Circulating currents in banks of transformers with star-delta connection

[diegoz14]

First of all i want to apologize for the grammatical errors, i’m not a native english speaker, but i really need help with this topic.

I’m an intern in the electrical company of my country, at the present time the company is focusing its efforts to reduce the technical and non technical loses in the distribution system, i was assigned the task of look into the behavior of star-delta connection transformers banks of high consumers, my job is basically use a amperimetric clamp and measure the currents of MediumV (34.5-13.8k)V side, LowV(480-120)V side and circulating currents, also power factor, impedance of the bank and position of the tap’s selector. The electrical system in my country has been left behind for decades making it a non-regulated system until now, i often find customers with circulating currents in the LowV side, 8 times higher than the most high line current, note that almost all clients have unbalanced loads and power factor up to 0.9, what makes this to happen? if you see how much current enters in the primary and how much is out trough the secondary you can see heavy losses, what makes the banks to have this kind of behavior? the currents between phases shouldn’t be line currents/√3 ? an unbalanced bank in delta will have phase currents higher than the line currents in an effort to “balance” the loads? I know that for a real analysis we need the angles of the currents, but with the information available, what conclusions can we obtain?

HERE YOU CAN SEE A GOOD EXAMPLES OF WHAT I’M TALKING ABOUT

client: ZIPPERs

total kVA: 225

discriminated kVA : 1*75+1*75+1*75 kva

PRIMARY PHASE A CURRENT: 7.5

PRIMARY PHASE B CURRENT: 7.3

PRIMARY PHASE C CURRENT: 7.2

SECONDARY PHASE A CURRENT: 22.1

SECONDARY PHASE B CURRENT: 10.1

SECONDARY PHASE C CURRENT: 15.8

SECONDARY NEUTRAL CURRENT: 34.3

BRIDGE AB CURRENT: 230

BRIDGE BC CURRENT: 215

BRIDGE CA CURRENT : 225

this is the “ZIPPERs” case, here we have a 3*75kva star-delta bank with a transformation relation of 13.8kV to 480/240 v , this bank is used to feed a poultry farm, if you see, the highest secondary phase current is just 22.1 A, meanwhile in the “bridges” that connects the individual transformers into a delta connection, the currents are up to 220 A , i don’t know the angles of the currents nor the voltages, this is why i can calculate the real efficiency, if a use only apparent power the efficiency of this case in specific is only 7.22% , and this is not the only case where the efficiency is below than 50%.

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