138kV Dead Tank Circuit Breaker Control Circuit Feeder Cable Sizing 1

[SouthSide08]

I'm curious to understand how others have determined the ampacity for sizing feeder cables to a 48Vdc control circuit of a 138kV HV circuit breaker. I have the information for the loads in the control circuit in which largest loads are the 2-trip coils and 1-open coil. All other loads (trip coil monitor, undervoltage relays, indicating lights, etc.) are non-significant in terms of load size. I have the information on the trip coils and open coil running duration, which is momentary (less than 1 secs). I believe that I don't have to consider these loads for my ampacity calculations since these aren't considered continuous. I see it similar to not sizing a motor feeder per the motor starting current. I would like to see some precedent or if there is something I may have missed. Appreciate y'alls comment in advance!

 

[ScottyUK]

Don't worry about the thermal capacity of the cable, worry about volt-drop between the tripping battery and the coils. You need to have sufficient voltage to close and trip the breaker even though the battery may be near the end of its discharge curve. My guess is you will end up with conductors rather larger than required to meet ampacity requirements.

 

[stevenal]

An IEEE/ANSI 48 Vdc controlled breaker should be able to close at 36 and trip at 28 Vdc per C37.06.

 

[bacon4life]

When looking at voltage drop, also be sure to check if there are circuit paths that may require round trips from the control house to the HV circuit breaker.

 

[SouthSide08]

Thank y'all for the feedback!

 

[SouthSide08]

I do have some follow up questions. I did the vdrop and there is no issue with it. The issue I'm looking at is that the vendor provided a 30A fuse in the control circuit. Is it common for HV breakers to have a fuse in the control circuit this high? I'm used to seeing 15 as the highest for control circuits for MV breakers and more commonly 10A and even lower. I don't remember worrying about the protection for my control cables ever. But I haven't had any protection this high on a control circuit. I'm located in the US and as far as I know the NEC mandates that a cable should always be protected by its protective device. Is my understanding correct? Because of the 30A fuse my control cable is larger than usual. Is this a common scenario for HV breakers?

 

[bacon4life]

Yes, I have seen several manufactures provide 30A fuses (or breakers) in the control circuit. In some cases, each 138 kV breaker is supplied from a 30 A position on the substation DC fuse/breaker panel, with the trip coil directly connected and the close coil connected through a 15 A or 20 A fuse/breaker.

Also, substation DC circuits have both protection in both legs because the batteries are floating.

 

[stevenal]

Not many 138 kV breaker controls would fall under NEC.

Overloading would be rare, the 30 will clear a fault, and the consequence of a trip failure due to a weak fuse is likely high.

What size wire do you have?

 

[RRaghunath]

The key point, as mentioned by Stevenal, is the need to avoid trip failure because of a weak fuse.
Will the cable size really increase if you consider fuse is there only to protect the cable against short circuit faults and not overloads!
Overload is better ignored as the operation of closing and opening coils is only momentary and occasional.

Rompicherla Raghunath

 

[SouthSide08]

Once again thanks for all the replies!! I'm taking all of your comments into consideration and have concluded that ampacity and protection of the control wires is not important in this case, as I've mentioned previously the vdrop is not an issue... @stevenal the cable feeding the circuit is a #8 (between the DC panel and the 30A fuse). The control wires switching the close and trip circuits are #14.