Power Distribution Unit problem

[mpparent]

All,

I have a PDU(600V-208/120V, 3PH), that has recently been showing signs of a strange problem. The unit has been tested and gone over in prior years, THD measurements, voltage/current measurements, etc., with everything in tolerance. Last year for instance, the voltage at the output of the xfmr was about 209V phase to phase. The input voltage was 600V. This year, w/ the same input voltage, the secondary measured at 214V phase to phase between all phases. Any ideas as to what's going on?? There's only about a 3% THD on voltage, and all of the neutral to ground voltages measure 0.

 

[gordonl]

The voltage measured at the secondary terminals of the transformer will vary with load. If you measure it once with heavy load, and again with light load you could see a 3-6% (or more) voltage variation due to the impedance of the transformer.

 

[mpparent]

Very true gordonl, however, unfortunately, the load has not changed......

 

[AAK]

Hi mpparent,
Did u mean the PDU was serviced last year ?
If so, maybe the tapping of the transformer was changed to give a higher secondary voltage. Or as gordonl suggests even small change in the load from last year causes such a slight change in the voltage (2.88 %).. Hope this helps. Best regards,
AAK

 

[polyphase]

The voltage drop through transformers is a function of the load power factor and the X/R ratio of the transformer. A transformer may have a voltage drop of less than 1% when loaded to 100% of its MVA or KVA rating with a unity(100%) power factor. The same transformer may have voltage drops of 4%, 5%, 6%, even 10% when loaded to 100% of its rating with a poor power factor load.

 

[mpparent]

Gents,
The PDU was serviced last year. The transformer taps haven't changed either. In response to the load pf comment, I believe the load power factors are around 85%+.

 

[dpc]

Are you sure the primary voltage has not changed?

 

[mpparent]

According to the maintenance guys I've talked with, the primary voltage hasn't changed. Also, ahead of the PDU's, exists a static UPS, so I'm pretty assured the voltage regulation is good.

 

[hinrg]

mpparent;
Did you check the voltage inputs to the static switch? Are they an A / B input from two seperate UPS systems or are they a UPS / utility fed. it sounds like you were on sub "A" one time and sub " B" on the other. Also, what type of meter are you using. The PDU metering can be out of calibration, you stated a 600 volt input on a 480 volt system. try using a good quality, calibrated power meter.

 

[busbar]

hinrg-- I may be out of turn with this comment, but I’m fairly sure 600V distribution is common in certain regions. I don’t see any posts having mentioned 480V.

 

[mpparent]

Busbar,

You are correct. This system is located in the U.S., however, as unusual as it may sound, it is a 600V system, not a 480V system as hinrg implies. The PDU in question is not a static switch PDU, if that's what you implied hinrg, it is however dual source. Manual operation of breakers is used to transfer between sources(the way the system is now, we can transfer from UPS to utility or from UPS to UPS, even though there is only one UPS system. The system is set up to be as flexible as possible for maintenance activity while still having the ability of keeping the critical load active). Our team uses Fluke digital meters.

 

[hinrg]

mpparent,
sorry about the improper response, i wasn't paying attention to details.