Generator Rated kVA & Parameters at Various Ambient Temperaure
Generator Rated kVA & Parameters at Various Ambient Temperaure
(OP)
Hello,
I have 14.24 MVA rated synchronous generator driven by gas turbine and it is rated at 42 degree C and all parameters are given based on this temperature.
If this generator will be running at 0 degree C. of site ambient temperature, the generator can cover more kVA e.g. let say 16 MVA as an example.
I think I should use the maximum possible generator MVA at lowest site ambient temperature (e.g. 16 MVA at 0 degree C.) for short circuit current calculation. If so, how about generator parameters? I guess reactance values (X"d, X'd, Xd, etc.) would stay at the same values regardless of ambient temperature, but resistance values (Ra, etc.) would be slightly lower than that of 42 degree C. This would affect generator X/R ratio for short circuit current.
If so, do you think I can use the following equation used for cable resistnace correction?
R' = R ( 234.5 + Tc )/( 234.5 + Tb )
R = 0.00213 p.u.
Tb = 42 deg.C
Tc = 0 deg. C
R' = 0.001806 p.u. (corrected at 0 deg.C)
This will increase generator X/R ratio and DC component value of short circuit current.
Would you guys please give me advices whether my approaches are appropriate for the maximum short circuit current calculation?
Thanks in advance.
I have 14.24 MVA rated synchronous generator driven by gas turbine and it is rated at 42 degree C and all parameters are given based on this temperature.
If this generator will be running at 0 degree C. of site ambient temperature, the generator can cover more kVA e.g. let say 16 MVA as an example.
I think I should use the maximum possible generator MVA at lowest site ambient temperature (e.g. 16 MVA at 0 degree C.) for short circuit current calculation. If so, how about generator parameters? I guess reactance values (X"d, X'd, Xd, etc.) would stay at the same values regardless of ambient temperature, but resistance values (Ra, etc.) would be slightly lower than that of 42 degree C. This would affect generator X/R ratio for short circuit current.
If so, do you think I can use the following equation used for cable resistnace correction?
R' = R ( 234.5 + Tc )/( 234.5 + Tb )
R = 0.00213 p.u.
Tb = 42 deg.C
Tc = 0 deg. C
R' = 0.001806 p.u. (corrected at 0 deg.C)
This will increase generator X/R ratio and DC component value of short circuit current.
Would you guys please give me advices whether my approaches are appropriate for the maximum short circuit current calculation?
Thanks in advance.






RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
David Castor
www.cvoes.com
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
The initial short circuit values have nothing to do with the turbine MW output. An unloaded generator delivers about the same initial short circuit current as a fully loaded unit. From a short circuit perspective, the MW output increase due to ambient air temperature is no different than increasing load by opening the throttle.
MW loading has secondary effects on short circuit currents, but if your system is that close to the ratings, you are in trouble already.
Just use the ISO ratings and nameplate values in your calculations. If you want to get conservative, use 85% of the given impedances because ANSI standards allow a 15% variation in estimated and actual impedance. That is similar to adjusting the transformer impedance to 92.5% x %Z since standards allow a +/-7.5% variation from the specified value. Also, run the short circuit calculations at 105% nominal voltage. Anyone of these variations has a greater effect on the calculated short circuit current than ambient temperature.
Sizing of the generator transformer and generator cables/bus is where the ambient temperature effect on gas turbine output needs to be considered.
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
Thanks for your quick advice and it makes me more comfortable. Do you think it would be good idea to use the max. generator ouput (16 MVA)at 0 degree C with the given parameters at 42 degree.C for my short circuit current calculations?
Dear dpc,
The reason why I am worrying a lot was that I will have four (4) generators directly connected to 11kV Switchgear (IEC), and IEC standard breaker (Std. time constant = 45ms) would not suit for my case due to high DC component especially for outgoing feeder breakers but I don't have any problems with peak and breaking currents. I will specify higher time constnat for %DC component.
In addition, if I would have TRV peak problem, as far as I know, a possible solution would have breakers with higher rated voltage (e.g. 15kV, 17.5kV, etc.). Would you have any recommended alternative solutions to lower TRV peak voltage?
But I don't expect I would have RRRV issue since generator surge capacitors will increase the rise time to TRV peak voltage.
Do you have any more advices regarding DC component and TRV concerns with my configuration?
Thanks in advance
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
You need to increase the breaker SC ratings, add some additional impedance somewhere, or run one less generator.
David Castor
www.cvoes.com
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
If the inspector has to get out the tape measure to check the required clearance, you don't have enough clearance. Doesn't matter what the measurement is, if it has to be checked there isn't enough clearance. Same thing applies here, if there is a need to massage the numbers you are starting with the wrong numbers.
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
Just for your information, I have 50kA breakers and its calculated DC component was about 33 kA_dc at this time while breaking current is about to 22 kA. I cannot increase SC rating of breaker since I cannot find out 63kA breaker for 11kV.
I just need to specify higher time constant for %DC component. i.e. t4=120ms. All other time constant does not suit for my case when breaker is delayed by protective relays.
I am now just wondering whether higher time constant (120ms) for breaker will affect breaker price.
Thanks for all of your advices again.
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
Thanks in advance.
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
I take it you are using IEC ratings and procedures.
You might want to get Conrad St. Pierre's book on SC calculations. It covers both ANSI and IEC methods.
David Castor
www.cvoes.com
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
Asymetric current includes the DC offset current.
Available Short Circuit Current is calculated from the impedance without regard to X:R ratio.
However, breakers are often rated in Available Short Circuit Current. But, a breaker that is rated at 20,000 Amps Available Short Circuit Current will be tested at a current level and phase angle corresponding to an expected XR ratio.
A breaker rated for a given Available Short Circuit Current will be tested and will be able to safely interrupt the corresponding actual asymetrical short circuit current in all but extreme installations.
Be sure that you are working with the correct values.
It is not appropriate to calculate the asymetrical current and use that value to select a breaker rated for Available Short Circuit Current.
It is appropriate to determine the X:R ratio corresponding to the breaker testing and compare that with the X:R ratio of the source. Further calculations may not be needed.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
RE: Generator Rated kVA & Parameters at Various Ambient Temperaure
You're right about the high X/R and the long time to first current zero, but please don't consider what some cheap & nasty designs by certain EPC contractors have done, which is to incorporate a timer to delay breaker opening for a few cycles until the DC component has died down a bit. The only reason for doing this is so they can use a cheap distribution breaker inlieu of a larger and more expensive generator breaker. My own opinion is that cost saving is a poor excuse for intentionally leaving a fault on the system when breakers are available which can clear it quickly.
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