Symmetrical components

[simatic7]

Hi all

As I was going through symmetrical components , I came across some question and hope that many of you could answer this.

1. In balanced network , only positive sequence current exists.

2. When there is earth fault in a phase , I assumed that zero sequence flows (considering star connection with earth). Positive , negative and zero sequence network are in series in the faulted phase and are equal in magnitude. Should positive and negative sequence (because of unbalance load in three phase because of fault) flow in other two un-faulted phase ?

3. When there is unbalanced load in all three phases , only positive and negative seq should flow in all three phases ?

I just wanted to know whether the statement two (2) and three (3) are correct or not. If they are wrong , what should be the correct modification.
regards

 

[ijl]

It seems that the questions are based on some confusion about sequence networks. One either calculates with phase currents and voltages in the physical ("phase") network, or one calculates with sequence currents and voltages in the sequence networks. It does not make sense to discuss about sequence currents in one given phase.

1) Correct.
2) The sequence networks are in series when calculating the earth fault current. The phase currents are calculated from the sequence currents using the (well known?) transformation matrix.
3) There may be zero sequence currents, if the (physical) network is connected to earth in more than one point.

The statement "negative or zero current flows somewhere in the network" means that when the phase currents are transformed to sequence currents, the resulting negative or zero sequence current is not zero. But one must know all phase currents for the transformation.

 

[ijl]

A correction/clarification (I was a bit too quick, or confused myself?):

The phase voltages and currents can be expressed as a sum of the sequence components. For example, for the voltage Va of phase a the sum is: Va = Va0 + Va1 + Va2, where the indices 0, 1, and 2 refer to the zero, positive and negative sequence components, and similarly for the other phases. Thus, actually, one can talk about the sequence components of the voltage or current of a given phase. But the point is, these are not generally needed. Only the sequence components of phase a are needed. These are generally called as _the_ sequence components, dropping the subscript a for the phase.

The sequence components of other phases can easily be calculated from the sequence components of phase a (_the_ sequence components), usig the definition of the sequence components. This means that if some sequence component is nonzero, then it is nonzero in all phases.

(Did I get this right this time?)

 

[jghrist]

By definition, the magnitude of sequence currents are equal in all phases. As ijl noted, you don't have to concern yourself with the different phases of the sequence currents.

The only thing I would add to what ijl said is that to get the phase voltages for the other phases, you have to shift the phase angles of the positive- and negative-sequence voltages. This is done by the transformation matrix that he mentions.

 

[simatic7]

Thanks friends . I am now clear and as far as I understood , it is rightly pointed out by ijl , by adding the word may in the statement 3, I appreciate all the answer as they were short and precise.
Thanks