Question: Why it is important to look always at the positive, zero and negative sequence at the time of MV or Higher voltages in the short circuit analysis?

I am electrical engineer graduated long time ago, having difficulty finding the right text on this regard. Currently I do study a file that go for coordination study of the project that give us enough information for the relay settings.

Truely yours,

TJ Samouh

 

Responding to TJ Samouh's Jun 4, 11:11 pm query... perhaps a historical perspective can provide the answer:

The calculation method, properly called Symmetrical Components, is a only mathematical "tool!"

Early in the history of Electrical Engineering, fault-currents for "balanced", 3-ph short-circuits were determined with classical
mathematics: Kirchhoff's laws; D-Y and Y-D impedance equivalency;
complex numbers; superposed currents; and 3-phase circuit "transformation" into single-phase circuits.

As circuit complexity grew, both in size and interconnection, the classical approach became tedious and often unwieldy. In 1918 C.L. Fortescue introduced the "Method of Symmetrical Components! It is a solution, preferable to those mentioned above, for solving problems having "unbalanced" conditions such as line-to-line, line-to ground (earth), double-line-to-ground,

Avoiding deep technical discussion, it is simply a means to "transform" an unsymmetrical system of voltages, currents, and impedances into symmetrical ones.Thus, a system of 3 unequal voltages displaced by unequal angles, can now be resolved into a set of 3 equal voltages displaced by 120°:

a) The first set of voltages referred to as the positive-sequence component having the original phase order, say A-B-C.

b) A second set of equal of voltages usually referred to the negative-phase component whose magnitude is different than a) and having a phase-order of A-C-B.

c) A third set of voltages, in phase with one another.

This method was used primarily for the unsymmetrical faults listed above, as well for operating motors under unbalanced electrical conditions. I never heard of a case where it is "always" used to solve symmetrical fault problems.

A key mathematical element for its success was the introduction of an operator, 'a', to rotate vectors 120°. Of course, it is analogous the complex-number operator, 'j', used to rotate a vector 90°!

Details are in Electrical Engineering text books used by power engineering students.

Regards, Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])