Transformer Loading

[dougjl]

I have some concerns about adding load onto a 1500 KVA transformer and wanted to find out what the accepted method of determing the allowable load to the transformer. It appears from the drawings I have of this and several other 1500 KVA transformers that the total H.P. + KVA attached to them exceeds 1500 KVA.

Is it okay to use actual current readings to determine tha actual load on the transformer. It seems that all the motors and other devices fed from the transformer would not be pulling full load amps all the time.

Thanks for any help!

Doug

 

[Newton1Law]

The total connected load applied to most transformers in the range of 1500 kVA would exceed the nameplate rating of the transformer. This is normal. There is considerable diversity in the load attached to a transformer. Depending on the type of load, i.e., electric heat, motors, induction furnaces, etc. there will be an approximate amount of the connected load that will be seen on the transformer at any given time. As an example, lets assume you have a process that requires 200 kW of lighting load that will stay on 24 hours a day. Heating load of 500 kW in total throughout the building but controlled in each area by separate thermostats. Since not all areas of the building will require heating capacity of the same levels or at the same time, it is reasonable to assume there will not be 500 kW of heating load on all the time. Since we know that not all heating load will be on 24 hours a day we need to figure out how much heat will come on and how often. A great deal of this type of work has been done in the past and average figures have been measured. For some types of space heating the demand at any one time is about 50% of the connected load.

To size the transformer then we have:

Lighting load – 200 kW ( no diversity - it’s on 24 hours/day)
Heating load - 250 kW ( only 50% of total at anytime normally)

Total transformer load = 200 + 250 = 450 kW. Assume a power factor of 90 % and we end up with a total load of about 450 kW/0.90 = 500 kVA.

The transformer should be a 500 kVA unit. The overload ability of the transformer for a short duration would allow you supply a greater load assuming a return to normal loading say within a day or so.

Hope this helps.

 

[230842]

To complete Newton1Law's answer, actual current reading is a very good (maybe the best) criterion to evaluate actual load, according to the followin rule:
Max. load per phase is 500 kVA
Let's say phase to neutral voltage is Vfn. (phase to phase voltage divided by the square root of 3)
Then, actual line current (amperes)must not exceed quotient between load (VA) and Vfn (volts) appart from overload conditions explained by Newton1Law, Julian