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I am trying to size a circuit breaker for a transformer.

The transformer is three phase 480 : 4160 V rated at 25 kVA. From my calculations there will be approximately 30 A on the primary and 3.5 A on the secondary winding.

Do the circuit breakers have to be voltage rated? Since the secondary winding is a medium voltage does that affect the type of breaker used?

I have found information on the NEC guidelines but I see that this is for the US is there any equivalent for the UK?

Will I need a circuit breaker for both the primary and the secondary winding?

Any help with this matter would be much appreciated.

Kind regards

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  • \$\begingroup\$ Is this for external or internal use? \$\endgroup\$
    – D.A.S.
    Commented Sep 6, 2018 at 9:15
  • \$\begingroup\$ The transformer is going into a housing and will be used subsea. \$\endgroup\$
    – Noj911
    Commented Sep 6, 2018 at 9:20

4 Answers 4

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Primary overcurrent protection protects the conductors and the transformer. Secondary overcurrent protection protects the conductors, not the transformer. Since this running opposite normal use, please be careful to remove any ground bonding to the 480 volt cores, and add it to the secondary 4160. You may actually be better off buying switch and fuses for the 4160 volts.

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Is the secondary connected to a grid like thing? If so it will need protection (which will be expensive, 4160V kit being medium voltage), otherwise just protect the primary.

I would be looking at either a type D breaker at 40 or 50A or so, or possibly an old school fuse!

Fuse links have the advantage of very high interrupting capacity and you can get ones with useful amounts of delay, so many minutes at 100% overload is something you can accommodate if the transformers thermal properties allow that (Most distribution transformers are large enough to allow Substantial short term overloads, and it is not like you are short on coolant, 25kVA is however on the small side for a distribution transformer.

Bear in mind also that a breaker in a sub sea pressure pot is not a lot easier to reset then a fuse is to replace, you are having to open the pressure housing either way (Which means hauling the thing up, and an annoying amount of wrench work on deck), and the fuse may actually be more reliable.

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  • \$\begingroup\$ Thanks a lot for your reply. Pardon my ignorance, but I've been looking at medium voltage rated breakers, for instance siemens do a 12KV rated breaker that has a continuous current of 800 Amps, if the current on my secondary is only approx. 3.5A that wont protect it? \$\endgroup\$
    – Noj911
    Commented Sep 6, 2018 at 9:53
  • \$\begingroup\$ Ah nope! And that thing will be expensive even by the standards of subsea kit. Your 4kV feeder is (if connected to some kind of grid like thing) going to need protection at the other end to protect the cable in the event of a fault, and that may be all you require at medium voltage, but do the sums on short circuit current and system discrimination before deciding that is the case. You will find that old school fuses really do have much to recommend them, look at someone like Ferraz Schawmuts range of medium voltage fuses, not cheap exactly, but nothing in this game is. \$\endgroup\$
    – Dan Mills
    Commented Sep 6, 2018 at 13:06
  • \$\begingroup\$ Ah right ok.In this instance the 4KV cable will be fed into another transformer onshore (this part belongs to our customer). So will the cable only require protection at the end where it feeds into the next transformer? Thanks for your help with this. \$\endgroup\$
    – Noj911
    Commented Sep 7, 2018 at 8:30
  • \$\begingroup\$ @Noj911 But which way does the power flow? If it can only flow in one direction, then you only really need to protect at the source, (and anywhere the cable size steps down), if it is a potentially bidirectional link (like say a wind farm tie to the local grid) then the cables and transformers must be protected from BOTH sources of supply. I would note that I am a little uncomfortable with someone who clearly is not a specialist playing with medium voltage design, here be dragons, and they are not all obvious (I would not design this because I know I do not know enough of the details). \$\endgroup\$
    – Dan Mills
    Commented Sep 7, 2018 at 9:34
  • \$\begingroup\$ The power flows from a wave energy converter back onshore. The senior engineers will be doing the design I've just been tasked with researching circuit breakers and was asking just for my own clarification. Thanks for your help though. \$\endgroup\$
    – Noj911
    Commented Sep 7, 2018 at 11:21
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5kV Class Breakers must be 60 kV BIL basic impulse level rated.

There are many other options with MOV's, magnetic, thermal , relay overload, remote manage etc.

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Size the breaker for the source

Whichever is the supply side of the transformer, that's what you breaker. I gather that the wave generator makes 480V (sounds very American), and you're stepping up to 4160V for the long haul to shore.

The reason is we breaker for the worst case, not the best case. The best case is the transformer never shorts out or anything like that.

You're really gonna want remote reset for that one

Of course, the real trick is getting energy to the breaker to actuate the remote reset. You could run a second set of wires, possibly in current mode so you don't have to care so much about voltage drop. Or you could embed some sort of SCADA system and use powerline signaling.

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