Positive vs Negative Sequence

[yesterlectric]

is it correct to call a system that has an ACB sequence under normal operation a negative sequence system? Or is that term only used to describe systems that have reversed their rotation during a fault?

 

[charlie b]

(Now attempting to knock some old dust from my brain cells) The terms positive, negative, and zero sequence are part of a method of mathematically modeling the behavior of voltage and current in normal and faulted conditions. The terms are not directly related to a motor's supply power sequence or its direction of rotation.

 

[kingpb]

Adding to Charlie's explanation, ABC or ACB is simply the order in which the phases reach there positive maximum. It is necessary to know this if you are trying to connect two different generating systems together.

 

[yesterlectric]

Yes I know this. They say when a circuit has a phase to phase fault the sequence reversed at the fault to ACB. So if ACB were normal, would it be a negative sequence? This stuff is beyond regular electrician education and I’m trying to learn it.

 

[jim dungar]

Yes I know this. They say when a circuit has a phase to phase fault the sequence reversed at the fault to ACB. So if ACB were normal, would it be a negative sequence? This stuff is beyond regular electrician education and I’m trying to learn it.

No.

As Charlie said the terms Positive Sequence and Negative Sequence have to do with the mathematical evaluation of a power system value. They have nothing to do with the direction of rotation nor the direction of power flow.

The problem is the way our industry takes short cuts with language, often times because the slang in an area has become 'normalized' by people doing training.

In your case you should be talking more about the direction of the power flow and not its sequence.

 

[LarryFine]

In the grand scheme of things, phase rotation is really arbitrary. There is no "global" standard.

What matters is relative rotation, that a rotation-sensitive load rotates in the correct direction.

Also, of course, that rotation directions of interconnected systems are matched to each other.

 

[charlie b]

The problem is the way our industry takes short cuts with language, often times because the slang in an area has become 'normalized' by people doing training.

Agreed. This is another example of what I like to call, "language getting in the way of communication." Words can have many meanings, many of which have nothing to do with the others. A motor powered with ACB sequence will run in the opposite direction as one powered with ABC sequence. Here, the word "sequence" has a meaning unrelated to the use of the same word in the phrase, "negative sequence."