voltage transformer 2

[rickei]

All voltage transformers what I've seen are single phase and then connected to three phase what you need (delta or wye). Is there some reason why not build three phase voltage transformer (using one core instead three)?

 

[dpc]

Are you talking about voltage transformers for metering and relaying?



"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[rickei]

Yes "dpc" I mean on measurement transformers.

 

[busbar]

I believe in the IEC world it is common to find a 3ø VT in one case {and maybe one core?}, where in the ANSI world, they are typically separate.

 

[dpc]

That's a good question. I've seen "three-phase" VTs for low voltage (600V) applications, but I'm not sure if these have a common core or not. I suspect the flux summation in the core may compromise the accuracy, especially if one phase has a signficantly different voltage.

At high voltage, the single phase units are probably more practical from a physical standpoint anyway.

I'm hoping scottf will be along to enlighten us both.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[waross]

If a three lagged core is used there will be a "Phantom Delta" effect. If a four wire wye primary configuration is used and the primary voltages are not equal, the phantom delta will transfer power from one phase to another trying to balance the voltages. This will burn out distribution transformers fairly quickly. If will also seriously compromise measurement accuracy.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

3-phase VTs have been produced in the US up to 115 kV level. I don't believe they used one common core, mainly for the accuracy reasons stated above, but I'm not 100% sure about that. I know there is a technique to compensate for a lot of the accuracy issues when using a common core for 3-phase.

There aren't many folks still making 3-phase VTs above 600V level currently. The main reason is that there is not much, if any cost savings in building a 3-phase unit for MV and HV applications. For applications up to 69 kV, single-phase dry-type units are much more affordable versus single-phase oil-filled and 3-phase oil-filled designs. The dry-type units are also generally smaller than oil-filled units and therefore space issues normally aren't a factor.

For applications over 69kV, oil-filled is the only option and 3-phase units start to become very difficult to build correctly, mainly because of the complicated insulation designed used in VTs. Manufacturing and processing become big challenges at this level and 3-phase options just don't make sense financially or logistically.