Synchronous versus asynchronous generators

Hi, it depends on the very detailed requirements.

Broadly speaking the synchronous generator features two advantages. First if designed properly it should have a better efficiency. I guess the asynchronous generator of that size and surface cooled will have an efficiency of about 96 to 97% whereas the synchronous generator most likely about one to two percentage points better.

Secondly the asynchronous generator loads the 11 KV line with reactive (inductive) power whereas the synchronous generator would be able to feed reactive (capacitive) power into the line.

Thank you for your reply.

The problem I'm facing is, we are having two induction generators of 350kW and 400kW. The prime mover is hydraulic turbine. The generation voltage of 415V is stepped upto 11kV and fed to the 11kV rural feeder. The load on this feeder is 600kW and is supplying power to eight villages. This feeder is subjected to power cuts for about 12 hours in a day. These being induction generators cannot supply power in isolation.

Hence if these induction generators are changed over to synchronous generators, what are the advantages and disadvantages? Will the synchronous generator give more energy in terms of kWH? any other advantages? please help

Thanks!

SAMUEL CHELLIAH

Hi,

the problem as far as I understand are the power cuts on the 11kV power line.

If you employ synchronous generators on the turbine to feed the villages during the power cuts (which is possible) you will encounter heavy problems when the external power on the 11 kV line resumes.

If this is predictable, than you can take appropriate measures. But if this is not predictable it is very well possible to destroy heavily the synchronous generators (broken shaft or worse). The damages which may be encountered depend mainly on the overall system paramenters (short circuit impedance of the transformer, length of 11 kV feeder line, further transformers on the way down of the 11 kV line, imdedances of the synchronous generators, size of shaft, used shaft coupling, etc....).

Appropriate measures to avoid these damages would be:

- the frequency of the synchronous generator must precisely meet the line frequency (speed controller of the turbine)

- the resuming power of the 11 kV - line must be synchronised with the induced voltage of the synchronous generators

If using asynchonous generators it is too well possible to feed the villages during power cuts. You simply need to magnetize the asynchonous generators which could be done either by a appropriately desined capacitor - bench or a small synchronous generator (about 200 kVAr) for the magnetisation of the asynchonous generators (hardly any power intake from turbine).

The bad news are, what concernes the possible damages basically the same holds true in case the 11 kV power resumes unpredictably.

The good news are: If all system parameters are known sufficiently precise the behaviour of the equipment can be calculated in advance quite precisely and possibly mechanical or electrical measures can be taken to avoid damages for both setups synchronous or asynchonous.

hope this comment will help you