5 Mega Watt UPS

It sounds as though you might need a bank of gensets for on-site power generation. I'm not sure there is such a thing as a 5MW UPS; the battery set would be huge.

Hehe...

Joke of the day.

Would love to see the battery bank!

5MW would be totally impractical.

5MW isn't a UPS. It's a sizeable lump of standby diesel generator.

The only MW size UPS I have seen was the size of a warehouse filled floor to ceiling with batteries and designed for powering a small power grid. I cannot remember how much it cost but I think it awas in the tens of millions (USD).

S&C can supply UPS's up to 20MVA but you will also need additional transformers as these store and operate in the KV range. They also offer low voltage UPS's up to 2.5MVA, but in either case you would need to split and distribute your load accordingly.

http://www.sandc.com/products/purewave/ups.asp

Rather than one big UPS I think you need to think smaller, such as adding either power quality filtering or smaller UPS's to selected loads such as variable speed drives. Diesel generators could be an option but may be cost prohibitive in this application for momentary voltage sags.

None of these solutions is going to be inexpensive however so a careful analysis and study of your power quality problems is VITAL in order so a suitable solution can be tailored to your specific problems and site.

How long are these voltage sags and to what level does the voltage drop?

What is the site load distribution (what is the total rating of your critical loads)?

Jack of trade thank you for your reply, we encountered voltage sag frequently say twice a week, ranging from 11 seconds, basically our Medium Voltage Switch Gear cut off if it sense undervoltage ranging above 6 seconds thus affecting LVSWG supllying power to Equipment Panels.

I think Diesel Generator is impractical for us because we dont encounter power failures only voltage sag, thats why im sourcing out if there is any available in the market with this kind of UPS. And if we encounter voltage sag you can not immediately energize the generators and still production will still stop, which unlike with UPS it will rectify voltage dropping and sustain it within few minutes.

Why is an 11-second sag sufficient of a problem to warrant consideration of a sizeable investment in UPS equipment?

11 seconds is quite long, but if your undervoltage is only occurring twice a week is it possible for you to increase your protection undervoltage trip time to allow your equipment to "ride through" the voltage sag without tripping.

This may seem overly simple but is the most simplest and cost-effective option based on the information given.

You would need to check to see if your local and site standards will allow you to increase the undervoltage protection time setting (should be ok), and that your equipment can handle the extended undervoltage duration (eg- variable frequency drive trip settings will not operate, cables can handle the increased current (as V drops I increases to compensate), etc) (which should also be ok but will need a bit of analysis). This solution should be be ok and is what I would try if in your position.

The only other solution I see are banks of expensive industrial HV (in the kV range) UPS's powering small sections only of critical plant each, and this could cost upwards of millions for small sections of a 5MW plant.

I wonder if you could segregate the sensing and control circuits on the VFDs and other starters from the power wiring. If so, you could put these more sensitive items onto a small UPS, say on the order of 5KW (rather than MW).

NEMA-rated motors are supposed to be able to start at 80% (I think) of nominal voltage; if so, they should be able to ride through your brief sags, so long as the electronics and controls remain stable.

We will supply online 5 MVA UPS from India..are you able to bear?.Here cost is criteria.We will design based on customer requirements .other wise you can segregate the critical loads from connected load of the plant.We will supply suitable capacity online UPS for critical loads.

25 years ago I was installing a hard-wired logic control system, very voltage sensitive. The then design engineer decreed that the 11KV OCB's should be fitted with UV relays. OK they only tripped a couple of times a month but that made us look for a cure. Any sections of the plant that a voltage dip (sag) could be problematical were provided with UV relays that tripped the E Stop for that section only. All the HV UV relays were disabled.

To try and keep a site of 2 Sq miles running during a thunderstorm was a nightmare. I got so frustrated running from one substation to the next I just got a chair, sat down and watched the storm. The chair was on a platform 100' in the air, I was soaked to the skin!

The bottom line is only install UV protection where it's absolutely necessary.

Most VFDs give you direct access to their internal capacitor bank (for adding reactors etc.)You can use these terminals to add infinite external capacitance to overcome ANY power disturbance providing you add the NECESSERY control and protection cirquits.
Just adding capacitors won't do for many reasons the main being that the original rectifying cirquit can't handle the initial charging current they require,so they must be charged by an extra resistance that is automatically bypassed when the final voltage is reached. Of course you must provide protection fusing and discharging cirquits etc. It;s the cheapest and most effective way to go. than using UPS and paying astronomical costs at this power level