"T" Winding transformers. 8

[waross]

A comment on the obsolete two phase systems in another thread reminded me of an issue that has puzzled me for years.
There was a time years ago when we saw a lot of "T" winding lighting transformers. These were typically 25 KVA, 480 Volts to 120/208 Volts.
Many years ago when three phase systems were replacing two phase systems, the Scott connection was able to convert two phase power to three phase power with two suitable transformers.
Around the 60's, someone realized that a similar connection with suitable ratios and taps could also be used to transformer three phase power to three phase power.
One of the advantages was two cores instead of three cores.
I have always wondered what other advantages there were, and also what were the disadvantages that led to these transformers going the way of the Dodo.
Anyone else willing to admit to being old enough to remember these transformers?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MatthewDB]

Square D still makes their smaller three phase units in Scott T. I think the cutoff is a few kVA.

I have seen a grounding transformer that was Scott T on a wind farm. It threw me for a loop, and I stared at the nameplate for a while until I realized how it would work.

 

[prc]

Background of T connection- The first major AC transmission in US (Niagra Falls-Buffalo-Newyork) was planned with two phases (90 degrees apart) by Westinghouse and the lines were laid up to Buffalo. Then the Oerlikon's 3 phase line ( Frankfurt-am-Mein) was commissioned in 1891. Copper requirement for per Kw of power transmission is minimum for 3 phase out of all polyphase systems. Charles F Scott (1864-1944)was then working at Niagra Project as assistant to Tesla. Considering the substantial copper saving and threre by money, Power company director board put great pressure on Scott to invent a solution to convert 2 phase to 3 phase ( as the two phase generators were already in place)so that power can be transmitted by 3 phase line. Scott made the brilliant invention- take out 3 phase supply from 2 phases using two transformers in 1894 and the first T transformers were commissioned in 1895-96.
Today T transformers are used in electric traction as track side supply transformers as also to feed furnace transformers. Most reputed are the 100 MVA 220/27.5-27.5 kV modified Scott connection used by Japanese for their Shinkansen (high speed trains) lines first commissioned in mid 1960's. Many other countries ( including India) are also using Scott connected transformers for supplying Rail lines.

 

[edison123]

prc - Your historic nuggets are priceless.

Muthu

http://www.edison.co.in

 

[Skogsgurra]

Agree. Some new things that I didn't know about there.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Mbrooke]

T-T is still very common in 3 phase 12kv-120/208 pole pigs. In fact I'd say 2/3 of them are T-T.

https://files.engineering.com/getfile.aspx?folder=73ca5566-92e1-4021-b558-6b4155a5fc3a&file=T-T.jpg

 

[Mbrooke]

Here is a better one from a CSP piglet:

https://files.engineering.com/getfi...50-46a9-a85a-667200269d95&file=4160T-208T.jpg

 

[prc]

Mbrooke- thanks for the info. Can you give some details-why in US this connection is popular and what is the advantage as waross asked? In India we never use it. Though piglets have come around recently here also, they are always single phase- either across two HV lines or between one HV line and ground. The ones used in US are with T connections on both primary and secondary sides, either with zero or 30 degree shift on secondary side. But in rail and furnace application T connection is only on primary and two independent secondaries ( with 90 degree shift between them) are available.
During my student days I always wondered how 3 phase voltage/current vectors are produced from two phases and admired this brilliant idea. I remained a fan of Scott. So jokingly I used to say, when you are pushed to wall (say loosing job) ideas will come out. Scott was president of AIEE during 1902-03 and resigned from Westinghouse in 1911 to join Yale University as Electrical engineering professor. On the 50th anniversary of AC transmission in US (ie 1936) he along with C C Chesney ( who was General Manager of GE Pittsfield for many years) wrote a 8 page paper for AIEE, "Early History of the AC system in America) In this he has elaborated the struggle they had go through for the acceptance of AC as both Edison (in US) and Lord Kelvin (in Europe)-great legends of those days- were vociferously opposing AC and advocating only DC for transmission and household use. He also narrated with details how William Stanley developed transformers in 1885 independently without knowing the patent applied by Ganz engineers in Budapest

 

[Mbrooke]

In the IEC world my understanding is that T-T is none existent as is open/closed delta secondaries. Basically anything outside of delta-wye grounded and the occasional Ygr-Ygr is unheard of in MV-LV.


In the US you have a lot more variants. One reason for delta secondaries is that historically industrial applications were 240 and 480 delta ungrounded, you could run the system with a phase grounded down and still have power. Similarly, you could still provide power via open delta-open delta- you could loose one transformer in a 3 phase bank- cut it out- and still have 3 phase power while a new unit was on its way. Similarly many overhead lines in the US were originally 2400 and 4800 ungrounded delta. You could still provide power with a phase grounded down. In short ungrounded power and delta-delta was all the rage and it worked well for its time- especially when reclosing was a lot more complicated and transformers were failure prone. The exception was 120/240 feeding homes. US Code requires that any neutral distributed and used for load be grounded down so you have to have a grounded system in homes.


Further, Delta-delta with the secondary center tapped allowed for 120 volts for lighting while giving 3 phase power for motors- this again made delta secondary attractive. As such shops, businesses, ect were fed delta-delta 120/240 3 phase 4 wire.

If someone had a lot of single phase and little 3 phase (say a gas station with only one or two 3 phase condensers or homes with the first AC units), you could set down a 50kva single phase pole pig and a smaller 15 or 10kva single phase pig for an open delta 120/240 3 phase 4 wire. In fact a POCO could set down a 100kva pig with a 25kva pig, the system could feed several 120/240 single phase homes and a 120/240 3 phase business. Played right it was economical and done frequently. A no brainier as most transformers were single phase and could be connected any way and "paired" any way no restrictions.


Then two things (discoveries) happened:

As load increased POCOs converted 2400 into 2400/4160Y and 4800 into 4800/8300 by adding an MGN. This allowed the same pole pigs to be re-used. However, a discovery was made over the years: wye grounded primary closed delta secondary transfomers became zero sequence units and as such banks would blow fuses on faults. Ungrounded wye-delta solved this, but latter that was also axed out by some POCOs as primary voltages increased.

Second it started to be known that ungrounded systems were actually less reliable then grounded systems in the long run. An intermittent, striking or arcing single phase fault in an ungrounded system often produced transient over voltages which damaged insulation. As such this forced people to move away from ungrounded systems. That and the fact most ungrounded systems ended up with standing ground faults anyways: the ground detector lights were ignored until they burnt out. Then forgotten.

Latter on another issue came about: ferroresonance, especially above 15kv. As such many POCOs banned delta primary and ungrounded wye primary in their operating practice only allowing grounded wye primaries. With the primary grounded wye, the secondary has to be Y as well not to have issues. And thus some POCOs now only give:

120/240 1 phase 3 wire

120/208Y 3 phase 4 wire

277/480Y 3 phase 4 wire

347/600Y 3 phase 4 wire (Canada)

Some wye only POCOs make an exception for Y grounded-open delta in that technically it does not conflict in any way (no ferro, no fuse blowing, ect).


Which leads us to T-T. Ferroresonance is unlikely at 15kv, and impossible at 5kv with typical system designs. Thus POCOs with extensive 15kv and under infastructure may chose to accept the small ferro risk associated with delta primaries. For 3 phase Y secondary power POCOs have the option of either 3 single phase pigs or a 3 phase pig. 3 phase pigs can technically be built:

1. 3,4 and 5 limb core

2. 3 single phase cores in one can

3. 2 single phase cores in can wired T-T.

However, here are some cons 1 and 2:

1. 3 limb units in wye primary can cause inductive tank heating with a blown fuse. 5 limb is a better solution, but more material. As such delta 3 limb is your friend for cost and weight over 4 and 5 limb.

2. 3 single phase cores in one can are the most ideal if you have a wye primary, however this is the most expensive option. If you have a delta primary a 3 limb will do the exact same thing as one set of 3 singles.

Now, we can compare a delta 3 limb with a T. The T when compared to the 3 limb is smaller and lighter, winning over all other 3 phase options. A 3 phase unit is easier to install, and looks better on a pole then 2 or 3 single phase units. No brackets required for example. Further T can easily be built in both grounded and ungrounded secondaries covering both 3 and 4 wire customers.

T may not be ideal for heavy industry or many 3 phase motors, but it is ideal for general 3 phase light and power customers.


End result being that utilities with 5 and 15kv class systems can obtain great economy by using 3 phase T-T units over all other options.

http://www.powerpartners-usa.com/wp...three-phase_t-t_product_spec_sheet.pdf?x39528

I wish I knew more on the subject, but this is my synopsis of how and why T-T is used by some POCOs.

Here is the 3 core version (Y-Y)

http://www.powerpartners-usa.com/wp...e-phase_triplex_product_spec_sheet.pdf?x30412

As you can see because the risk of ferro goes up above 15kv, T-T and delta is generally not offered due to POCOs avoiding ungrounded primary connections at higher voltages.


Now in the IEC world 22 and 33kv Dyn11 is all the rage, but my understanding is that because the units are typically larger (500kva) and feeding many more customers of wide variety (resi and commercial) it is unlikely that a transformer bank will encounter loading below 5 or 10%. The load will dampen ferro on any blown fuse event.

 

[prc]

Thanks a lot Mbrooke for your learned exposition!

 

[Mbrooke]

PRC, no thank you I learned a lot from you. Also Edison was right about DC being superior- but thats another thread.

 

[marks1080]

Thanks prc and Mbrooke!

 

[edison123]

Thanks Mbrooke.

I was right?

Seriously though, my dad's master's degree thesis was proving how DC was the better option for transmission. After over 50 years, India had its first HVDC transmission.

Muthu

http://www.edison.co.in

 

[waross]

Also Edison was right about DC being superior- but that's another thread.

Did you mean Tomas Alva Edison?
Does your statement hold up when the technology available to Tomas Alva is considered?
I understood that one of the disadvantages of DC for transmission bak then was the low limit of maximum voltage due to commutator flash over.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Thanks everyone for the informed responses. I have just learned more than I suspected in regards to "T" transformers.
I was not aware that the first use of "T" transformers was for the three phase transmission of two phase power.
Fascinating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Yup- Edison. My statement is debatable for the late 1800s, but who says that Edison didn't look to the future as Tesla did? DC is superior in many ways. AC transmission has significant limitations. But the reason why AC won is due to the lack of high frequency switching and rectification at the time.

For example, he considered its sustainability long before this became a popular issue. “We are like tenant farmers chopping down the fence around our house for fuel, when we should be using Nature’s inexhaustible sources of energy—sun, wind, and tide,” he remarked in a 1931 conversation with Henry Ford and Harvey Firestone. “I hope we don’t have to wait until oil and coal run out before we tackle that.”

https://spectrum.ieee.org/green-tech/buildings/dc-microgrids-and-the-virtues-of-local-electricity

This article mainly champions for microgrids, but DC is still the best solution even on a continental level.

 

[marks1080]

Mbrooke - I was kinda hoping you'd start another thread

 

[Mbrooke]

Oh- my mistake then. Never mind.

 

[waross]

I second marks1080's suggestion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

No one is going to fix how expensive it is to step up or step down dc anytime soon. It is tolerable for microgrids due to the losses being less painful than any mitigation. Move any substantial amount of power and you got to move it a long ways 300-400 km to break even with AC transmission.



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