8/2/2019 Temporary Paralleling of Transformer for Changing Over
Supply
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Temporary Paralleling of Transformer for Changing Over
Supply
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. Bothtransformer'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 notthe practice before.
Would appreciate any ideas and discussions on this.
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.
So long as the HV supply to the transformers have no break
between them, no problem. Its
something Ive done many times.
If you havent 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 l ive. (It took afair amount of time to put
right though)
Mechelecs comment about fault levels is valid, but so long as
the paralleled time is kept to a
minimum it shouldnt be a problem. Youd 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
This is a procedure that you really need to be careful with and
understand the implications of doing.
8/2/2019 Temporary Paralleling of Transformer for Changing Over
Supply
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as it has been mentioned the available fault current will
increase when running with two
transformers in parallel, this is very concerning and you should
be looking to run like this for as short
a time as possible.
Also as you are increasing the available fault energy you have
to be aware that if you do get this
wrong and make a mess then there is a very good chance that
things will go pear-shaped. Your
panels may not be rated sufficiently to deal with this extra
fault current.
Phasing and sequence...... This is another risk with 3ph systems
although your two sources may be in
the right sequence, you may find however the phasing may be out.
This would mean that you would
close on and have a fault right there and you would be at the
mercy of the grid protection.
Your two sources are they referenced ? if not then you may find
that frequency differences would
cause trouble, the system would try to sync lock up through your
'breakers.
Power companies may get very jittery about this as it may
develop issues with their own network
and cause problems to them.
How critical is your process ? What are you supporting and can
it survive a momentary dip ?
Have worked on a system where we retro fitted a UPS to a bus
panel and then carried out the
transition so that no power was lost. This can be undertaken by
some specialist companies that deal
with this sort of procedure - Aggreko ?
