Tansformer power usage 3

[weldon]

I'm in maintenance at a graphic printing plant. Currently I am trying to find ways to lower the electrical bill. We have 14 480-volt transformers in the plant, they run constantly even when the machinery is not running. The units are idle two days every weekend and three full weeks a year. Is there any way to calculate the power consumption of these transformers when they are setting idle? I feel they should be shut down when not in use and brought back on one at a time to alleviate any electrical spikes at the meter.

 

[RRaghunath]

Weldon,

I appreciate your idea. Yes, ou can measure the no-load power by connecting watt meter on the primary. This, if done for one transformer, may be you have a typical value that is applicable for all the transformers (they are all identical!).

Or else, you can ask the manufacturer for a copy of the factory test report which will include this information.

For switching off the transformers, you can draw up a scheme using the auxiliary contacts machinery power supply CBs or switches for automatic switching off. If your interest is to switch off only during weekends, manual action could be simpler.

Hope you have got atleast part answer.

Wish you all the best.

 

[jbartos]

Suggestion: The idle power transformer kW consumption can be calculated from the transformer efficiency that is probably on the nameplate. kWloss=(1-efficiency)xkW. kW is approximately equal to transformer nameplate rating in kVA. Check with the transformer manufacturer tech support regarding the weekend turn-offs. There are other aspects involved, for example temperature dilatations (contractions, expansions), moisture condensation, etc. These could have an impact on the transformer life expectancy.

 

[busbar]

It is somewhat in vogue these days to trim energy use, and success can be dependent on diminishing numbers. Cutting loose some transformers may also reduce “standby” energy use, often present as transformer load but not mentioned in equipment literature. It’s difficult to say without some measurements.

A healthy portion of no-load losses are reactive [id est, of low power factor] so savings will to a large degree dependent on the utility rate structure, and plugging your measurements into varying facets of that rate. I would study the ranging capabilities of whatever portable load-logging instrument, to avoid “buried in the mud” inaccuracies.

Consider starting with one of the larger loads, and one of the smaller loads, and ramp from there. Depending on the served process, Monday mornings {after-weekend repowering} can be heartburn for maintenance personnel.

 

[mpparent]

Another thing to consider is the possibility of the consolidation of loads, eg if you have 5 xfmrs, try consolidating to 2 or 3(you would have to do a load study to determine this). The benefit of this is that transformers like to be loaded, and unfortunately, transformers are typically loaded to only about 30% of their capacity. Two things would happen, you would have a higher efficiency on the transformers you have operating, which would help out on their losses, and also, you would recoup the losses of the others, since they would be non-functional. Now having said that, it would probably be wise to do a pay-back analysis to see if it's worth investing in a possible infrastructure rework.

Mike

 

[nishatabad220]

wat is the idea of restricted earth fault protection

 

[mansooratdelphi]

weldon
you have not mentioned the VA or KVA ratings of the transformers. Low ratings are usually no problem when switching on or off.
I assume that the 14 transformers operate 14 independant machines. It may be possible for the machine operators to get used to switching OFF an ON as required.
regards

 

[DanDel]

I performed a survey which included monitoring the idling current for several 1500-2000kVA transformers several years ago. The largest amount I found was approximately 1.5% of the rated kVA, which included mostly reactive power, not kW.
I wouldn't expect much savings from shutting down idling transformers for two days out of the week. The small amount of current flow used in the transformer is mostly comprised of reactive power (kVAR), and the only kW used by the idling transformer is dissipated as a small amount of heat, which in turn is a cheap way to keep moisture out of the windings. The investment of time and materials used to de-energize and then sequentially re-energize these transformers would be greater than any savings, and the added stress to the transformers will definitely reduce their service life. The reduction in service life for multiple re-energization increases greatly with the size and primary voltages of the transformers. Check your electric bill for low Power Factor charges for a start to lower the energy cost. Keep in mind that ANY Power Factor charges are excessive, since they all can be eliminated with capacitors.

 

[electricuwe]

jbartos,

you should consider that there are two different values of transformer losses:

No load loss (losses in core)

Load losses (losses in the windings)

In an idle transformer only no load losses there will be only no load losses.

The calculation you suggested in your post dated Jan 5th does not take care of the different kind of losses. Usually no load losses are quite low compared to the transformers rating and I fully agree to DanDel.

 

[weldon]

Thanks for all the information and input. It doen't sound like turning off the idle transformers would be much of a savings and may even cause extra wear. The plant has cut the budget as much as possilble without laying anyone off. This is the main reason I am looking into saving on the power bill. I have installed fresh air intakes on the A/C units that run in the winter to keep the compressors off as much as possible. I am in the process of installing additional air tanks and repairing air lines to cut down on the air compressors running time. I know all plants have different power needs, but is there any general cost cutting ideas anyone would have? DanDale, what do you mean that all power factors can be eliminated with capacitors?

 

[DanDel]

Weldon, check your electric bill. If you have a charge (penalty) for low power factor, this can be offset by installing capacitors in the system. The best installation of the capacitors is different for different applications, but first see if the utility is giving you a low PF penalty. Check out our web page -

http://www.neete.com/

and click on the 'Power Factor/kW/kVA/kVAR Relationships' text in the left column for a quick explanation of PF.

 

[mysterymike]

Correcting power factor is a great way to reduce bills, by avoiding penalties.

Also, depending on who your power authority is and how they bill, there is usually a peak usage charge, which large motors can create large peak usages when they start. If you have any large HP motors not on drives, look at simple softstarts to eliminate inrush currents.

Also, often overlooked in power savings but critical, are zero-sequence harmonics. These are created when the three phases are not balanced, and they generate (or can generate) large neutral currents, which are wasted but you are charged for in some cases. Solution one is to balance loads on all phases on all branches as best as can be done. Solution 2 is some companies (PQI, Commonwealth Sprague and probably many more) have zero sequence harmonic filtering systems.