH of the following faults;

(i) Failure of the excitation unit of one machine;

(ii) Sudden shutdown of one machine due to, steam or fuel supply, shortage.

Principles of synchronisation.

Incoming generator frequency must be equal to the bus bar frequency.

Incoming generator voltage must be equal to the bus bar voltage.

The incoming generator and bus bar voltage must be in phase and in same phase rotation.

KW power. Governor setting of alternator are adjusted to achieve KW load sharing.

kVAr Load Sharing

This is achieved automatically by the AVR units which adjust the excitation after synchronising so that each machine shares kVAr and generates the correct voltage.

Excitation failure.

When excitation is lost on a generator the effect on the system will depend on whether it is operating singly or in parallel with other machines.

For single operation it will be obvious that the system voltage will collapse.

However, when one or more healthy machines are connected in parallel, the system voltage may fall only slightly, if at all.

This is because the excitation on these other machines will increase and will offset any tendency for the voltage to fall.

The faulty machine will continue to supply kilowatts by operating as an induction generators.

Unfortunately, this reactive current is seen as a large circulating current flowing between the faulty and the healthy machines.

This current could eventually cause damage to the faulty machine but, or more immediate importance, the over current condition could cause tripping of a healthy machine.

[Failed generator will run as induction generator drawing from the other. Rise in current in both AEs causing overcurrent trip on failed generator and overload.

Over current trip on other generator. ]

Failure of fuel supply

In case of steam or fuel supply shortage the generator speed will drop frequency will drop and load will be transferred to other generator.

In case load on 2nd generator is more than 110% then preferential trip will occur or else overload trip will activate and there will be total blackout i.e. depending on load on AE