Calculate Phase current 3

[STEIN69]

Can anyone tell me how to calculate phase current on a DELTA
connected Transformer, Primary side?

 

[rbulsara]

Firstly, you need to understand or clarify what you call 'phase'.

If you are talking about the current in each 'phase' winding of the transformer (internal to the transformer) then the current in each 'winding' arranged in delta would be the rated current divided by 1.732, at full load.

1.732 is square root of 3.

If you mean the current in each of the three conductor feeding the tranformer, it is the rated current of the unit at full load. This is also the line current.


At any other loads, the current in delta winding is line current divided by 1.732.

 

[eemotor]

In a delta connected transformer, each line is connected to two transformer phases. Thus, each phase current is equal to Iline/sqrt3. <br>You can also find out the phase current by using VAphase/Ephase

 

[RalphChristie]

In addition to above comments:

P = 3^0.5 x V line-line x I line

where:
P = Power rating of trsf in VA
3^0.5 = square root of 3
V = line to line voltage
I = Full load current

You can use this formula for either star or delta and on either side of the transformer.

For delta connection:
V line = V phase
I line = 3^0.5 x I phase

For star connection
V line = 3^0.5 x V phase
I line = I phase



Example

100kVA trsf, 11kV/380V delta primary /star secondary

Primary side
P = 3^0.5 x V line-line x I line
100 000 = 1.73 x 11000 x I line
I line = 5.249A

I line = 3^0.5 x I phase (delta connection)
5.249 = 1.73 x I phase
I phase = 3.03A

Secondary side:
P = 3^0.5 x V line-line x I line
100 000 = 1.73 x 380 x I line
I line = 151.934A

I line = I phase (star connection)
I phase = 151.934A