Rising the Xfmr Tap Causing HV side to be down 8

[SMB1]

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!!

 

[edison123]

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

 

[SidiropoulosM]

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).

 

[stevenal]

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.

 

[ScottyUK]

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.

 

[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

 

[edison123]

Sidiropoulos,

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

 

[jghrist]

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.

 

[SidiropoulosM]

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.

 

[stevenal]

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.

 

[freddycor]

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

 

[jbartos]

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.

 

[jbartos]

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.

 

[edison123]

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

 

[jbartos]

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.

 

[busbar]

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.

 

[nag400]

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.

 

[edison123]

nag400,

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

 

[CKent]

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!!!

 

[SidiropoulosM]

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.

 

[ScottyUK]

Edison123,

[blue]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[/blue]

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!