Current Detectors For Mho Relay

[nightfox1925]

There is a phase current detector setting associated with the MHO distance relay we are having. This detector is used to enable (supervise) the MHO distance element.

What is the standard norm for setting this? Can a setting below the maximum line load current will be acceptable?

 

[cranky108]

Yes it can be set below max load current. But it is not suggested. However, I have seen older zone relays that worked well without it (although vibration was a problem).

If you are having problems with weak in-feed, where the current at one terminal may not be sufficent to pickup your MHO relays, you should consiter a communications scheme to trip this terminal from another substation. This is as a supplement the the local MHO relays.

 

[nightfox1925]

Thanks cranky108. Do you have any suggestion on the basis of setting this for me to ponder?

Anybody who was involved with this are very welcome to provide their experience.

 

[davidbeach]

One family of relays has long had user settable supervisory overcurrent elements for the distance elements. The latest relay in the family has all those settings in firmware, set at what used to be the minimum setting. They've found that the supervisory overcurrent set too high was more likely to cause a lack of operation than a too low setting was to cause an overtrip. Use the load encroachment elements if you have concerns about the distance element picking up on load.

 

[cranky108]

Your right in that a current detector set to high is much more of a problem then with it set to low.

Use fault studies, phase to phase for phase units, to calculate settings. I'd try to use about 2/3 the calculated lowest fault current value or lower(Just a general guide).