Harmonics in pole power transformers?

We don't want to sense current, only the presence of 240VAC from the transformer. It is meant to alert the customer that they need to call someone from the utility to come out because either the HV line has gone dry or the transformer has failed. It would make no sense to send their own staff because they aren't allowed to work on anything up the pole or even before the meter box.

These are remote installations where a truck roll is expensive no matter who has to come out. So if the wrong crew shows up they have a big cost wrote-off.

We can't. It's all automatic and often legacy equipment that some might even call pre-historic.

Outside of a metal breaker box, there's no h-field with no current, and no e-field leaking out. The ambient e-field will be dominated by other stuff, like other circuits and light bulbs in the room.

Maybe it can't be done.

Suppose you had a small, coin-sized magnetized thing. Would you need an electrician to open the breaker box, stick it to the cover or the side inside, and close it? You don't need an electrician to open the box and flip breakers.

There is no room. Just that last pole, the transformer, a meter and breaker box and the pumps. Other than that maybe some cattle.

We'd be counting on the ever so small leaking E-field because with the pumps turned off that's all there is going to be. Hence my question whether we could use harmonics to see a change versus the E-field coming from the distant HV line. The engineer taking care of the software side is very good at extracting signature signals to hear "the grass grow".

Possible. That's why we are engineers, to try out until we are 110% sure it truly can't be done. If it turns out it can be done the company has a leg up on the competition, big time :-)

No, but many outdoor boxes have a gasket seal which you technically are not allowed to compromise. What we were mulling is a 2nd transmitter inside the box. But that would probably become a show-stopper.

As John says, you don't have a B field and the E field is shielded out by the box.

How about gluing the sensor to the inside of the box, the receiver to the outside, and using two piezo buzzers to communicate?

Cheers

Phil Hobbs

If the pumps aren't always running, what triggers the pumps to turn on and off?

Would it be acceptable to determine that power has failed when the command to turn the pumps on fails?

Michael

could be a simple mechanical float switch?

how about at small load at the pump and a current sensor?

maybe the load could a powersupply for something that injects a lf/hf on to the line that you can detect at the box?

-Lasse

Oh! Good idea. I thought it was something electrical, powered by something else.

That is a good idea too! With a circuit in parallel with the pump, keep a lead-acid battery charged, or a supercapacitor. When power fails, send the owner a text message or something.

:D

Michael

Put a small strip of double-sided PC board between the power conductors coming into the breaker box and wrap earthed foil around the outside of the conductors for a short distance each side of it. A high-impedance differential op-amp would detect the voltage difference between the two sides of the board when there was a voltage difference between the two conductors, but would be unaffected by common-mode voltages or an external field.

(With slightly changed geometry, this could be used as an undetectable telephone-tapping device.)

Offhand, I can see some problems with using harmonics:

1. Most of the distortion (harmonics) are cause by non-linear loads, over which you have no control. Worse, they vary. If you can see transformer induced harmonics under all that junk, I would be impressed by your DSP programming.
2. Core saturation is a function of load current, which can vary. I suspect that the distortion is much less with a light load, than with a heavy load. If the transformer fails while it is lightly loaded, you probably won't see a change.
3. Increased distortion from core saturation is only a function of perhaps one failure mode, over current. There are plenty of other failure modes, such as coolant loss, various leakage faults, phase balancing, space weather, etc that do not necessarily generate a corresponding increase in distortion.
4. Grid tied (solar) generators are really noisy and tend to produce harmonic spikes on the power lines. The better inverters have line filters to reduce the problem, but most do little or nothing.
5. HomePlug and various carrier current communications schemes apply RF onto the line. While this is unlikely to affect low order harmonics of 60Hz, it still should be considered and tested.
6. If you put your finger on the probe tip of your oscilloscope, you'll see a fair approximation of the type of junk found on power lines. Seeing small changes in harmonic content is going to be difficult.

It's more reliable than trying to detect xformer distortion. If your purpose is to anticipate a transformer failure, there's nothing better than temperature. Think about attaching a stick on label that changes color at some critical temperature: Or just slop some thermochromatic paint on the xformer case.

HV line failures and fuse failures will produce an instantaneous loss in xformer output voltage. I would think you could by just monitoring the local line voltages and waveforms.

Back when I was building server farms, we were learning all about reliable power. PG&E would pay for damaged equipment, but it was my responsibility to prove that they screwed up. So, we invested in power line impairment equipment. As I recall, it was made by Dranetz, but I don't recall the model. It gave us a paper tape printout of just about anything that might go wrong on the power line. It paid for itself in about 4 years and inspired PG&E to replace some failing equipment. I still do some of that at various mountain top radio sites.

This is probably overkill for what you need, but comes close to what you're trying to detect. It might be useful to look at some of this equipment, see what they can detect, determine how they do it, and see if it can be used. If you invent a new method of doing all that by induction or no wire contact, I want to invest in the company.

If the meter is an old electro-mechanical one there will probably be a useful B field leaking from the voltage winding, even when there is no load.

Also, the input and output wires are well separated at the meter terminals which could make it easiler to attach metal foil for voltage sensing.

John

It's funny everything has to be real good, but hiring an electrician is just out of the question.

funny project.

What about using a phase monitor relay? Here's one at random from Grainger

Check around for the relays that are used with automatic generator transfer switches. These monitor/detect the utility power (and phase, which is whe re I got the idea to mention this), so that a running generator won't trans fer over to a dead utility supply.

If I recall, there are some models of these relays that require no separate power source.

If I haven't understood your requirement, well then,.. nevermind.

Old-fashioned (and many new) electric meters leak mag field like crazy. Stick some little pickup thingie to the glass.

Try this maybe:

Our hope is that enough E-field leaks out to be detectable. But ideally we'd like to distinguish it from other 60Hz fields from far away and that may be via harmonics that are hopefully stronger out of a transformer than on the other fields (usually from HV power lines).

That's our plan B, sans the buzzer since low bandwidth RF would go through at that short distance. But it's expensive, might nto be palatable to have two units.

My experience is different. There is always a compromise between idle losses and cost in a transformer and most of the idle losses are due to saturation. Lots of detectable harmonics. But I don't know if it's the same with pole transformers, hence my quesion here in the NG.

No, it's always there. This is sometimes why you see US equipment failing in Europe, core saturation (before SMPS came about).

Different out in the boonies. There's only AC motors out there. Or sometimes no load and we must not get an alert upon no-load.

Not allowed.

However, we cannot plug in anywhere and there may not be any current.

Now that is a good idea! As long as there is a meter.

All in conduit and inaccessible.

All kinds of triggers: Float switches, people, timers, et cetera. It's beyond out influence and we can't rely on any of that.

Nope :-(

We can't have anything installed beyond sticking a plastic box somewhere in the outside because that would require truck roll (electrician).

Very normal. Calling out an electrician can mean a 2-3h truck drive.

They always are but in the end generally work as desired :-)