De-rating of steam turbine generator

[leoliu]

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

 

[Alex68]

Hi leoliu
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

 

[leoliu]

thanks, Alex68. My question is not very clear in my last message.
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?

 

[Alex68]

The rotor during its heating caused by a negative sequence current is not interested in knowing the origin of the unbalancement.
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.

 

[leoliu]

thx. But I was told that there exists "Derating factor versus unbalance voltage (or current?)" curve in standard. Where can I find such standard curve?

 

[davidbeach]

The curve is as given by Alex68:

The curve equation is

K = t * (I2/In)^2

where usually
K = 10s
I2/In = 0.08

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.

 

[ccjersey]

What influence does the connection of the generator windings have on the rotor heating. Will delta be more or less succeptable than wye connected sets to load imbalance. Is there a way to measure the negative sequence current or its effects on the rotor to detect damaging temperatures before it becomes severe.

Thanks,

Jim

 

[davidbeach]

The winding connection would have no impact, if negative sequence currents are present, they will heat the rotor. The good new is that any decent numerical generator protective relay can measure negative sequence current and alarm and/or trip for levels in excess of set points. Most numeric feeder relays will also provide negative sequence current elements.

 

[leoliu]

I got new understanding on this question. Motor has "Derating factor versus unbalance voltage" curve, but generator does not have such curve. However, generator has protection curve associate with "negative sequence current" as told by Alex68. The reason is that the terminal voltage of generator is almost always balanced in practice due to delta/wye connection of step-up transformer or delta connection of generator windings. Anyway, there is no unbalanced voltage and current, but negative sequence voltage and current at generator terminal.