De-rating of steam turbine generator
De-rating of steam turbine generator
(OP)
Hi, everybody:
Does anone know what de-rating factor should be for a steam turbine generator due to unbalanced load? thx.
Does anone know what de-rating factor should be for a steam turbine generator due to unbalanced load? thx.






RE: De-rating of steam turbine generator
your question is not clear for me.
Anyway an umbalanced load produces the flow of an negative sequence current. This current produces a contro-rotating field that overheates the rotor components.
Thus an negative sequence current creates heating problems. Every generator is characterised by a inverse-time curve indicating the maximum admissible inverse seq current.
The curve equation is
K = t * (I2/In)^2
where usually
K = 10s
I2/In = 0.08
Regards
Alex68
RE: De-rating of steam turbine generator
I agree and understand that your relay curver is for "Short-time" heating during system fault conditions. My concern is for "continuous" negative sequence rating of turbine generator (23 kV, 750 MVA) due to unbalanced load. I heard that the rating is around 10%-15%. Two questions: 1) Is this number reasonable? 2) If the negative sequence current (e.g. 20%) exceeds the rating, the generator has to operate in low MVA output (< 750 MVA). what is the de-rating factor?
RE: De-rating of steam turbine generator
Usually the data sheets of a generator include also the curve of the maximum admissible negative sequence current. Looking at that curve you can know the maximum permanent current of negative sequence.
I have never heard that you can de-rate the generator to permit a greater anbalanced current.
The negative current produces a contro-rotating field. This field induces a current in the rotor with double frequency. This phenomenon is independent from the generator load.
RE: De-rating of steam turbine generator
RE: De-rating of steam turbine generator
The generator manufacturer would have to provide the value of K. I2 is the negative sequence current present and In is the generator nameplate current (I2/In is I2 in per unit). There is no "usual" value for I2/In, it is what ever it is, and the more unbalance there is, the more I2.
RE: De-rating of steam turbine generator
Thanks,
Jim
RE: De-rating of steam turbine generator
RE: De-rating of steam turbine generator