Generator de-rating - 60 Hz to 50 Hz 1

[raynjulsTX]

Anyone remember the theoretical derating factor from a 60 Hz system to a 50 Hz system. I know that individual manufacturers vary, but I can't recall the basis for the derating.

 

[BEPC13]

If I remember correctly from school it is just 5/6. Whatever the rating of a machine is at 60Hz the rating at 50Hz would be 5/6 of that.

 

[edison123]

For the same specific electric loading (amp-conductor per meter) and specific magnetic loading (flux density), the output of the same machine is directly proportional to its speed.

I vaguely remember some thumb rule about output being 20 MVA per metre of core length for 3000 RPM machines (50 Hz).

 

[electricpete]

In an ideal world, you would derate your voltage by 5/6 to keep your flux the same. In that case for comparable power factors your output would be limited to 5/6.


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[edison123]

pete,

In, generators, the voltage will derate by 5/6, with the same flux density (i.e. without doing anything to the winding).

What you say will apply to the motors however.

 

[ScottyUK]

The efficacy of the shaft-driven fan will drop by the sqaure of the reduction in speed, so the cooling effect will also reduce. Thus the machine maximum permissible stator current will be restricted. Similarly, the maximum rotor current will be restricted. This will reduce the practical reactive capability of the machine further than the simple reduction in voltage would suggest.



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[edison123]

hi scotty,

I mentioned both output and volt drop (see my posts). with reduced output, naturally kvar capacity too comes down with correspondent reduction in rotor current.

 

[jbartos]

Comment: Since 50Hz and 60Hz generators tend to have different output nominal voltage, the 5/6 rule will also be affected by the generator output voltage.
Often seen mathematical relationship for generated open circuit voltage over the magnetic circuits as:
Vtt=fundamental rms terminal-to-terminal voltage=(2xpi/sqrt2)xFIxfxNx(KpxKdxKsxKfi)
where
FI is fundamental flux per pole in Webers
f is line frequency in Hz
N is total number of turns in series between terminals (equals 2 x series turns per phase for 3-phase Y-connection)
(KpxKdxKsxKfi) constant factors

 

[Rodmcm]

If you derate the generator voltage then you can maintain the flux the same and hence the generator output. For example a 60HZ 13.8kV unit will have the same output 50HZ 11.5KV.

 

[jbartos]

Suggestion: Keeping V/Hz within the machine rating is recommended, namely, F(kV/Hz)= 13.8kV/60Hz = 0.23kV/Hz = 11.5kV/50Hz ~ about the same constant