Sending Control Signals over Power Lines
Sending Control Signals over Power Lines
(OP)
A little while back I was trading emails with a PE who claimed to have come up with a way to effectively transmit data over 60Hz (or whatever frequency for that matter) power lines. I have kind of lost touch since then but wanted to get some feedback on thoughts as to whether or not this would be feasible and what kind of regulations or restrictions might be placed on doing so.
One of the big things I'm wondering about is power quality - how much 'divergence' (THD?) from a pure sine wave is one allotted and what standards and terms should I become familiar with to quantify this? Additionally, what is the relationship between THD on for example, the low side of a utility transformer (where control signals might be useful), and the high side of that transformer? Would the reatance of the transformer act to filter out fast rise times on the secondary? I'm picturing putting a square wave directly into the low side of a transformer (no 60Hz signal), would the high side of the transformer essentially be the signal having passed through a bandpass filter?
Additionally, I have run into applications where engineers decide that they need to run control signal wires separate from power wires, and it sounds like the major driving factor behind this is due to electromagnetic interference that is introduced in the signal cables when in close proximity to the power cables. Are there any additional restrictions or codes that govern this or is it simply good engineering practice, depending on the case..?
One of the big things I'm wondering about is power quality - how much 'divergence' (THD?) from a pure sine wave is one allotted and what standards and terms should I become familiar with to quantify this? Additionally, what is the relationship between THD on for example, the low side of a utility transformer (where control signals might be useful), and the high side of that transformer? Would the reatance of the transformer act to filter out fast rise times on the secondary? I'm picturing putting a square wave directly into the low side of a transformer (no 60Hz signal), would the high side of the transformer essentially be the signal having passed through a bandpass filter?
Additionally, I have run into applications where engineers decide that they need to run control signal wires separate from power wires, and it sounds like the major driving factor behind this is due to electromagnetic interference that is introduced in the signal cables when in close proximity to the power cables. Are there any additional restrictions or codes that govern this or is it simply good engineering practice, depending on the case..?






RE: Sending Control Signals over Power Lines
htt
RE: Sending Control Signals over Power Lines
RE: Sending Control Signals over Power Lines
My company did it for Sea Land Systems. We transmitted refrigerated container data over all the 460VAC power lines on ship and shore.
There are no THD issues with respect to data over power lines as you are talking about a small HF modulated signal on top of a very large noisy LF power signal.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sending Control Signals over Power Lines
I am wondering what might be causing the issues with interference with some of the bands dictated by the FCC. It seems as though a number of groups are up in arms over this and this is the big stumbling block right now for this technology. It also looks like it is suprisingly mature, in the sense that there are already a number of people on this board who have mentioned they've used something like it in their design or have come across it in work.
There's two design considerations I would like to learn more about concerning these technologies:
1. Cause for the creation of interference of other bands that groups are complaining about. There shouldn't be any 'new' EM waves created on a large scale that I would think - it sounds like the carrier is 20-200 kHz but it's modulated on a power cable, which (especially for HV/MV systems, I know they are'nt all this way especially as you get into the distribution level) there should be a bit of insulation that will mitigate these effects, I would think. And if it's just a matter of getting the right insulation in place (my Emag is a bit sketchy), then I don't see this as being a limiting factor at all. Of course from the sounds of the complaints I think I'm off base on this.
2. Data rate/ Throughput of these systems. I can't even really begin to comfortably talk about communication systems and use the right terminology, but I'm hoping to build some intuition for what kind of constraints there are on transmitting data (speed of transfer, bandwith, etc) when using a carrier wave on top of 60Hz wave, vs just transmitting the data directly over the line (using the line JUST for the data).
RE: Sending Control Signals over Power Lines
A popular technique is to use dual frequency signalling with alternate frequency pairs in the band below 150 kHz. Another one is the "Turtle" technology, which is in the 90-95 and 105-110 Hz bands. Turtle is very slow, a typical meter reading takes around 27 hours.
There are problems with both techniques. The <150 kHz is being disturbed by frequency inverters and, to get the signal through, many systems now operate above their allowed power levels as "temporary" solutions. That makes the total radiation from these systems a problem for some tele and radio activities.
The Turtle system used to be very reliable. But the use of frequency inverters to couple wind turbines to the grid has introduced interharminics that sweep the 90-95 Hz and 105-110 Hz bands continuously. My understanding is that Turtle - for obvious reasons - does not check and retransmit every character sent. Instead, the whole telegram is checked when transmission is complete (after 27 hours). Since it has been subject to interference many times during the 27 h period, it is mostly (always) so corrupt that is is scrapped.
I have been involved with several cases here in Sweden where about six thousand meters (seems to be a practical limit for one Turtle system) cannot be read because of this. Measurements always show either frequency components sweeping in and out the communication frequencies or, more common with the larger turbines, an elevated noise floor with fairly constant energy/Hz across the signalling frequency bands.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Sending Control Signals over Power Lines
Guess they'll turn off all your power on June 31. Stock up on candles.
jimgineer; The bandwidth of power line communications is absurdly low for broadband to a bunch of customers. One fiber will carry vastly more data with much less trouble OTShelf.
The right of way is always the issue. If a power company wants to get into the data business they should either lease right-of-way to someone who knows what they're doing, or run copperless fiber tower to tower, or buried and not involve their HV systems.
This is just what the railroads did, ran fiber next to the rails on their right-of-ways. They actually used locomotives to try to plow the trenches.
Furthermore, I believe there is a glut of bandwidth because the lots of fiber was run to cover projected needs, then we came up with wave division multiplexing which upped the bandwidth capacity of any fiber more than 10x.
So what's the purpose of your data?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sending Control Signals over Power Lines
Can you give any more information on how exactly this 'turtle' system transmits data using 90-95 and 110-115Hz bands? Also, does a transformer effectively shield against transmission of control signal (essentially isolate), but still (obviously) transmit power to the other side? Or does it depend on the signalling method to be able to answer this question.
From a utility perspective, if they wanted to transmit data on a power line they would prefer not to have to buy repeaters to get the signal through the coils, to the other side. I guess I'm thinking about the reverse of this-I don't want to transmist any data to the other side of the transformer ---
RE: Sending Control Signals over Power Lines
http://www.data-linc.com/powerlcm/lcmapp.htm
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Sending Control Signals over Power Lines
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sending Control Signals over Power Lines
RE: Sending Control Signals over Power Lines
Is there also energy transfer from the effect of the transformer acting as an air core transformer?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sending Control Signals over Power Lines
RE: Sending Control Signals over Power Lines
I've always understood pf correction you to place the capacitance in parallel with the line. I know this is not fully related to what we were talking about before, but still.
RE: Sending Control Signals over Power Lines