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#1

Sequence impedances

How I can calculate of negative impedance if positive impedance is given for a short transmission line circuit?
08-15-2014 05:46 AM
Top #2
For a transmission line, the negative sequence is equal to the positive sequence impedance because it is a linear, symmetrical and static circuit element and the impedance is independent of the phase order of the source voltage. Transformers, reactors and shunt capacitors also behave in the same way. Rotating machinery will typically have a different negative sequence impedance because the direction of the shaft will oppose the negative sequence voltages.
08-15-2014 08:00 AM
Top #3
I agree.
They are equal.
08-15-2014 10:04 AM
Top #4
Hi Jim

I agree with you, as long as the transmission line if fully transposed. Without transposition, a transmission line is not exactly symmetrical.
In general, this might not make much of a difference, but it could be important for protection (for instance).
My point: you have to assume it is fully transposed and consider the negative sequence impedance to be equal to the positive sequence impedance, if no further information is available. But, then, the proverbial grain of salt should be added to this impedance information.
08-15-2014 12:48 PM
Top #5
Thanks Leonardo. I agree that without transposition, a T-line is not exactly symmetrical. For a shot line as described, the neg-seq is essentially equal to the pos-seq. The exact solution is more extensive and requires a matrix representation that accounts for ground wires and other nearby circuits that have mutual coupling.