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Loading resistor for an Open Delta VT
3

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.
 

RE: Loading resistor for an Open Delta VT

In ASEA installations the open delta gap was bridged with a 60 ohm 20W resistor. IIRC this provides a path for I think it was the third harmonic component to be shunted out. There was also an effect using this to prevent ferroresonace. Google that...

rasevskii

RE: Loading resistor for an Open Delta VT

More than likely that winding is for connection as "broken-delta" and not "open-delta".

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

If you can find a copy of an AIEE paper by Karlicek & Taylor on "Ferroresonance of Grounding Potential Transformers on Ungrounded Power Systems" AIEE Power Apparatus & Systems,1959, they presented a formula:

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

I agree with Scott, it would be broken delta rather than open delta. Open has two windings connected phase-phase on both primary and secondary while a broken delta has three winding, wye on the primary and delta on the secondary with one corner broken for the insertion of instruments. If you aren't using the broken delta, why not reconnect it in wye and make use of it?

RE: Loading resistor for an Open Delta VT

David-

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

What a waste of a perfectly good resource.  As we've been upgrading our protection, we take the old broken delta winding and reconnect in wye so that we have two voltage sources and can apply one voltage to the set 1 relays and the other voltage to the set 2 relays.  The broken delta has always been a field connection, just like the wye, so just a wiring change to get between the two.

RE: Loading resistor for an Open Delta VT

veritas,
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

David-

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

David-

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

Fuse failure on the secondary is one reason.  NERC/FERC are heading in a direction where it appears that multiple secondaries will be considered as redundant.  Whether that is completely true or not is another matter, but if we can meet redundancy requirements (in the pipeline, not yet issued or enforceable) it is much more effective to use the two secondaries.  With limited budgets, it is better to get new/better relays onto more positions than to double up all the VTs.  We also turn on other protective elements when the relay detects a loss of potential condition.

RE: Loading resistor for an Open Delta VT

From a manufacturer's perspective (which is mine) it seems it would make just as much sense technically (and save a lot of $ in some cases) to use 1 secondary winding and then fuse out different circuits as needed from the junction box. Secondary fusing is rarely integral to the VT itself.

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

(OP)
Hi scottf,burntx2

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

I agree with Scottf.I have seen in my country broken delta( we call it opendelta here.But let us respect US sensibilities!) PT secondary is used for detecting single line grounding in unearthed delta systems.Winding is rated for 110/3V per phase,so that in the event of earthing one primary line,110 V will appear across the broken delta which is used to actuate relay.Resistances can be connected across the broken delta or line to earth in case of star connected winding.Calculations for R for variuos cases are given in Cahier technique No190 "Ferroresonnance", at Schneider Electric web site.http://www.schneider-electric.com/sites/corporate/en/products-services/technical-publications/knowhow.page

RE: Loading resistor for an Open Delta VT

(OP)
prc - many thanks for the Cashier Technique tip. Was very helpful.

Thanks also to all those who contributed to this thread.

Regards.

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