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Buried Tertiary Winding
2

Buried Tertiary Winding

Buried Tertiary Winding

(OP)
What size in MVA is a buried terciary winding?.

For loaded delta there is a rule of thumb of 1/3 of the MVA rating. What is the rational for that?

Thanks

RE: Buried Tertiary Winding

Could be that 30 % is a typical third harmonics magnitude.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Buried Tertiary Winding

The main reason is that there can be very high zero-sequence currents in the delta for ground faults on the wye-connected systems.  If the delta winding is too small, it can be damaged during a ground fault.  

 

David Castor
www.cvoes.com

RE: Buried Tertiary Winding

Looks like IEEE is trying to standardize this. See page 11 of http://www.transformerscommittee.org/meetings/F2010-Toronto/Minutes/F10-Performance.pdf

CODE

In the event no continuous thermal duty for the stabilizing winding can be established from
the user's specification, the manufacturer shall design the stabilizing winding considering
the circulating current resulting from a full single phase load in the largest main secondary
winding (for more than one secondary winding), and infinite bus supply on the primary
winding. This is taken to be 33.3% of the transformer rating.  

I also remember reading somewhere that early transformers needed a larger stabilizing winding because the core steel created significant harmonics.  As core materials improved there was less need for a large tertiary.  The normal currents in our 115/230 kV autotransformer tertiaries are quite low. Unfortunately I can't find where I read it.  

RE: Buried Tertiary Winding

2
cuky,for loaded tertiary, tertiary rating shall be the load or 1/3 main winding rating which ever is high.For buried tertiary, minimum 1/3 main winding.

The rational for above is that in case of a line to ground fault on primary or secondary,ampere turns in tertiary winding due to fault current in tertiary, will be 1/3 of that in main winding.In order to get sufficient mechanical withstand strength in winding 1/3 MVA is assigned to tertiary.In case you specify 1/3 MVA  only there is a chance for some manufacturers adopting very high current density in tertiary( thin copper section, thereby jeopardising the mechanical strength of teriary winding.)

The above situation can arise also when only one secondary phase is loaded to its full rating.Since tertiary voltage per phase will be root 3 times more than primary or secondary( for a 1:1:1 voltage trf)  we can find that current in tertiary will be 0.33 pu to get same MVA to compensate secondray AT.In this case tertiary require continous rating while for the earlier case it require only momentary rating to withstand forces.So if the manufacturer has capability to build short circuit withstand strength, he can go for lesser rating also.

MVA required for tertiary to take care of harmonics in excitation current of transformer is negligibly small today ( yesterday also !)It never exceeded 2-3 %( today <0.5%) of full load current.
So there was no need for larger tertiary in early days as excitation current was never near to 1/3 full load current,anytime in the history of transformers.

Auto-transformers can manage with smaller tertiary because equivalent rating of auto transformer winding is smaller than a two winding transformer of same line MVA.
 

RE: Buried Tertiary Winding

(OP)
Thanks friends,

If I understood well, the tertiary winding should be rated to handle 1/3 of the MVA regales if is for loaded or unloaded application.

Regarding the alternative of using the transformer tank in lieu the tertiary winding, any ideas associated with harmonic mitigation, overall transformer performance and cost impact
 

RE: Buried Tertiary Winding

Learned two things: 1) not my table  2) tertiary winding has many duties.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Buried Tertiary Winding

With three phase, 3 limbed construction (ie up to around 150- 200 MVA, core type transformers)we can eliminate stabilising tertiary( ie tertiary provided only for stabilising neutral and not for loading)winding as the tank will act as a virtual tertiary.Slight heating of tank surface will happen with single phase loading.But advantages are reduced cost( 8-12 %) lower losses, better short circuit withstand strength etc.Many countries ( eg India) have eliminated stabilising tertiary in that range of transformers for better reliability. There are countries ( eg Italy and Turkey)where tertiary eliminated in still larger units eg 400 MVA 400kV auto-transformers.
Harmonic mitigation is no longer a problem. In olden days, 3rd harmonic currents in the excitation current used to go down to earth through neutral and interfere with tele communication circuits. Today's wireless communication system with better filters are least affected by such currents.Magnitude of excitation current also came down with better steels,though percentage of harmonic currents in excitation current has gone up due to improved W/kg figures for CRGO.

RE: Buried Tertiary Winding

(OP)
This is an excellent explanation.

Thanks

RE: Buried Tertiary Winding

I've read the reasons above, and find most of them a bit to complicated for a simple concept. The individual phase capacity for a three phase transformer is 1/3 the nameplate. You want to match your tertiary to the phase value so the transformer is undamaged when supplying a single phase load at this full capacity. And since damage curves are based on capacity, you then get to use the same damage curve for the tertiary that you do for the other windings for SLG faults.

RE: Buried Tertiary Winding

(OP)
Hi Stevenal,

For transformer damage curve, I understand that the delta winding LG in the delta is 0.58 pu relative to the wye winding (see table below).

How this equate and impact the delta tertiary winding size of 1/3 of total MVA for unload or loading application?


                                     

RE: Buried Tertiary Winding

The single phase capacity of a wye connected transformer or generator winding is 1/3 the rated capacity of the winding.
The single phase capacity of a delta connected transformer or generator winding is 2/3 the rated capacity of the winding.
Look at the single phase rating as a ratio of the three phase rating of a three phase generator converted to single phase with a delta or double delta connection.

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

RE: Buried Tertiary Winding

Transformers can use internal reactors to reduce the short circuit values in the tertiary.  Is it just easier or more reliable to havea high rated tertiary than add a reactor?

It seems kind of weird to talk about single phase loading a power transformer.  Are we really only talking during single phase faults?Does anyone load power transformers on a single phase basis? Our typical ground fault protection in distribution would trip at about 1/4 of rated capacity for a single phase load.  Our transmission banks usually balanced withing a few percent.  

RE: Buried Tertiary Winding

Transformers should be able to withstand phase loss without overloading. I see nothing weird here.

The 1 and 1/sqrt(3) in the chart are pu currents. And they are not winding currents, but line currents (where you put the S&C fuse they're selling). To get the pu  VA, you must multiply by the voltage in pu times the number of involved windings. For the three phase fault, multiply by the line to ground voltage, 1 pu, times 3 windings. For the l-g fault, this is the line to line voltage, or sqrt(3) pu times 1 winding.  3 and 1 pu. If you want the three phase case to be your base, divide both by three to get back to 1 pu for three phase. 1 and 1/3 pu.

RE: Buried Tertiary Winding

Let me put in a simpler way.It is true that in power transformers, rarely single phase loading will come.It is the single phase L-G fault on secondary that  will be critical to cause 1/3  fault current to flow in tertiary in a 1:1:1 transformer.Let us say, phase fault current on star  secondary is 3I, line to neutral.This AT will be compensated by I circulating in delta tertiary + 2I flowing in the faulted star  phase winding of primary.2I will be coming from the other two phases of primary, I each ( these AT in un faulted primary phases  will  be compensated by the I circulating in closed delta winding.) In transformers, at any moment, total AT in a phase shall be zero.

 1/3 MVA of seconday is compensated by 1/9 rating of tertiary + 2/9  line rating of primary.So 1/9 line rating is used for tetiary phase winding. The damage curves are not relevant here, as it is not time that is not important.The mechanical damage to winding will happen during the first 1-2 cycles of fault due to dynamic forces from assymetrical current wave.

Waross,I did not follow your statement.Whether delta or star, the per phase rating of winding is 1/3 line MVA.

It is a  practice to use reactors ( either inside the delta or external to closed delta ) to increase the impedance to other windings,there by reducing the fault current inside tertiary winding.But some utilities object to it considering it as a weak link.

During the first 30 years of transformers( remember it was patented in 1885)nobody thought it necessary to provide a stabilising delta tertiary in star/star transformers. Concept of symmetrical components developed in US ( it was the longest paper -more than 100 pages- ever published in AIEE)made engineers to go for stabilising tertiary and during  initial years the tertiary rating was only 5-15 % of line MVA to take care of third harmonics in excitation current. Failure of tertiary during LG faults made engineers to go for 1/3 line MVA later.But French engineers were reluctant to follow US and UK practices and went ahead with star/star transformers without tertiary most of the time.

Last one story.One of the Indian utility ordered some 50 MVA 220/110 kV 3 phase  auto-transformers on GE,Canada in early    1960's.Specification details were sketchy. Indian engineers were sure that stabilising winding will be provided in such large transformers- those days it was a very large unit.Canadian manufacturer, reading spec between lines, did not provide tertiary.Transformers were put in various substations.In one area, soon after commissioning the transformer, telephone cables started to fail.( manual switching days of telephony)Problem got solved by replacing the transformer with another one with stabilising tertiary!

RE: Buried Tertiary Winding

"It is true that in power transformers, rarely single phase loading will come."

And when this rare event does come, it should not cause damage.

"The damage curves are not relevant here, as it is not time that is not important.The mechanical damage to winding will happen during the first 1-2 cycles of fault due to dynamic forces from assymetrical current wave."

IEEE C57.109 disagrees. The mechanical and thermal damage are expressed on time/current curves, with the mechanical damage dog leg stretching from 2s to 8s. If damage from a through fault were to occur in 2 cycles, our 3 cycle breakers with 1/2 cycle relays are pretty worthless.

 

RE: Buried Tertiary Winding

When a single phase load is connected to a delta secondary, The phase that the load is connected to will supply 1/2 of the current. The other two phases may be thought of as an open delta and will supply 1/2 of the current. This works out to 2/3 of the transformer KVA rating.
A wye winding with a load connected from line to neutral will supply 1/3 of the transformer KVA rating.
 

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

RE: Buried Tertiary Winding

(OP)
Hi Stevenal,

It is not intuitive to me how the damage curve of the main transformer winding is related with the minimum size of the tertiary particularly went there is not official guideline from In this subjet. The issue is more uncertain to me after reading that the IEEE Transformer Committee is studying sizing  the tertiary winding  around 20% of the total MVA.

Regarless if the size is 20% or 30% of total MVA for loaded or unloaded winding application is significant large that will have impact on the total price of the unit and is associated with the stresses develop during fault scenario.

RE: Buried Tertiary Winding

Perhaps I misled by stating the damage curves are for windings. The damage curves are really for transformers, predicting the time/current where any damage will occur based on the MVA rating of the transformer. Think of the weak link of a chain. If 1/3 MVA tertiaries have been used successfully in the past with the existing curves, I predict that lowering it to 20% will make the tertiary the weak link that will fail below the curves used now for SLG faults. Don't know if Mr. Vijaian considered this in his calcs.

Waross,
"Buried" means the connections are not brought out. There is no way to put a single phase load on the buried delta.  I'm speaking of single phase loads and faults on a wye connected winding.  

RE: Buried Tertiary Winding

Hello Steven. Thank you for the correction.
Yours
Bill

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

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