Automatic transfer switch 1

[Rabalac]

Hi,

I am an electrical engineer at a telecom facility. We want to improve the reliability and redundancy of ac power system. Previously we had one diesel engine generator, however, we purchased another genset (ATS and generator).

I told the installer that I wanted the standby system to set up in such away that generators run alternately. And if one generator fail the other one should come online and carry the load.

The installer is saying that because we did not purchase an alternating ATS it cannot work like that. I dis agree.I think that it must be possible to build a control system to achieve that operation or maybe there is a kit out there that can be used.

Does anyone have any experience with this sort of thing. I neeed some advice.

 

[peebee]

Anything is possible with enough money. But the alternating system you describe is certainly an unusual one.

Much more common would be a system where both generators started on any outage, and the first one up to speed would assume the load. This might be a cheaper and more reliable system for you.

You would need to provide much more detail on the intended electrical arrangement of your two ATS's in order for us to provide a more detailed response.

 

[dpc]

If you are concerned about reliability, you will want to try to start both generators and be happy if one actually starts. If both start, you can always shut one off if the outage is long.

 

[jghrist]

I'm no expert, but I don't think that the two generators can be run in parallel unless the proper paralleling controls are used. If the OP just purchased a generator and ATS meant for single generator backup, will this work?

 

[dpc]

That's a good point, jghrist. If no paralleling equipment is provided, it may be necessary to start only one, with the other as a backup if the first doesn't start. This could probably be manually alternated via selector switch every month or so.

Must be Monday morning.

 

[rbulsara]

rabalac:

By your own assertion you want to improve the reliability and redundancy. If that is true, you are going about it in a terribly wrong way.

You need to hire proper consultants and have them design a system. You can't just buy a piece of equipment and hope a contractor will make it "work."

System you describe is not at all considered reliable. The alernating ATS is a totally differrent kind of animal and very unusual.

I concurr with other posts above.

 

[lansford]

Re: "You can't just buy a piece of equipment and hope a contractor will make it "work."
Unfortunately for us consutants, the above statement is very very true and the way it is in my world. Everyone is our competition.
JIM

 

[gepman]

If your generator(s) are required under Article 700 or Article 701 you will probably have a difficult time and spend a lot of money getting any custom controls listed for your application. Your generator set probably falls under Article 702 so you can make custom controls without much problem and your requirements should be reasonably easy to accomplish presuming that all the necessary I/O for the control system is available in your two generators and ATS's.

As others have said, the time to design the system is BEFORE you purchase the equipment, NOT after.

 

[cxphim]

Hi

If reliability and redundancy is important then the best solution is to start both generators and run them in parallel. to do this you need to haw load sharing capabilities and auto synchronizing. this solution is almost as safe as you can get using diesel generators. to go to the step of rebuilding the system you need to look at the statistic of your power suppler. how likely is it that there will be a power outage? if you look at 5 years how many times and how long time each time. then you look at your generators in the same time frame. then you get a picture of how safe your system is. but most important is what is the consequence of loosing the power, economic, health and safety and so on. but these are all things I guess that you already haw consider since you are installing generator NR 2. but to sum it up. If your generators Haw electronic governor's (speed control) there is many products on the marked that can be implemented to your system Woodward, Deep sea, Deif, these are all manufacturers that haw products that can do bout your original solution an my solution. also a big advantage of my solution is that you can get automatic back synchronising of the net. and you can do scheduled testing whit load without any blackout. To giwe more spesific answer we all need more info about yor system.

 

[catserveng]

We did a number of similar systems for some dairies a few years ago, the decision of which unit to run was based on available fuel, but really wasn't a big problem. The sites had a small gas engine running off digester gas, and a similar size diesel in case they needed to run for awhile but didn't have enough digester fuel. They didn't mind open transition transfers as long as the cows got milked and the milk handled properly

If I am understanding you correctly, you have two independent units, each capable of supporting the facility by itself. You want to be able to have both installed and be able to select which one will start and carry load in the event of an outage, if the selected unit does not start and close within a desired time, you want to start the other unit and have it close to the building load. Is this correct?

If so, this is a fairly simple exercise, intercept the engine start signal between each ATS and it's respective engine/generator, and install a control system, we usually use a micro PLC with a local selector switch.

The reason this appears unusual is because no one wants to spend a lot on money for redundency. It sounds like you are proposing a complete redundent simple open trasition backup system. The US Military has had a us do a few of these for some test sites operating in remote areas, to date no problems if used as intended.

You can also make the controls start both engines, and only close one ATS, but requires more work and getting into the control wiring of the ATS, and should include some additional safeties.

Or you can go with a small switchgear lineup and controls for paralleling as described above, certainly more flexible, but also more expensive and complex. The nice thing about the system I think you want is the ability to perform maintenance on the generator and ATS and still have a standby for you facility.

Hope that helps

 

[burnt2x]

Peebee,

"Anything is possible with enough money."

to some market limitations though!


Rabalac,
You could go for a selector switch for simplicity, as mentioned above. I guess you only need to find the right persons to do what you want!

Maybe, just maybe, you want is a logic that alternately start the engines that you have, and it is possible.
It's a matter of having a logic that monitors which engine ran last (maybe a flip-flop circuit or a wired mechanically-interlocked keep relays) and lines up the other engine for the next start, and vice versa. An auto-start reset cycles the starting order if one genset fails to start. (Some time ago, I happened to buy a UL listed engine-driven fire pump with an EATON control that alternately starts the two starter motors of the engine!This is a different case but I guess the idea is the same in your problem)

I assume you have enough UPS power for the duration of the transfer. My experience during a time test of 2MVA back-up genset auto-transfer lasted 45 seconds due to longer engine run-up. The voltage level check set to go at about 70% of rated supply volts. Smaller gensets could be a lot faster.

 

[waross]

I suggest the following logic most easily implemented on a PLC but possible with relays.
1> An alternator circuit to point at the set scheduled to run next.
2> A power failure detection circuit that will send simultaneous start signals to both sets.
3> Circuits to detect good power from each set.
4> If the next set to run is the first with good power available, inhibit the other transfer switch and then transfer the load to the set.
5> If the UNscheduled set is the first with good power start a timer. (10 or 15 seconds?)
6> If the scheduled set produces good power within the time period, proceed with the transfer as at step 4>.
7> If the timer times out inhibit the scheduled transfer and transfer the load to the unscheduled set. Set an alarm to indicate a failed or slow stat.
I would also start a second timer at step 5>. (3 or 5 seconds?) If this timer timed out I would send an alert "Slow Start on Number 'X'".
The unused set may be shut down after a short period, or left running so as to be ready in the event that the first set on-line fails.
The last task will be the logic to detect the return of the mains and transfer back to the mains.
If you can accept slow open transition transfers that is a lot simpler and dependable than any other scheme.
And thanks to catserveng for his suggestions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Why thanks Bill,

Actually we had a pretty simple system, using the capability of the ATS's to do most of the thinking. We took the engine start command, utility status and generator status (usually slaved off the indicator lamps on the front of the ATS).

Since the ATS sensed loss of utility we just used the engine start outputs thru a selector switch, in case we needed to bypass the controller, then feed them into the PLC with the other status inputs. The priority selection was a simple switch, input high meant the "#1" unit went first, input low meant the "#2" unit went first. For a military job we added a flip flop circuit to swap priority after every run (that was more of a prime remote application).

Utility would fail, both ATS sent start command, PLC allows selected start to pass thru right away, PLC got notification of succesful start by looking at emergency supply status input, ATS will only switch to live source, so it switches, other ATS is still calling for start because utility is still out. If logic doesn't see the selected unit come up in desired time, it blocks start of first unit, the passes thru start of second unit. We know status of both units and of utility from two ATS inputs. Seven inputs and two outputs, we used a PICO smart relay on a job for a small marine plant with a similar requirement. We have also added interlocking relays and in some cases voltage sensing relays (dead bus type) for added protection to prevent any possiblity of an unwanted/uncontrolled sync.

Lots of ways to do it, usually I'm a little lazy and try to use all that was supplied by the EG and ATS suppliers, then just add what I need to get by.

I like using ATS's and controllers from Thomson Technology, since they usually have dry contacts for the status inputs described above, but we have also applied these type of systems to GE Zenith, ASCO, GENERAC, and CAT ATS.

We are starting to see some interest again as a lot of our customers are dealing with run time restrictions due to air pollution requirements, and the idea of different fueled redundent standbys is looking attractive to some customers.

Last job we did we used an AB MicroLogix 1100 PLC, and used the data logging feature to allow the customer to have a log of generator operation. Hardware cost just under a $1000, Programming labor was flat rated $2500, install labor for controls over existing engine and ATS system, about $1500, at the end we did a load bank, total job was just over $5000. Site had a 200ekW CAT unit about 8 years old with a ASCO ATS, and other unit was a 150 ONAN with an ONAN ATS that was older (not exactly sure of vintage). This customer went this route because the ONAN was old and had some problems in the past, but was buried in his facility, and the CAT was a trailer mounted unit they brought in in the summer when outages were likely, otherwise it was off site at another facility.

Mike L.

Again, not a perfect solution, but it has worked well if it meets the operational requirements of the customer, and fairly cost effective in most cases.

 

[Rabalac]

Thanks for your responses. I do not want a parallel system as Catsereng realized. Just for record I did not purchase or spec the gensets others did and I am trying to get the setup to work.

Thanks for your responses. CATSERENG, I am going to do some research into the ideas that you have given me.

 

[Rabalac]

Catserveng,

in my propsed system I wowuld want one ATS to chose between two gen. This output would feed another ats, say ats 2. Ats 2 will chose between gen power and utility.

Have you ever setup anything like that?

 

[ScottyUK]

Rabalac,

Why not have one bus with multiple infeeds switched under automated control? Someone was designing an outrageously complex switching scheme in a thread just recently. Might be worth a read.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

For the ultimate in safety with mechanical interlocks, your plan is best.
However, if either transfer switch fails, you may be dead in the water.
The best redundancy will be achieved by using one ATS for each generator. If one switch fails you can run on the other until repairs are made.
This is too close to call without on-site knowledge.
I have been asked several times to implement your scheme and rejected it as not suitable for the site. It was too complex for the third world where trained repair people were not readily available. In these instances I used one automatic transfer switch and a manual transfer switch to select the generator to run and a selector switch to send the start signal to the proper generator. Three installations have been running for about 10 years now with no problems and happy customers. A fourth installation ran well for about 5 years until one of the generators was moved to another location.
This was site decision not a technical decision.
In your case I am sure you have qualified service people available. You are on site. It's your call how to configure it!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

Rabalac,

I remember somebody telling me once to "KISS it" boy! (Keep It Stupidly Simple). 'twas my drill sgt, I guess! With that, I suggest "SIMPLICITY" as the design criteria.

IMHO, telecom companies usually do away with power reliabilty problems by using high-end and reliable backup UPS systems. Having said that, the company's personnel have enough window to go to the engine room, initiate genset start and put the genny online (manual transfer), whichever unit he/she chooses. That setup does the job. No hassles, safer, and doesn't require burning more bucks for a little comfort provided to the operators of the telecom company.

Your latest scheme for ATS (choice on the 2 gensets) is good. Just don't make things too complicated. More parts in means more parts to think about when maintaining them.

 

[catserveng]

Rabalac,

I'm pretty much in agreement with Bill's last comments. Either way will work, how you implement a system on your site depends on a number of factors.

It's obvious your current installer isn't willing to think outside the "box", he may have good reasons for it.

You have been presented with a number of opinions from many facets of this industry, now your best bet would be to find a local electrical engineer with a smaller size generation background, and ask him to look at your installation and operation and give you some expert advice that takes in not only your perspective and desires, but any code or regulatory issues applicable, and he may have a local source for the expertise you need to implement the system you want.

Hope that helps.