Zone Sequence Coordination

[stevenal]

After doing some relay testing, I've been doing a study of zone sequence coordination. Seems we inadvertently tested right at the pickup point used to transition the shot counter. Instruction manual suggested settings involve a time overcurrent pickup element that essentially drops out at the same point where it picks up. We watched the shot counter jump to the last step as relay locked out after tripping only once when we expected multiple reclosures/trips. It occurs to me that this testing situation is not that far from real world. While a fault current that provides exactly 10.000 A secondary might be unusual, it is easy enough to imagine a fault that crosses the pickup point many times before clearing.

An instruction manual from B says nothing at all about zone sequence coordination. Recommended settings from C appear to work like those of A. Manufacturer D uses a time overcurrent pickup to transition, but current must drop by 10% of pickup prior to the next transition. So after implementing the 10% bandwidth of D using logic equations back in A, it looks like we have reclosing that will coordinate with downstream reclosers for any possible fault current that does not evolve enough to cross over our 10% bandwidth prior to clearing.

 

[stevenal]

No comments? Thought I might hear from DavidBeach at least.

 

[davidbeach]

Steve, was it one of my relays? If so, please call. I'm not sure I recognize the term "Zone Sequence Coordination". Can you provide more detail of your testing?

 

[stevenal]

No, but I included your manuals in my research. Zone sequence coordination (one manufacturer calls it sequence coordination) allows an upstream device with fast and slow curves to coordinate with a downstream device that also has fast and slow curves. Fast coordinates with fast, slow with slow; devices sequencing together so that the upstream device never trips first on its fast curve when the downstream device is on a slow curve.

As far as the testing: a permanent fault was simulated via both secondary and primary injection with similar results. Testing right near the zone sequence pickup point would cause relay to skip one or more open intervals sometimes going straight to lockout on the first operation.

 

[davidbeach]

Steve, I guess I should have recognized "Zone Sequence Coordination" as that is what we call it in the manual. So, your current jittered across the pickup point used to increment the counter on the upstream relay, driving it to lockout instead of tracking the shots of the downstream relay and breaker (or downstream recloser) and staying with it shot for shot, correct? If that is correct, I'll see if I can find out how our relays work in the condition you were testing.

 

[stevenal]

That's correct, shot counter incremented with no downstream device present or simulated.

 

[davidbeach]

Steve, OK, I'll see about making some inquiries. Just out of curiosity, why would you have the zone sequence coordination turned on if there is not a downstream device to coordinate with? Any hints as to who's relay you were testing?

 

[stevenal]

Davidbeach,

This was just the testing phase, so no actual feeder was yet connected. But I also like my settings to be flexible enough to add a recloser at some point, or for operators to reconfigure the system so that any substation feeder breaker that does not normally have a downstream recloser may be switched so that it does. No downside to having it turned on if no incorrect sequencing occurs.

They come in either blue or blue.