Calculating Maximum Demand

Quote "if i calculate KW for each line separately in start of every minute by a device that have a clock, synchronized by GPS, so i can sum up KW of these 3 line and then calculate Max Demand based on it. "

If I am understanding you correctly, your method appears to be right. Each sub will have its own 15 minute demand during the day but that may not be the maximum system demand. The max system demand occurs at some point in time during the day. So you will need to record each sub demand and pick the time when the sum of the three subs is the greatest. There are many recording meters that can do this for you.

what type of meters can do this for me?

Pls elaborate your power scenario. Do all three supplies are synchronised or isolated? Do each line supply to different mills or areas? Do all the three areas work simultaneously?

about power scenario, i must say that the factory receive all it's power from one supplier and because of that, the supplier needs to calculate one max demand.

but the problem is that, the factory receive all it's power by 3 line from 3 substation.

every substation have it's one meter to calculate demand for that line but it is not synchronized by the other meters for other lines.

the factory try to share required power on it's input line to minimized it's demand.

I think we must have all demand data for all line in one place to calculate max demand.

or having a meter that can share data with other meters to do that!

reza-ra

Check here http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLL_en&q=electrical+power+monitoring+recorders

Google power monitoring recorders

Maximum demand is measured in kVA which is UxIxSqrt(3) and no power factor since =1.00 here.

The way the Maximum demand works: There are Two items used: the Total Time of Measurement T1 and the Periodic time for integration tp. These differ from metering company to other, or from country to country. Let us say that T1 = 15 minutes and that T1 has been divided into 30 sec intervals. That makes 15 x 60 / 30 = 30 intervals or periods of measurement.

Then: during every 30 sec, the meter measures the maximum demand kVA and records it. Each time this is measured, for every interval of 30 sec thereafter, the M.D. value is recorded untill the 15 minutes are elapsed. Then the Highest value of all these periods is recorded and displayed. The separate registers for the 30s intervals are zeroed and the process repeats.

Every 15 minutes, the M.D. value is Updated to the highest only.

At month end, the M.D. register is read and billed then the register is zeroed to start a new month billing period.

Now Your Problem: If I Understood you, You have 3 Supply lines to your factory. If the 3 supplies are supplying at the same time 3 different area of your factory, then they are billed separately (3 metering devices, one for each separate zones. In this case, the 3 Maximum demand values will be added or billed separately with the kWh values for 3 bills.

If the 3 supplies are used to supply the same Zone (same circuit with the same machinery) and used to switch over to each supply when the others failed, then the maximum demand should not be added but the maximum of the 3 meters will be considered.

On the other hand, all depends on who are the 3 suppliers: 3 different power suppliers or the same company? And how the arrangement has been done with them.

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Thanks for your complete response.

But the problem is that we have one supplier, and the factory receive it's power from many lines because they needs all lines power at the same time.

there is no switching, the factory use all 3 line power for it's production together.

and this is because the factory have many improvement in last years, and may need more power in feature.

I am Glad to have been of help.

And as explained, the 3 maximum demand will be added by the supplier.

If you could put all the lines into One of bigger capacity (transformer, Supply source...) so as to force the supply company to meter you from one single source, then you will lower the maximum demand if the 3 zones supplied do not have their peak demand at the same time (which will be most of the times)!!

Also, Try to start the biggest motors (or loads) before the smaller ones and in sequence so as not to have too much peak demand.

This will also help reduce the maximum demand.

Check out what is known as a totalizer. Your three sources probably each generate pulses for the meter, collect the pulses over a 15 minute period, and determine the demand for that period. A totalizer just adds the pulses from each source and then you can compute the total demand for the plant. It will almost always be less than the peak from each source, unless they just all happen to hit their peak during the same 15 minutes, which is possible if the entire plant load is tied together and the three sources act as a network. If different parts of the plant are isolated and served by a single source, then they probably won't occur at the same time.