grading of overcurrent relay 3

[veronica19978]

When grading the overcurrent relay of an earthing transformer(zig zag) with other overcurrent relays downstream,what is the criterion to be followed?is it to be made more sensitive than others i.e it should trip before other overcurrent relays dowstream or it should be treated as ordinary overcurrent relay that will trip after the downstream relays(graded over them)?.Please help if anyone can.What if instead of OVercurrent relay it is earth fault relay?Thanks in advance.

 

[slavag]

Hi Veronica19978.
According to our expirience not needed any grading of overcurrent protection of zigzag with other overcurrent relays.
Actually phase current of zigzag very very small.
You need provide grading according to the EF relays of others. And of course must be grading of EF relay of zigzag with otheres.
Hope that help.
Good Luck.
Slava

 

[Marmite]

I agree with Slavag. Another point worth making is that as the earthing transformer provides the earth reference for the system, you wouldn't want it to trip before the other protection has had time to operate and clear the fault. Once it trips the other earth fault protection is effectively disabled. It needs to trip before the associated neutral earthing resistor/reactor is damaged by the fault current.In my experience earthing transformers are rated for a short time current for 30 secs, and are generously proportioned to withstand the associated heating and magnetic forces. NER's are typically rated for 10 secs.
Regards
Marmite

 

[CoryA]

I have heard of several installations using a continuous-rated neutral-grounding resistor, and an appropriate zigzag transformer, to allow the system to operate with a ground fault. In either case, as Marmite suggested, ground-fault protection on downstream devices should be set to operate first. The affected feeder is then removed from the system, personnel know immediately where to direct their fault-locating efforts, and the rest of the feeders can continue to operate uninterrupted. Protection at the zigzag transformer could then function as back-up ground-fault protection, and operate if the downstream device is not able to clear the fault. This is often accomplished by increasing ground-fault trip-delay settings on successive upstream devices.

---
Cory Anderson

http://startco.ca/

 

[veronica19978]

According to our expirience not needed any grading of overcurrent protection of zigzag with other overcurrent relays.
Actually phase current of zigzag very very small.

So as far as overcurrent relay grading is considered,all other relays should be graded over o/c relay of zigzag transformer.Is that what you mean?please clarify.

 

[slavag]

Morning.
No needed any grading of zigzag overcurrent relay with other relay and other overcurrent relays with zigzag overcurrent.
Please pay attention on the Marmite's and Cory's post.
Reason of zigzag is virtual ( sorry for term) ground reference. That menas, if zigzag will trip fast, the other earth fault protection will not operated.
Very important point what is your application:
zigzag is directly grounded, via resistor, via petersen coil?
It's a point of zigzag protection, becouse this ground fault current is a maximum "load" current of zigzag.
Your set of overcurrent protection must be higher then this current and time of ground fault protection is longer than
ground fault time of others.
Please don't take in account overcurrent protection of zigzag in grading of overcurrent protection of downstream and upstream feeders.
And of course, don't forgot about:
"In my experience earthing transformers are rated for a short time current for 30 secs, and are generously proportioned to withstand the associated heating and magnetic forces. NER's are typically rated for 10 secs."

Best Regards.
Slava