Correct Answer - Option 3 : 57.7%
Concept:
In star connection
VL = √3VPh
IL = Iph
In delta connection
VL = Vph
IL = √3Iph
VL = line voltage
Vph = phase voltage
IL = line current
Iph = phase current
In transformer
\(\frac{{{V_P}}}{{{V_S}}} = \frac{{{N_P}}}{{{N_S}}} = \frac{{{I_S}}}{{{I_P}}}\)
Explanation:
As phase voltage is equal to line voltage in Δ-Δ, this connection requires more no of turns per phase when compared to Y-Y of same voltage rating.
For same voltage rating the Y-Y requires lesser number of turns/phase.
Therefore, the Y-Y bank has less capacity compared to Δ-Δ bank and it is 57.7% of the capacity of Δ-Δ bank.
Important Points:
- Delta – star connection type three-phase transformer is used for both large and low voltage rating transformers.
- Delta – star transformer is used at the generator side to step up the voltage levels and Star – delta transformer is used at the load side of distribution systems to step down the voltage levels.
- Star – star connection transformers are used for small, high voltage transformers.
- Delta – delta connection transformers are used for large, low voltage transformers.
