UPS Power Distribution Unit....Necessary??

[crank71]

We are looking at a redesign for our control center UPS system. We have the option of duplicating the existing setup, which has been troublefree or making some cost savings by installing a ferro transf. & 42cct panel in place of a PDU.
99.9% of the time the PDU does no more than act as a distribution panel...but we have to plan for the 0.1%...
This a brief rundown of the system:
Under normal operation our critical SCADA loads are connected to a Digital Equipment Corp. PDU (circa 1986), which houses an isolation trans, 42cct panel board, some metering/protection functions...etc. The PDU is fed from an 18KVA UPS (circa 1997) and we have battery (circa 1997, 10 year design life) backup of around 2 hours at full load. A maintenance bypass is installed incase the UPS fails, utility fails or both. A 85KW 120/208V gen will supply power directly to the PDU. The iso. transformer in the PDU cleans up the gen power before distributing to the loads.
The PDU's I've looked at (MGE & Liebert) seem overkill for what we need.
I was envisioning a 42cct panelboard and a ferro transf in its place. Any comments?

 

[mpparent]

Sounds like overkill to me...I'd stick w/ your present situation.

Mike

 

[rbulsara]

I am not sure you are on the right track or have the correct understanding as to what do PDU /UPS or Ferro-resonant transformer (like Liebert's Datawave) do.

You are partially correct when you say PDU act as a distribution panel, 99.1% time. In fact I acts as such 100% of the time. In addition it has a transformer that has dual role. One is to transform (usually) 480V to 208/120V (or like if your voltages are different). Second is to provide isolation. The isolation is more important when the UPS is in bypass.

Now the part that PDU 'cleans up' power is completely wrong. The conditioning of power is done by the UPS (not PDU).

So I am not sure what a ferro transformer is doing it for you!!!. In fact you are lowering the 'conditioning' and battery back up capability and hence the reliability of the setup. You are going backward in time. If I am not mistaken you cannot have batteries with a ferro unit. Ferro is just a power conditioner, which a UPS also is, but only protects against momentary voltage dips and spikes, not against a power outage.

You should keep the UPS. UPS gives you the ability to have batteries to ride through duration between a power outage and the generator start!!! A ferro does not! There is nothing wrong with replacing PDU/transformer with a stand-alone transformer and wall mounted panelboards. If the UPS is 208/120V output, you may not need the transformer even, but it is good to have it for isolation when on the bypass.

 

[rbulsara]

Or if you are saying, you want to keep the UPS and just replace the PDU with a ferro transformer, that will be real waste of money to gain nothing.

If your load are really critical, hire some experienced consultant for advice and design.

 

[crank71]

I didn't mean that the PDU acts as a conditioner, just that it was meant to isolate the loads from the generator when run on bypass. I had been told by a couple of vendors that a PDU was overkill and that we could replace it with an isolation transformer and panelboard. I just wanted to throw it out to the forum.
Thanks for the post rbulsara...

 

[alehman]

The PDU is a convenient, compact and expensive way to package a transformer and panelboards. They are commonly designed to sit on a raised floor system, with flexible whip cables to the computer equipment exiting from the bottom. They work well in densely packed data center applications.

You can get almost any normal transformer options such as shielding, harmonic cancellation, ferroresonant, etc. in a PDU or in a separate transformer. The benefit of a ferro is voltage regulation. Ferros are typically used where there is no UPS to regulate voltage. With a UPS upstream, there is probably no need unless as you say, you are worried about your generator when operating on bypass. You must judge how important that is based on sensitivity of your load and frequency of operation on bypass.