High Voltage lines near transmitters

[guilio2010]

So I’ll start with a basic question. If I had a transmitter near a HV power line (say 50’), is it possible that the power line could induce a voltage on the transmitter or power/comms wiring and cause false values?

If I took a scope and noticed that the voltage readings increased as I walked towards that transmitter, is that an indication? Probe were held in open air. Is that an indicator to determining power line interference on equipment?

Thanks,
guilio

 

[wayne440]

(This reply has been heavily edited - are you working on radio transmitter, industrial instrumentation transmitter or ???)

Walking around with a "scope" and looking at whatever stray voltage the leads happen to absorb is probably a waste of time.
It might be helpful if you will describe your transmitter and the symptoms of the problem.

 

[guilio2010]

Thanks wayne440. I agree on many devices operate fine near these lines which is why I am not saying it’s root cause but when static discharge is reported I have to consider it a source to investigate.

As for need more detail, that’s where I have to reply back with what details do you need in order to provide more useful feedback? I have never considered or dealt with high voltage (HV) lines before so I can provide a lot of details but is it relevant to what an HV SME needs? Hence, why I started with the basic question to start the conversation.

Thanks,

 

[wayne440]

We were typing at the same time. What is the application of the transmitter? industrial instrument or radio?
If radio milliwatts or kilowatts? Frequency range? Local or remote control?

 

[guilio2010]

That’s again wayne440. I see you edited your post.

Transmitter is a guided wave radar. Communicating RS485. The transmitter is mounted to a metal tank 30’ in higher. We current run 20/4 wire, shielded. 2 wires for power, 2 for communication. This is the end device in a daisy chain wiring. What we are seeing is the transmitter spiking to the high level and showing an interface level. The problem is there is nothing in the tank and we verified. I took a scope reading of the power, channel, and shield and the signal looks fine from what I can tell. (Normal high/low signal - and typing this I should record a full polling to verify all transmitters signal to compare as the controller polls one transmitter at a time). The tank is the closest to the power lines. I took my probe and randomly touched equipment with my probe (not saying it’s correct) but was reading voltage off metal components that should not have any voltage on them. As I moved away, the voltage readings on the metal components reduce to nothing.

Let me know if this help or if you have anymore questions.

 

[wayne440]

Unfortunately, I have only a very general understanding of that sort of transmitter. I work with public safety radio (police, fire, EMS, 911 etc), hence my earlier assumption of a "two way radio" type transmitter. Is this a new development in an existing installation? - in other words has it worked properly before? If it has, try to find out what has changed- for instance has someone grounded or ungrounded the signal cable shield?

 

[guilio2010]

No. We just commissioned the site and started at day 1 and would randomly clear but would stay HIGH level for most of the time. I increased our polling rate to see clear and alarm times and how often. Historically if we had issues like this it was from our VFDs causing spikes in the signal and it was clear that was the issue as the spikes matched the carrier frequency of the drive. Like I said, I should have taken a longer poll of the communication to see if that transmitter shows signs of different amplitudes than the other 7 on the same communication (I may do that tomorrow). We have some additional data to look at but I just can’t discount the HV lines and voltage readings on other equipment. Even the sign along the road that is really close to the lines showed a voltage reading which makes me want to thing if the reference voltage may be shifted in some way.

Thanks for the reply. Again, just trying to get an idea.

 

[wayne440]

I will watch this thread and probably learn something. My understanding of RS485 is that it uses the differential between signal lines to carry data. If your cabling is well shielded twisted pairs, it should be relatively robust against outside influence. Good luck.

 

[cuky2000]

HV power lines under normal operating condition at a low frequency of 60 or 50 Hz typical do not produce EMI interference. However, defective line hardware, such as connector, insulator, ring, fittings, etc., could be converted in corona noise-generating sources of wave in the MHz range that might exceed the critical value and produce ionization of surrounding air followed by corona discharge and radio interference even if the target equipment is away from the power line (say 50 radios).

A common noise mitigation strategy to protect transmitters is to provide a good shielding technique fallowing the manufacturer recommendations and acceptable industry practice to minimize potential interference outside the acceptable range of the transmitter.

 

[guilio2010]

Thanks Cuky2000.

We have shielding around our conductors but measuring the shield noise at our isolation ground, I read nothing. I would expect that if I was getting that much interference, similar to what I was getting with our VFDs, I would see this. I’ll have to check our other components, conduit as an example. It’s only this one transmitter also. We already replaced it, re-calibrated it with same results.

I guess to ask further, why would I be reading voltages on our equipment (piping, galvanized walls, RMC, grounding conductors) and why are those voltage stronger on equipment closest to the transmission line? Even the signs not connected to our equipment are reading voltages.

 

[itsmoked]

What approximate voltage are the lines and what is the approximate distance between the lines and your sensor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guilio2010]

itsmoked, I remember talking to the power company before during the construction of the location and I don’t remember the exact voltage (or find my notes) but i remember them mention a value above 300kV. 345kV sticks in my mind but I don’t know the voltage ranges so I could be wrong. These are transmission with the lattice metal structure.

Distances, I’ll have to wheel it off to get accurate but I have drawings showing ~75-100’. Visually it looks closer but I know we had to keep a certain distance from the structure anyways.

 

[itsmoked]

Whoa. OK so not some distribution lines. That kind of voltage and only a 100ft will definitely impose voltage potential on metal things.

Normally you would have metal tanks specifically and specially grounded. If they're not then they will have 60Hz waveform imposed on them and conducted to anything electrically, (conductivity), connected to them.

If they are appropriately grounded, wires leading to them will still act as antennas (lousy ones) that will intercept the cycling fields generated by those transmission lines.

If you're asking if the actual power fields will directly interfere with the in-flight RF transmissions - NO.

Does your transmitter take in the 485 signal and process it then convert it to RF and send it or does it simply take in the characters and modulate the radio signal directly with no interposed processing?

If it does do some processing, like packet assembly and/or checksumming, does it do any 485 responses like bad comm checksum reporting?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guilio2010]

We have grounding all over our equipment, bonding, and then we take it back to earth ground bed. For our transmitters, we have a specific isolated ground that all shields connect to and then that will tie into our ground bed. Even with the ground, I see the 60Hz wave with the scope. What’s the best way to measure this? One probe on the metal part, common in open air? Just want to make sure I am doing that part correct.

As for the signal, this does not convert anything to RF. It’s connected directly into our controller 2 wire and level measurements are sent to the transmitter head from the cable and the data is transmitted over the RS485 wire. My plan is since the transmitter is spiking false readings is to get on the tanks and start measuring voltages on the ground conductor, shield, conduit, etc...possibly break connections and see if I can isolate the noise if that is the case.

The controller will provide us with bad comms error if communications fails or times out, however, we receive a valid comms status the entire time which makes me want to say something is affect the transmitter cable that detects the level. When we had comm issues with the signal on our HART transmitters, it was noticeable, we had comm failure status and we knew looking at the scope what it was.