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!!

 

[RRaghunath]

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).

 

[SidiropoulosM]

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.

 

[SMB1]

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

 

[edison123]

SidiropoulosM,

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

 

[davidbeach]

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.

 

[jbartos]

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.\\\

 

[SMB1]

jbartosls. 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.

 

[edison123]

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.

 

[busbar]

Aside — Transformer labeling should clearly reflect tap characteristics.

 

[SidiropoulosM]

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.

 

[edison123]

sidiropoulosM,

Yes, would you please post your simple example ?

 

[SidiropoulosM]

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.

 

[edison123]

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.

 

[jghrist]

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.

 

[edison123]

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 ?

 

[SidiropoulosM]

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

 

[edison123]

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?

 

[SMB1]

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.

 

[nag400]

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