If I had to select between Individual Three Phase Potential Transformers vs Three Potential Transformers in one case for a 600V system (which doesn't require many inches of spacing between phases to avoid arcing). I know the later saves space and about 15% of money. But, should I worry about mutual inductance, mutual interference? The interference I'm referring to, is measuring in one phase also contributions from the other phases to it, instead of the accurate voltage present in that phase alone. Also there seems to be Three Potential Transformers in one case which share a common core and those which are basically individual PTs glued together. Any difference from interference point of view? Is there any bigger inaccuracy in readings in having open delta PTs vs 3 winding PTs?

  • \$\begingroup\$ What kind of interference do you fear between individual phases of a three phase system? \$\endgroup\$ – Uwe Apr 11 '19 at 20:46
  • \$\begingroup\$ @Uwe I have updated my question. By interference I mean. I'f you are trying to read with a PT the voltage of one phase and the voltage reading is not precise due to the contribution or voltage in other phases \$\endgroup\$ – VMMF Apr 12 '19 at 10:28
  • \$\begingroup\$ A three phase power system with asymmetric load may have three different phase voltages anyway. The kind of transformer would not change that. There are many cascaded transformers within the power grid. Do you want to use the Potential Transformers for voltage measurement only? \$\endgroup\$ – Uwe Apr 12 '19 at 10:45
  • \$\begingroup\$ I have a control relay which must monitor the voltage in the different phases. If the voltages are unequal due to unbalance I want to make sure the control relay is able to accurately measure the voltage of every individual phase without having a disturbed reading because of whatever voltage there is on the other phases. The proper selection of the PT is key in this. Thay is why I'm asking what would change if I select one over the other \$\endgroup\$ – VMMF Apr 12 '19 at 10:56
  • \$\begingroup\$ If there is a difference for both versions, it should be specified within the datasheets. I would compare the specifications carefully line by line. \$\endgroup\$ – Uwe Apr 12 '19 at 11:03

There are four options:

  1. A three-phase VT wound on a three-limb core.
  2. A three-phase VT wound on a five-limb core.
  3. Three separate single-phase VT's.
  4. A voltage measuring device that measures your 600VAC directly without a VT. (Where I come from, direct voltage meaurement, without a VT, is used up to at least 440 VAC.)

Of these, I would recommend that you avoid VT's wound on a three-limb core, as they are not capable of reproducing residual voltages. This is because there is no magnetic path in a three-limb core for any residual flux to flow.

That is, if your voltages VA + VB + VC do not balance to zero, a three-limb VT will not reflect this imbalance. This sounds like it is critical for your application.

The extracts below are from a typical drawing I prepared for HV directional overcurrent/earth fault protection.

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