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?
I am answering this from a practical perspective and what is used in industry. Usually, open delta is kept shorted precisely due to the same point you have raised - inaccuracy. In modern power stations, use of open delta is only used as a back up for the same reason.I cannot comment on the mutual inductance if all the three PTs are kept in the same case. Rather, I would be more concerned on the down-time in case of a flash over, in which case you would be spending more money and plant down time till it is replaced with a new one.
I think, there will be some measurement error, but quoted value I have no idea
Leo Devasagayam
Thank you very much for your answer. The Feeding transformer topology is a Grounded Wye-Grounded Wye 12.47 kV : 600 V. The load is 3 phase exclusively, in this case big motors. If the 3 PTs in a case share a common core vs if they don't. Is it less accurate?
If you are running true 3 phase motors then having a 3-phase transformer that has a common core is no different to the motors themselves, so should not be an issue.
Make sure however that the transformer is rated to handle the motor’s initial start up currents, with individual isolation switches at the motor side.
If the motor start up currents are too high they can saturate the transformer core’s magnetic properties and then Very Bad Things Can Happen !!!
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