Practice Test Question: Negative Sequence

[EricGoetz92]

The question reads:

"What can you do to create a negative sequence when testing a 47 relay?"

The choices were apply 3x pick-up on a single phase, apply 1.732x pick-up on two phases, or reverse the rotation. All three of these would produce negative sequence voltage, but if you want to test a reverse rotation element (what 47's are typically used for), then the easiest thing to do is to just reverse the rotation. But technically all three of these can be used to create a negative sequence when testing a 47 relay. Why is only one of them correct?

[EEBSMSPE96]

The question reads:

"What can you do to create a negative sequence when testing a 47 relay?"

The choices were apply 3x pick-up on a single phase, apply 1.732x pick-up on two phases, or reverse the rotation. All three of these would produce negative sequence voltage, but if you want to test a reverse rotation element (what 47's are typically used for), then the easiest thing to do is to just reverse the rotation. But technically all three of these can be used to create a negative sequence when testing a 47 relay. Why is only one of them correct?

Answer:

Many times negative phase sequence (A-C-B Rotation) is confused with negative sequence (current (I2) and voltage (V2)) quantities. Since the question refers to the testing of a 47 relay (which responds to negative phase sequence aka negative phase rotation i.e. A-C-B rotation), the correct answer to this question is "reverse the rotation" of your test source. Positive phase sequence is A-B-C Phase rotation which is what "normal" voltages and currents are on the grid. Hope this helps.

[storm89]

if you want to test a reverse rotation element (what 47's are typically used for), then the easiest thing to do is to just reverse the rotation.

Looks like you answered your own question my friend.

[EricGoetz92]

Answer:

Many times negative phase sequence (A-C-B Rotation) is confused with negative sequence (current (I2) and voltage (V2)) quantities. Since the question refers to the testing of a 47 relay (which responds to negative phase sequence aka negative phase rotation i.e. A-C-B rotation), the correct answer to this question is "reverse the rotation" of your test source. Positive phase sequence is A-B-C Phase rotation which is what "normal" voltages and currents are on the grid. Hope this helps.

That does make since. I was to hung up on symmetrical components and over thought it (which they were clearly anticipating since they made the other two answers viable options to make negative sequence voltage).