Backenergize/Backfeed Step up transformer 1

[Myz3]

Have done some reading for two winding transformers that energize from lower voltage side of power transformer (delta-Y) will see issues at lower voltage side, namely higher inrush current, high neutral current, have higher voltage drop. Some solutions were should energize with full winding by increasing to max tap, and employ controlled switching/POW to reduce inrush current.

Having to check back energize/backfeed step down transformer(sorry for the confusion of the title), particularly AutoTransformer 275(Yn)/132(Yn)/33(D)kV. Energizing from TV (tertiary voltage) up to HV/LV I believe has not been mentioned so far. What are the concerns to be highlighted(are they the same as two winding transformer)? And what ways to mitigate?

 

[waross]

Problems arise when a delta/wye transformer is back fed.
With a wye input and a delta output you have a choice of two possible effects.
1. Overheating if either voltages or phase angles are not equal. (Wye neutral connected.)
or
2. Switching transient overvoltages. (Wye neutral floating.)
Case 2 may be mitigated by POW switching.
A three legged core may present phantom delta effects similar to a wye/delta.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

FWIW, in my utility it is an almost universal rule that when planned switching is taking place to energize / place on potential any type of transformer, regulator or phase shifter, the device must be on neutral tap.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

Myz3- I understand your question like this. In a 100 MVA 275/132 kV transformer with 20 MVA 11kV tertiary what are the issues for energization from 11 kV.This is similar to the question-can one energize a 100 MVA transformer through a 10 MVA transformer from LV side.
1) Inrush current as a % of rated current of tertiary will be much higher in this case resulting in voltage dip. Reason-Tertiary inner most winding near to core and hence inrush current is more.

2) Since energization is from LV side, need not worry about overvoltage on 132 & 220 KV terminals from transferred surges from tertiary.

3) Since you are not going to load the transformer, keep the transformer on maximum tap or rather don't allow any part of regulating tap winding floating. This is to avoid part winding resonance on floating tap winding that may cause a dielectric break down there.

 

[Myz3]

Thanks for your reply. Appreciate your time in looking at my question and concerns.

I'm quite new to transformers, only a handful year of experience. Figured maybe to point out that the HV & LV (Wye) are solidly grounded while Delta has neutral through zigzag transformer.
Also during backenergize, the transformer won't be loaded (You guessed it right prc) and the HV side would not be connected to grid.

Problems arise when a delta/wye transformer is back fed.
With a wye input and a delta output you have a choice of two possible effects.
1. Overheating if either voltages or phase angles are not equal. (Wye neutral connected.)
or
2. Switching transient overvoltages. (Wye neutral floating.)
Case 2 may be mitigated by POW switching.
A three legged core may present phantom delta effects similar to a wye/delta.

Is this for two-winding transformer? I'll need to take a look at phantom delta effects.

FWIW, in my utility it is an almost universal rule that when planned switching is taking place to energize / place on potential any type of transformer, regulator or phase shifter, the device must be on neutral tap.

At this point, in my utility is also doing the same, where the device is on neutral tap. Have heard that in middle-east, they would adjust tap to highest tap to reduce inrush currents (but I think this applies to two-winding transformers, not sure about autotransformers).

Myz3- I understand your question like this. In a 100 MVA 275/132 kV transformer with 20 MVA 11kV tertiary what are the issues for energization from 11 kV.This is similar to the question-can one energize a 100 MVA transformer through a 10 MVA transformer from LV side.
1) Inrush current as a % of rated current of tertiary will be much higher in this case resulting in voltage dip. Reason-Tertiary inner most winding near to core and hence inrush current is more.

2) Since energization is from LV side, need not worry about overvoltage on 132 & 220 KV terminals from transferred surges from tertiary.

3) Since you are not going to load the transformer, keep the transformer on maximum tap or rather don't allow any part of regulating tap winding floating. This is to avoid part winding resonance on floating tap winding that may cause a dielectric break down there.

Yes, you understood my question. I can't tell if my question is similar to 'energize a 100 MVA transformer through a 10 MVA transformer from LV side', as I can't wrap my head around the difference in autotransformer and two-winding transformer in terms of the 10MVA from LV side. Was this mentioned in this forum before? will have a look at it.

1) Since tertiary windings is sourced from distribution system, high inrush in tertiary winding would cause voltage dip to distribution. Understand for EHV, voltage changes is usually local, so if adjacent substation is far (electrically/physically), the distant adjacent substation would not experience the voltage change. Now would this apply to distribution, given long distribution lines?

2) Phew, I understand it should be the same (no transferred surges to 132/220kV) if a second autotransformer is energized parallel to first autotransformer at 33kV side.

3) Curious to know if there are methods to keep from regulating tap winding floating?

 

[prc]

1)If you are serious with transformers, strongly suggest to have a book like "Transformer Engineering by Dr S V Kulkarni " and study. As I understand,this forum generally does not support students or help in home work problems. The other day, when I mentioned in another thread that transformer copper losses will go up with increase in temperature and overheat, an expert over ruled saying it is not correct as increased temperature will increase winding resistance and hence transformer current will come down! It is difficult to explain.

2) When a 100 MVA is energized from HV say 132 kV, the current will be 5-10 times the full load current ie almost 1000 MVA for a few seconds. So if you are charging through a 10 MVA from 33 kV grid, say 33/132 kV it can over load and damage 10 MVA. In your case, tertiary seems stabilizing delta and its rating may be 20-30% of HV/MV rating. Being innermost winding means, inrush current will be higher still as times of tertiary rating it may be OK.

3) To avoid,floating tap winding, energize at maximum tap ie full regulating winding in circuit.. Frankly, this is not done to reduce inrush current. Of course it will reduce slightly. Attached paper may be interesting and clear some of your queries.

4) Let me learn from you-How you are putting that "quote" portion in the text. I could not find the correct " switch"

 

[crshears]

Quote trick:

>highlight text to be quoted, enter Control + C to select

>when composing reply, type the word "quote" with square parentheses on either side of it, then behind it click Control + V to copy text [editing as needed], then type "/quote" with square parentheses on either side of it

>use Preview to confirm the results will look as you intend before clicking on Submit Post

>those are the steps I followed to create this:

Let me learn from you-How you are putting that "quote" portion in the text. I could not find the correct " switch"

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

Thank you crshears. What you mean by square parenthesis? ["quote"] quoted matter["quote"]? or [quote: quoted matter] Some how it is still not coming Ok.Sorry being clumsy.

 

[crshears]

Like this [ quote ] but without the spaces inside before the quote and like this [ /quote ] but without the spaces inside after the quote

In other words [ quote ] at beginning and [ /quote ] at end, but without the spaces

I can't type it exactly character for character without the spaces; if I do the text auto formats to

at beginning and

at end



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

Thank you crshears! I got it.

In other words

 

[waross]

Looking at the icons across the top of the reply panel you will see a cartoon character with a text bubble.
Hover on the character.
You will see the word Quotes.
This will open a dialog box with the word "Who".
Whatever you enter here will replace the colline ) in Quote: in your quoted entry.
Click on enter.
You may then use Control V to paste your clipboard into the quote.
[ignore][ignore]

an example.

[/ignore][/ignore]

an example

Your cursor will be within the quote and must be re-positioned.
You may edit either your header or the body of the quotation.
I have used the [ignore] command twice so as to show the use of the ignore command.
If you use the open command, [ignore ] without the close command, it will display as normal text.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Hello prc.
While my work with transformers has been mostly as a user,an installer and as a trouble shooter, yours has been as a builder and tester.
My experience testing may be days compared to your years.

an expert over ruled saying it is not correct as increased temperature will increase winding resistance and hence transformer current will come down!

Was that me?
May I tell the whole story and ask you how my experience may affect large power transformers?
Thank you in advance.
I gave a group of students an assignment:
Verify the rated Percent Impedance Voltage of a transformer by testing a transformer.
The transformers available in the lab were small dry type transformers.
Memory fails but somewhere between 500 VA and 2000 VA.
The Percent Impedance Voltages determined by testing were all lower than the nameplate values by an unacceptable error.
How much error is unacceptable?
When the students complain that they are not getting good results I consider the error to be unacceptable.
Why.
Part two of the lesson.
I understand that factory testing of % imp is done on transformers at operating temperature.
The students had been testing transformers at room temperature.
I then had the students check the cold resistance of the transformers with an Ohmmeter and calculate the increase in resistance that would occur when the transformer was at rated temperature.
With this adjustment, the re-calculated % imp was acceptable but not exactly as the nameplate rating.
The small remaining error was consistent with the ratio between DC resistance and AC effective resistance.
I believe that the students learned much more and retained much more than if I had just covered % imp and testing in a lecture.
Any comments that you care to add in regards to power transformers are welcome, prc.
I know that comparing these tiny transformers with power transformers is not apples and apples.
Respectfully.


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

 

[waross]

cr, Check out the icons across the top of the reply panel. Note: for some of these commands you have to scroll down and down to find results.
Going right from the Image icon are the following Quick Commands.
Chain Link: Enter a URL. Link will be displayed on the page. The word Link may be edited to the Title of your choice.
Happy face: This brings up an Emoticon menu. (Scroll down.)
Omega: This brings up 10 pages of extended characters and an entry to the Unicode Character Table for a potpourri of thousands of Emoticons and Special Characters.
Cartoon Characters with text bubble: Quotations.
Next to the right: Coding: This allows sample code to be entered but not executed.

10 REM help! I'm stuck in a loop
20 go to 10

Box tied with ribbon: Spoiler:

Spoiler

Hover the cursor to read the [ignore]spoiler.[/ignore]

Next to the right: Clean: Deletes highlighted text.
?: Help: Scroll down. This brings up many but not all commands and shows usage, formats and examples.

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

 

[prc]

1) Thank you very much Bill for your kind guidance to this computer illiterate.

2)

Was that me?

No sir! You are a super expert and we are learning from you something new every time you write.

I was only mentioning misconceptions that people hold regarding some common issues. Any one who went through the experiment that you gave, will never forget that basic !! Wonderful way to learn.


3) Transformer load losses consists of I²R loss in copper winding + stray losses in metal (steel) + eddy losses in copper winding. First part- increases with increase in temperature.Second +third part (consists of 10- 30 % of load loss )comes down with increase in temperature. Reason: First Part-Winding I does not depend on winding resistance. Second and Third part- With increase in temperature, steel resistance goes up and hence eddy current induced comes down and net I²Rloss in metal comes down. Over all, load loss goes up with temperature. Guaranteed load losses corresponds to a temperature of 75 C in IEC world and 85 C in IEEE world. In factory losses will be measured at ambient temperature and will be corrected for above reference temperatures using the correction formula given in standards.