Feeder protection

[tctctraining]

Utility has some standards and practices they use to set the breaker settings such as using 50% of L-G fault end of the line values for pick up and similar rules.
What criterias are used to set the feeder phase pick up and ground pick up settings. Any good book on this subject. I am not a protection guy ,so excuse my basic question.

 

[SMB1]

Yes,
for feeder protection:

1. instant. element 50-phase: The objective of this element in feeder application is to protect the whole feeder by detecting the minimum phase fault current.

2. time delay element 51- phase: The objective of this element is to protect the feeder or cables against abnormal loading. I> 1.5 I full load

Since there are no relays beyond this relay; a small time delay is normally used taking in consideration "Cold load" which is a short-time increase in load current that occurs when a distribution feeder is reenergized after an outage.

3. instant. 50-earth: The objective of this element in feeder application is to protect the whole feeder by detecting the minimum ground fault current.

4. 51-Earth : The objective of this element is to detect the feeder ground fault. Since the pick-up value is independent of the load, the relay operates only during ground fault. So, the setting is selected to be sensitive.
As stated in section 5.3.2.a:
(For feeder's earth-fault relays, the TAP value is determined taking account of the maximum unbalance which would exist in the system under normal operating conditions. A typical unbalance allowance is 20%.)

To avoid operation on possible unbalances in a normally balanced circuit, a good rule of thumb is to set the ground relays not less than 10% of the maximum load current.

 

[tctctraining]

Thanks SMB1 . It seems they are extracted from a book or a reference. would it be possible to refer me to that source?

Thanks

 

[dpc]

Check here: faq238-1287

For distribution feeder protection, you can refer to J. Lewis Blackburn's Protective Relaying book. Also, Cooper has some good technical references for feeder protection that they might give you.

 

[cranky108]

There is another way of looking at relay settings.
51 Phase set at or below 67% of end of line phase to phase fault level.
50 Phase set at about same level as 51 phase used for hot line work, and normally not turned on.
50 High set Phase set to pick up for very close in faults, but not always applied.
51 Ground set at 50% of end of line phase to ground level, or 50% of 51 phase set level, which ever is lower.
50 Ground set at about the same level as 51 ground used for hot line work, and normally not turned on.
50 High set ground set to pick up for very close in faults, but not always applied.

Who cares about load, as if it is to close to settings then distribution planning has a problem.

 

[davidbeach]

Wow, must be nice to have such a simple system. On long feeders it can be possible for fault current at the end of the feeder to be quite low and not significantly different from total circuit load, but the installation of an other substation is not practical. Reclosers can help. Don't know that general statements about the use of instantaneous elements can be made, too much variation in use and setting, some times too much variation even with in a single system.