If the primary line voltage rating is 3.3 kV (Y side) of a 25 kVA, Y-Δ transformer (the per phase turns ratio is 5: 1), then the line current rating of the secondary side (in Ampere) is _________.

Concept:

In a three-phase system,

- The current passing through any two lines of supply is called as line current
- The current passing through a supply line is called as phase current
- The voltage across any single phase is called as phase voltage
- The voltage difference between any two phases is called as line voltage

Important Point:

In a star-connected three-phase system,

VL = √3 × Vph

And IL = Iph

$I_{ph}=\frac{V_L}{\surd 3Z}$

In a delta connected three-phase system,

VL = Vph

IL = √3 × Iph

$I_{ph}=\frac{V_L}{Z}$

Where,

VL is line voltage

Vph is phase voltage

IL is line current

Iph is phase current

Explanation:

Primary side rating: 3.3 kV(Y), 25 kVA

Phase current in primary side $=\frac{25}{\sqrt{3}\times 3.3}=4.37\mathrm{A}$

Phase turns ratio = 5 : 1

Phase current in secondary side = 5 × 4.374 = 21.87 A

Line current in secondary side $=21.87\times \sqrt{3}=37.88\mathrm{A}$