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How to make sure the rated VA (Burden Capacity) of CTs by simple
site test ?Matthew Luther You pretty much CAN'T test the max stats
of most CTs on site. It's simply impractical
to generate up to 500kV depending on what type of CT you're
using. What is normally done, is to performa saturation test, using
a Meggar or Doble test set. These are not cheap, but they last
forever. My guyshave one that's 15 years old, and we use it a lot
every year.Philippe Mertens Hi Thomas, as you may know, CT's are my
special field of interest ;-)I have to agree partially with Mr
Luther; when the CT's are installed in a transformer you can't just
push1000A trough the transformer bushing to check the CT secondary
or when they are live on a substationinsulator post, it's very
dangerous to even think about trying it. When I would order a
substation I wouldrequest a 10A test-windings on all CT's so then
you can actually test the full capacity of the CT's on-siteby
injecting in the fully distributed 10A test-winding. But when the
spec does not ask for test windings, youwill not get test windings
on your CT's and veryfying the burden by direct measurement will
not bepossible.
There is however a calculation possibility. When there is no
primary current, you can measure the
secondary resistance Rct and take the secondary magnetising
curve so you can see at which voltage theCT saturates = Vsat. Note
that measuring the DC resistance of the CT will magnetise the
CT-core and forbig cores there may be a huge time constant so you
have to wait untill the measurement has stabilised.I'll give an
example so you can check the calculations for your application.
Imagine you have a 2000/2ACT with 20VA and 5P20 and you measure Rct
2.6 Ohm and Vsat 400Volt. You need to correct Rct to75C instead of
ambient temperature of the CT so multiply the measured Rct by
((235+75)/(235+Tamb))with Tamb in C.F.i. the ambient is 34C then
the 2.6 Ohm at 34C will be 3.0 Ohm at 75C
If you open the secondary circuit, you can also measure the
secondary connected load resistance Rb.Imagine Rb is 2.5
Ohm.Calculate the burden Rb for which the CT is designed; Since P =
R . I and the rated secondary current is2A, The rated load R of the
CT is 20VA/4A = 5 Ohm.
At this point you already know that the actual load of 2.5A is
much lower than the load for which the CT is
designed so that must be enough to be sure that the CT will not
saturate. But you also want to check thecurrent output.
Since our secondary circuit has a voltage source Vsat and two
resistances in series: Rct and Rb we cancalculate the secondary
current that will flow at the saturation point with the measured
values; I=Vsat/(Rct+Rb). In our case at saturation point there will
be 400V/(3+2.5 Ohm) = 72.73A. I guess this current of72.73A is a
clear indication for the protection engineer if the CT's will have
enough current output. If wecheck what the designed output will be,
we take the design value for the load (=5Ohm) and the
designedsturation current which is 20 x the nominal secondary
current for a 5P20 CT = 40A. At the designed knee-point voltage of
the CT there will be 40A secondary current and 5Ohm load so the CT
knee-point voltagemust be at least 40A x (3+5)Ohm = 320 Volt and we
have measured a knee point voltage of 400V whichis more then the
designed value.I hope this can help you further ThomasMatthew
Luther I like my method of using Meggar's and Dobles to test CT
saturation better. :DPhilippe Mertens I agree on the CT saturation
point but you still want to evaluate if the output will beenough
and that during fault conditions the CT will not saturate so you
need burden or current to checkwith protection scheme.
Correct?Matthew Luther It's more important to know that the CT
won't saturate before fault conditions, thanduring. Even if it
saturates during a fault, as long as that saturation is high
enough, you will still trip. I thinkverifying that the output of
the ct is exactly to spec is a little over board. If you can't
trust yourmanufacturers to get that right, you shouldn't be buying
from them, and their is inherent wiggle room in allof these
measurement both for tripping and output. I think that verifying
the CT saturation curve beforeinstallation matches the
manufacturer's provided curve and/or meets your requirements is
more than
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7/30/2019 Va Burden Testing
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enough. If you haven't installed it yet, you can check the
ratios too, pretty easily. Honestly, that's enough,in my
opinion.
Now if you're the manufacturer, I definitely think that all
these tests and calculations need to be performedas quality
assurance every so often. It's definitely worth looking into how
your manufacturer performs QAif you don't have a long relationship
with one, or even if you do.thomas kunjukunju Hi Phillippe,Thanks
for your simple and proper explaination about matter.I hope it will
help our group membersalso.More over I appreciate your ablity and
interest on matter.M. etin OLAK Hi Luther,
I am agree with Philippe.The point is secure your protection
plan rather than trusting to manufacturers.
And, please dont forget; the site tests are carried out in order
to detect anomalies may caused fromtransportation to site and
erection (of course, the manufacturer phase should be cleared
before).
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