I'm a little confused, grounding the X0 point on a WYE primary would not give you a ground reference to the secondaries??
Also I thought the ferro resonance problem was only when the a WYE primary with a common core transformer had the X0 bonded to a WYE source grounded conductor which can cause high circulating currents in this primary neutral bond and conductor when there is any current imbalance on the secondary, I've seen a few of these mistakes cause this bonding conductor to burn up as well as cause the transformer to run very hot.
From what I understand a primary WYE made up of separate tanks which doesn't have a common core would not have this problem?
Maybe I'm missing something or have it wrong?
I do understand that using a WYE primary on a delta primaries will provide a ground reference for the deltas just like a zig-zag so it is very common for utilities to use WYE primaries on 3-bank transformers to provide a close to point of use grounding when the tanks are fed from a delta primaries, our local primaries here are 12.4/7.2 kv WYE's the only deltas are the local substation 69k feeds as well as the 138kv, and the 345kv line that feed the area sub stations 69kv feeds from the generating plants.
Grounding a primary wye does not ground reference the secondaries, but it does change the impedance of the primary under a fault condition involving a primary wire fault, such as a phase coming down in an over head line on the neutral. Think grounded wye broken delta setup like you mentioned to detect ground faults. A phase grounding down on the primary causes a potential to change across a broken delta, normally a near zero voltage potential changes to a much higher one. This potential will activate a relay or light to indicate a ground fault. In a closed delta, its shorted together which will cause a current to circulate within the secondary delta winding. That circulating current causes the impedance of the secondary windings to change, which in turn is transferred over to the primary windings. The 2 primary windings on the non faulted phases will draw more current while the primary winding on the faulted phase actually tends to feed back into the cable/wire. In a nut shell the primary fuses end up seeing more current then they are supposed to during a fault. If conditions are right that can cause the primary fuses to blow. Also, if the line has single phase protection, ie fused cutouts protect the line in question, if a phase becomes grounded and the fuse blows but the other 2 remain closed as they normally do with only one phase faulted, the primary wye will actually back feed power into the faulted phases. The secondaries along with the building load can back feed the transformer on the faulted phase through its secondary, where power will be stepped up and appear on the primary. The potential on the primary can then back feed the downed phase. If the fuse on the bank does not blow (it may or it may not) that can leave a downed conductor with a dangerous potential, sometimes enough to electrocute someone or start a fire. Ironically, if the phase does not remain grounded, but rather just opens up, the bank will continue to supply 3 phase power without any issue at all, as long as the load doesn't go above about half of the cans original rating. This is often desirable to do when a grounded wye primary is involved. Ferroresonance is also not an issue be it a 3 phase can or 3 single phase cans in a bank, as long as the primary has a solid ground.
Ferroresonance can occur on any ungrounded primary connection be it individual single phase cans or a 3 phase unit. I believe common cores can play a roll, but Im not sure to what extent, but with out a doubt is has been seen to occur in 3 phase banks made of single phase units. An ungrounded wye is the most susceptible, as well as delta but its seems to happen slightly less in delta primaries. Ferroresonance is not a concern when the transformer is loaded, its impossible to occur on a transformer that is fully loaded. Even at only 40% load is near impossible, mostly rare at 10%. With out any load though the risk is high, and is usually triggered by an open phase or single phase switching. Ferroresonance is a type of resonant condition when transformer inductance, cable capacitance and the like start to resonate with one another. Im not a perfect expert on the subject though, I know just the basics, some one could explain it better. All I know is never energize a transformer unloaded when conditions are ripe, always add a load bank to be or some building load that's resistive to be safe.
Some utilities will actually add a cut out to the bank that they will close when energizing the bank, then open the cutout to leave the primary neutral point floating, that way both problems can be eliminated. Others just keep it simple and disallow all deltas for new services.
Utilities only use primary grounded wye deltas for grounding purposes when its necessary to derive a neutral source when another is non existent. They are used in substations when the secondary of the distribution transformer is delta, and use the grounding transformer neutral either to detect ground faults on the feeders or if the unit is big enough to have a neutral for phase to neutral loads out on the feeder. California utilities are big users of zig zags and GrY deltas to detect ground faults on their feeders since about half the substations have delta delta transformers 66kv to 16 or 12kv. The remaining uses are like noted in the beginning of the post.
If your curious about the connections in utilities the usually go like this: Generator is wye grounded through a resistor to limit fault current. 23kv is stepped up to 345kv via a delta wye GSU. 345kv is step down to 115kv or 138kv via a grounded wye autotransformer, often having a delta tertiary for circulating currents. Some utilities may use this tertiary for loads too. 115kv is stepped down to distribution be it 7/13.8kv Y 13.2/23Y 19.9/34.5Y ect via a delta wye unit, usually rated 40MVA when serving suburban towns. The secondary is solidly grounded and a multi grounded neutral is brought along with the poles. Customers are usually served by a grounded Y grounded Y giving 120/208 or 277/480. Sometimes a delta wye is used instead if the other is too unfavorable for the application at hand.
My 2 cents.
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