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]

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 ?

 

[ScottyUK]

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:
[blue]"So increasing the turns ratio and holding the (HV)secondary side at a fixed voltage must lower the (LV) primary voltage.[/blue]

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?

 

[edison123]

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

 

[ScottyUK]

Kumar,

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

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Your text

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you can also use

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regards,


Scotty.

 

[stevenal]

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,

 

[jbartos]

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

 

[prc]

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.