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Context

A plastics factory with installed power of about 500 kVA. Most of the consumption is represented by motors and resistances.

Problem

There are several (8+ a day) power interruptions, lasting usually less than 50 seconds each. There's a diesel power plant and an automatic transfer (of which they have damaged two already). Usually the plant "enters" when the power is reestablished, which doesn't solve the problems caused by the interruption and instead causes the transfer to suffer. In very unstable days, they prefer to override the transfer and have it in "manual" to avoid damage. Interruptions beyond 18 seconds require a complete stop of some of the machines and the consequent generation of considerable waste.

Proposed solution

There are large UPS (500+ kVA), often used to serve data centers and the like. Their prices range around USD 200k for a 500 kVA one. The idea is to set up the transfer to take on for larger outages (say in excess of 120 or 150 seconds) and to have the UPS as the main source for shorter ones.

Doubts and Questions

I'm not sure if the inductive nature of the load and their start peaks can be effectively handled by the UPS. I don't know if it's robust enough for the task at hand (in lack of a better word). What protections should I consider for it?

In one UPS specs sheet I read that it can handle 150% load for one minute (that will be 750 kVA) and 125% (625 kVA) for 10 minutes. Does that mean that it can keep the plant running for those times? What should I consider to avoid the UPS not performing to expectation?

Do you see a better solution?

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  • $\begingroup$ Start everything on the genny the switch to the UPS once running. $\endgroup$
    – Solar Mike
    Commented Feb 7, 2020 at 14:00
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    $\begingroup$ For that kind of money (200K) you can buy your own electric generators and experience no interruptions at all, as long as you don't run out of fuel. :-) $\endgroup$ Commented Feb 9, 2020 at 0:14
  • $\begingroup$ Presumably generator fuel is more expensive than grid power, when it's available, or else they would be running on generators all the time (in fact there wouldn't be a grid). $\endgroup$ Commented Feb 10, 2020 at 17:11
  • $\begingroup$ As for your question, have you considered asking the company that makes the UPS? For a $200k sale, I'm sure they'll be more than willing to answer all your questions. It's not like a $20 device for a hobby project where you just have to buy one and see how well it works. $\endgroup$ Commented Feb 10, 2020 at 17:11
  • $\begingroup$ @user253751 yeah, sure. But it's likely their answers could be a little bit biased. That's why I came to the experts ;) $\endgroup$
    – PavoDive
    Commented Feb 11, 2020 at 10:15

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I realize it might be a bit moot with the delay in this answer, but I would never advise an industrial plant to install a high-capacity UPS for process loads. The loads that your plastics factory are running are not what these UPS systems are designed for. Even with a 500 kVA UPS, think large but rather "regular" loads: hospitals, schools, data centers, etc.

What they need to do is get to the root cause of why they are experiencing 8 intermittent power failures per day. If this is an issue with the utility provider, then those connections and solutions need to be found at that level. A gen-set with ATS cross-over is a great solution for periodic power failures on critical loads, but 8 switches a day may be too many depending on the equipment.

What can help the plant to return to operation faster is to have automation equipment, computers, and critical/life safety loads on an industrial UPS and to keep the process loads on generator backup. This allows your plant to not have to "reboot" each time the power goes out and get back running faster. If the cause of the power loss is brown-outs, and too high of a demand from the plant, you should be staggering the start of higher motors to avoid this.

Now, on the flip side, if you have ended up installing a high-capacity UPS, I would love to know what you went with and how it is functioning!

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