When operating synchronous generator at a power factor lower than 0.8lagg, or at leading power factor, aparrent power must be derated, can you explain pls how these derating factors can be obtained from generator capability curve, in general how can you read the capability curve and obtain operational informations out of it?

 

Are you asking simply how to read the curve? If so........ looking at the attached curve you find you Megawatts on the X-Axis (for example 25) . Draw a line up until you reach the cooling air temperature (40C in this example). Draw a line to the left (green in this example) to the to the Y axis to read the allowable Megavars which would be about 31 in this example. This would give a derated apparent power of sqr(25^2+31^2) = 39.8. Note that if the cooling air was 63C you would be limited to about 24 Mvars (pink line) so under those operating conditions sqr(25^2 + 24^2) = 35.6. Not sure if this is what you were asking but if not could you restate the question.

 

Thanks MWO, you answer was so nice. It helps me a lot as a practical example which include how to de-rate in different ambient temps.
If you add to your answer how to estimate derating factors for operation with small lagging power factors e. g 0.7,0.6 lagg, will be so nice, my friend.
Any how thank you very much

 

MWo
I forget to add this:
According to your example, the MW(25),and pf(0.8)is kept constant, MVArs only will change according to temperature change.... True?

 

You cant keep the pf and MW constant while changing the Mvars. If you look at the supplied capability curve you will see the pf lines are linear. The pf is the ratio of the real power to the apparent power. The tangent of the arccosine of pf is the ratio of the of the Mvars /Mwatts. In other words for a given operating condition the Mvar, Mwatts and pf are fixed. If you change one, another must change also. In answer to your question on the graph, for a given pf you would go up the power factor line until you hit the temperature curve. Straight down to the x-axis would be your max Mwatts and straight across to the Y-axis would be your max Mvars for this particular pf. If you change either the Mwatts or Mvars without changing the other it would be a different power factor. I hope this helps

 

I also have a spreadsheet i have built which allows you to define the 40C curve and the 63C curve below the 0.85 PF corresponding to my example curves I gave you. The spreadsheet treats this curve as part of the segment of a circle and does the analytical geometry for you to define the center and radius of the circle. A third sheet has a place to enter an operating temperature between the other two and it interpolates the circle for that temperature. It allow you to enter the Mwatts and it will calculate the Mvar limit for you. Playing with this might help you in your understanding. I could E-mail you a copy if I had your address.

 

Thanks MWo
I think the same thing is true for estimating operation points for power factors which is less than. 85 pf. I mean there is especial capability curve also with many radial lines representing power factors plus there is envelop for the safe bounderies within which you can operate your synchronous generator represented by a thick line, and number of many semi circles, so as you said you can go down the radial line representing ur more lagging pf until it hits the the thick envelop line representing the safe bounderiesvand there you will find the factor by which you can multiply to find your derated kva, I think if I can rember this capability curve is prepared by Stamford, I will send if found it.
Thank you alot

 

I also have a spreadsheet i have built which allows you to define the 40C curve and the 63C curve below the 0.85 PF corresponding to my example curves I gave you. The spreadsheet treats this curve as part of the segment of a circle and does the analytical geometry for you to define the center and radius of the circle. A third sheet has a place to enter an operating temperature between the other two and it interpolates the circle for that temperature. It allow you to enter the Mwatts and it will calculate the Mvar limit for you. Playing with this might help you in your understanding. I could E-mail you a copy if I had your address.

MWo... Below is my. Email:
[email protected]

 

MWO
I send you my email which is:
[email protected]

 

@Wilimohi,

Depending on which version of Mark* turbine control system the machine uses (I presume the machine has a Mark* turbine control system) there may be a reactive capability curve on the HMI which dynamically places the real-time operating point (based on MW and MVAr) on the graph to let the operator know where the machine is currently operating. Some even use generator temperature sensors (usually RTDs) to draw the temperature curves to make it more obvious for the operators/supervisors to understand where the machine is presently operating. (Of course, this presumes the operators are trained to know how to read and interpret the reactive capability curve.)