Advantages and Disadvantages of Transformers connections 6

[mnnc]

Hi, I would appreciate it if anyone who can explain what are the advantages and disadvantages of transformer connected delta on the secondary side? connected Wye on the secondary side?

 

[Cooplineman]

Hi waross.

Thanks for the response. I worked on AEP and Ohio Edison (First Energy) property when I used to work for contractors. I have worked storm damage in about 6 different states and have never seen the practice of tying H2 to neutral. If you would have asked, they would laugh you off the property I guess I'm just curious why anybody would have a standard to do so. I'm sure you can probably answer that, but it would seem to have to be really important because of all the negative issues that you previously pointed out that occur in these circumstances. In my mind, the only way that you could do this half safely would be to have a 3 phase breaker for line protection and to oversize the bank, causing unnecessary inductive reactance, so that if you lost one phase, it wouldn't burn up the other two transformers.

I can definitely see why they would want to go to an only 87% efficient open wye/open delta for their standard wye/ delta bank needs.

Your thoughts?

 

[waross]

Hi Cooplineman
I suspect that the practice had it's origin in the days of 2400 volt primaries, with Delta/Delta transformer connections.
In some areas these systems were upgraded to 4160 Wye and the secondaries left as is. The old texts all gave the advantage to the Delta secondary because. "In the event of a transformer failure you can supply the plant at 58% capacity on Open Delta." 86% x (2 transformers out of 3). Many generations later, with improved transformer construction and experience with Wye dependability, you can add, "With Wye connections the transformers don't burn out nearly as often."
A couple of generations ago, when there were a lot of Delta/Wye conversions going on, I understand that some transmission circuits were changed from Delta to Wye to increase capacity. There may have been some new transmission installations also. The transmission lines were arranged with floating neutrals to save wire and had severe problems with 3rd harmonic heating. The problems were completely solved by the addition of a neutral wire to the circuit.
There was enough publicity and enough case studies circulating in the industry that I am sure that many older engineers avoided floating neutral Wye primary connections completely.
I hope someone with more harmonic experrience than myself will give us a post on possible third harmonics in a distribution bank with a floating Star primary.

 

[RalphChristie]

I have never seen a star-delta transformer on distribution level - but I have to admit that I have not worked on that many places.

I have seen star-delta transformers on transmission level, the neutrals were solidly grounded due to degraded isolation towards the neutral point.
One place (distrubution level) where we use it is on our trolley systems. (DC-overhead pantographlines for some of our mining-trucks - to reduce diesel consumption) We use Dd0 and Yd1 transformers, 11kV/1200V, to create a 12 pulse DC-system. The neutrals are left floating - thus not connected to ground.

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[waross]

Hi RalpgCristie,
Good to hear from you. I worked in an old plant in the early 70's that used Delta 440 volt secondaries, but I don't remember weather the primary was Wye or Delta. Could have been either.
The Broken Delta
With one exception, I first came in contact with problem Wye/Delta about 12 years ago.
I ran into a serious Star/Delta problem about 30 years ago. It was a booster transformer for the glass furnace in a bottle factory. Molten glass conducts electricity (And frozen water doesn't).
I forget the rating of the transformer but it was bigger than an outhouse and smaller than a bus. It was connected Wye/Delta.
In the transformer secondary junction box, the ends of the windings were brought to six large copper bus plates. the delta was formed by connecting each pair of plates with a short piece of bus bar about 6" x 3/8" From each of the six plates several 1000 MCM cables fed to electrodes in the molten glass. We were called in because the transformer was overheating.
There was a primary voltage unbalance that was causing the problem.
We removed one of the connecting plates between phases which left half of the load connected to each winding end. This was not Open Delta, all three phases were in use, but one corner of the delta was open with one half the load on each side of the break. I've never seen it in a text but I've always thought of it as "The Broken Delta." P.s. the transformer temperature dropped to normal.

 

[waross]

Sorry about the spelling mistake, RalphChristie,
I guess that my fingers and my head are a little thick tonight.
BTW I wasn't aware of the floating primary neutral until wfowfo pointed out the REA/RUS practices. And thanks to stevenal for the link to the RUS standards.
yours

 

[mc5w]

Waross,

You are completely WRONGO about star-delta distribution transformers. Ohio Power, all of the divisions of First Energy, and Duquesne Light Company use star-delta transformers to supply both 3-wire 3-phase and 4-wire delta 3-phase. ( Duquesne Light Company does not have 4-wire delta but the others do. )

The primary neutral point of a 3-transformer start-delta transformer is NEVER EVER connected to the supply neutral or to ground. This completely blocks circulating current in the secondary except for the 3rd harmonic. This is a very successful connection that is used all over the United States. In fact, this is a STANDARD connection on wye primary systems. The primary neutral is only used to bond lightning arrestors and the secondary neutral together to meet UL, NESC, and NEC requirements.

If you look closely at a 3-transformer wye-delta bank that supplies 240 volts corner grounded, 120/240 volts 4-wire delta, or 480 volts delta, the neutral point of the transformer primary is fully INSULATED from ground of the system neutral.

I have also seen a 7,620Y13,200 volt 3-wire primary 4-wire delta 3 transformer bank where one of the transformers is 3 times the size of the other 2 and it is NOT burning up. The greater magnetizing current of the larger transformer turns into a zero sequence circulating current in the secondary and then flows through all 3 primary hot wires.

NOBODY every builds a transformer bank with some of the primary tap settings different from the others.

 

[mc5w]

I forgot to mention that this delta-wye bank that Ohio Power has with 1 transformer larger than the other 2 feeds a large restaurant in Canton, Ohio. This transformer bank has withstood 2 instances or the supply substation losing a phase on the 138,000 volt side of the substation.

Both Duquesne Light Company and Ohio Edison have 2,400Y4,160 volt distribution systems and 3-wire wye primary 3 or 4-wire delta distribution transformer banks work fine on these systems.

You third world utility has more problems than that there are delta-wye distribution transformers. The things were probably hooked up wrong in the first place.

Interestingly, a 4-wire wye primary 3-wire delta secondary is sometimes used as a grounding autotransformer to refit say a 13,200 volt delta secondary substation winding so that it will act as a 7,620Y13,200 volt source. Ohio Power sometimes will do this with a 138,000Y79,674 primary 69,000Y39,837 secondary 13,279 delta tertiary transmission transformer. A 7,620Y13,200 volt 4-wire wye primary delta secondary transformer is connected to the delta tertiary to obtain a neutral for 7,620Y13,200 volt distribution. In some instances a small 7,620Y13,200 volt floating neutral wye primary 4-wire delta transformer is the only load on the 13,200 volt delta tertiary and the small transformer supplies internal power for the station relays and operating controls.

Also, when a 3-transformer bank is operating 3-wire wye primary 3-wire delta secondary a single phase secondary load divides 2/3 into the transformer that directly connects to the single phase load and the other 1/3 of the current through the other 2. There is a similar relationship for 120 volt loads on a 120/240 volt 4-wire delta secondary.

Wye-delta transformer banks DO NOT present problems if you are using proper design, installation, and repair procedures such as cutting the other 2 phases and grounding all 3 before repairing a downed wire.

Sounds a lot to me that when this third world customer upgraded a wye-delta bank from 2 transformers to 3 they forgot to isolate and insulate the neutral point of the primary neutral.

Cleveland Public Power also uses the wye-delta configuration on their 7,970Y13,800 volt distribution and they use both 2 transformer and 3 transformer banks. They do not have any problems with these.

Oh, First Energy, Ohio Power, and Duquesne Light Company have considerable amounts of rural distribution and do not have these kinds of problems.

 

[waross]

Hi mc5w
A couple of misunderstandings.
1> I did not mean to suggest that tap settings be different in normal practice. I meant to use the example to illustrate the effect of unequal primary voltages.
2> I don't have a problem with one transformer being larger than the others. In fact I expect to see it whenever there is a large single phase load and a small three phase load fed from an Open Delta bank.
3> I was not aware that RUS standards allowed floating neutrals but thanks to some friends on the forum, I now have the RUS standards down-loaded and have learned something.
4> What is the voltage to ground of of your neutral connection under phase loss and single phase conditions? This is considered a safety issue in some jurisdictions and as a result neutrals are always grounded without exception.
4> This is an international forum. The whole world is not bound by US standards.
I wonder about the effect of third harmonic currents in a floating Wye/Delta bank. That is a question best answered in a new thread. I'll probably ask it some day.
I see three choices.
1> Use a Floating Wye/Delta connection and be aware of safety issues and third harmonic issues.
(Note; In a system that has always used floating neutrals it's probably not much of an issue.
In a system that has always used grounded neutrals regardless of transformer connections, and human nature being what it is, you would probably have a few fatalities before the work crews 'Internalized' the message that what was safe for 30 years is now lethal.)
2> Use a Grounded Wye/Delta connection and destroy transformers and refrigerators.
3> Use a Wye/Wye connection and be aware that your larger loads will be supplied with 208 Volts instead of 240 volts. There are enough millions of 230 volt motors running happily on 208 volts that it's probably acceptable.
Re Withstanding single phase conditions; We had two short outages yesterday. The short outages are invariably single phase. The three phase outages are usually more serious and take a lot longer to fix.
Is anyone out there from Florida? I'm wondering what FPL does with the neutrals on Wye/Delta banks. (Should that be FLP?)
yours