Calculation of Equivelant Impedance
Calculation of Equivelant Impedance
(OP)
I'm attempting to calculate Zeq at the buss level. I have three generator impedances (Z1, Z2, Z3) connected in parallel and the cable impedances (Z12, Z23) connecting each of the generator busses. How do I calculate Zeq at the buss? Are they all in parallel or something else?
Z1 nodes 0-1
Z2 nodes 0-2
Z3 nodes 0-3
Z12 nodes 1-2
Z23 nodes 2-3
Z1 nodes 0-1
Z2 nodes 0-2
Z3 nodes 0-3
Z12 nodes 1-2
Z23 nodes 2-3






RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
1) If both the ties are closed then Zeq=Z1||Z2|Z3 [if the tie impedance is zero].
2) If both the ties are open then if a solid three-phase shortcircuit will occur on the middle[no.2] bus bar then Zeq=(Z1+Z12)||Z2||(Z3+Z23)
3) If both the ties are open then if a solid three-phase shortcircuit will occur on the
No.1 bus bar then Zeq= (Z1)||{[Z2||(Z3+Z23)]+Z12}
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
RE: Calculation of Equivelant Impedance
For the starters here is a hint: No current would flow through Z12 and Z23 when the respective tie breaker is open. How do you suspect the source of Z2 (or Z3) will contribute to the fault on bus 1, when the Tie 12 is open?
You need to start with a correct and complete impedance diagram. What you have drawn is representation of a one-line diagram. There are no voltage sources and no "common" (reference) bus is shown.
With all due respect, you need to refer to some text books and standards such as IEEE Violet Book™, IEEE Std 551™-2006— Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems (used to be part of the Buff Book) and/or the Red Book to learn how to prepare impedance diagrams and how to perform short circuit calculations.
Before that a refresher in electrical circuit network analysis (mesh or loop analysis) would help as to how to solve currents and voltages in a mesh.
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculation of Equivelant Impedance
Thank you for your valuable remarks. I am very sorry for my European representation of impedance diagram but I still prefer the IEC 60909 for short-circuit calculation.
If the diagram was as simply as seems to me now to be I don't see what the problem was.
By the way, the dashed line provided with 2 arrows on the ends represent, in my world, a cable with 2 potheads connected DIRECTLY to the busbar and the tie breaker connect two busbars at the end of each busbar. If the connection between busbars is the cable connected to the breaker it is a very simple case without any problem. From the O.P. with my poor English I understood wrong. I am sory for the inconvenient.
RE: Calculation of Equivelant Impedance
IEEE or IEC, your diagram is still wrong and incomplete. Electricity does not flow differently in Europe, certainly not through an open breaker!.
Your own post:
which is correct and hence the two diagrams on the right make no sense. The cables are in series with the breaker, you are showing them in parallel.
You do have a problem!
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculation of Equivelant Impedance
1) Your quotation is from 7JLAman4 post of 28 Aug 09 10:41.
2) The impedance diagrams [both] are according to schematic diagram from left hand.
3) I agree –and I apologize again-I didn't understand well as I thought the cables are in parallel with the breakers-and not in series as usual.
I quote from my own post of 28 Aug 09 0:34:
"If I well understood your scheme –three generators connected to three bus bars itself interconnected by means of two tie breakers SHUNTED by cables –then [developing what dpc said]:"
"Shunted" does mean "parallel", does it?
RE: Calculation of Equivelant Impedance
If the cables shunt the breaker, the breaker is out of the circuit. The circuit is always closed via the cables. Opening or closing the breaker will have no effect.
Rafiq Bulsara
http://www.srengineersct.com