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rogerj1 (Electrical) (OP)
17 Jan 07 16:34
To all,
What seems to be the latest and greatest scheme in Negative Sequence Protection?  In our practice, we use SEL relays operate on a negative sequence overvoltage element (59Q).  This method is designed to protect a power transformer from "single phasing".. or the loss of a primary phase conductor.  It is my understanding, that when a Delta-WyeGnd transformer looses a primary phase, there will be one secondary phase at nominal voltage, and the other two secondaries at 1/2 nominal voltage. (in PT secondary, this is 120V, 60V & 60V seen at the relay).  In this condition, we have the 59Q pickup for a duration and produce a trip. We also implement an undervoltage blocking element to prevent false tripping in the case of a lost secondary voltage (ie. a blown secondary fuse from a PT, PT maintenance, etc).  Generally, we reset this condition once a 52a contact has closed back in, but, we have had some problems with this as well.  I'm interested in using 51Q schemes and wondering if any of you might have some thought or experience on the matter.  Thanks your help and the forum!  
dpc (Electrical)
17 Jan 07 18:51
I use it very rarely.  It creates a huge confusion factor and only helps much for line-line faults, which are not that common to begin with.  

It has merit, but I've almost never seen it used.  Of course, YMMV.
SphincterBoy (Electrical)
18 Jan 07 9:40
We use it for protecting a feeder circuit w/o a neutral conductor.

The reason?  The feeder is a long line, and the phase overcurrent relays would not pickup any faults, even when the lines were broken and on the ground (still energized).

We theorized that with negative sequence protection, it might provide more sensitive detection, w/o nuisance tripping.

So far, it has functioned much better with the negative sequence than w/o.

Here is a link to a good paper on the subject:
stevenal (Electrical)
18 Jan 07 11:08
Like zero sequence for LG faults, - sequence can be set sensitively for LL faults without worrying too much about load conditions. It must be coordinated with downstream protection during LG faults, though.

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