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Rising the Xfmr Tap Causing HV side to be down
8

Rising the Xfmr Tap Causing HV side to be down

Rising the Xfmr Tap Causing HV side to be down

(OP)
Hellow, every body
In our dispatch center i have seen that when the dispatcher raises the tap form the HV side, the following happened:
1.    The voltage in the secondary increased (Which is OK and expected)
2.     The voltage in the primary (HV side) reduced ???

(N1/N2)=(V1/V2)
V2=(N2/N1)*V1

From this relation we can see that the primary voltage should not change?

So, Why does the HV side drop?

the dispatcher was not able to answer my qestion!!

RE: Rising the Xfmr Tap Causing HV side to be down

That happens because of the increased VAR flow from the source on the primary of transformer to the load on the secondary side.
The increased VAR has to travel through various inductive components that exist such as the generator, transformer, OHL/Cable etc. between the source and the transformer primary terminals.
Farther the source from the transformer location, greater the drop in the voltage at the primary terminals.
The formula you mentioned is right and can be applied in a straight forward way only if the source side is rigid (infinite source).

RE: Rising the Xfmr Tap Causing HV side to be down


SMB1, maybe you could re-check your facts. Raising the high side tap increases the transformation ratio. This should not affect the high side voltage very much, except for the incremental voltage drop due to the additional var flow, as rragunath suggests. But the low side voltage should be lower, not higher. After the tap change you have a new transformation ratio, with more step-down than you had before.

RE: Rising the Xfmr Tap Causing HV side to be down

(OP)
Thanks rraghunath and SidiropoulosM for your reply
and now it is clear for me.


 

RE: Rising the Xfmr Tap Causing HV side to be down

SidiropoulosM,

By raising HV side taps, as you say the transformation ratio is raised. Consequently, the secondary side voltage will also rise.

RE: Rising the Xfmr Tap Causing HV side to be down

Raising the high side taps would lower the low side voltage.  As you raise the high side tap you are raising the nominal high side voltage while leaving the nominal low side voltage the same.  All else equal, raising the high side nominal voltage will leaving the actual voltage the same reduces the low side voltage.

RE: Rising the Xfmr Tap Causing HV side to be down

Suggestion to SMB1 (Electrical) Aug 31, 2003
Hellow, every body
In our dispatch center i have seen that when the dispatcher raises the tap form the HV side, the following happened:
1.    The voltage in the secondary increased (Which is OK and expected)
2.     The voltage in the primary (HV side) reduced ???

(N1/N2)=(V1/V2)
V2=(N2/N1)*V1
///V1 primary stays constant, i.e. V1=constant
N2 secondary turns stay constant, i.e. N2=constant
N1 primary turns were raised, e.g. 2.5% totaling to N1raised=N1 + 0.025 x N1=1.025 x N1
Therefore,
V2=N2 x V1 /(1.025 N1) = (N2/N1) x 0.09756 x V1
i.e. the secondary voltage is 0.09756 lower.\\\
From this relation we can see that the primary voltage should not change?
///Yes, the primary voltage stays constant since there was not change upstream in this voltage.\\\
So, Why does the HV side drop?
///It does not drop. It stays constant.\\\
the dispatcher was not able to answer my qestion!!
///It may have been too easy for him. Therefore, you have to try harder in electrical engineering.\\\

RE: Rising the Xfmr Tap Causing HV side to be down

(OP)
jbartos:Pls. see rraghunath post

1. The Tap is in the HV side
raising the tap in the HV side reduce the turns (N1)
so, as a result:
                  - V2 increase
                  - V1 reduced because of the voltage
                    drop caused by the increas of the I1

Now, to see this let us apply the turns(N) Amp(I)
N1 X I1 = N2 X I2

N1 decrease and I1 shuld increase
I1 is nothing but the flow of the reactive power as rraghunath said this flow raise the voltage drop in the HV side.
 
   


RE: Rising the Xfmr Tap Causing HV side to be down

The tap position indicator (TPI)is arranged in such a way that higher the tap position indicator shows, less is the no. of turns included in the HV side. So for a given voltage on HV side, raising the tap (as indicated by TPI)will raise the LV side voltage. Conversely, to maintain a constant LV side voltage (which is the normal goal), higher tap no. is selected for a falling HV side voltage and a lower tap no. is selected is selected for a rising HV voltage.

RE: Rising the Xfmr Tap Causing HV side to be down


Aside — Transformer labeling should clearly reflect tap characteristics.
  

RE: Rising the Xfmr Tap Causing HV side to be down

Edison123: Please do a manual calculation, do a power flow digital simulation if you must, but do come to the correct conclusion: The low side voltage is LOWER after raising the high side tap.

This is actually quite simple. If you still don't get it, let me know and I'll post a simple example.

RE: Rising the Xfmr Tap Causing HV side to be down

sidiropoulosM,

Yes, would you please post your simple example ?

RE: Rising the Xfmr Tap Causing HV side to be down


Example:

We have a 500/230 kv transformer. The high side has taps at 475 kv, 487.5 kv, 500 kv, 512.5 kv, 525 kv. The high side is currently set at the nominal tap of 500 kv. The high side system voltage is 500 kv. The low side voltage is 230 kv.

Now we disconnect the transformer from the system and we change the high side tap to 525 kv. The transformation ratio has changed:

from 500/230=0.460

to

525/230=0.438

We reconnect the transformer to the system. The high side system voltage is still 500 kv. This voltage does NOT change, it's determined by the upstream system conditions. The low side voltage is now 500*0.438=219 kv.

The above should be clear to everyone. If you still doubt it, I can post the results of a power flow simulation from my office tomorrow.

RE: Rising the Xfmr Tap Causing HV side to be down

Sidiripoulos,

I appreciate your post and understand it well. Could you please post the tap position nos. in your transformer each of your HV taps ? (for example for 475 KV, what is your tap no. ? for 525 KV , what is you tap no ?). The normal practice I have seen is 575 KV will be named tap no. 1 and 475 KV will tap no. 5. So as you increase the tap nos., the transformation ratio will rise raising the LV volts. Is it possible, in your place, the tap positions are numbered in a reverse manner ? Just tyring to understand.

RE: Rising the Xfmr Tap Causing HV side to be down

Seems to be a matter of semantics.  I have seen off-load taps designated A-B-C-D-E or 1-2-3-4-5 going from higher to lower high side voltages of 105% - 102.5% - 100% - 97.5% - 95% of nominal.  I would say that raising the taps would be changing them from a lower to a higher voltage, though, not changing the tap position from a lower to a higher letter or number.

RE: Rising the Xfmr Tap Causing HV side to be down

jghrist,

The idea behind providing taps in the HV winding of a power transformer is to maintain a steady LV voltage. So it is a logical step to name the tap nos. in a manner that implies raising the tap (from higher HV volts to lower HV volts)would mean raising the LV Volts. This I believe is the international norm. The case cited by you (105% as tap no. 1, 102.5% as tap no. 2 etc) also follows this convention. So, how could raising the tap would mean lowering the LV Volts ?

RE: Rising the Xfmr Tap Causing HV side to be down

Focus on understanding the physics first. You can then number the taps any way you like.

RE: Rising the Xfmr Tap Causing HV side to be down

SidiropoulosM,

Thanks for your advice. Refer your first post.

First, you claim "Raising the high side tap increases the transformation ratio". Then you state "But the low side voltage should be lower, not higher ....."

Care to follow your own advice?

RE: Rising the Xfmr Tap Causing HV side to be down

(OP)
edison123,
I think we all know agree that raising the tap changer HV side reduces the no. of turns in HV side and increases the ratio.

RE: Rising the Xfmr Tap Causing HV side to be down

Members,...
if appropriate in this context, let me put my views by practicle example of a transformer The transformer is rated 13.2 kV /480 V. (Almost the same has been mentioned by jghrist)

Volts   %    TAP
13860 (1.05)  1/A
13530 (1.025) 2/B
13200 (1)     3/C
12870 (0.975) 4/D
12540 (0.95)  5/C

As in the above case.. increasing the tap on the HV side is to increase the LV voltage...
In otherwords the transformer should deliver a nominal voltage of 480V if kept on Tap 5/C prrovided the input votlage is 12540 V. I feel the same standard is being followed everywhere and i do agree with edison123.

Regarding the provision of taps on the HV side..it is to have a better control over the lower voltage /load side. As i said above.

regards,
nag400

RE: Rising the Xfmr Tap Causing HV side to be down

Thanks SMB1 and and nag400. It is the patronizing comments in this post that got my goat.

RE: Rising the Xfmr Tap Causing HV side to be down


Nag400: if by "increasing the tap on the HV side" you mean "going from 1A towards 5C", then you are correct. If you mean "going from 12540 towards 13860", then you are incorrect.

Remember: Increasing the high side tap (in the direction of increasing voltage) LOWERS the low side voltage.

(sorry, I couldn't resist another patronizing comment).

RE: Rising the Xfmr Tap Causing HV side to be down

I think folks are confusing the load tap changer with the deenergized tap changer. SMB1 is using an LTC (remember it is controlled by the dispatcher) The LTC, which can be physically located on either side, is used to regulate the secondary voltage. To do so, it adjusts the turns ratio in whatever direction is needed so that "raise" will increase the secondary voltage. "Raising" the deenergized tap changer to a higher primary voltage setting will reduce the secondary voltage.

RE: Rising the Xfmr Tap Causing HV side to be down

Stevenal,

I think this is a generator transformer connected to the grid. If so, the secondary is connected to, theoretically, an infinte bus. You can't raise its voltage. So increasing the turns ratio and holding the secondary side at a fixed voltage must lower the primary voltage. Practically, a big generating site can influence the local grid voltage but the effect on the primary voltage is still present.

The AVR of the generator will try to compensate for the drop in primary voltage by increasing excitation and raising generator terminal voltage. Current will increase as a result. The infinite bus will absorb reactive power exported by the generator.

RE: Rising the Xfmr Tap Causing HV side to be down

ScottyUK,

SMB1 has mentioned the tap changer is on the HV. I agree with stevenal that it is a on-load tap changer given it is a load dispatch centre

RE: Rising the Xfmr Tap Causing HV side to be down

Sidiropoulos,

Going by your last post, you seem to be catching with this post at last. Keep it up.

RE: Rising the Xfmr Tap Causing HV side to be down

I find it interesting that the consensus on semantics seems to be the opposite of mine.  I think in terms of ±% taps and would say that going from +2½% to +5% would be raising the tap even if the tap position indicator goes from 2 to 1 or B to A.

What if the tap changer has a reversing switch?  A typical OLTC distribution substation transformer in the US will have the tap changer in the secondary with 16 tap positions of 0.625% and a reversing switch to provide ±10% regulation.  The indicator positions will be numbered 1-16 plus N.  If raising the tap means going from a low numbered position to a higher numbered position, then you could be either raising or lowering the secondary voltage.

RE: Rising the Xfmr Tap Causing HV side to be down

Jghrist, thanks for bringing some common sense back to this thread.

Edison123, the only reason we are here is to learn from each other. I have a feeling you still haven't got it.

Remember: Raising the high side tap to a higher voltage setting LOWERS the low side voltage.

If you still doubt it I can post digital simulation results.

RE: Rising the Xfmr Tap Causing HV side to be down

Jgrist,

I wasn't aware there was consensus.

Tap position indicators built to ANSI standards show all 33 positions. Going from lower 16 to lower 15 is done with the "raise" control just like going from raise 15 to raise 16. We know the reversing switch is there, but that is all. To me these are both "raise" operations. Moving a deenergised tap is very different.



RE: Rising the Xfmr Tap Causing HV side to be down

Dear All:

After of read all the opinions of everyone, I think that the most important thing is understand the theory of operation ( according the original post ) of this very simple issue. I think that everybody ones that understand this ( and have a little of experience in field )kwnows what to do if need to move the tap changer. And the less important is if the taps position names are numbers or letters.
Or what do you prefer? All inside or nothing out?

Saludos

RE: Rising the Xfmr Tap Causing HV side to be down

Suggestion: It is clearer to refer to turns, meaning raise the turns number N. Some textbook use this approach. Raising taps is sometimes interpreted as raising turns or adding tap turns.

RE: Rising the Xfmr Tap Causing HV side to be down

Suggestion: Visit
http://www.beckwithelectric.com/lc/controls/apptip/Tip8.pdf
for tap adjustments between two transformers. One must follow a definition of tap raise.

Tap raise is not defined in current IEEE Std 100 "Dictionary".
The closest definition is Tap Change.

RE: Rising the Xfmr Tap Causing HV side to be down

SidiropoulosM,

Thanks for your offer of educating me (digital or not). I am very clear in my mind about transformers and tap changing operations. In my time, I have repaired and rewound transformers up to 125 MVA, 220 KV rating. So, let us stop this bickering in the interest of others. However, if you should feel strongly (!) that any of my posts here with reference to original post are factually incorrect, I will be delighted (?) to hear about it.

Regards,

Kumar

RE: Rising the Xfmr Tap Causing HV side to be down

Suggestion: Visit
http://www.beckwithelectric.com/infoctr/appnotes/App13.pdf
for a tap raise implying the secondary voltage V2 raise.

Reference:
1. Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 14th Edition, McGraw-Hill, 2000,
Section Load-Tap Changing from page 10-27 on uses tap change or tap changing expression, no raise tap or lower tap.

There appears to be no commanding resource/reference for Raise Tap and Lower Tap except the actual tap changing hardware and definitions by the manufacturers.

RE: Rising the Xfmr Tap Causing HV side to be down


Mainstream IEEE transformer standards C57.12.00, C57.12.70, C57.12.80 and C57.12.90 seem to be mum on “voltage trimming” tap details in power transformers.
  

RE: Rising the Xfmr Tap Causing HV side to be down

Dear SidiropoulosM,
The concept is clear as stated earlier , But with regards to the digital simulation you are referring to using any of the standard packages.Let me put an example here.
The things will be as follows.(if i am referring to similar power system analysis packages including the one i have)
In the transformer data -> you enter the minimum tap voltage as (let us say for the same exaple i have stated.)
Min Tap Voltage : 12540 V with tap no as 1 and
Max Tap Voltage : 13860 V with tap no as 5
('coz .... the packages accepts like that and which i am currently working on also has the same way of reprasentation)
Then in the simulation studies to increase the LV voltage the tap to be set will be 1 rather than 5..and hence in this case you are decresing ratehr lowering the tap no. to get higher secondary votlage and vice versa.
But as for as the field conditions are considered... We order them to "raise the tap" to increse the secondary voltage and "lower the tap" to decrease the secondary voltage depending on the grid voltage condition and not the reverse.

RE: Rising the Xfmr Tap Causing HV side to be down

nag400,

thanks for clearing up that "digital" mystery of which I have heard ad nauseum in this post.

RE: Rising the Xfmr Tap Causing HV side to be down

2
Cheer-up Guys!

You have done some wonderful exchange in this thread.  Even a novice Engr in the field of trasnformer operation would very well be enlightened by the 'facts' and 'views' each one of you have shared.

As one of you may have put it, we may all have the solid knowledge regarding the voltage transformation given the nominal tap ratio and the corresponding tap settings.  I guess it really is just a matter of how you 'denote' the increase/decrease in tap.  BTW is there a standard notation for that?

Theoretically, SidiropoulosM, Jghrist et.al., are correct that "Raising the high side tap to a higher voltage setting LOWERS the low side voltage".  Personally, I view it the way Jghrist does, that is +/- taps.  Meaning I denote the nominal tap (say 220kV) as 'nominal'.  Then a '+' in tap of 2.5% (225.5kV) as +1 step (meaning increase/higher tap) and so on...Convesely, a '-' in tap of 2.5% (214.5kV) as -1 step (meaning decrease/lower tap) and so on...However, I could also see the point of Edison123, nag400 et. al., because it seem very convenient that when you need to increase the voltage of the secondary you just increase the tap of the primary (which corresponds to a lower primary voltage tap) and because our PXFs are denoted that way...we have 220kV primary voltage with 17 steps +/- 10%.  Nominal is at tap 9 with 220 kV.  Therefore, tap 8 will be 222.75kV, tap 7 will be 225.5kV and so on, while tap 10 will be 217.25kV, tap 11 will be 214.5 kV.  This sometimes causes some confusion particularly if you're talking with non-technical guys such us the substation crews.  I guess it just a matter of being in the same 'plane' with the person you are talking or dealing with.

Well, good job guys!!!  Let's all keep it up...

BTW, could you guys please share your brialliant ideas regarding phase/series reactor installation...I have found one thread in this Engineering forum but not much have been said.  I'm also interested with the inquiry sent by the originator of that thread...Thanks!!!

RE: Rising the Xfmr Tap Causing HV side to be down


CKent, you've summed it up very well. Your refreshing comment is timely, we needed some fresh air around here. Many disagreements arise from using different definitions and I think this was one of those. You say that some wonderful exchange has taken place in this thread. This is the whole point of these forums. Personally I learn a lot through my participation here, especially from people with field experience (busbar, Dandel, jghrist, edison 123 and many others). Sometimes we overdo the provocative and intimidating comments but we all learn something at the end.

RE: Rising the Xfmr Tap Causing HV side to be down

Edison123,

ScottyUK,

SMB1 has mentioned the tap changer is on the HV. I agree with stevenal that it is a on-load tap changer given it is a load dispatch centre


I don't think I suggested that the OLTC was anywhere but the HV winding! It would be unusual to see the OLTC in the LV winding of a large transformer.

Once again I guess I'm guilty of 'terminology ignorance' - I had assumed that load dispatch centre was a grid control room from where the generation and power flow of the grid is determined, rather than a physical part of the system such as a substation. So apologies for my ignorance and thanks for educating me!

RE: Rising the Xfmr Tap Causing HV side to be down

ScottyUK,

My post you refer didn't imply that you meant OLTC was on the LV winding. I just re-stated SMB1's post. As`a matter of fact, in my post to aslam2000, I did highlight your mentioning the taps are not provided on LV winding due to current limitations.

Hey, what happened to those posts you and I had made to aslam2000. They are all gone.

But you do agree that the transformer in this particular case is not a generator transformer but a step down transformer in a substation (or load dispatch center or whatever), don't you ?

RE: Rising the Xfmr Tap Causing HV side to be down

Edison,

I absolutely agree that the transformer in question is not a generator xfmr, and so the comment about the AVR response doesn't apply in this case.

I stand by my comment on voltage when the ratio is changed:
"So increasing the turns ratio and holding the (HV)secondary side at a fixed voltage must lower the (LV) primary voltage.

Havn't a clue where the posts to Aslam have gone. Wierd, huh?

So, is a load dispatch centre what us Brits know as a bulk supply point or primary substation?

RE: Rising the Xfmr Tap Causing HV side to be down

Scotty,

I fully agree with your statement in blue. BTW, how do you get to do your posts in colour ?

In India too, a load di(e)spatch centre is a primary substation.

Kumar

RE: Rising the Xfmr Tap Causing HV side to be down

Kumar,

Example of colours below, but use square brackets [ ] instead of my ( )

colour on: (blue)

Your text

colour off (/blue)

you can also use

(b) and (/b) to toggle bold text.
(i) and (/i) to toggle italic text.
(u) and (/u) to toggle underline text.


regards,


Scotty.

RE: Rising the Xfmr Tap Causing HV side to be down

I guess I need to state a little more clearly why I claimed this was an LTC. A "dispatcher" to me is someone on the radio, phone, computer, or switch board; not the one wearing the gloves. If he "raises the tap" rather than ordering the operation, sounds to me like he performed it personally. LTCs are equiped with motors and controls and are easily controlled from switchboards or SCADA. Deenergized tap changers are not usually equiped with motors, but must be cranked by hand.

To those who couldn't find standards, this is from C57.12.00:

The neutral position (the position in which the LTC circuit has no effect on the output voltage) shall be designated by the
letter “N” for load tap changers. The raise range positions shall be designated by numerals in ascending order,
corresponding to increasing output voltage, followed by the suffix “R,” such as 1R, 2R, etc. The lower range positions
shall be designated by numerals in ascending order, corresponding to decreasing output voltage, followed by the suffix
“L,” such as 1L, 2L, etc. (this applies to the relationship between two windings of a transformer only, such as the H and
X windings). In the event of system requirements, such as reversal of power flow, regulation of input voltage (LTC in the
primary winding), or any unusual conditions, nameplates shall have raise-lower designations as specified by the user.
This applies to two-winding transformers only.

Me again. Clear enough until they got to the exception. How is input voltage regulated? Does an LTC located in the primary winding used to control secondary voltage qualify for the user designated exception? Kinda silly exception, since the whole standard is subject to user designation override.

Some more IEEE C57.12.10:

6.4 Position indicator
... The position indicator shall be marked in accordance with the following:
a) The normal (rated low-voltage) position shall be located on the vertical centerline of the dial,
preferably at the top of the dial, and shall be indicated by the letter “N.”
b) The raise range, when referred to the regulated (low-voltage) circuit, shall be located on the righthand
portion of the dial. A larger letter “R” (Raise), appearing only once, shall be located in the
right-hand half with an arrow indicating the direction of raise. The sixteen tap positions in the raise
range shall be marked, and a number shall appear opposite at least every fourth position. Number
16 shall be the highest voltage position (see Figure 5).
c) The lower range, when referred to the regulated (low-voltage) circuit, shall be located on the lefthand
portion of the dial. A large letter “L” (Lower), appearing only once, shall be located in the lefthand
half with an arrow indicating the direction of lower. The sixteen tap positions in the lower
range shall be marked, and a number shall appear opposite at least every fourth position. Number
16 shall be the lowest voltage position (see Figure 5).

Later,

RE: Rising the Xfmr Tap Causing HV side to be down

Suggestion: On load tap changers can become involved by having a series transformer applied to the transformer winding, e.g. visit
http://www.waukeshaelectric.com/ref_library/pdf/transformers_loadtap/UZD_Load_Tap_Changer.pdf
for such OLTC
http://www.wilsondencol.co.uk/taps.pdf
for tap changer locations, etc.
http://www.geindustrial.com/products/brochures/dtr.pdf
http://www.geindustrial.com/products/manuals/dtr/dtrguide.pdf
for: Raising the tap implying the tap increase
etc. for more info

RE: Rising the Xfmr Tap Causing HV side to be down

When you say raise the tap in a trf ,it means different results at different parts of the world and even in the same region depending on the manufacturer.Unfortunately this is an area requiring some standardisation so that altercations as we have  seen in this thread can be avoided.Till then ,let us talk the subject referring to the rating and diagram plate of the trf.
The situation on this topic can be summarised as below:
As stevanal explained,in US and regions where ANSI is followed,the OLTC will be on LV side ie tappings provided on LV winding for LV variation of +,-10% in 32 steps and Raise taps will be numbered 1R to 16R with 16R being the highest voltage tap.Lower taps will be numbered as 1L to 16 L with 16L showing minimum voltage.But the same trf in Europe and India (sometimes in US too )will be numbered as 1 to 33 taps with 17 tap being rated one 1 tap being highest voltage tap and 33 tap the lowest voltage one .(In IEC countries it will be more like +15% to -5% in 16 steps with 1st tap +15 % tap and 17 th tap -5 % tap )In US in addition there will be a DETC on HV side for HV variation of +,-5 % in 4 steps ,numbering being 1st tap highest voltage and 5 th tap lowest voltage tap .
But in IEC countries, the OLTC will be on HV side for HV variation (since they go for resistor ,high speed OLTC against US going for reactor type OLTC)of +5 % to -15 % in 16 steps so that constant LV voltage can be maintained for varying HV voltage.(no1 tap for highest HV voltage and no 17 for lowest HV voltage )But exceptions are common,in India one utility regularly buy 100 MVA 220/66 Kv trfs with OLTC on LV side for LV +15 % to -5 % variation.
Confusion can arise when another arrangement is folowed.Utility may ask for HV oltc ie oltc on HV winding for variation on LV side ie variable flux regulation .With constant HV voltage, HV tap changer will be operated to get Lv voltage of +15 % to - 5 % in 16 steps with no 1 tap  giving lowest Lv voltage and no 17 tap giving highest LV voltage.Pl see the contradiction in tap numbering with respect to the constant flux regulation tapping. In reality,power system will never be in ideal situation ie constant HV or LV voltage and normally a mixed flux regulation will be happening in many situations of tap changing in trfs.
Coming to SMB1's observations-a tap change cannot alter the incoming voltage value as it depends on other parameters.When tap is  raised on HV side,no of  HV turns come down and  for constant HV voltage, per turn voltage goes up and LV voltage goes up.But for  the same load,current on HV side need  not change .Lv current will only come down as the  LV voltage goes up and regulation will be slightly lower due to this.But in actual situation this will be minor and not noticeable.

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