Help needed û Adjustable constant AC current source: a viable circuit or a valid equipment supplier 1

[Millbo]

I have a question or perhaps more of request for information and or suggestions please from those who know electronics more than I do.

My Issue at hand:
I need an adjustable constant AC current source to simulate the secondary of an energized hotspot CT to verify the winding temperature simulation systems set point on power transformers. The test duration is 45 minutes and the results are plotted out to look at the stabilization.

The current to be applied will range from 1 amp up to 5 amps depending on the secondary ratio of the CT and the type of simulation system installed. Some simulation systems use a simple adjustable resistor to trim the current, another type uses jumpers to remove sections of a resistor while the more complicated systems uses a current balancing autotransformer in combination with an adjustable resistor.

I’ve used everything from the F series relay test sets down to five 100W light bulbs controlled by a dimmer switch but due to the changing resistance in the heater that the current passes thru the operator has to constantly make adjustments to keep the current at its desired set point. The constant current ability of the secondary current injection sets (Doble F2700) have performed the best of anything I’ve used. It’s a simple field verification, but I can hardly justify the cost of a 12 relay test sets at $30k each. I just don’t need all the extra bells & whistles they come with.

The WTI system in a nutshell:
The winding temperature indication (WTI) system uses the secondary current of the “hot spot” CT to apply a bias to the winding temperature indicators bulb, via resistance type heater elements, to offset or increase its reading to match the temperature gradient that is determined in the FAT heat run. The electronic types of temperature monitors use a split core CT that’s clamped around a jumper that is shorting out the hotspot CT. In these cases I’ll pass current thru this jumper.

In the past most just assumed the factory has this set and don’t bother verifying its setting but several years back we had a chemical refinery loose power in the middle of their process due to the WTI applying a 40°C gradient to the 49T instead of the intended 12.9°C, needless to say they were not happy about this. Ever since, I’ve made it a rule to verify this set point.

Everything that internet search engines and other forums have offered was either DC systems or AC systems that only offer a mA current range.

Summary:
1. I need a 1 to 5 amp adjustable constant AC current source.
a. I want to stay as close to the wave form of a CT secondary as I can.
2. It needs to be able to maintain its set point regardless of changes in load impedance.
3. It needs to be able to operate continuously for multiple tests.


I would greatly appreciate anyone who knows of either a viable circuit arrangement or a legitimate supplier.

Thank you and have a great Holiday Weekend,
Mike

P.S.
For my own education, how is this AC current kept so stable in those current injection sets?

 

[VEBill]

The current to be applied will range from 1 amp up to 5 amps

One trick that you might find useful is to use multiple turns on the primary of the CT. This could reduce the total current that you need to control and probably make things generally easier.

The lower current means you could use thinner gauge wire, so physical space should be a non-issue.

If it was me, I'd dress the turns well away from the core (except where they pass through the CT, obviously) so as to avoid secondary effects that might affect accuracy.

--

An adjustable (stable) voltage source into a fixed resistance is the low cost method of sourcing a controlled and variable current. Avoid hot items (light bulbs or resistors being driven hard) as they'll swing on you. Keep things cool and thus stable.

The lower current of multiple turns should make this easy.

 

[ScottyUK]

Ametek who took over California Instruments have a couple of fairly powerful current sources available.

Not 100% sure but Pacific Power may well have something of use too.

 

[electricpete]

One trick that you might find useful is to use multiple turns on the primary of the CT. This could reduce the total current that you need to control and probably make things generally easier.

It seems like the op intended not to use the ct but instead to inject currently directly to the circuit fed by the ct secondary, which would decrease test current requirement compared to primary injection you're describing (even with a boatload of turns) I think.


=====================================
(2B)+(2B)' ?

 

[Millbo]

VE1BLL
Thanks for the replay but I may have been a little vauge in my initial post.
The bushing CT that I'm simulating the secondary current for is installed inside the transformer and I don't really need access to it. I'm just simulating it's output current.

To perform the commissioning test I spoke of, with the transformer de-energized I just lift the hotspot CT secondary wires in the control cabinet, usually S1 & S2 and land my current source wires in their place.
- I also remove the ground jumper that is on the non-polarity lead for this test.
Then I inject the predetermined current into the terminals where the CT wires were and verify the temperature increase on the winding temperature gauge.

The resistors I spoke of are part of the circuit I need to energize to verify their adjustment.
They are the wire wound ceramic type that have exposed turns that a slider is moved along them to "trim" the total resistance and thus the bias current that is applied to the heater element.

I've attached a typical circuit that I have to verify the "matching unit" settings.

ScottyUK
Thanks for the info and links.
I currently have some Ametek dry well calibrators (CTC-140 I think) but have not looking into what else they carry.


Thank you for your suggestions,

Mike

 

[Millbo]

electricpete,
A secondary current injection set is what I use to apply my test current to the circuit in my first reply.
I remove the CT from the circuit and am just simulating the secondary of that CT as if the transformer was at full load.

I chose to use these injection sets being they maintain a very constant AC current, ulike a variac which requires constant adjustments back to my initial target current.

My problem is I don't need all the extra functions they offer like frequency adjustments, timers, contacts, aux contacts...etc.
I'd like to just have the current module out of one and a way to control it.

I'm looking for a steady 60 Hz AC current that will stay at the set point I tell it e.g. 1.98 amps, 4.96 amps...etc.

I'll be the first to admit that electroics is not my strong suite but I'm trying to educate myself as I go.

Thanks for your reply,

Mike

 

[Millbo]

This will need to be something that is both rugged and can survive field conditions.
They will be hauled around in our test trucks with the other test equipment.

I've exhausted every avenue I can think of.

Thanks all and have a great holiday weekend,

Mike

 

[VEBill]

I'm "expert" at misinterpreting posts. Thanks for the clarification.

A variable AC current source could be created from a variac (adjustable autotransformer) feeding a low voltage transformer (e.g. 115v to 6.3v, Amps as required) feeding a fixed, suitably de-rated, power resistor. These are all 'garden variety' parts that are (for example) probably in my basement.

Measuring the current, if your DVOM doesn't do AC amps, use a calibrated series resistor.

Everything would need to be documented and carefully calibrated for your purposes.

Or rent an AC Current Source.

 

[VEBill]

Oh, I missed your last post. Disregard the rental suggestion.

 

[VEBill]

http://tmi.yokogawa.com/us/products/generators-sources/standard/2558a-ac-voltage-current-standard/

60 A AC ?

 

[VEBill]

Ah ha! Google: MCTA-5

Is that close to what you need?

 

[Millbo]

VE1BLL,
That current source sounds like it will fit the bill.
I'll send them an email for some additional information.

I appreciate the link and the leg work on this one,

Mike

Also,
I also ran across a company called Lectrotek out of India.

http://lectrotek.com

They carry a device called AMPCAL which sounds nice but I've never heard of this company. Any experience with them?


Question:
What is meant by the phrase "Microprocessor-based AC Current Source"?

Thanks again and everyone have a wonderful Christmas Holiday.

 

[itsmoked]

What is meant by the phrase "Microprocessor-based AC Current Source"?

That usually means the AC is synthesized electronically. It also infers it will be 'easily' adjustable.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Millbo]

Thanks itsmoked.
So does this mean its a modified wave signal?

 

[ScottyUK]

'synthesised' likely means it is more sinusoidal than the utility supply which often has a slightly flattened crest and may have other distortions present. It shouldn't be the traditional six-stepped 'quasi-sine'. That's really old technology. The worst you'll see might be a little high frequency switching hash on the output.

 

[LiteYear]

For my own education, how is this AC current kept so stable in those current injection sets?

FWIW, one way of doing it is to first derive a high precision AC voltage signal (eg. drive a tuned filter circuit with an oscillator), and then feed that signal into a voltage follower. The output of the voltage follower is connected across a fixed resistor and the load placed in series. The controlled AC voltage across the fixed resistor creates a controlled AC current. More precision can be achieved with feedback, but even without it this circuit can be effective. 5A is a little tricky though, because the fixed resistor is always in circuit, so needs to be both high precision and high power.

TI have a app note on such a circuit (although they show the simpler case of a DC source):

http://www.ti.com/lit/an/sbva001/sbva001.pdf‎