Loading resistor for an Open Delta VT
Loading resistor for an Open Delta VT
(OP)
Hi
I have a 50VA, 11kV VT with one primary and two secondaries. One secondary for metering and the other an Open Delta winding not to be used. I have heard that this winding should not be left open but be loaded with a resistor.
Anybody tell me why and what the value of the resistor needs to be?
Thanks in advance.
I have a 50VA, 11kV VT with one primary and two secondaries. One secondary for metering and the other an Open Delta winding not to be used. I have heard that this winding should not be left open but be loaded with a resistor.
Anybody tell me why and what the value of the resistor needs to be?
Thanks in advance.






RE: Loading resistor for an Open Delta VT
rasevskii
RE: Loading resistor for an Open Delta VT
The idea is that a resistor placed across a broken-delta will reduce the chance for ferroresonance.
Under normal system conditions, there is little to no voltage across the resistor. However, during ferroresonance, one phase voltage will rise, thus creating a voltage across the resistor, which adds burden to the VT secondaries, helping to dampen down the resonance.
RE: Loading resistor for an Open Delta VT
RBD = 100 X La / N2; in ohms
where:
RBD = broken-delta resistor
La = PT primary inductance, in millihenrys, and
N = turns ratio
RE: Loading resistor for an Open Delta VT
RE: Loading resistor for an Open Delta VT
In the IEC world, often times the 2nd winding provided for broken-delta connection isn't designed to be loaded under nominal conditions. In other words, the accuracy rating of the 1st winding does not consider burden being placed on the 2nd winding (broken delta winding).
Also, the ratio is normally something like 110/3V instead of 110/sqrt3 V.
They normally refer to this winding as the residual voltage winding.
RE: Loading resistor for an Open Delta VT
RE: Loading resistor for an Open Delta VT
Since you already have the VT specs, why use that data in computing for the minimum broken-delta resistance (also known as the "Burden Method")!
If your VT secondary is 100/3 V, then (110/3)^2/50VA= 27 ohms (if V = 110/1.732, then Rmin = 80 ohms)
RE: Loading resistor for an Open Delta VT
Not really a waste of a resource, since the VTs are cheaper to manufacture when only having to consider the burden of the main secondary winding.
In the IEEE world, the accuracy/burden ratings are total rating divided in any means between the secondary windings. I.e. 0.3 WXYZ means the VT will maintain 0.3 accuracy class when loaded up to 200VA in any split between the secondary windings. The IEC world works differently in that each secondary has it's own burden rating and performance must be guaranteed when all secondaries are loaded to the rated burden.
Burnt2x-
Normally the resistance value should be calculated based on the thermal burden rating of the VT and not the accuracy burden rating. For instance, the VT under discussion here might have a thermal burden rating of 500VA or so, resulting in a higher resistance value, which would normally result in better damping.
RE: Loading resistor for an Open Delta VT
Forgot to ask about your comment regarding adding another "voltage source". I'm a bit surprised that you would consider voltage from another secondary winding of the same VT as a separate voltage source, given that a fault on one secondary winding impacts the second secondary winding just the same. There really isn't any added degree of redundancy added by using another winding, past maybe an inter-winding open-circuit, which is so rare that I've never actually seen it happen in the field.
RE: Loading resistor for an Open Delta VT
RE: Loading resistor for an Open Delta VT
For HV VTs, multiple secondary windings are standard, but not so for MV and LV VTs. In some cases for MV and LV VTs, it's more economical to install 2 separate single secondary VTs, versus one VT with two secondary windings.
RE: Loading resistor for an Open Delta VT
This VT has been manufactured to IEC 60044-2. It has a thermal rating of 250VA. As correctly mentioned it has a threephase secondary winding with one corner "broken". The line-to-line voltage is 110V so Vph is 110/1.732 = 63.5V.
The way I see it the Zbase is 110^2/250VA = 48.4ohms. Current base is 250/(1.732*63.5) = 1.312A and so the resistor wattage should be = 1.312^*48.4ohms = 83Watts.
Correct?
Thanks for all the postings so far. I find them very interesting and educational.
Regards.
RE: Loading resistor for an Open Delta VT
RE: Loading resistor for an Open Delta VT
Thanks also to all those who contributed to this thread.
Regards.