Detect presence or absence of 7200V grid?

[wpns]

Anyone know how I could measure the presence or absence of a 7200V AC Grid, maybe with a non-contact device? Our island gets a 3-phase 7200V connection from the main island, and their meter is in a large box (with, I assume, traditional Potential Transformers) far from any network connection.

The power has been unusually unreliable lately, and I've been asked if there's any way of detecting when the power fails (pretty easy, 1/3 of our island goes dark) and returns (so we can stop running our island generator) without a 5-minute drive out to the meter box.

We could try to get permission from the PoCo to tap into the meter connections (I can only assume there's 240 there) and power a radio transmitter, but who knows how that would go.

The power lines are (three) coaxial wires, so wrapping a sensor around the wire probably wouldn't get any useful signal...

We could also detect the 2400VAC grid after it passes through a set of three transformers to feed our island (where there is a network connection), and we obviously should use Potential Transformers, but they are thousands of dollars each, we need three of them and a box to put them in, etc. I was hoping there's a better solution...

Any thoughts? Thanks in advance!

 

[Quattrohead]

Sounds like you are losing a phase, I want to see what's going on inside that box because it looks like it might have substantial water damage.

 

[TM48]

It looks like the PoCo needs to do some inspection and maintenance.

 

[timselectric]

What's the voltage of the generator?
I assume it's connected through a transfer switch?
Can you monitor the grid source at the transfer switch?

 

[wpns]

Sounds like you are losing a phase, I want to see what's going on inside that box because it looks like it might have substantial water damage.

Yes, the power lines are laid on the sand on the shallow bank, and propellor damage is a regular thing. We lose a phase, we turn on our generator, they come and patch two phases together, we turn off our generator, we lose another phase, we turn on our generator, they patch all three to the one functional one, we turn off our generator, hopefully they fix the cut wires before we lose the last one. And the main island lost _all_ power and both internet providers a few days ago, so it's not always our feed.

Fortunately, we have no 3-phase loads, so we don't need all three grid lines to be 120 degrees out of phase with each other.

Yes, rust is a continuing issue with standard steel grid transformers and high voltage boxes. I want to know what's going on in there too, but I don't want to open it up. Did I mention a marine environment?

 

[wpns]

It looks like the PoCo needs to do some inspection and maintenance.

You're not wrong.

 

[wpns]

What's the voltage of the generator?
I assume it's connected through a transfer switch?
Can you monitor the grid source at the transfer switch?

240Vac 3-phase, stepped up to our grid voltage of 2400V with three other transformers. We've got a third-world substation out by the generator.

Yes, transfer switch from the generator or grid to our island loads.

Yes, we can monitor the grid source, but it's either 7200 or 2400 VAC, and I don't want to use an opto-isolator and a 360K resistor.

 

[Sweet Tatorman]

How about some form of sensing using non-contact capacitive coupling?

 

[v_green57]

That meter has wireless capability and monitors all three phases. Is it at the rusty switch cabinet you show in the image?

Who has responsibility for reading the meter? Can you get wireless access to it if you ask them nicely?

Looks like it *might* be this one:

Vision LTE Cat M1 Modem Meter - Vision Metering

Vision Metering is proud to introduce a low cost solution for using the public wireless systems to meter residential, commercial and industrial customers. The

www.visionmetering.com

Uses LTE Cat M1 wireless. Makes sense, given your description of your island infrastructure.

 

[wpns]

How about some form of sensing using non-contact capacitive coupling?

Yeah, except it's coaxial, so there's a ground (shield) around the conductor, so there's {probably? hopefully?) no leakage.

 

[wpns]

That meter has wireless capability and monitors all three phases. Is it at the rusty switch cabinet you show in the image?

Who has responsibility for reading the meter? Can you get wireless access to it if you ask them nicely?

Looks like it *might* be this one:

Vision LTE Cat M1 Modem Meter - Vision Metering

Vision Metering is proud to introduce a low cost solution for using the public wireless systems to meter residential, commercial and industrial customers. The

www.visionmetering.com

Uses LTE Cat M1 wireless. Makes sense, given your description of your island infrastructure.

Hmm, the PoCo _ought_ to know, I'll see if they have that infrastructure set up or if we send them a meter reading every month.

 

[v_green57]

The cabling you are describing is direct-burial URD:

Medium Voltage Primary Underground Distribution | Wire & Cable | Products | Southwire

Improve your power distribution with Southwire's range of MV cables, including southwire medium voltage cables.

www.southwire.com

The "shielding" isn't 100% like actual coax, so it does leak some EM field that can be detected.

Would advise against monitoring the cable directly, though. Dangerous.

If I am piecing things together correctly, the PoCo already has CT's on the three phases and that's what's feeding the meter. Far safer to monitor those.

That is the technique I used to use when doing load studies on private primary distribution systems.

 

[v_green57]

Another "no-touch" method to monitor things would be an IP camera pointed at the meter (if this is even physically possible). Would need a small solar system to power it and whatever wireless link you would need to get it back to your network.

Crude, I know, but works.

 

[v_green57]

Looked at the image of the meter more closely - "DataOnDemand Radio - RF"

Digging around on the Vision metering site:

Vision XT - Vision Metering

The XT Platform is the “fully-loaded” kWh meter offered by Vision. In addition to having all of the standard features of the ST platform, it also provides

www.visionmetering.com

Data On Demand - Vision Metering

DataOnDemand provides Real Time Pricing, Demand Control and Management, Co-Generation coordination through control of customer-owned generators and provide

www.visionmetering.com

Radio with Data on Demand

Vision Metering’s Data On Demand system provides real time energy consumption values using a robust 900MHz RF star network. Each endpoint is metered using a Vision meter equipped with Data On Demand radio and communicates directly with a RadioGate. The RadioGate collects the data and delivers it to the EndSight data collection system on the end-user’s network.

 

[wpns]

Remembered that my new clamp-on ammeter has a Non Contact Voltage detector, went to see if that would work, discovered we have one. Who knew?

 

[v_green57]

Remembered that my new clamp-on ammeter has a Non Contact Voltage detector, went to see if that would work, discovered we have one. Who knew?

Be careful with that. It is reading ELECTROSTATIC fields, not ELECTROMAGNETIC. Can be prone to false or no trigger.

Put it on LOW and stir it around in your hair or a nylon piece of clothing. The static electricity might set it off. My TIF's go off when I do this.

Of course you're in a much more humid location than I am, so YMMV. Humidity snubs static fields.

 

[Cmiller]

Would it be possible to install 3x small 2400V to 120V "control" transformers? (I have zero experience at such voltage levels, just thinking out loud here...) Something real small, purely to give a 120V reference for a more readily available meter. You could have this installed/set up right at the transfer switch, tied into the "main" side, of course...

Something like this.


Obviously that is a used unit, and I don't know anything about setting up something at those voltage levels, but perhaps there are even smaller ones available, since you wouldn't need anything big enough to run anything other than a metering device...

I guess maybe you are hoping to set something up pretty cheap, since the biggest goal is to not need to drive 5 minutes to check up on the current status of mains power. However, I imagine you could soon offset the cost in fuel savings for the generator.... If it means shutting down 10 minutes earlier, due to not driving 10 minutes round trip, that would add up with time!

Out of curiosity, how big is the generator? Sounds interesting!

 

[wpns]

Would it be possible to install 3x small 2400V to 120V "control" transformers? (I have zero experience at such voltage levels, just thinking out loud here...) Something real small, purely to give a 120V reference for a more readily available meter. You could have this installed/set up right at the transfer switch, tied into the "main" side, of course...

Something like this.
View attachment 286484

Obviously that is a used unit, and I don't know anything about setting up something at those voltage levels, but perhaps there are even smaller ones available, since you wouldn't need anything big enough to run anything other than a metering device...

Thanks for that, a lot more palatable than the $1-2K I was finding elsewhere!

I guess maybe you are hoping to set something up pretty cheap, since the biggest goal is to not need to drive 5 minutes to check up on the current status of mains power. However, I imagine you could soon offset the cost in fuel savings for the generator.... If it means shutting down 10 minutes earlier, due to not driving 10 minutes round trip, that would add up with time!

There’s a rumor that our generator costs less than our 65 cents per KWHR to operate, even at our landed cost of fuel near $10/gal, but I’m nearly certain that the people making that analysis are not taking into account generator, maintenance or depreciation, or the cost of replacing our 30-year-old generator when it fails.

Out of curiosity, how big is the generator? Sounds interesting!

210 kW, and our grid load exceeds that at times (Christmas to New Year’s is a good example when the island is full and everyone is taking showers and charging golf carts at the same time), so I’ve been after them for a decade to replace the generator, but it’s too expensive.