Fault current on a 11'000 transformer

[normanton]

Hi guys, haven't posted in a while. Working at a hospital now, we have 2 11KV transformers on site. We only run 1 at the time , the other is just used as a backup (close a bus coupler to bring it in). Being a nosy git, I asked about running both transformers at the same time. Obviously this would effect the fault current. My query is wether our switchgear is big enough to deal with the increase in fault current if they were to run in parallel. They have an impedance of 5.58(transformer 1) and 6.16( transformer 2). Anybody know the calculation to work out what the fault current would be. I know you'd have to work out the FLA first but all the calcs I'm doing seems to be throwing out numbers which are way off. Any more details you need about the transformers let me know

 

[Knobhead]

I’ve had the same problem with MPFC during switchgear maintenance.

We knew we would exceed the fault level during the change over. Two of us working together could swap over in seconds, one closing the bus-section the followed by the other opening the transformer feed.
You would be very unlucky if a heavy fault occurred in the few seconds for the change over.

One thing we always did was close the 11KV bus-section and open one of the incomers first so that there would be no circulating currents. It’s a bit of a rigmarole but the safest way.

The LV ACB’s the and the 11KV VCB’s had Castell two out of three key interlocks. Naughty me, I have a couple of master keys. (Always locked in my desk draw.)

Why are you cycling the transformers? They are much better off live all the time, all be it on light load. You do have two lots of losses to pay for, cheap compared to a transformer going bang just when you need it.

 

[Knobhead]

Just spotted something in your first post. The impedance is different.

Got to go out now, will get back to you later.

 

[DPG]

I’ve had the same problem with MPFC during switchgear maintenance.

We knew we would exceed the fault level during the change over. Two of us working together could swap over in seconds, one closing the bus-section the followed by the other opening the transformer feed.
You would be very unlucky if a heavy fault occurred in the few seconds for the change over.

One thing we always did was close the 11KV bus-section and open one of the incomers first so that there would be no circulating currents. It’s a bit of a rigmarole but the safest way.

The LV ACB’s the and the 11KV VCB’s had Castell two out of three key interlocks. Naughty me, I have a couple of master keys. (Always locked in my desk draw.)

Why are you cycling the transformers? They are much better off live all the time, all be it on light load. You do have two lots of losses to pay for, cheap compared to a transformer going bang just when you need it.

You could do it even quicker if you use the 1000 Amp push-fit Wago connectors. Daz

 

[Engineer54]

What are we talking about here 11KV switchboard or LV switchboard?? I'm going to surmise that the 11KV side of things are going to be supplied by RMU's rather than a MV switchboard.

Never seen two transformers connected to a common LV switchboard with differing impedance values, they are always pairs of identical transformers. If were talking 1000KVA TX's then were talking around a 20KA fault current, so if a pair of 1000 KVA TX's are to be run in parallel then you'll need say a 45/50KA rated switchboard.

Why on earth you are only using one of the TX on a bus coupled switchboard i don't know?? All sounds a bit heath robinson to me, especially as you say this is a hospital installation. I'm almost frightened to ask, how your standby generator(s) and essential supplies fit's into this set-up!! lol!!

 

[Knobhead]

What size are the transformers?

Without some information the fault current can't be calulated.

 

[normanton]

What size are the transformers?

Without some information the fault current can't be calulated.

Sorry about the delay. Haven't been online in a while.

The transformers are 2500KVa. And I actually made a mistake in the first post. We run both transformers at the same time each feeding half of the hospital. There is a bus coupler connection between the two then.

 

[normanton]

We have two 750KVA generators and we use a load shedding system. Which to my knowledge is controlled by the BMS and brings an area in at the time, starting with theatres, ITU etc

 

[Brightspark2]

Whats the story with the red and white sockets in a hospital?...


One colour for medical equipment one for normal?...different supplies?...

 

[normanton]

Different hospitals differ slightly. But in general, red sockets are fed from "essential" dist boards and white sockets "non essential" boards. Basically anything of importance will be connected to the essential supply. In the event of mains failure the red sockets/switches will come back online via the generator. White sockets/switches won't come back on until mains is restored.

 

[Engineer54]

Sorry about the delay. Haven't been online in a while.

The transformers are 2500KVa. And I actually made a mistake in the first post. We run both transformers at the same time each feeding half of the hospital. There is a bus coupler connection between the two then.

Are you Sure they are 2500 KVA ?? I'd like to know what the bracing KA level is on that LV switchboard??

 

[normanton]

Will double check tomorrow. But I'm 99% sure they are..

 

[normanton]

Forgive my stupidity but I'm unfamiliar with the term bracing ?

 

[Engineer54]

Forgive my stupidity but I'm unfamiliar with the term bracing ?

Not a stupid question, as few electricians will get to work on Main Switchboards and the like, let alone know much about the design criteria. It actually means what it say's. ...lol!!

Bracing is basically the amount and type of fixings that attach the bus bars and any cable connections to the switchboard to stop them buckling under a fault current condition. You maybe be very surprised just how flexible (if not molten) a 3200A bus bar can become under fault conditions. It's part and parcel of the overall fault level withstand, the switchboard was designed and built too... That's a very simplistic explanation by the way!! lol!!

 

[normanton]

Every days a school day! Lol. Never realised that to tell you the truth. Something to bear in mind tho