positive, negative and zero sequence

[bisonee]

I took a basic power course back in college and I work for a A/E consulting firm so we dont deal with it alot, but could someone simply tell me what each of the above mean and how this is involved in a 3 phase system.

 

[davidbeach]

Arbitrary values of magnitude and phase angle for three-phase voltages or currents can be broken down into three symmetrical components, a positive sequence where there are three "phases" each with the same magnitude and separated by 120 degrees, a negative sequence where the three "phases" are in the opposite order from positive sequence but still all three are of the same magnitude and are all separated by 120 degrees, and a zero sequence where all three "phases" are of the same magnitude but all have the same phase angle.

So we went from three magnitudes and 3 phase angles to 9 magnitudes and 9 phase angles, seems like it just complicates things, but in reality it is so much easier, mathematically, to work with the balanced symmetrical components than with the original arbitrary values that it worth the effort.

A balance three phase system is all positive sequence. Negative sequence and zero sequence components are the result of system imbalances, including faults. Many fault types can be distinguished simply by looking at the symmetrical components. Zero sequence is created by faults to ground.

 

[dpc]

In addition to what David said, the benefits and usefulness of symmetrical components relate to analyzing unbalanced three-phase (actually polyphase) systems. These are mathematical constructs so don't get too hung up trying to find real-world analogies.

 

[cflatters]

Look for the book on 'Symmetrical Component Theory' by J Lewis Blackburn. Also type in 'Symmetrical Components' in Google, recall reading a good article from Basler (John Horak I think)

 

[bisonee]

Thanks