About differential protection 2nd,3rd,5th harmonic blocking..? 4

[rayas91]

hi friends,

I was entering set values of differential protection relay.

I realized that there are only 2nd and 5th harmonicc blocking.

Why dont we use 3rd or other harmonic blocking ?

Thanks

 

[catserveng]

Here is an excellent discussion of harmonic restraint and blocking by SEL,

https://cdn.selinc.com/assets/Liter...armonic_KB_20061019_Web.pdf?v=20160503-160741

Beckwith, GE and otehrs also have some very good atricles relating to this subject.

Hope that helps, MikeL.

 

[rcw retired EE]

Short answer- differential protection on transformers will trip on transformer energization because the supply side CT's see the magnetizing inrush current and the load sides don't. Magnetizing inrush current has 2nd harmonics. Restraining or blocking on 2nd harmonics prevents false trips on energizing.

5th Harmonic currents occur when a transformer is overfluxed / over-excited. Additional magnetizing current goes in but doesn't come out. Again, blocking keeps the unit on-line, preventing a false trip.

Only reason to block on third harmonics is if the high harmonic content falsely actuates the differential protection. But then you have an expensive differential relay that is turned off most of the time by the load currents. You only want to block when an off-normal, non-destructive condition is momentarily present and could cause a false differential trip.

 

[RRaghunath]

I agree with rcwilson.
Blocking of differential protection on detection of 2nd or 5th harmonic content evolved based on the need to prevent relay malfunction for non-fault events.
3rd harmonic is similar to zero sequence current that appears during earth faults and we don't want to block the protection.
Having said that, when the transformer is delta connected on one side, the delta makes the reflected zero sequence currents from star side (for earth faults on star side) to circulate within the delta. Thus, these currents which come to the relay through CTs on star side do not appear in delta side CTs.
This is addressed in the relay settings by selecting the zero sequence filtering appropriately (so that star side also the zero sequence currents are filtered out).

Rompicherla Raghunath

 

[rayas91]

catserveng,rcwilson,RRaghunath thank you all.

Your explanations and file are very clear to understand about harmonics..Thank you again.