Does a component exist to?

Thread Starter

Benbobben

Joined Jun 8, 2022
8
Hi,
Sorry if this is the wrong board. I want to create a piece of test kit to test Automatic voltage control (AVC) relays for power transformer tapchangers.

The AVC works as follows:
When the voltage is greater than desired the AVC closes a contact that energises the transformer tapchanger motor. The transformer taps down to a lower output voltage. When the voltage is lower than desired the AVC closes a contact that energises the motor in reverse therefore tapping up and increasing the voltage.

Typically the transformer has 17 different taps. The AVC knows which tap it is on by applying a voltage (typically 10v dc i think) across a series of resistor. The resistance ranges from 0 to 6400 ohms in 400 ohm steps and are brought into the circuit on a
Set of contacts that rotate as part of the tapchanger mechanism.

Essentially I need to create a variable resistance that is controlled by a 'step-up' or 'step down' input. For example the relay issues a step up command by closing a contact, the contact closes a signal to said device which increases the resistance by 400ohms. The signal will be part of the test kit ideally a dc voltage.

Any thought of how to do this would be much appreciated. I've tried google but can't see anything suitable.
I can think of a way by using relays bit it involves masses of wiring.
Thanks
Ben
 

michael8

Joined Jan 11, 2015
410
It looks like it can be done with 5 relays which each switch in a power of two * 400 ohm resistor. There would need
to be a binary up/down counter to drive the relays etc.


tap 0 resistance 0 relays []
tap 1 resistance 400 relays [0]
tap 2 resistance 800 relays [1]
tap 3 resistance 1200 relays [1, 0]
tap 4 resistance 1600 relays [2]
tap 5 resistance 2000 relays [2, 0]
tap 6 resistance 2400 relays [2, 1]
tap 7 resistance 2800 relays [2, 1, 0]
tap 8 resistance 3200 relays [3]
tap 9 resistance 3600 relays [3, 0]
tap 10 resistance 4000 relays [3, 1]
tap 11 resistance 4400 relays [3, 1, 0]
tap 12 resistance 4800 relays [3, 2]
tap 13 resistance 5200 relays [3, 2, 0]
tap 14 resistance 5600 relays [3, 2, 1]
tap 15 resistance 6000 relays [3, 2, 1, 0]
tap 16 resistance 6400 relays [4]
 

crutschow

Joined Mar 14, 2008
34,281
You could also use two low on-resistance, 8-channel high voltage multiplexer (example) to switch in the desired resistance from a string.
The string could be sixteen 400Ω resistors in series.
A binary up/down counter could generate the Mux address from the pulse signal.
 

Thread Starter

Benbobben

Joined Jun 8, 2022
8
Thanks for the replies.
This is all fairly new to me by the way, been a while since I've done any kind of electronics.
With the multiplexer I can see how a string of resistors could be switched in.
Could you recommend a counter that would work?
I'm guessing it will just be a case of matching the counter outputs with the logic control signals of the multiplexer? So the right resistance is switched in on the right count?
 

LowQCab

Joined Nov 6, 2012
4,023
A Schematic would be very helpful right about now .......
And, an explanation of what You want, or expect, and why.
.
.
.
 

michael8

Joined Jan 11, 2015
410
Do you have a pointer to a manual on the interface between the AVC and the tap changer & motor?

This would be a manual/specification for the AVC or the tap changer motor. (Or whatever holds
the 400 ohm resistors).

Is this tester for one specific model of AVC or general?

It's not clear to me how/what voltage the AVC uses to signal the motor on the up/down wires? Or what
voltage the AVC applies to the string of 400 ohm resistors to sense their resistance value?

For a "generic" tester I'd lean to real contacts and use relays, not semiconductors.
 

Thread Starter

Benbobben

Joined Jun 8, 2022
8
The motor control supply is always 110v ac which is derived from an auxilliary transformer within the tapchanger, this is then wired to the up/down contacts of the AVC and back to the motor control circuitry.

I cant find any info on the voltage applied to the resistor chains. I would guess at 24v

Normally I will be on site for 5-6 weeks testing various relays and high voltage switchgear. The transformer on site will typically be in use and can only be wired up the th AVC relay during a shutdown.
Therefore it would be nice to have piece of kit to simulate a tapchanger, this would allow me to test the relay before the outage when there is no pressure on time.
 

LowQCab

Joined Nov 6, 2012
4,023
We know less than You know about this particular piece of equipment.
A Schematic is a requirement,
unless You happen to run into someone who's actually worked on the same model.
.
.
.
 

michael8

Joined Jan 11, 2015
410
What are you trying to test in the Automatic voltage control (AVC):

basic function?
AVC commands tap step up/down as voltage changes.

tolerances on: inputs / outputs / timing
voltage sensing causes tap changes as proper voltage(s)
restance tap sensing (600 +/- x ohms)
timeing: delays on tap changes up/down as voltage varies around trip point

Likely there are other things which could be measured. Also the tests
could be automatically repeated over time (and temperature?).
 

Thread Starter

Benbobben

Joined Jun 8, 2022
8
First Picture shows part of a typical motor, I need to apply a signal source to N2B, the tap raise/lower (N7/N8) signals need to initiate a change in the resistor chain to simulate a successful tap up/down.

Pictures 1 is an old Three Position Indicator essentially a potentiometer which has a moving dial displaying tap 1-17 on.
Picture 2 shows a newer type of AVC which applies an auxiliary voltage from the relay, the relay then senses the tap position.
For the relay to accept a tap has completed succesfully:
>Tap is initiated by relay.
>Tapchanger taps up/down (takes roughly 10S)
>The relay must see a contact is closed in the tapchanger that feeds back into the relay(Called Tap in Progress)(Fed from same source as relay power supply)
>Sensing via resistor chain confirms tapchanger has reached correct tap.
In Progress.PNG

Hopefully this is a bit clearer now.

What are you trying to test in the Automatic voltage control (AVC):

basic function?
AVC commands tap step up/down as voltage changes.

tolerances on: inputs / outputs / timing
voltage sensing causes tap changes as proper voltage(s)
restance tap sensing (600 +/- x ohms)
timeing: delays on tap changes up/down as voltage varies around trip point

Likely there are other things which could be measured. Also the tests
could be automatically repeated over time (and temperature?).
Essentially on site testing of applied settings to confirm correct operation of relay including:
Volts of out of bandwidth initiates tap up/down. (There are also voltage reduction commands that decrease target voltage by 3 & 6% so testing at these voltages also.
Timing of said taps. ( There is a 80 S delay for initial tap and intertap delay depending on the time the transformer takes to make 1 tap, also for very high volts there is a 4 second delay for tapping down)
Various alarms such as watchdog, tapchanger runaway (where tap up/down signal is permanently applied), volts out of bandwidth for >15mins.

I have all the equipment to test the relay I just want something to simulate the change of tap position by responding to raise/lower inputs.

Thanks
Ben
 

michael8

Joined Jan 11, 2015
410
So in the real case the 400 ohm per tap resistor is really a variable resistor connected to the motor?

What's the expected driving voltage to the variable resistor (or it's power rating?).

I'd think that to simulate a relatively smoothly changing resistor is a bit more complex then just switching in the final value.
 

Thread Starter

Benbobben

Joined Jun 8, 2022
8
Found a photo ive taken previously, the contacts are made on the back of the disk containing the resistor chain.
I don think it will matter if the change of resistance is a bit coarse. The contacts on the actual tapchanger tend to be a bit poor anyway
 
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