Daisy Chain vs Feeder Bus

[JimmyJunior]

Our principal administrative building is a 4 story office building served by six substations. These consist of 28 year old Westinghouse 12.47kV/480Y/120V and 12.47kV/208Y/120V double-ended substations. All of the transformers except for 3 are Westinghouse originals. All of the substations are housed in rooftop electrical rooms.

The substations are interconnected by daisychaining from two individual 12.40kV feeders. This system has given us satisfactory, relatively fault free, 24hr, service throughout the 28 service years.

Recently, a proposal to modernize the building's electrical system by replacing the substations and using two feeder buses instead of daisychaining has been tendered. Each feeder would connect to a bus. Each bus would in turn feed each substation.

The cost of doing this today would be astronomical.

Another proposal has been tabled that recommends refurbishing the substations by replacing older components with more modern components (transformer and breakers). The substation bus structure is basically sound and can remain in service for another 30 years with proper maintenance. Our agency would be able to do this as money becomes available. The cost would still be high but not as much so as the other proposal.

Is daisychaining out of style?


Thank you,

 

[jghrist]

Can you provide a better description of what you mean by "daisychaining?" Is this bringing the two feeders into the first substation, then using a two- or three-way switch to connect to the next?

I would be extremely reluctant to spend astronomical amounts of money to replace a system that has provided "satisfactory, relatively fault free, 24hr, service throughout the 28 service years" just to be "in style."

 

[Zogzog]

28 years is not old for this type of equipment, in fact in my opinion it is better equipment than you can buy today.

Overhauling and possibly upgrading the trip units to more modern types (depending on what you have now) is your best bet and much cheaper than buying new.

What do you have now 15kV DHP's and 600V DS breakers?

 

[rcw retired EE]

Jimmy Junior- By "daisy chaining" do you mean the 12.47 kV cables from the source to the first substation terminate with the cables that loop to the next substation without any switch or isolating means between cables? Usually, the transformer's HV disconnect switch's line terminals are used as the cable junction point.

This is a cost effective design with minor cable fault isolation issues. If a cable fails, load can be transferred to the other feeder at each double-ended substation. There is no means of quickly isolating the bad cable section and returning the rest of the feeder to service. But power to the facility is not affected except during the initial failure. Unless you are anticipating cable failures, the system should be suitable for continued service with proper maintenance.

Replacing transformers because they are 30 years old is not a good reason to spend money, unless other concerns are present like eliminating oil-filled transformers to save on fire insurance, or load growth has exceeded a transformer’s rating.

Replacing 30 year old switchgear may not be cost effective compared to just upgrading the trip units or adding some intelligent metering. Unless there are corrosion problems or other known issues with the equipment, if it works don't fix it.