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I’m aware of the importance of the neutral float and tie down cut out. I’m been told that with the float closed, and one tub goes bad that the other two will be forced to try and compensate for the one that is not working. What I don’t know is WHY. Why will this happen only when the float is closed in and not if it is open.
If I understand your question right, I believe it's all about circulating current and imbalance, with the 3rd phase out and the high side floated you have it due to imbalance with each pot grounded or the floater grounded it doesn't see the imbalance and is acting like an open delta , hence the reason to temporarily ground down the floating neutral on closed delta backs when energizing to combat ferroresence , an engineer could probably explain it better as this is just what I have always been told, I was also told there is a reason it is called Electric Theory and not Electric Fact.:D
I’m going to discuss the light/power delta bank, with a lighting transformer and two power transformers. Voltage is 120/240/208..
this bank should operate normally with a floating primary neutral. Tie the neutral down (close the floating cutout - a fourth cutout) while putting the bank in operation, then open the neutral or cutout for operation. You can Close the neutral to de-energize the bank for testing or maintenance, but NEVER close the neutral in a trouble situation when one of the primaries is already open. There can be primary level voltage on that floating neutral in that situation, and you should Open the other transformer fuses with a load buster before doing anything.
Floating the primary neutral does two things;
One, it helps prevent circulating current. The bank has unbalanced load and the floating neutral allows the bank to balance itself through the three phases. (It’s common to see a larger lighting trans. and two smaller power trans. Like a 50kva lighter and two 25kva’s for the powers)
Two, if you lose one of the power transformers, the bank’s secondary voltage will go haywire trying to work as an open delta (loss of one phase) without a primary neutral. The customer equipment can’t work with those voltages and will be shutdown and the customer will call in.
If you leave the primary neutral closed, the bank WILL continue to work as an open delta (because the primary neutral is connected). The voltage will stay the same, but you will lose about 30% of your load capacity and possibly burn out the other transformers. The customer will not call because there is no change in voltage. They could operate this way for days or longer and burn out more equipment.
in a trouble situation, a lineman can isolate a bad power transformer from the bank, then close the primary neutral, have the customer shed unneeded load, and operate as an open delta until the company can get out there to replace the bad power trans. Have done this many times.
In my experience the size of the transformers and their impedance values and the loading are key factors. While energizing grounding guards against resonating caused by matching capacitive reactance with the inductive reactance of the design. I have seen that many times on different voltages on banks and cables. Under a zero load condition mostly with the ground floated. Add secondary load it can help one issue but it may cause another one if faulty equipment is involved. Temporary measures of offloading to keep the freezers running will work no question,58% capacity of three phase or 76% of two transformers. Quite often the time it takes to isolate and fix the bank another transformer can be delivered to the site. In my opinion these banks only confuse customers, meter people, and do not save anyone money in the longer scheme of system design. Just my opinion of 46 years. Hope it helps.Quote:
Originally Posted by copperlineman