an advice for uninterruptible power supply

We need an advice:

Here in the University we have a medical equipment for experiments which is powered with 480V and have a load of 37KVA

The problem is that here in my country the public power supply is very unstable and for this reason we get a power generator (Diesel machine) of 100KVA. The generator have a time to start of about 1 minute after power down. This time to start produces frequent failures in the equiqment which is very sensible to power change. I'ts necessary to have continuous transparent power supply for the equipment.

Can you recommend some general solutions for this problem?

thanks
nebisman

Hmm, tough one...
This must be a x-ray generator based machine, at this consumption. One simple sollution would be 20X24V/100amps accumulators, coupled in series, which offers 480Vdc. Are you sure about the voltage? Isn't it 380vac/3phase? But that depends on your machine. Most of wall powered equipment requires AC power. However, some of them can be DC powered. For example, a power supply which have a 50Hz transformer as a primary energy transformer cannot be DC powered, but a SMPS power supply can, because at the input, after EMI filters, an AC to DC converter (bridge+condensator filter) always exists in SMPS. Check the schematic of your equipment, and if the power suppy (supplies, all of them) is SPMS, then you can put DC on it. Also, you will have to use a high power switching relay ( nominal current at specified power rate seem to be around 77 amps), so the relay must have switching capability of at least 100 amps. Also, between the relay and equipment you must insert a serious filter, to compensate the glitch inserted by the relay. It is a general idea, do consult other people and take care...your equipment seem to be expensive.

The power supply of equipment is 480 three phase VAC. Yes this equipment is very sensible and expensive and I guess we request a system that maintains constant AC voltaje, still if power is down and retain the power until the power generator (three phase too) is active. The central idea is that equipment receives continuous AC three phase power supply all time.

Thanks for your repply and more ideas are very grateful.

thanks nebisman

Hi,
In this case, an inverter would be required, after those accumulators, something similar in principle of operation with an inverter from a single phase classic ups. Maybe as switching elements you can use IGBTs, they are capable of high power. The main energy supply can be the same, 20X24V to supply witn 480V and then use the inverter to obtain ac/3p from dc, or you can use fewer accus and in this case, the inverter must boost up the voltage. In this case, larger currents would be required in primary section. Here is another idea: such an inverter can be used online all the time, inserted between source energy (either accus or rectified main power supply) and your equipment, because if the power grid have this poor quality, I am sure that besides power interruptions you also have large power fluctuation around its nominal value. Btw. in which country are you?

use a professional grade UPS system with a short backup time.

there will not be any shortcut solution when you are playing with some thing like 40 Kva.

Hi,
I agree with rauol. We can talk about principle of operations and how it is possible to do it, but at the end of the line, when you are taking into consideration other non technical issues (the value of your equipment, the responsibility, etc.), the best idea is to buy an already designed equipment or to adress to a specialised company for this (i.ex. Siemens automotive). 40kva is not an usual ups, and I think it is not a good idea to build it yourself and make experiments on such expensive equipment like a x-ray based one.