Paralleling 34.5 kV feeders

[EEJaime]

Good day,

I have someone in our office whom has called for running two sets of 500 kcmil copper, shielded, 35kV cables in parallel to serve several loads in a radial configuration from the 34.5kV main service switchgear lineup. In my 25+ years, I have not seen anyone run parallel conductors for these type of medium voltage feeders. The loads themselves vary in size and type from large motor loads, to general building loads to aircraft ground power, (400Hz), loads. His reasoning was that he wanted to get 100% of the rated ampacity of the circuit breaker feeding each load. (I don't know specifically the load relationships to the breaker sizes).

My question is in the basic concept. I may not have seen this but is this a common practice in the industry? Are there potential problems with parralleling these feeders, ((2)-5" PVC conduit each with 3#500kcmil conductors)? How should the terminations be handled? How should the shields be earthed? Etc.... This just seems like a bad idea to me, but I don't have a specific technical reason for my misgivings.

Any comments would be appreciated.

Regards,
EEJaime

 

[dpc]

It's not common, but not unheard for large express feeders. Using it for feeding multiple drops on a single feeder might get a little complicated.

A single 1000 MCM Al cable can carry a lot of kVA at 34.5 kV. Putting that much load on a single breaker might not be desirable - it will limit the ability to sectionalize and isolate problems in the future.

This is an industrial, not a utility system?

David Castor

http://www.cvoes.com

 

[EEJaime]

Correct DPC,

It is for the distribution system for a major airport expansion. The airport authority in essence is to be it's own utility on-site. They currently are fed by the utility, but are in the process of acquiring the system from the utility and this is the first major expansion under their new scheme.

Thank you for your reply,
EEJaime

 

[dpc]

For an airport I think I would want to limit the load on any one breaker to something a lot less than you are looking at with double-circuit 34.5 kV. For such a critical facility, designing to maximum loading on a circuit breaker could be false economy in the long run.



David Castor

http://www.cvoes.com

 

[EEJaime]

I agree. However we do have redundant feeders and double ended-tie breaker configured substations and low voltage building main switchbooards with the ability at two levels to isolate any section of feeder. I was merely asking about the physical practicality of paralleling these conductors.

Thank you DPC-good day,
EEJaime

 

[waross]

I have seen 750 MCM distribution cables installed in parallel. Some circuits were fed with single 750 MCM, some were fed with two parallel 750 MCM.
A sub station was overloaded. A second sub was built several miles away. About four circuits passing the new sub were cut into and fed from the new sub. Two of the existing circuits back to the old sub were used as standby circuits to back feed the load on either sub from the other in the event of a failure or planned outage.
The feeds ran buried for almost 1000 feet before connecting to the overhead distribution system. The distribution circuits were single conductor 750MCM. The inter-tie circuits were two parallel 750 MCM per phase.
Cable size is generally dictated by load or anticipated load, and voltage drop considerations.
In the case of circuits of 600 Volts and below, fed from molded case circuit breakers, the minimum cable size may be dictated by the rating of the trips in the breaker.
However I was under the impression that distribution circuit breakers, under engineering supervision, were controlled by a relay with settings appropriately for load, line and system conditions and that the maximum rating of the breaker was often much greater than the settings on the protection relay.


Bill
--------------------
"Why not the best?"
Jimmy Carter