Power Distribution and Control System Chat

Starting a thread here for power distribution systems, including medium voltage system, and PLC-based control systems.

This is for the hard core power/controls folks out there in power land; otherwise consider this rather tedious and boring reading.

The question was asked, why don't I use 24 VDC in my PLC setups, and that needs some clarification.

A typical PLC comes with a power supply for the processor and all the boards connected to that chassis.
Those DC power supplies seem be very robust and reliable.

You can install several different types of cards in the chassis, such as 4-20mA for analog inputs and outputs, RTD modules for monitoring temperature, cards for monitoring vibration, and relay cards.
There are other types of cards available for other functions, but the above items are the functions I use exclusively on the projects I design.

For output contacts, I always use Form C double-pole, double-throw isolated relay contacts, to give total circuit isolation.

For the relay contacts, you can use a 24VDC power supply and route that through the relay contacts, or you can use 120VAC without a power supply, and route that through the contacts.

I never use a 24VDC power supply with the relay contacts because the power supply is an additional point of failure, and the control systems I design are for wastewater, which is considered critical infrastructure that needs to be very reliable.

The conductor lengths/distances in the two treatment plants where I typically work is large; each plant serves approximately 500,000 people.
I would not want to risk trying to use 24VDC over long distances, due to the voltage drop.

I use a pretty beefy 4-20mA twisted shielded pair, such as a #14 AWG, and those will work over a very long distance, such as at the lagoons, which may be 1/2 mile long.

I researched why some people use 24VDC, and one website mentioned safety concerns.
120VAC does cause a lot of deaths, and is a dangerous voltage because it causes heart fibrilation.

In a large wastewater plane, with large 4160 volt motors, 23kV power distribution, lots of 480 volt motors, extemsive hazardous locations such as Class I, Division 1, etc., the danger from 120VAC in the control systems seems minor.

For motor control centers, I include a terminal/relay cabinet on the end, so that a technician can troubleshoot all of the control circuits without having to open a bucket and be exposed to a potential 480 volt arc flash.
The control wiring between each starter bucket and the control panel can be pre-wired at the factory, and so you don't end up with a birdsnest of control wiring strung all through the motor control center.

In other locations I have had contractors string control wiring over the top of bare 480 volt MCC busses, and so I prefer to factory wire the MCC's.
I use 600 volt wire insulation for all low voltage wiring, with "low voltage" being any voltage less than 600 volts.

For control wiring that has to enter a classified location, I use an intrinsically safe relay in the control panel.

For process control in a relatively small area, there may be some advantages to using 24VDC for relays, but I personally would not use it for any application.

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For PLC systems, I require a non-proprietary system/software, and there cannot be any hidden or inaccessible code.
Allen-Bradley compact-logix is the standard for stand-along processes at the plants I work at, with Emerson controls used for SCADA systems.
I use coated cards where available from A-B, due to corrosion problems at wastewater plants.

I prefer simple ladder-logic, with perhaps an occasional simple function block.

The plants where I work have multiple sub-systems, such as dewatering, return sludge, aeration, fine and coarse screenings, de-grit systems, disinfection, etc., and typically each subsystem has one or more PLC's which operate that system as a stand-along process.

Then there is an overall plant PLC control/SCADA system that functions as both a monitoring/display at workstations all over the plant, plus workstations in the control room.
The SCADA system is controlling the overall plant process, and it often modulates the other stand-alone systems as required for the appropriate flows, oxygenation, etc.

There are large SCADA screens around the plant, you can see every motor in every process, and see which motors are running, what their flow rate is, the percentage open of modulated valves, levels, temperatures. There is no control from these overview screens, but they show at a glance what is happening with all of the equipment in the plant, and give total input and output flows for the plant.

The SCADA also allows data capture from all over the plant, which goes into algorithms and trending programs.

Communication outside of buildings, and around the large site is via fiber, which eliminates the danger of overvoltages from lightning strikes from propagating down control wiring around the site.

Medial converters are used to convert back to standard ethernet for connection to the PLC's.

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The overall control strategy of the main plant that I design for is "Keep things as simple as possible".

Use as few components/programming as possible for any given controls design.
I use fuzzy logic where applicable, since it is simple and very effective for many processes.

Use redundancy in power distribution and controls, when possible.

Keep all software non-proprietary, and open sourced, ie: open programming that everyone can access.

I try to stick with factory wiring diagrams if possible.

Sometimes I have to integrate several systems together with controls, interlocks, safeties, etc., and this requires a custom configuration.
The custom configurations are tricky, since one has to consider every state that each system may find itself in, and figure out how that will affected the other connected systems.

One system required interconnecting multiple large hydraulically actuated sewage valves with their respective 1,500 hp synchronous motor, and with the magnetic drive that couples the motor to the pump, for speed control, and interconnections to the controls for the 4160 v synchronous motor, plus controls back to the SCADA system.

If something goes wrong, and a valve closes, it can deadhead a pump, or if the valve opens without the motor running, it can backspin the motor and damage it, etc.

One has to come up with a matrix of possible conditions, and provide enough interlocking for a safe shutdown of the equipment if something malfunctions, or what happens if power is lost and then restored. How do you stop the process in a safe manner, and stop all the water flow without damaging the pumps.

You can cause millions of dollars of damage in just a few minutes if the controls don't work correctly in all situations/scenarios.


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GreenTwin said:

The question was asked, why don't I use 24 VDC in my PLC setups, and that needs some clarification.

Click to expand...

I really enjoyed reading your post it was insightful and it included your own personal experience.
But just for clarification you do use DC in your panels, (Supplied by the PLC rack / brick) you just prefer not to use 24 vdc control relays. Ok i understand that but ........ ( You know what they say about curiosity and cats)
I have a few more questions if you don't mind me asking ?
What about 24 vac control relays? Was it the use of DC switching power supply instead of the old heavy transformer and diode power supply that turn you away? I had some customers that prefer the isolation that is better with a old transformer type.
( I use to own a UL-508A listed custom control panel shop sold it to a ABS pump distributor in 2001. We design and build lift station control panels for municipalities on the east coast. )

GreenTwin said:

I require a non-proprietary system/software,

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You are the first person I ever head that from. So all your ladder logic programs are open source?
I wish I new some one like you when I had my panel shop. With a copy of your non-proprietary ladder logic programs and some minor changes I could of save a lot time on the sewer plant I did here in Fl. ( Don't take me serious I am just having a little fun with you. I understand what you mean. I knew of a programmer who use to progam a timed bug in his programs he called it job security. If I am not mistaken I think he will be secured for another 3 years if you get my drift)

I have used RSLogix , Studio5000, Modsoft , Unity , Melsoft , Easysoft and Direct Soft programing software in all my endeavors in the shop.

All of the control relays I use have 120VAC coils and contacts.

I occasionally see others use 24VDC in their PLC systems, but have not seen that on a large wastewater plant, and I think it is due to the long conductor lengths. Thinking back about 24VDC, I never have used it, and never considered using it, probably initially because it it required another device (power supply with a finite amount of capacity), and practical voltage drop considerations.
There is no practical limit regarding how many 120VAC circuits you can have on a PLC system, and one plant may have many hundreds of those circuits, with no power supplies anywhere.
Power supplies generate a lot of heat, and take up space in the control cabinet.

Some folks I think like that they can use battery backup with the 24VDC, but without AC power available, nothing matters, since you are not able to pump wastewater. One plant that I work at has 5 incoming utility circuits, each at 1200 amps, 23kV, and those have automated SF6 switching.

We use dual interruptible power supplies (120 VAC) to back up each PLC cabinet, and keep the logic functioning.

Inside the plant, all the 5kV switchgear is main-tie-main configured.
At the south plant, they use double-ended substations, which are very nice, with a walk-in switchgear section.
The north plant has dual 5kV feeds on the substations, but no redundancy on the 480 volt side, and only one transformer.

The north plant has a standby generator for the influent pump station critical loads, 800 kW diesel, and we are adding a 2.75 MW diesel at 5kV to power halft of the influent pump station.

As far as non-proprietary, I should clarify that.
What this means is that the integrator must furnish the owner with printed and digital documentation of all of his programming, and there cannot be any hidden or inaccessible code.
There cannot be any oddball code either, and generally it must be ladder logic, since there is a very wide understanding of that, and it is simple to understand and follow the logic.
Generally the ladder logic looks much like the manufacturer's equipment schematic, where possible.
This refers to the independent PLC systems in the plant.

I did some work for a production plant, and they needed a much more sophisticated program, and that setup did not use ladder logic, but used a much higher level program with many pre-programmed batch functions specific to that industry.

For overall plant SCADA and control, Emerson is used, and Emerson uses an open program, ie: I or anyone else with the password can access all the code in the plant directly, and modify the code if desired and authorized.

There are some integrator companies who will not let you access the code; they just make things run, but if you want to change something, only they can make that change, and you have to blindly accept what they do.
I don't work with proprietary integrators, since you lose control over the process when you hand that much off to a 3rd party.

With open programming, you could change to a different software/system if desired.

The Emerson PLC systems are in large cabinets, and the ones we use have dual processors that will hot-switch automatically if one fails. Very reliable system, with a very big company for support.

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Rule no.01 of effective project management is "Do not lose control of the project to anyone".

Integrators will try to do their own thing, as well as contractors, and even internally in the old days our CAD guys (back before we did our own CAD) would try and rig their drawings so only they could access and modify them (good ole job security).

I spell it out in the specifications exactly what is required, and I state that in the pre-bid meeting that it will be enforced.

The contractors I generally work with have a lot of experience in industrial/municipal, and they know what I want/need, so usually they don't try to pull something on me, but the construction process has to be monitored and documented very carefully every step of the way, else things can and will spin out of control.

Sometimes like herding cats, but one gets use to it.

There are some studies about the design/construction process, and they mention "the time/effort triangle", where the bulk of the planning and thought work should occur early in the project, as as things progress, everything comes together seamlessly and without problems, finishing at the point of the triangle.
Projects that don't adhere to the triangle approach are an absolute nightmare, and I make sure my projects always adhere.
Sometimes I get involved with others who don't use the triangle, and not only does it produce poor design, but it is impossible to be efficient, and impossible to make a profit when working that way.

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This may be a waste but just in case
For anyone who is reading this just out of curiosity that does not know what the acronym PLC, SCADA and RTU stand for.
PLC stands for programmable logic controller.
RTUs are referred to as remote telemetry units
SCADA stand for Supervisory Control and Data Acquisition.

GreenTwin said:

Some folks I think like that they can use battery backup with the 24VDC, but without AC power available, nothing matters, since you are not able to pump wastewater.

Click to expand...

The panels that I have designed and built for the State of Fl. the RTU systems had to include a 24 hour back up battery for loss of 120 VAC power.
That is why we used dc relays. The RTU systems for duplex lift stations used radios that required 12 vdc so we used 12vdc relays.
The RTU had to be able to send back info like Station number ,Pump station power failure , Individual pump failure
(Pump 1 fail / Pump 2 fail), High water alarm , Pump station control panel alarm and seal fail.

For a triplex station or panels that required the use of SCADA / PLC system we had to incorporate a UPS that had to be able to supply
power for 90 min. minimum for data acquisition. These stations are also required to have emergency diesel generators backup.

GreenTwin said:

As far as non-proprietary, I should clarify that.
What this means is that the integrator must furnish the owner with printed and digital documentation of all of his programming, and there cannot be any hidden or inaccessible code.

Click to expand...

We always supply the customer with four printed as built drawings, one copy was laminated on both sides and glue on the inside of the control panel door. Two floppy disk (Yes that's how long ago I was doing this.) with all documentation and three copy's of the program, one copy was write protected.

GreenTwin said:

My boss bankrupted a contractor for using the wrong pulling compound on a medium voltage cable installation for a very large facility.
The contractor used grease on very expensive 23kV cables, and degraded the cable covering/insulation.

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Unfortunately this happens more often than one realize with contractors. They try to save a buck and cause more problems. I bet between both of us could write a book about some of the nightmares we seen. On the plus side they do not stay in business very long.

GreenTwin said:

The contractors I generally work with have a lot of experience in industrial/municipal, and they know what I want/need, so usually they don't try to pull something on me, but the construction process has to be monitored and documented very carefully every step of the way, else things can and will spin out of control.

Click to expand...

The key there is experience in industrial/municipal here in Fl. they are few and far between.
I always made sure we had a field technician or a field engineer at all major start ups where we supply the panels.
We have supply panels from Fl. to N.J. and believe it or not in Alaska also, but they were built for Pritchard Brown emergency gen sets.

GreenTwin said:

Sometimes I get involved with others who don't use the triangle, and not only does it produce poor design, but it is impossible to be efficient, and impossible to make a profit when working that way.

Click to expand...

GreenTwin you sound like a man after my own heart.

GreenTwin said:

The question was asked, why don't I use 24 VDC in my PLC setups, and that needs some clarification.

A typical PLC comes with a power supply for the processor and all the boards connected to that chassis.
Those DC power supplies seem be very robust and reliable.

You can install several different types of cards in the chassis, such as 4-20mA for analog inputs and outputs, RTD modules for monitoring temperature, cards for monitoring vibration, and relay cards.
There are other types of cards available for other functions, but the above items are the functions I use exclusively on the projects I design.

For output contacts, I always use Form C double-pole, double-throw isolated relay contacts, to give total circuit isolation.

For the relay contacts, you can use a 24VDC power supply and route that through the relay contacts, or you can use 120VAC without a power supply, and route that through the contacts.

I never use a 24VDC power supply with the relay contacts because the power supply is an additional point of failure, and the control systems I design are for wastewater, which is considered critical infrastructure that needs to be very reliable.

The conductor lengths/distances in the two treatment plants where I typically work is large; each plant serves approximately 500,000 people.
I would not want to risk trying to use 24VDC over long distances, due to the voltage drop.

I use a pretty beefy 4-20mA twisted shielded pair, such as a #14 AWG, and those will work over a very long distance, such as at the lagoons, which may be 1/2 mile long.

I researched why some people use 24VDC, and one website mentioned safety concerns.
120VAC does cause a lot of deaths, and is a dangerous voltage because it causes heart fibrilation.

In a large wastewater plane, with large 4160 volt motors, 23kV power distribution, lots of 480 volt motors, extemsive hazardous locations such as Class I, Division 1, etc., the danger from 120VAC in the control systems seems minor.

For motor control centers, I include a terminal/relay cabinet on the end, so that a technician can troubleshoot all of the control circuits without having to open a bucket and be exposed to a potential 480 volt arc flash.
The control wiring between each starter bucket and the control panel can be pre-wired at the factory, and so you don't end up with a birdsnest of control wiring strung all through the motor control center.

In other locations I have had contractors string control wiring over the top of bare 480 volt MCC busses, and so I prefer to factory wire the MCC's.
I use 600 volt wire insulation for all low voltage wiring, with "low voltage" being any voltage less than 600 volts.

For control wiring that has to enter a classified location, I use an intrinsically safe relay in the control panel.

For process control in a relatively small area, there may be some advantages to using 24VDC for relays, but I personally would not use it for any application.

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Click to expand...

You might be interested to know that Hartford Insurance refused to let me use solid state design in a critical safety control panel. All relays all 120 volts all hard wired. Some DC was allowed because a battery room was available. Old school design but effective. The whole purpose was to force an organized sequenced shutdown.

If one is to survive in the business, one really has to learn fast, be savy and aware, and have a good situational awareness at all times.

I was lucky to join a company in 1989 that designed manufacturing plants all over the south-east USA, and many were 1,000,000 (+) square feet, and ran in the hundreds of millions of dollars.
The "old man" I worked for had 50 years of medium voltage and power distribution experience, and I can say for sure that I would not have made it without his expert guidance in everything I did in those early days.
He was not always a nice fellow, since in the end he got senile, but he did know his stuff very well.

There is a certain comradery among those who have made it in the business, such as wce4.
Sort of like going down in the trenches, doing the hand-to-hand combat (metaphorically speaking of course), coming out alive and 1/2 sane afterwards, and battle-hardened for life.
The proverbial saying comes to mind "What does not kill you makes you stronger"

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GreenTwin said:

There is a certain comradery among those who have made it in the business,

Click to expand...

It's a crappy job but we prefer it that way.
This gave me a chuckle when I read it for the first time. It was printed on a lift station pump supply company T shirts. Unfortunately I don't remember who the supply company was.

Last question did you ever work on a control panel with a bubbler system for liquid level measurement ? I was told back in days when I was active that treatment plants where shying away from them.

Just a quick note I will not able to reply for the next few days. Enjoy the eclipse looking forward in seeing your video.

The bubblers are here to stay, and they are alive and well.

They installed a nice ultrasonic, or something similar, with an 8 inch PVC tube down into the wet well.
Does not work well at all, due to things like floating trash, or the real killer is foam on top the influent.
Gives a totally wrong reading, and with lots of foam, it pegs out the pumps at 6,000 hp, resulting in an empty wet well, and pumps trying to run dry.

They still have the ultrasonic, but they went back to the old bubbler system, since it never fails, and always gives a rock solid reading.

For those not familiar with the bubbler level measuring device, it is a tube that leads down into the wet well, and compressed air is pumped down the tube.
It uses a pressure transducer at the top of the tube, and convert pressure to an analogous level.
From the internet: Immune to surface foam, pH, conductivity, temperature, turbulence, and solids content.
There is a compressed air regulator that pressurizes the tube, with a differential flow rate controller to maintain a constant flow rate regardless of fluid level in the wet well.

The tube is basically self-cleaning due to the air flowing out the end of the tube.
The electronics can be mounted far away from the waste stream, in a clean location.

The north plant used four 1,500 hp syncrhonous motors for influent lifting, each with magnetic drive coupling the motor to the associated pump. The mag drives are as large as the motors; perhaps 8 feet diameter, and about 8 feet tall.
Mag drives are an old technology, but give perfect and reliable speed control, which is ramped up or down depending on the bubbler reading.

The south plant uses these crazy electrodes in a pot of electrolyte, and they control those motors by raising and lowering the electrode in the pot. I am not sure the horsepower of those motors, but I guess they would be induction type (I will have to ask about those; I am not as familiar with the south plant).
Motor voltage is 4,160 V.
This old technology also works well, but is rather crude.
The installed a variable speed drive on one motor, and have had a lot of trouble with it.
Solid state devices are sensitive to temperature, corrosion, voltage spikes, and overcurrent.

VFD's are the trend these days, but that is one device that they continuously have problems with all over the plant.
For one pump station in a nearby city, it had about 5 pumps at 60 hp each.
I use a suppressor on the incoming service, on the line and load side of each VFD, on the control wiring in the VFD cabinet, etc.
This is the only pump station in that city that never fails after a big lightning storm.
They called me up and asked me why their other pumps stations kept failing, and I said "Because I did not design them.".

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WRT the relay voltage issue, most aircraft, which admittedly are smaller than large manufacturing plants, use 28VDC for relays and distribute 115VAC 400 Hz 3-phase for most electronics and other uses. 28VDC is important because it is below the value that is considered a safety risk in the MIL Spec world and therefore does not require the rigorous guarding that higher voltages require. I would think that today, anyone laying out a large control system would use optical or low voltage low current distribution to remote locations (to save power, reduce EMI related potentials, and to keep the size of wiring smaller), CAN bus is typical though there are other protocols.

I do fiber between buildings/structures, and around lagoons, etc., but once you get down to the PLC level, you have islands of strategic PLC's that control their own zones, such as aeration, return sludge, disinfection, etc., and those are generally hardwired.

For open-close valves, and there are generally a lot of those, the owner likes to seen open-close confirmation on the SCADA screen, so I use limit switches on both ends of the travel. Simple things like limit switches don't really lend themselves to a fiber solution.

For security, the trend is to use a media converter at each camera, with fiber between them, since they are scattered all over the site, and the poles are pretty much lightning magnets.

I have seen smart devices that you can link with ethernet, but in a highly corrosive wastewater plant, the biggest problem I see is electronics failures. There is nothing quite as utilitarian or flexible as a good PLC system, with the bulk of the wiring being be tin-plated copper.

For the tight confines of an aircraft, I could see where low voltage relays would add a lot of safety, and they would be practical given the short distances involved.

Communications between PLC's and between remote modules and PLC's is generally ethernet, since the speed has gotten so high, such as the giga speeds. With a big system, the high speed keeps things very close to real time.

So called "long runs" in real life use interposing relays.
End of the b/s.

We don't use those at the plant, but I guess at some point they may become necessary in some situations.

For control systems on opposite sides of the plant, we tie with fiber, so it is a radial approach with the PLC's and their hardwired I/O, and then all the PLC's are all tied with a fiber loop (with redundancy).

I have heard of the repeating relays from the old telegraph days.

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