Power Quality

[LARGrizzly]

What is it?

I know I can monitor it, but what can I do about it?

What do you hear about power quality in your area?

I am in the Northeast and all I hear is negative. We're having a funny glitch thing happening with our PLCs. The boss told me to investigate a UPS for each of the PLCs. With 80 or so PLCs in the plant I don't know if this is a cost effective plan. Maybe there is some other power conditioning device available that is cheap in volume so we can protect our PLCs? Any suggestions, or am I barking up the wrong tree?

 

[Bung]

Unfortunately, industrial plants can be their own worst enemy when it comes to ower quality problems. PQ issues are generally the result of something messing up the voltage waveform, resulting in bad effects for other things.

Each customer doing his own little bit to affect the waveform accumulates until everybody has a problem. Also, each device causing a problem has a bit of a "sphere of influence" (talking very generally) - the further away (electrically, not necessarilyy geographically) you are, the les the influence, which is why looking internally for sources of distortion usually carries the biggest gains.

Best thing to do is to get a PQ expert in to do some measurements and find out where the problem is, and what it is. UPSs / power conditioners have their own problems for power quality, and if you are not careful you could end up with a similar problem sommewhere else (or maybe the plant next door gets it instead.

Sorry I haven't been very specific, but it is a kind of "how long is a piece of string" type question.

Bung
Life is non-linear...

 

[nag400]

Dear LARGrizzly,

These questions arise and people will think of electricity only when these type of non-trivial problems arise. and its obvious that electricity / power will be cursed.. sorry for being ironic but it is the fact everyone is aware of..
May be the following suggestions will provide you some tips of the power variation.
Let us define quality supply
i define it as ripple / harmonic free, constant frequency and constant voltage source.
PLC device being sensetive are always may not sustain small variations in power supply hence needs quality supply.
Going for UPS is the quick choice and decision everyone makes of. In that case go for an on-line UPS which will be active through out the period of operation. Go for a higher capacity UPS which will monitor the whole sysem instead of individual.
CVT's at higher ratings if possible can be used.
If there are surges in the system go for a filtering device (inductors and capacitors).
Before i suggest anything, Analysis has to be carried out regarding the load, load pattern, Supply level etc. or may be you can even explore to have a captive generation !!!
-nag400

 

[LARGrizzly]

Thanks.

I didn't realize that my question was as general as it was. I called the local utility and they started asking questions that I didn't have answers to. It was then that I realized I had bitten off more than I can chew. (Please excuse the cliche) They try to look for patterns, how long its been going on, what our service level/usage is, how often it happens, etc., etc.

Unfortunately I haven't seen any of the symptoms. The boss said get a UPS so I can only repeat the things he said he has seen. We have a PQ monitor here, I guess I'll have to try to learn something about it and hook it up. Thanks again for your responses.

 

[cuky2000]

Dear LARGrizzly,

The enclose site provide you with a checklist that could help to identify and characterize the PQ problem in your facility. I suggest checking if your local utility has similar information available.

May not be a bad idea start an experiment with one unit to evaluate the effect of dedicated UPS for the most critical PLC. There are cost effective choices in the market for small units similar to the one use for PC that could be an alternative to explore. Fill free to invite UPS vendors, but try to keep an independent judgment.

Beware that some UPS could create additional harmonics upstream. Therefore, the impact of installing large amount of UPS or bigger unit should be considered.

Good luck.

PQ Check List:

http://www.pge.com/002_biz_svc/002c1b_checklist.shtml

 

[electricpete]

It might be helpful to contact the PLC manufacturer to see if they have reports of similar problems. Is there known weakness to voltage spikes, dips, etc. Most likely you won't get a response to that question but it doesn't hurt to fish. Next question: What are the spec's that are required for the incoming power. They should be able to answer that question. If nothing else then you have something you can compare to what you measure (or what power company estimates) to help prove that power quality may be causing your problems.

By the way, is it possible that the PLC's are having problems unrelated to power quality?

I'm sure you've already considered most of this. I apologize if all this is obvious.