Loading the Tertiary Wdg of 3-wdg Power Transformers
Loading the Tertiary Wdg of 3-wdg Power Transformers
(OP)
Good day!!!
We have been using 3-phase, 3-wdg power transformers connected YG-YG-D, with voltage ratings of 115/34.5/13.8 kV and capacity of 83MVA...Our standard now is to use the tertiary winding only as a stabilizing winding, with one point of the delta grounded...
However, we are planning to utilize the tertiary wdg now to serve the station transformer, normally connected D-YG...
Does anybody have any experience on this? what must be the proper connections to avoid operational problems such as ferroresonance or whatever? should one point of the delta of both the station XF and tertiary of the main XF be grounded or not? or should we leave the grounded point of the delta tertiary grounded and then have the tertiary of the station xf ungrounded?
the station XF shall be connected thru UG power cables with approx 20m from the tertiary of the main XF.
Another scenario that I'm looking into is the possiblity of using an open delta pole mounted transformers as station XF instead of the 3P padmounted, which would significantly reduce the cost. Any comment on this?
We have been using 3-phase, 3-wdg power transformers connected YG-YG-D, with voltage ratings of 115/34.5/13.8 kV and capacity of 83MVA...Our standard now is to use the tertiary winding only as a stabilizing winding, with one point of the delta grounded...
However, we are planning to utilize the tertiary wdg now to serve the station transformer, normally connected D-YG...
Does anybody have any experience on this? what must be the proper connections to avoid operational problems such as ferroresonance or whatever? should one point of the delta of both the station XF and tertiary of the main XF be grounded or not? or should we leave the grounded point of the delta tertiary grounded and then have the tertiary of the station xf ungrounded?
the station XF shall be connected thru UG power cables with approx 20m from the tertiary of the main XF.
Another scenario that I'm looking into is the possiblity of using an open delta pole mounted transformers as station XF instead of the 3P padmounted, which would significantly reduce the cost. Any comment on this?






RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
I like the open delta solution. It is not symetrical, but it avoids a few issues with delta secondaries. It is cheap and repairs are cheap.
When open delta is mentioned, there is often a chorus of "But the regulation is poorer than with a closed delta."
Yes, the regulation is slightly poorer, but when you do the numbers on it the difference is not serious. Open delta transformer banks are often oversized for the load, and so the actual voltage drop under load is seldom an issue.
respectfully
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
Review transformer nameplate and spec data to verify that the tertiary is adequately sized for intended load.
Determine if tertiary is internally corner grounded. If not, I'd probably leave it ungrounded if possible.
Obviously any time you use a winding, you run the risk of damaging it on a through fault. That risk-benefit analysis is up to you. Make sure to factor in a lead time of well over 52 weeks for a replacement transformer. But they brought out the leads to bushings for reason - so you could hook it up.
As for the open-delta option, why not just add a third pole-mount and make it a real three phase power source assuming you have the third phase available on the primary. As far as I'm concerned, open-delta systems and three-phase motors are a really bad combination.
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
With regards to the fault, as I've mentioned in my post, the station XF will be connected thru an UG power cable (about 20m). The padmount XF has internal single phase fuse with gang operation capability. The set up almost indicates that the occurrence of fault may be minimal, 'but' may never be totally eliminated. So, yeah, issues of fault should also be given consideration particularly, if ever, the fuse should operate first before the transformer differential. There is no room to mount for fuse at the tertiary of the xfs. Before, we use to install the station XF near the power XF but with the new station design including the fire/safety sprinkler system, the station XF were forced out near the power XF.
Padmounted is initially the preferred equipment as it will significantly enhance the physical design of the substation. Unlike when you install a pole mounted XF inside the substaion.
So really, the issue is since we have no experience yet with this set-up, we are studying what should be the proper connection of both the delta windings, whether we should corner ground both or not or just one of them, in order to avoid operational problems like ferroresonance, third harmonic occurrence, etc., that may otherwise occur with improper connection.
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
I don't see why ferroresonance would be a concern, especially if you are doing three-phase switching.
This is a fairly common installation, and three-winding transformers in general are widely used and have a long history.
The drawbacks of three-winding transformers have already been mentioned - you are putting all (or a lot) of your eggs in one basket. Replacement of a three-winder is generally more difficult than a two-winding, but you already had that problem. By using the tertiary winding to power loads, you are increasing the risk of transformer failure due to a throughfault. But that's what transformers are for - to transmit power.
As Grace Hopper once said "A warship is safe in the harbor, but that's not what warships are for."
I do recommend putting the tertiary winding and the incoming feeder, if possible, within the transformer differential zone.
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
is there a simple way to determine or compute if a particular system is susceptible to ferroresonance?
RE: Loading the Tertiary Wdg of 3-wdg Power Transformers
Scroll down toward the bottom of this page and find a reference to a paper on ferroresonance by John Horak.
Long secondary feeders and light loading are contributors to ferroresonance.