When would you most likely use a negative sequence relay?

[Tseppish]

For which of the following would you most likely use a negative sequence relay?

Correct answer is determining if a generator is supplying an unbalanced load.

Wouldn't it be preferable to use a Zero Sequence to monitor load balance, and a Negative Sequence to determine and monitor phase rotation?

[GavinK5]

Negative Sequence Relay manuals are the best source for information on this topic. The following comes from some GE literature, links are included below:

When a generator is subjected to an unbalanced fault or load, its stator current includes a negative sequence component. This current sets up a counter-rotating flux field in the machine which causes double frequency currents to flow in the rotor iron and slot wedges, resulting in local heating.

Since negative phase sequence overcurrent relays were first introduced some 20 years ago, this type of protection has become standard for the majority of generators.

Such relays were developed in response to the recognition that the negative sequence current components of unbalanced faults could cause dangerous overheating in critical parts of the generator rotor and that therefore a practical device for protecting the generator was a relay which directly measured this quantity causing the problem. Its function was and is to protect the machine in the event line relays or circuit breakers fail to perform as expected.

Generator negative phase sequence currents can result from any unbalance condition on the system including untransposed lines, single phase loads, unbalanced type line faults and open conductors.

https://www.gegridsolutions.com/products/brochures/sgc.pdf

https://www.gegridsolutions.com/products/applications/ger3181.pdf

[EricGoetz92]

For which of the following would you most likely use a negative sequence relay?

Correct answer is determining if a generator is supplying an unbalanced load.

Wouldn't it be preferable to use a Zero Sequence to monitor load balance, and a Negative Sequence to determine and monitor phase rotation?

Negative sequence current increases as the current imbalance rises, but zero sequence doesn't necessarily increase. You are correct in saying that negative sequence current occurs for a reverse rotation, but it is kind of assumed that you wouldn't just magically get a reverse rotation on a generator after it is has been running for a while. Reverse rotation detection (paired with a sync check relay) is more a permissive to close than "running" protection. However, you can have an unbalanced load without having zero sequence current depending on the nature of the unbalance. For example, let's say the generator is supplying a wye connected motor that is protected by a fused contactor. If the C phase fuse blows and the motor starts to single phase, all of the current entering A phase is leaving B phase offset by 180 degrees. So there is no zero sequence component to it (since there is no ground current), but there is an increase in the negative sequence current.

[JKwentua]

Negative sequence current increases as the current imbalance rises, but zero sequence doesn't necessarily increase. You are correct in saying that negative sequence current occurs for a reverse rotation, but it is kind of assumed that you wouldn't just magically get a reverse rotation on a generator after it is has been running for a while. Reverse rotation detection (paired with a sync check relay) is more a permissive to close than "running" protection. However, you can have an unbalanced load without having zero sequence current depending on the nature of the unbalance. For example, let's say the generator is supplying a wye connected motor that is protected by a fused contactor. If the C phase fuse blows and the motor starts to single phase, all of the current entering A phase is leaving B phase offset by 180 degrees. So there is no zero sequence component to it (since there is no ground current), but there is an increase in the negative sequence current.

Reading your response raised a question for me, now if you were to replace your negative sequence relay with a negative KVAR relay with the same exact scenario what kind of do you think the pre determined fault would be? would it cause the relay to trip or would it by pass the relay because the KVAR relay only monitors one current and three voltages?

[EricGoetz92]

Reading your response raised a question for me, now if you were to replace your negative sequence relay with a negative KVAR relay with the same exact scenario what kind of do you think the pre determined fault would be? would it cause the relay to trip or would it by pass the relay because the KVAR relay only monitors one current and three voltages?

Whatever effect a blown primary fuse has on the negative KVAR relay wouldn't really be important. A negative KVAR relay is tripped on a reverse VAR flow and a negative sequence relay trips on a high negative sequence symmetrical component. No one would ever use a negative KVAR relay to protect against a blown primary fuse and the only way a negative KVAR relay would trip due to a blown primary fuse is if a ton of other protection was disabled so the negative KVAR relay was the last line of defense. Even then, it might not trip because I don't know the typical KVAR flow during a single phasing situation of a machine that typically has positive KVAR flow.