VT specification for very low burdens.
VT specification for very low burdens.
(OP)
Hello,
I am trying to prepare a VT specification and have come across what I believe is a pretty standard issue:
IEC 60044-2 sets the accuracy clases for burdens ranging from 25% to 100% of the VT rated burden. However the burden of new protection and metering devices that have to be connected is so small that it does not even reach 25 % of 10 VA, which (I believe) is the lowest of the standard VT rated burdens.
For these cases, what is your view on the most suitable way to specify the VT? I thought of the following posibilities:
a) Add dummy load in paralell to the instrumentation to raise the burden and specify VTs with low standard rated burden (10VA).
b) Specify non standard VA rating (as low as 0.5VA) and contact manufacturers. However this will probably be very costly.
c) Specify the VTs using C57.13, which seems to cater for 0VA burdens. However this wouldnt be in line with the rests of the substation specs which is defined in IEC terms.
Your view is much appreciated.
I suppose that this must be a fairly common problem, however I haven´t managed to find any thread advising on this issue. If this is the case, my sincere apologies!
Fernando
I am trying to prepare a VT specification and have come across what I believe is a pretty standard issue:
IEC 60044-2 sets the accuracy clases for burdens ranging from 25% to 100% of the VT rated burden. However the burden of new protection and metering devices that have to be connected is so small that it does not even reach 25 % of 10 VA, which (I believe) is the lowest of the standard VT rated burdens.
For these cases, what is your view on the most suitable way to specify the VT? I thought of the following posibilities:
a) Add dummy load in paralell to the instrumentation to raise the burden and specify VTs with low standard rated burden (10VA).
b) Specify non standard VA rating (as low as 0.5VA) and contact manufacturers. However this will probably be very costly.
c) Specify the VTs using C57.13, which seems to cater for 0VA burdens. However this wouldnt be in line with the rests of the substation specs which is defined in IEC terms.
Your view is much appreciated.
I suppose that this must be a fairly common problem, however I haven´t managed to find any thread advising on this issue. If this is the case, my sincere apologies!
Fernando






RE: VT specification for very low burdens.
I have always assumed (sorry, Jeff) that the ratio is specified at zero load and that the accuracy is maintained up to rated burden. I often use VT:s for recording purposes and the recorders usually have a 10 or 1 Mohm input resistance, or very close to zero load.
I have never had any problems with measurement discrepancies when compared with data from other equipment in the system.
I have thought like this: If you have .5% at 10 VA, then your accuracy at zero load is either close to 0% or, if the loading effect is compensated for, a .25% too high value at zero load, which implies 0% at 5 VA and -.25% at 10 VA. In either case, that error is too small for me to worry.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: VT specification for very low burdens.
Your feedback is much appreciated. Surely your take is absolutely fine for the application you mention.
However, one of the VT secondaries i am trying to specify is used for Tariff metering and surely any deviation from the rated VT accuracy will raise concerns. I believe that the VT manufacturers compensate voltage drops in the VT by slightly modifying the VT turns ratio ( I might be wrong though), so if the accuracy is (say) 0.2, at nearly zero burden might be higher...
RE: VT specification for very low burdens.
Regards
Marmite
RE: VT specification for very low burdens.
RE: VT specification for very low burdens.
I never do tariff things. Only "difficult and impossible" problems.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VT specification for very low burdens.
Don't do this with tariff CTs though!
RE: VT specification for very low burdens.
That´s very interesting issue you highlight: "The interdependence of the VT secondaries". From reading IEC 60044-2, I had understood that the "accuracy vs burden" of one secondary should be achieved with the rest of the secondaries loaded from 0 to 100% of their rated burden. If that was the case you wouldn´t need to load secondaries other than the tariff metering ones to achieve the accuracy stated by the manufacturer. Isn´t this actually the case with commercial VTs?
Thank you!
RE: VT specification for very low burdens.
I'm lazy, I prefer to use a known test point (or near enough as it depends on what resistance value I need and how close one of the standard ones is).
RE: VT specification for very low burdens.