Voltage monitoring from log-distance

Hello Dave,

Sending the values as text gets involved. At the minimum you would need a micro controller. IP requires a rather luxurious uC and lots of programming.

Can't you just use a voltage to frequency converter such as the LM331, transmit the values as a pulse rate and then convert back to a voltage at the receiving end? You could also just use a counter at the receiving end since the pulse rate would be proportional to the input voltage.

Regards, Joerg

Fluke "Hydra" data logger or equal. Basically this is a multimeter with 21 inputs. It scans the inputs at a preset rate and displays the voltage on each one, and also outputs this to an RS232 serial port. You might convert to RS485 or use a commercial serial-to-Ethernet converter to make the long haul. Or, if you don't need automated processing at your end, point a cheap webcam at the front panel. Total cost several hundred dollars US.

You can fake the above by buying a $50 multimeter that has a serial port and coming up with your own hardware to switch it around to the different inputs. You will need to replace the multimeter battery occasionally as well - have it measure its own battery as one of the inputs. Again, convert to RS485 or Ethernet for the long haul. Total cost a couple of hundred dollars US.

If you can reduce it to a pass/fail condition, you might hook relays across the various voltages and wire the contacts together such that a closed circuit means all is well and an open circuit means trouble.

Put a PC at the remote end with an analog I/O card. Write a program to read the inputs and transmit it over Ethernet. Total cost several hundred dollars US. Or, buy a small embedded system with analog inputs and Ethernet output (Z-World?) and do the same for a couple of hundred dollars.

Matt Roberds

Some high-end digital meters have RS-232 output as an option, which could be hooked up to a modem...

I would use two windows PC's (NT based OS)connected through Ethernet , TCP-IP. The points to be monitored will need signal conditioning which can be as simple as relay, or some threshold detecting circuitry isolated by an opto-isolator chip.These can be inputted to the computers Parallel port. A simple program could be written in VB to monitor and transmit the data in real time. The receiving PC will also have a VB program to decipher the received data. A big advantage using PC's is that you can modify the program or hardware as your needs change.

If your data is critical, and your budget allows, look into PLC (Programmable Logic Controllers) over Ethernet. One such supplier is:

Is this a one off project or are you looking for a larger number ?

Do you have AC power available or just battery/solar power at teh source ?

Are there more then one monitor station between the source and the host ?

Fairly low power wireless network nodes can be created to pass data from one node to the next without a high power transmetter at each node.

IP networking, outdoors is a disaster looking to happen.

Unless you can protect the IP wiring from static or lightening.

This requires a wireless solution.

donald

scada wrote:

PLC is the highest cost for this simple requirment.

You would have to use the proper protection with any outdoor aireal run. You could use Fiber optic cable with that short of a run. Use Ethernet to Fiber converters on each end to bring it back to copper.

You

Fiber

The Ethernet solution would of been my preferred method, but I was assuming their was an established network

Another way to go would be to run the twisted pair between both points and use a modem on each side connected to each PC through the serial port (RS232). I have used this very successfully at low baud rates, up to 9600 without problems.

You've all given me some good ideas. I've decided to throw out the ethernet/IP method for the complexity issues already mentioned.

More details: What I'm trying to monitor is power conditions at a three-phase power diesel generator. I would like to monitor the voltages and power consumption remotely, transferring the data to another location where I would digest the data and display it on a web page graph. I have all of the 'back-end' web site stuff figured out. It's just the data collection part. I don't really want a PC in the generator shack for lots of good reasons. I have two of these generators to track plus I'd also like to monitor a few single-phase locations.

The generators are about 1-2 miles from the computer doing the web site computations.

Since I can get dedicated twisted-pair wires, I've thought about RS-422 or such, but I'm still stuck for the analog-to-digital conversion. I've looked at PIC solutions that use temperature sensors, but I haven't found anything yet to measure voltages.

Does this information bring anything else to mind?

Thanks again for all your help.

Dave

Ah! Lots of people want to do this, so you can get devices that have everything you need in one box. They usually even have some kind of RS-422 or RS-485 serial port on them. Google for "power quality monitor" or "three phase power monitor" or similar - for instance:

Some of the PICs are capable of doing it, but you would have to come up with a safe interface to the 3-phase power on your own. If you're not sure how to do this, you're probably better off buying a box that already does what you want and hooking up to its serial port.

Standard disclaimers apply; I don't get money from any of the companies mentioned.

Matt Roberds

What you are trying to do is routinely done by every utility in the world; they call it SCADA. The first thing you need to do is find out what type of monitoring instrumentation is already installed in your generator switchboards. Worst case would probably be that you need to install new transducers on existing PTs and CTs. This is not a trivial matter, special wiring methods are required to maintain high reliability and a technician familiar with switchboard wiring materials and methods should be used for the installation. Power distribution CTs for instance have no internal burden resistors, disconnecting the secondary under load could be lethal and can be expected to cause equipment damage. There are loads of other special considerations unique to switchboards as well.

There are quite a few options once you have low level transducer signals available, but the easiest would be to use a transducer with a built-in RS-485 port such as the AMETEK DMPS-PM Multi-Function Digital Power Meter:

Lots of other transducer options are available also, or you may be able to use transducers already installed in your swbd without adversely affecting the protective relaying. The nature of your questions leads me to believe you should get someone with switchboard experience to review your switchboard wiring diagrams and identify the optimal way to extract the signals of interest; once you know the exact nature of your available signals you can start to consider how to get them into RS-485.

In light of this new information, your best reliable bet would be to use a PLC. The fact that you need analog interpretation and not just digital will add some $ to the design. Popular industry protocols include Modbus RTU, Modbud ASII, Profibus to name a few. Equipment manufactures include Siemans, Square D, Acromag, Pheonix Contact, and the list goes on. I believe Newark.com has a low end PLC that has something like 24 I/O for around $200, that includes software. I don't recall if it has analog. There are others also around that price. The trick is to find one that has analog capability in your price range. The outputs of most of these will allow for RS422.

As others have mentioned you will need PT's and CT's to monitor the AC Lines. Depending upon the PLC input modules you may need signal conditioners as well.

Of course if you must roll your own, the PIC is a good place to start. I designed a Modbus master-slave system that worked well, using a PIC chip. That design was strictly digital, no analog. However adding analog to such a design using Modbus protocol is feasible. Keep in mind the Modbus RTU & ASCII are free to use, Modbus Plus is Licensed. Good luck, keep us informed...