Temporary Paralleling of Transformer for Changing Over Supply

[calvint]

I have two 415V busbars separated by a bus section which is normally open. Busbar 1 and busbar 2 are normally fed separately by transformer 1 and transformer 2 respectively. Both transformer's HV side are from the same 11kV board. Both transformers are of the same manufacturer, model, ratio, tap and vector group.

My present arrangement when carrying out maintenance on either transformer is to first switch off the transformer and its LV board before closing the bus section CB. This causes momentary supply interruption to the loads connected.

Since both transformers are actually identical, can I actually just close the bus section CB first thus temporarily paraleling the transformers before switching OFF the particular transformer I would like to service? Are there any dangers or considerations? Since both are identical I think it should be ok but my colleagues are of a different opinion as this was not the practice before.

Would appreciate any ideas and discussions on this.

 

[mechelec]

If the bus section is a fully rated circuit breaker and the rest of the panel has been maintained to manufacturers specification then you will be afforded some protection if there is a problem but as you have listed all the major considerations you should as you say be able to parallel the two transformers. They could be run like this indefinately if there is some form of directional protection installed as well. You will need to consider the fault level though as this may be increased whilst the parallel is on and your LV gear may not be rated.

 

[Knobhead]

So long as the HV supply to the transformers have no break between them, no problem. It’s something I’ve done many times.

If you haven’t paralleled the LV before make a check of the bus-bar phasing. I once came across a board where 2 transformers had correct phase rotation but were 120 degrees out of phase IE
Red -> Yellow
Yellow -> Blue
Blue -> Red
Switching “dead” never showed the problem, fortunately we tested before going for it live. (It took a fair amount of time to put right though)

Mechelec’s comment about fault levels is valid, but so long as the paralleled time is kept to a minimum it shouldn’t be a problem. You’d be very unlucky for a heavy fault to occur just at the moment you carried out the switching operation.

ACB's are ideal fore this kind of operation but some fuse-switches are rated at load make / load break, check up first

 

[normanton]

We've had the same problem at my hospital. By closing the bus section you're increasing the fault current dramatically. Albeit for a few seconds. You'd have to be extremely unlucky for a fault to occur in that brief moment. We carry out the procedure in the same way as you do.

 

[Engineer54]

Depends if the two supplies are from the same source and/or are phased out correctly as Tony has stated. Generally speaking if everything is paired correctly and phased out correctly then it's fine to run the switchboard with the two TX's in parallel, providing the switchboard is capable of withstanding the combined fault levels that the TX's can deliver into a fault.

For normanton

The fault levels on 2 X 2.5 MVA TX's are going to be huge compared with standard Switchboard KA fault level ratings, that normally cater for TX ratings from say 2 x 1 MVA to 2 x 1.5/1.6 MVA. All our LV Switchboards are over rated at 50 kA as standard, but that won't normally be the case. 2 X 2.5 MVA TX's paralleled will more than exceed 50 KA....

Just noticed this thread is over 3 years old!! lol!!