timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
A question about short circuit MVA (MVAsc). If we have a generator of 2 MVA, then what will be its MVAsc ?
Short Circuit MVA
- Thread starter timm333
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
Not enough information to answer...You need to know the generator's subtransient reactance.
Generator MVAsc = MVA/X''d
timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
Thanks. So if a 2 MVA generator has Xd'' = 0.2, then MVAsc = 2/0.2 = 10 MVAsc, this makes sense. I am just being curious that why MVA and MVAsc are not the same. In other words, if the generator is capable to produce 10 MVAsc, then why its useful output power is only 2 MVA (and not 10 MVA)?
the 2 is into a load
the 10 is limited only by its reactance, the windings will be destroyed if sustained
similar to a xfmr
rated mva 1
impedance pu is 5%
sc mva = 1/0.05 = 20
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
[FONT=Georgia, Bitstream Charter, serif]The subtransient (x") time period lasts from about one to three cycles, which is from 16.7 milliseconds to 50 milliseconds (assuming a power frequency of 60 Hz). So if you consider useful power output for about 20-50 milliseconds, then you are good to go. [/FONT]
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Yet another reason is that the running MVA may well be limited by the size of the prime mover (engine or turbine for example), while the rotating mass of the system will allow the MVAsc to be limited only by the generator portion.
A vendor of cheap wind turbines once famously rated his wind generator at roughly twice the sustained production for the reference wind speed on the grounds that once it got up to speed and you closed the output circuit it would deliver the higher power "for awhile" as the system slowed down to a new equilibrium speed.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Also why large generators were once considered impractical because the misinterpretation of energy transfer conditions caused the designers to try to make source impedance and load impedance equal!
timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
The first page of the link above from the website of Cummins states that R for generators is negligible. So does it mean that the generators have no X/R ratio like transformers? When we say, for example, the X/R on the primary side of the transformer is 15, does it mean that all of this X/R is due to the cable between generate and transformer (without any contribution from generator)?
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
At first, since there are two series of reactance: saturated and unsaturated in my opinion x"d saturated it is the minimum reactance in order to calculate the maximum short-circuit current.
From ANSI Standard C37.010- see:
http://arcadvisor.com/faq/motor-transformer-xr-data
a 2 MVA X/R=25 but for transformer of same 2 MVA rated X/R=7.5.
This ratio, for generators, refers to synchronous reactance. See:
https://circuitglobe.com/synchronous-reactance-and-synchronous-impedance.html
From ANSI Standard C37.010- see:
http://arcadvisor.com/faq/motor-transformer-xr-data
a 2 MVA X/R=25 but for transformer of same 2 MVA rated X/R=7.5.
This ratio, for generators, refers to synchronous reactance. See:
https://circuitglobe.com/synchronous-reactance-and-synchronous-impedance.html
- Status