Tap Changer/AVR Question

[nightfox1925]

We have a 72kV-4.16/7.2kV, 3 winding transformer with ON-Load Tap Changer (LTC) on the primary winding. The tap changer is controlled by a Beckwith 2001B AVR whose PT input is derived from a 2 x 4.2kV-120V, Vee-ve on the 4.16kV bus end. The transformer primary tap changer is designed to be +/-10% with 7.2kV at nominal tap and 8 x +10% and 8 x -10% tap position. The +10% (79200V) tap is at tap position no. 1 and the -10% (64800) tap is at tap position no. 17.

I would like to know what is the industry definition if you say "Raise" and "Lower" on tap changing/AVR terms.

If you say "Raise", does this mean that you are:
1) Raising the "Tap Position".
2) Raising the primary voltage (transformation ratio).
3) Raising the secondary voltage.

 

[galleon50]

Raising is refering to the tap position and not the actual voltage

 

[nightfox1925]

Thanks galleon50.

I have attached a portion of the tap changer motor control diagram from MR:

I am doubtful of my interpretation on the bar just below the motor symbol. Based from what I am seeing, I have my interpretation as follows:

1) Main Contactor K1 is for "RAISE" and K2 is for "LOWER".

2) The "Raise" direction will move the tap position from Tap Pos 1 to 33 (in steps).

3) The "LOWER" direction will move the tap position from Tap Pos 33 to 1 (in steps).

4) K3 is a brake contactor and A1 is a capacitor brake.

If somebody has encountered the same MR diagram, I would appreciate any comments.

It was commented in the field that the AVR "RAISE" connection to the Tap Changer Control was moving the Tap Position towards Tap Pos 1 (thereby increasing the primary voltage winding)and decreasing the secondary (regulated) voltage instead of increasing it.

 

[dpc]

My expectation would be that RAISE would move to a higher primary tap and lower the secondary voltage.

Either way, it should be possible to set it up to work properly.





"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[bacon4life]

Most of our transformers have low side LTCs, so it is easy.

On the few high side tap changers we have, field relabeling was done that now corresponds with voltage instead of winding ratio. On new purchases, we even ask the the manufacturer to do all the internal labels so that raising the tap = raising the secondary voltage = lowering the ratio.

 

[slavag]

Hi Nightfox1925.
Actually, if we say "RAISE", we meant RAISE of secondary voltage for the stepdown transformer.
From TCH mnf. point , it's moving from 1 to ...position.
Best Regards.
Slava

 

[prc]

Industry practice is to put tap no 1 on the highest voltage tap ie winding position with maximum number of turns.
So when we raise the tap, it means tap is moved from lower numbered tap to higher numbered one -means primary receiving voltage has dipped due to load and we are raising the tap to keep the same secondary voltage on transformer.For the same primary voltage if you raise the tap,it will result in higher secondary voltage with overfluxing due to higher per turn voltage in winding.This is an abnormal situation.

When taps are provided on secondary,tap 1 will have max number of turns and maximum secondary voltage.When load increases on secondary, terminal voltage dips due to regulation and to keep the same secondary voltage at load terminal,tap is lowered on transformer.

 

[nightfox1925]

Thank you gentlemen for all your prompt technical comments. Looking at the MR specs, it appears that the tap changer unit and control provided by MR at the transformer conforms to dpc's comments.

The "Raise" command operates the tap changer to go to a higher primary tap or goes to Tap Position #1 (79200V or +10%).

The "Lower" command operates the tap changer the opposite way.

Having said that, I am now checking this against the Beckwith 2001B (+M-2067 Adapter) Panel)AVR to see how the AVR "Raise/Lower" is taking place. The AVR manual can be downloaded at:

http://www.beckwithelectric.com/tapchangers/m2001/m2001c.htm

I am trying to get the idea of the "Raise/Lower" function of the AVR to ensure that it is in harmony to the MR tap changer operational requirements. The manual seems to discuss a lot of technical sales talk rather than direct technical operational description....can anyone help me out..I might have missed a lot in the process of undertsanding the manual.

 

[nightfox1925]

I got lucky, based from the AVR Spec, the LED indications are defined as follows:

RAISE LED - The red LED indicates that the voltage is BELOW the lower band edge and the timer has started timing for the tap changer RAISE operation

LOWER LED - The red LED indicates that the voltage is ABOVE the upper band edge and the timer has started timing for the tap changer LOWER operation

With these indication descriptions, I believe that the AVR "Raise" would mean increase the voltage and the "Lower"
would mean decrease the voltage.

Since the "Raise" operation on the tap changer would move the tap to the higher tap winding ratio (Towards Tap #1), I can say that the "Raise" output of the AVR should be wired to the "Lower" circuit at the Tap Changer control circuit at the transformer. Eventually, a label should be indicated in the transformer tap changer panel to indicate:

"Lower Tap Winding Ratio" (raise secondary voltage)"

"Raise Tap Winding Ratio" (lower secondary voltage"

Any comments?

 

[tlrols]

Good question...and the answer is all three. There are a few conventions used for what it means to "raise" or "lower" a tap on a tap changing transformer.

I have personally dealt with at number of different ways:

1. Total tap position: Used with tap changing on 500/230 kV transformers with the tap changer on the 230 kV side. Tap position 1 is the lowest possible voltage on the 230 kV, tap position 17 is the highest possible voltage on the 230 kV. This was used by dispatchers and the SCADA displays.

2. Modeling software: Usually shows total tap ratio on either the system base or the transformer base--they ARE different. The total tap ratio follows the convention (usually)of increased tap ratio equals decreased secondary voltage (assuming that is where the tap changer is).

3. Follow the voltage: Another method, that dispatchers tend to like is raise means the controlled voltage goes up, i.e. +1 to +8 for example and lower means the controlled voltage goes down, i.e. -1 to -8 for example. Ironically this is usually opposite to what the integer tap position is in the modeling software. The 0 tap position was the neutral point on the tap changer with it then only being affected by the fixed high and low taps.

Since there really is no universally accepted convention we usually write our instructions and procedures to state that tap changers must be operated to raise or lower the controlled voltage--that way operators get a freee test to figure out they are going the wrong way. If the TCUL's are automatic we test them before they get released to operations--just too many opportunities for error.

 

[slavag]

Agree
It's my understanding too.

 

[dpc]

There are (at least) two different systems - the tap changer mechanism itself, and the LTC controller.

I haven't looked at the Beckwith documentation in a long time, but I would think they would cover this situation in the instruction manual. It's unusual to put the LTC on the primary, but it is done. The only issue might be matching up the tap position # in the LTC control with the actual position. But as long as it know where neutral is, you should be OK. I'd give Beckwith a call- they build retro-fit LTC controllers for a living, so I'm sure they can help.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[nightfox1925]

Thanks dpc. I am also attaching the actual transformer tap nomenclature for our discussion reference:

The tap changer raise is actually raising the turns ratio (goes to tap Pos. #1) which will in turn lower the secondary voltage.

The AVR Raise pertains to increasing the secondary voltage. We are proposing to swap the raise and lower wiring on the tap changer local control panel and perform a controlled functional test via secondary voltage injection on the AVR with the control voltage on the tap changer and motor voltage ON and the transformer main power OFF to verify operation. Transformer turns ratio test will be performed during the functional test.

 

[NAZ55]

I agree with prc's definition. It doesn't matter if you call it potato or putaato, as long as you understand the concept and reasoning behind it.

Also, some time increase in transmission nominal voltage can also make you want to move tap positions to keep your secondary at the desired level.

I have always preferred regulators over LTC at the distribution level, since failure in one phase doesn't mean you have to take transformer/line outage

 

[pwrtran]

nightfox1925
Tap1 gives you the lowest secondary voltage, and Tap17 gives you the highest.
To make it simple:
Low voltage seen at secondary bus --> Raise Command Sent --> move the tap to a higher number of position
High voltage seen at the bus vice versa

 

[prc]

pwrtran,we cannot generalise like that.It all depend how and where the tap changer is provided.As per current practice, tap changer is provided in four configurations viz,
1) Tap changer on primary for primary voltage variations for constant secondary voltage ( standard in Europe,India,China etc)
2) Tap changer on Secondary for secondary voltage variation, for constant primary voltage.( US standard and rarely in other areas too )
In both above types, the flux density in the core will remain the same( as per turn voltage remains same ) at all taps and hence called constant flux voltage regulation.
3)Tap Changer on primary for secondary voltage variation for constant primary voltage.( Sometimes used in India,US)
4) Tap changer on Secondary for primary volatge variation for constant secondary voltage ( For Traction supply transformers in UK & India)
With 3 &4, the flux density in core will vary wuth tap changing and hence are called variable flux voltage regulation.

But in all cases , tapping with maximum effective turns in circuit is called tap 1 and the one with nminimum effective turns is called 5,9,11,13,15,17,19,21 etc