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Ekow Kwofie
09-21-2013 01:36 AM

33kV double feeder circuit

In operating a 33kV double feeder circuit with one on each side of the bus connected to a transformer each and to an 11kV bus. Which is better?
1. Coupling the 33kV buses and decoupling the 11kV buses.
2. Decoupling the 33kV buses and coupling the 11kV buses thereby neither paralleling of feeders nor trafos.
3. Coupling both the 33kV and 11kV buses at the station thereby parallel the trafos as well as the feeders.
09-21-2013 04:09 AM
Top #2
Michael Novev
09-21-2013 04:09 AM
Answer to your question depends on several factors. What is your 33KV feeder rating. If one feeder cannot carry whole load of both transformers, paralleling will not gain much. If you feeder fails second one will be overloaded and trip. If your supply is from different stations in case that you parallel 33KV feeder you parallel both stations. Make sure that they are synchronized. Also if one of them is less loaded or have higher rating transformers (lower system impedance) it will take most of the load. If neither of above conditions is a concern you can parallel 33KV.
Second question is transformer rating. If each of transformers is able to carry the whole load of 11KV feeders then parallel them. If not failure of one transformer will lead to overload and trip of the second one, hence you lose the whole station.
Last question is equipment short circuit rating. If you parallel 33KV, system impedance will decrease (two parallel impedances). Same if you parallel transformers. Make sure that your 11KV breakers are able to carry the short circuit current.
09-21-2013 06:13 AM
Top #3
Raymond Lee
09-21-2013 06:13 AM
Fo you have a scan of showing the bus configuration showing all breakers and disconnects ?

Tie the low side, if you have bus A & B protection. Feeder protection tapped of the LV bus is unaffected, but fault current is now fed from both sources, check fault capability of LV bus, feeder breakers and cables.

HV tie only needs to be closed if you have issues at the source end, then you can feed both former from 1 line, then the LV is opened.

Need prints, common issue on this site (no offense).
09-21-2013 08:40 AM
Top #4
Omer A.Osman
09-21-2013 08:40 AM
Dear Dkow, eventually depends on your system, is it a dedicated station or not. But let me clarify the following:-
A)- For primary side (33kV).
a. Definitly your substation constitutes part of the 33kV network, that the two feeders as termed by you are known as "loop in " & " loop out" that is the Ring Main Unit RMU. No bus coupler is recommended (mandantory) at 33kV bus, accordingly.

b. Usually, the MV switchgear at the primary "RMU" either "QRM", "TRM" or 4P MV SG.That is to say TRM for "LOOP IN", "LOOP OUT" and one Trafo Feeder. While the QRM / 4P are used for switching station in which " LOOP IN" , "LOOP OUT" and two Traf. Feeders will be found.

c. The Network should be in ring connection while it is operated in radial connection as per local authority recommendation. radial connection is achieved by opening the Ring at some point (the nearest substation to the middle of the ring, as possible).(ie either the loopin or loopout of that substation to be disconnected ).

B)- for Secondary Side (11 kV Network).
a. It is strongly recommended to provide a bus coupler at the secondary side (11kV Bus in your case) for the down stream control, flexibility of operation and for maintability purposes.
b. The control philisophy of such arrangement (the incomer-1 , incomer-2 & the aforementioned Bus-coupler) is recommended to be TWO OUT OF THREE with the appropriate electrical & mechnical interlocks, ie only two breakers / isolators to be on simulatenously so as to avoid parallel operation of these transformers & capability to utilize both transformers without any risk, taking into consideration that the ring legs have to be connected to distinct bus-section of substation (11 kV in your case).

Note: Loop in is the incomer from the precede station.
Loop out is the incomer of the next station.

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