Loading the Tertiary Wdg of 3-wdg Power Transformers

[CKent]

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?

 

[waross]

I would be more concerned with reliability issues using the main transformer. The tertiary winding may not be designed with the capacity for extra loading. Also, a fault on the station system may damage the tertiary winding and render the transformer unusable.
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

 

[apowerengr]

CKent, I agree with waross the open delta is both easier and more common - you handle the regulation issue with a leading connection and planning for +/- 10% variations as the transmission system varies. I'd speculate your power transformer specs call for a 30-40% rated tertiary winding, as such your 100kVA or less station power open bank won't pose a loading issue. I am currently working on eliminating a customer's 8 mile utility distribution feeder supplied off the tertiary of a 138/69/13.8kV Ygrd/Ygrd/delta transformer - a much worse situation from a reliability standpoint. Do you have room to mount power fuses at the tertiary bushings or will there be some exposed length of conductors? Often, mounting provisions are made on the power transfomrer tank for the station service/aux type transformers supplied by tertiary windings.

 

[dpc]

This is done fairly frequently. You need to:

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.

 

[CKent]

You're right apowerengr, our 3-wdg XFs are normally rated about 33% of the XF rating...and loading of the station XF will surely be considered in the total load of the XF..so yes, dpc, tertiary is adequately designed for the intended load...the capacity is not an issue.

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.

 

[apowerengr]

Cable will ultimately fail (as all insulation does) assuming you need some supporting structure for the cable transition from overhead to underground (assumes the tertiary bushings are either side or top-mounted) put the fuses there. You'll most likely need a differential relay capable of taking three sets of inputs which would also improve your protection. I suppose you could add supplemental CT's at the station service padmounted transformer on the primary side - then you'd have a differential zone which includes the station service transfomrer underground cables - minimizes risk of a fault damaging the power transfomer......let the bayonet fuses handle secondary faults.

 

[CKent]

Aside from the protection, we are more concern if there will be issues of ferroresonance or harmonics when we either ground or unground the delta windings of both XFs.

 

[dpc]

Harmonic issues will be no different than now, assuming your new load on the tertiary is not terribly distorted.

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.

 

[CKent]

thanks dpc... our 3P padmounted XF have 1P fusing with gang operation capability...although we cannot discount the probability of one/two fuse blowing out which could then lead to 1p/2P tripping or switching...if we use our pole mounted DTs as station XF, 1P fusing is the only option...either case, is there a cause for alarm concerning ferroresonance...I believe though that ferroresonance is not likely to occur at voltages lower than 25 kV...

is there a simple way to determine or compute if a particular system is susceptible to ferroresonance?

 

[dpc]

http://www.basler.com/html/dwntech.htm

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.