Short Circuit MVA vs. Voltage Drop
Short Circuit MVA vs. Voltage Drop
(OP)
Gentlemen,
There should be a relationship between short circuit MVA at any point in a distribution system and the voltage drop that occurs when a motor is started at that same point in the system. Is there a straight forward relationship between the two or does the voltage drop also depend heavily on the system X/R ratio?
Thanks in advance,
Raisinbran
There should be a relationship between short circuit MVA at any point in a distribution system and the voltage drop that occurs when a motor is started at that same point in the system. Is there a straight forward relationship between the two or does the voltage drop also depend heavily on the system X/R ratio?
Thanks in advance,
Raisinbran





RE: Short Circuit MVA vs. Voltage Drop
Short circuit MVA helps you determine the source impedance, which is part of the total impedance, along with the supply feeder impedance up to the load terminals while considering the VD. How significant could the effect be? It depends on the magnitude of the current and the power factor and other values involved.
Rafiq Bulsara
http://www.srengineersct.com
RE: Short Circuit MVA vs. Voltage Drop
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If we learn from our mistakes I'm getting a great education!
RE: Short Circuit MVA vs. Voltage Drop
For a short circuit away from the transformer, the circuit impedance is now determined by the vector sum of
A> The sum of the transformer and conductor resistances and B> The sum of the transformer and conductor inductive reactances.
The X/R ratio of the transformer does not apply to a short circuit at a distance from the transformer.
As Scotty says, it gets complicated for a generator. Add the speed stability of the prime mover to the action of the AVR. Then consider whether the machine is self excited, self excited with a current boost module, PMG excited or excited from a source separate to both the electrical output and the prime mover of the gen set.
Bill
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"Why not the best?"
Jimmy Carter
RE: Short Circuit MVA vs. Voltage Drop
In this method, series and parallel MVAs are combined using the same formulas as capacitors. In the case of motor starting, sources use the SC MVA and the motor uses the starting MVA.
RE: Short Circuit MVA vs. Voltage Drop
If you compare your solutions to rigorous solutions you may find errors. MVAs are not all created equal as your method assumes. You are ignoring the power factors of the MVAs.
Compare the % regulation of a transformer to the % impedance rating to see how MVAs are not equal.
I learned that on this forum several years ago. Was that you that helped me Rafiq?
Or it may have been Lionel or DickDV.
At full load and unity power factor the calculated voltage drop based on the transformer impedance will be more than the actual voltage drop which will be based on the transformer regulation.
Try replacing >- load MVA
With >-- load MW + load MVAR
and replace
(load MVA + source MVA)
With >-- (load MW + load MVAR + source MW + source MVAR)
Did I get that right E-Pete?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Short Circuit MVA vs. Voltage Drop
I use the MVA for a quick check of volt drop & fault, not for a detailed analysis. I use an impedance calc for the details. I have noticed some difference in the results but nothing earth shattering.
I'll add the power factor to my MVA quick calcs and see if the results get appreciably closer to the impedance calcs.
RE: Short Circuit MVA vs. Voltage Drop
At short circuit conditions the transformer X + R comprises the total impedance and is described by the percent impedance.
Check a few transformer specs and compare percent impedance and regulation figures.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Short Circuit MVA vs. Voltage Drop
Me? I do not recall. I generally do not believe in short cuts, I only understand V=IZ.
I did not check validity of mivey's MVA method, but I would assume that ignores R and one has to convert the nameplate MVAs to a given base value.
Rafiq Bulsara
http://www.srengineersct.com
RE: Short Circuit MVA vs. Voltage Drop
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Short Circuit MVA vs. Voltage Drop
Here are the formulas to get the MVA values needed for a motor start check:
System: MVA = MVA_base / Z_system_pu
Line: MVA = kV^2 / Z_Ω
Xfmr: MVA = MVA_rated / Z_pu
Motor: MVA = kVA / X"d
RE: Short Circuit MVA vs. Voltage Drop
Bill
--------------------
"Why not the best?"
Jimmy Carter