External vs internal summed CTs

[mbrooke]

What are the pros and cons of externally summed CTs for low impedance bus bar protection vs a unit that does it internally (like a B90 or SEL487B)?

 

[Bugman1400]

Back in the day, the pros included a reduction in wire and a reduction in the number of CT inputs to the relay. The cons include failure to diagnose an offending ckt for a through fault and increased saturation (and subsequent higher voltage) for strong buses.

 

[mbrooke]

Back in the day, the pros included a reduction in wire and a reduction in the number of CT inputs to the relay. The cons include failure to diagnose an offending ckt for a through fault and increased saturation (and subsequent higher voltage) for strong buses.

Am I correct to assume that current utility practice is to use paralleled CTs only for radial lines? Or can looped transmission lines also be included?

 

[Bugman1400]

For new construction or an upgrade, I would expect looped (networked) subs to use a dedicated CT input for each line (radial or source). However, I have seen new distribution subs parallel the CTs from two feeder ckts. In that case, I believe the number of feeder ckts exceed the number of CT inputs of the relay.

 

[mbrooke]

For new construction or an upgrade, I would expect looped (networked) subs to use a dedicated CT input for each line (radial or source). However, I have seen new distribution subs parallel the CTs from two feeder ckts. In that case, I believe the number of feeder ckts exceed the number of CT inputs of the relay.

So the way to go then is a B90 or SEL-487B. Just seems easier to parallel them and use a single over current relay instead of 3 bus relays- 6 if the sub has redundant protection. Plus your not limited to 21max inputs.