Electrical Loads Assessment

1. You know the voltage of the machines, but you have not told US.....
2. CB ratings are usually amps, but cannot be compared with kVA loads without voltage.....

Sorry,it is just where i live in the world voltage is 380 (Line-Line)

thanks for your concern

Thanks, numbers are :-

1. 71 kVA x 15 machines = 1065 kVA.
2. (1065/0.38)/1.732 = 1618 amps /phase.
3. 1065 kVA on one 1500 kVA transfo is OK.
4. Machines run at 60 kVA, distinctly less than 71 kVA rated. 15 x 60 = 900 kVA.
5. Even if all machines fed through bus coupler (1600 amp), no problem, considering 2. and 4.
6. Machines started separately, not together - 8 x full load is 568 kVA. Note 568 + 900 = 1468. Compare transfo rating 1500 kVA - no voltage drop or rating problems.

Is question answered OK?

thanks alot...

If the (15) machines are divided about equally on each side of the bus tie breaker, this ought to work. (The bus tie breaker needs to support only whichever fraction of the loads is a bit higher than half.)

Quote Eagle Wings "one last question, there are records of energy (Kwh) on medium side that shows consumption / month it can be accurate that i divide this energy by no.of working days by working hours to get "P"in Kw.."

No. It will give you a number but it likely won't be the peak. Suggest you get a recording meter to determine the actual peak load. If you are metered on a kw demand billing, the utility will have that information.

Example demand 1000 watts for 1 hr is 1000 kwh and a peak of 1000 watts

demand 1 watt for 1000 hrs is 1000 kwh and peak is 1 watt.

Both give you the same kwh's but significant difference in peak demand.

We are expecting the arrival of 8 machines (we already have 5 )and 15 would be identical.

I am in no way capable of answering your question, however I just wondered about your statement in italics, to me it does not work out mathematically????

8 + 5 = 13.

15-13 = 2.

13 + 2 = 15 (Identical)

2 machines will come later, may be next year.

Mathematically solved.

Yep..............real convincing!!!!!!!

My knowledge in electrical engineering is fairly limited, to say the least.

...........it also appears that my knowledge of computers is on a par with my electrical knowledge...........I pressed the wrong button..........twice............forgive me, it must have been a seniors moment...............or as my grandson reminds me........."old timers"

Now getting back to what I wanted to ask..............Is what you pointed out in your answer to the original question, aligned to the power factor in any way? I just felt it may have.

i really appreciate the answer ,for me it was a bit weird this big difference between nominal and "actual"

it was a concern in the plant if we need buy higher transformers or not

the calculations based on total connected load (specially they need keep 100% redundanccy between two transformer with bus coupler)

doing algebric sum without any diversity factor makes it necessary to replace the 2*1.5 MVA with 2*3 ,we have other loads fed not only these machines

could the energy consumption make any sense?

i noticed the monthly Kwh over a year and highest value was 314,000 Kwh

meter placed on the MV ,it is the drawn by the two transformer ( and i can assume they are equally loaded)

could this help to determine the "reserve" on each one ?then compared to the new demand to check?

hope you can help again

thans

I'm sorry to say this, but you don't seem to have any idea what you are telking/asking about.

You did not say there were other loads of any consequence! This was major omission, it is the total load that matters!

Regarding the adequacy of transformers:-

1. it is the Volt x Amps and the power factor which decide the voltage drops due to regulation and motor starting. The kWh alone does not reveal this.
2. Diurnal and Seasonal variations of load and ambient temperature are important for transformer life and overload capacity.
3. A transformer to IEC76/BS171 is rated at 40 Celsius - in many locations such a temperature is rarely reached. Roughly speaking, the life expectancy of decades is based on a mean ambient of 15 Celsius/Oil temperature 75, over life.
4. Overload capacity of oil-filled transformers is considerable and the effect of overload on life may be negligible if overloads are when the ambient is low or are compensated by periods below full load. There was a section in BS171/IEC76 dealing with overload ratings and their effect on life.
5. Your transformers will normally run at ~50% rating, it is realistic to consider a degree of temporary overload when only one transfo is operating, so long as you do not get motor starting trouble due to voltage dips. In any case, the duration of a motor start is a "fleabite" to an elephant in heating up an oil-filled transfo.

Assuming the extra loads to the motors (which you did not give, please advise) hold no deleterious effect, I believe you are in a situation where some thought about your present and future load patterns and some finding, recording and logging of information could avoid an expensive and disruptive transformer change (not to mention switchgear, cables etc).

You need to find out all the metering you have in place which could give you relevant information and cost of extra meters or recorders - their cost will be far less than new transformers. You must have existing records of when and for how long motors have run, how many together etc - if not, it is information you ought to have for maintenance. A few pointers :-

1. kW-hour readings each hour every day should give you a load pattern, the longer you record, the more reliable and useful. It is very important to talk with the operations/production department so you know when readings are usual/abnormal. Some thought to the loads you have could give a good idea of how many motors are running and the corresponding kVA load. Separate instruments for your "housekeeping" load, distinct from the motors you mention, would help (kW-h at least, may be in various locations of which you are not yet aware).
2. Transformers may have oil or winding temperature indicators you can read regularly - these have indicators, alarms and protective shutdowns because they have great effect on life.
3. A simple thermometer in a proper housing on your site will give ambient temperatures. You may already have records. There may be a monitoring site nearby from which useful historic data is available - contact your national meteorological institute.
   

really appreciate the answer ,for me it was a bit weird this big difference between nominal and "actual"

it was a concern in the plant if we need buy higher transformers or not

the calculations based on total connected load (specially they need keep 100% redundanccy between two transformer with bus coupler)

doing algebric sum without any diversity factor makes it necessary to replace the 2*1.5 MVA with 2*3 ,we have other loads fed not only these machines

could the energy consumption make any sense?

i noticed the monthly Kwh over a year and highest value was 314,000 Kwh

meter placed on the MV ,it is the drawn by the two transformer ( and i can assume they are equally loaded)

could this help to determine the "reserve" on each one ?then compared to the new demand to check?

hope you can help again

thans