47N vs Sync Check (25) for transfer scheme

[rockman7892]

For an automatic transfer scheme in switchgear with SEL-351S relays connected to wye-wye incoming PT's on 34.5kV switchgear is there a need to monitor both 47N and 25 sync check functions on coming sources? sync check is required due to closed transition (there are also bus PT's) and a 47N is typically used for Neg Sequence. since 25 function is typically only monitoring one phase i'm assuming it cannot detect phase reversal or loss of phase on other two phases that is monitoring and thus 47N function is required?

Can a 59Q function be used in similar fashion to 47N (SEL-351 only has 59Q) and are there any pro's/con's to each?

 

[cranky108]

Because you are only bringing in one phase for sync-check, you can't use this input but for one phase. So while you can monitor that phase for under voltage, you can't see negative sequence under voltage.
That said you can monitor the base side (with three inputs) for negative sequence.
So which side do you look at as your base side?

 

[rockman7892]

cranky108

Relay will have 3-phase input on line side of breaker and single phase PT input on load side for sync check. My understanding of sync check function in most relays is that it only compares one phase regardless if 3 phase input voltages are brought to both sides?

With primary side having 3-phase voltage input that would be considered base side. With that we can use the 59Q function in SEL relay to monitor Neg Seq voltage. I believe that is what you are saying?

 

[RRaghunath]

We use L-L voltage and not L-N voltage signal for synchro-check. This is the practice in MV systems (up to 33kV).
Use of Negative Phase sequence voltage check at bus level (it is the bus voltage signal coming to relay) doesn't make sense as any loss of phase voltage would be known through other consumers anyway. Also, if one of the poles of synchronising breaker fails to make contact, this protection element will not be able to prevent closing of CB.
Frankly, I have not seen this practiced anywhere.