cycle transformer power to save energy ?

[Andy32821]

Hi Everybody,

The facility I am at (large convention center) has dozens of 480 volt 125 kva 3 phase power transformers to supply power to exhibitors. Even with no shows the unloaded transformers are energized costing about $800/ day in energy costs. The heat then must be removed by HVAC costing even more.

I would like to cycle these off when there is no event. This would mean turning the unloaded transformers off/on via a breaker about three times a week. In some cases certain transformers may be off for months.

The transformers are in clean air-conditioned tunnels and humidity can be controlled.

Am I going to save money with this scheme or is pre-mature equipment failure of the transformers and breakers going to out weigh power savings?

Thanks,

Andy

 

[jraef]

You seem to have covered the bases by knowing that humidity could be a factor, but be forewarned that breakers are designed for limited operations. You may be better served to add latching contactors on the primary feeds to avoid costly breaker failures when you really need the power. Think of how your conventioneers will feel if you go to switch that breaker on one morning and it fails.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[hottip]

There are two issues here:
1. You can try to approach the energy waste by switching the transformers on/off manually or you can buy an energy management system which would do this for you. We need to probable to revise the maintenance procedure (to decrease the time between the maintenace intervals)
2. The other issue is that you can try to negotiate with the energy company to buy energy on a daily basis (I do not know if this is working in your country). The advantage of doing this is that you can have different energy tariffs for different days (depending on the energy you consume in a peak day with exhibitors and in a day with no exibitors) and will save a lot of money for you.

 

[EEJaime]

Cycling the transformers, especially if they remain off for extended periods of time will probably save you money. If a transformer is unloaded, the only energy costs you will be saving will be the losses across the coils. So a simple payback analysis will tell you more. If you were able to rotate the units which are cycled off so that all the transformers are excercised, it would help in keeping the entire system maintained. Is there any way to install a non-fused disconnect switch in the primary feed to the transformer? This would save the c/b's from having to be manually operated.

$800.00/day at say $.15/kWh= 5333.33kWh / 24 hours is 222.22kW. If your losses are in the neighborhood of 5% at 0.8pf, you're talking about 5500 kVa in transformation. About 45 of the 125kVa units you mentioned. Wow, I guess you could say you work in a LARGE convention center.

When you have one of your larger shows, are all the units in use? What is the average number of exhibitors per show? Maybe you could alternate the branches used from one show to the next. I don't know if this is of any help, or if I just muddied the waters further. But it would be interesting to track the use and costs over a year. Your building must have an Energy Management System. It might be a good investment for long term records and maintenance to install monitoring modules on the feeds to the transformers to track exactly what the consumption is and make adjustments accordingly.

Just more things to consider. Good luck.

 

[SierraJ]

Your proposal seems very interesting and besides the usual skepticism for the possible transformer failures (and one could argue about failure to down stream equipment also), I would urge you to make an engineering cost analysis and try it in a limited basis.

I think you have an ideal scenario for doing a pilot project and test yourself the pros and cons of your proposal. You will be able to confirm (with a minimum investment):

1.) failure rate of switched vs. non-switched transformers and any other related down stream equipment (panelboards, contactors, starters, etc.)
2.) energy savings
3.) etc.

I would select a "test area" (single building, a single quadrant within a building, etc.) and install (if not already available) single throw heavy-duty safety switches in the supply side of the dry type transformers. Also develop an SOP for the test area to clearly define when to turn on or off the subject transformers, etc.

Otherwise I would try to obtain (from other forum members or from the transformer vendor) any type of study or paper on transformer failure due to prolonged de-energization.

Keep us posted on any progress of your idea.

 

[stevenal]

Your proposal may save more than energy. Industrial customers are usually billed for poor power factor, and unloaded transformers are the worst.

 

[Andy32821]

Surprisingly our power factor is so good we get a credit from the power company. (I am at a loss how that happens.)


I calculate saving about $250,000 a year with $30,000 worth of labor. (Savings include power and HVAC chilled water reductions. Not that much compared to $13,000,000 a year in utilities, but a good hedge against the rate increases.

We will try ½ the facility for a while and see how it goes.

Thanks for everybody’s input.
Andy