Low Frequecy Operation Of Steam Turbine

By low frequency do you mean that the generator output frequency (Hz) is low or that it does not run very often?

Most steam turbines can run safely if not at optimum efficiency over a fairly wide range of speeds.

If the problem is low output frequency from the generator, then the generator and everything the generator is driving will run slower and get hotter than normal for the same KW load.

If you are grid tied, i.e. connected to the local utility, the utility line frequency will determine the system speed/frequency.

The same issues with heating and slow operation will be present if the utility is the cause of the under frequency operation. This is why most installations are protected with under as well as over frequency protection relays.

If the utility is running under frequency, your plant will probably separate from the utility to prevent these problems while causing others. If you're plant is not capable of maintaining frequency when it is not tied to the utility, that is another problem!

I hope I have helped, if not, OH WELL!

Have FUN!
TT3

At TT3 said, if your turbine is grid tied, there are almost no possiblilities of going off grid frequency and hence low frequency operation is only of theoritical interest here.

However in case you are stand-alone then there is a possibility.

Any way for either cases, what you heard is true, to a certain extent.

Near the rated frequency, a bit on either side does not matter much. The profiles of buckets and nozzles are designed for the rated frequency (and thus velocities). going off this value may affect the streamline flow of the steam, which will be harmful only in extreme cases and in normal cases you may have a bit loss in efficiency.

However goinn too off the rated speed may have other problems. The components of the turbine have their own resonant frequencies. These are kept quite off the rated frequency in design stage (you may be aware that the turbines during field test are quickly speeded up through some speed ranges).

These resonant frequencies become more significant (and hence dangerous) in case of larger components (eg LP Blades). These due to large overhangs tend to act like tunig forks with disastrous effects.

The under-frequency operation of a turbine can pose problem to its last stage blades. At the same time, it can can cause overheating of the end coils of the generator. now a days due to invention of better materials, the turbines can operate safely over a wide range of frequency i.e. 48.5 Hz to 51.5 Hz.

The low frequency operation can also cause over -excitation of the generator because the v/f ratio gets disturbed and over-fluxing may result.

One must always be careful to avoid overfluxing which can lead to harmonics and premature degradation of the coils in the generator.