Ground Fault Settings 1

[hibb]

We have been having debates on how sensitive to set ground fault relay settings (instant. and time delay) for overhead distribution feeders. Any good guidelines or practical experience to share would be appreciated.
These forums have been tremendously helpful around our office.

 

[jbartos]

Suggestion: Visit

http://www.geindustrial.com/pm/notes/artsci/

and download this GE publication. It is a good start. There are other books, e.g. Blackburn, Warrington, etc. that have been posted in this Forum.
Also, there are General Electric Co. publications associated with GE protective relays and their applications.

 

[SMB1]

Time delay GOC should be set 10% or less of the max 3ph palanced load. However, for expected unbalance in the load meusurment for the unbalaned shall be made under max load condition then the setting will be 150% of the unbalance.

For the inst setting you should cover 100% of the line if it is not requiered to coordinate with down stream device such as fuse , reclouser .. etc.

 

[zkoita]

From my literature review and understanding, these are my comments.
The settings depends on the application. For a balanced load 10-20% of the system rated current is sufficient. For hazardeous areas, the leakage current should not exceed 15% of the rated current of the circuit or 5A, whichever is greater. In power distribution, the ground fault relay is set 20-80% of the system rated current.
I think you should fisrt predetermine your ground fault level, and then set your relay.

Any comments will be appreciated.

 

[RRaghunath]

Normally currents available in case of ground faults involving overhead distribution feeders are highly restricted by the terrain along the route. Hence, it is a practice to provide sensitive earth fault protection for OH distribution circuits with a typical setting of 2% pick up and a long time delay of about 5 seconds. This is in addition to regular earth fault protection that can be expected to operate for low resistance faults and set at around 10% pick up.

Instantaneous protection can be provided for long lines where in the ratio of fault currents for faults close to the source end for those at the far end is atleast 2.

 

[james001]

My way of doing the settings are as follows:
**Set the phase pickup at max load or 110%
**Set the ground pickup at 50% of the phase pickup (make sure the ground pickup is greater than the largest tap fuse)
**Instantaneous setting depends on your application. If you have a downstream fuses, would you want to trip the main breaker or let the fuse blows for any faults along that tap point. It can affect other customers. It all depends on the customers. Also do you have reclose options?
** Time dials need to be adjusted accordingly.

james

 

[rtrimblepe]

james001,

Would you defince what you mean by "** Time dials need to be adjusted accordingly"?

Thanks.