Under Frequency Protection

[Guardiano]

Hi,

I am actually on a site where a 2,5 MW 400 V 50 Hz steam turbine used to operate on underfrequency(below 45 Hz) very often without tripping.Actually the turbine operates in islanding mode and provide power to a plant.The turbine is rated 25 Bar and when steam pressure dropped below 20 Bar,the turbine lost speed but continue to supply the load as there is practically no under frequency protection on the alternator.Nobody seems to be aware or rather does not want to consider this problem.An under frequency protection is of no interest to them as it will be tripping the generator and cause downtime to them.
I know that operating a steam turbine below rated speed,at least below a threshold frequency,could have resonance effects on the turbine blades and obviously detrimental effects on the life of the turbine.How far is this argument true?

Guardiano

 

[m3ntosan]

Have a look into the low pressure stage turbine and see if there are any blades left.
How do they manage to keep the auxiliaries working at 45 Hz anyway?
Any overexcited equipments enjoying the 45 Hz frequency?
Any vibrations?

May you grow up to be righteous, may you grow up to be true...

 

[byrdj]

You should contact the manufacture of the Last Stage Buckets (LSB). this may or may not the the original equipment manufacture (OEM). Given the unit is realitively small, the LSB design may have considerable margin for off frequency operation. The OEM should provide a chart given the accumulated time at differnt off frequency levels.
I have seen stations with relay logic that started timers to document the time operated off frequency

 

[cranky108]

Dumb question: What does the frequency relay trip?
Does it trip the main breaker only?

This maybe what the system was designed for, to allow enough auxiluty power to bring the plant down.

 

[Guardiano]

Thanks all for the comments.Yes,it is a back pressure turbine and no visible vibration is noted at the low operating frequencies.It is a very old turbine,probably manufactured in the early seventees.I understand that the turbine might have been designed to operate very near to 45 Hz but for very short period of time.I am looking to set the under frequency as follows: 48 Hz 5 secs,47 Hz 3 secs,46 Hz 1,5 secs and 45 Hz 0,5 secs.Any comment?

Guardiano

 

[byrdj]

the LSB manufature's "Curves" I have seen for off frequency operation were more exponential, with design frq being infinity and bucket's next resonance being zero.

therefore 48Hz would be 120sec, 47hz 30sec 46hz 3 sec 45hz instanstanous

But I also was thinking the frequency deviation between bucket resonances are usually like +/1 2 to 3 hz. I'm not a rotating element expert but I think that is called the Campbell curve

 

[slavag]

Hi.
from my expirience, 47Hz 10sec is a lower setting of U/f protection of steam turbines.
Good Luck.
Slava

 

[electricpete]

Additionally there may be a concern for overexcitation of the generator if field (and output voltage) is not reduced proportionately to frequency, since flux is proportional to volts/hz and you are decreasing the denominator (hz)

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(2B)+(2B)' ?