3phase Transformers

Example:

Three single-phase 1 MVA 10 kV/230 V, 50 Hz transformers are connected in (\Delta-Y) to form a three-phase transformer bank. Determine,

• a) Turns-ratio
• b) Line-to-line voltage ratio
• c) Power Rating

Assume the total leakage reactance of each-single phase transformer referred to HV-side is (X_{eq}= x_1 + x_2' = 10~\Omega)

Construct single-phase equivalent circuit of the 3-phase transformer bank

• d) on per phase-delta basis
• e) on per-phase Y basis

Suppose that primary and secondary leakage reactances (actual values) of each single-phase transformer are given as: (X_{1}= j 5 ~\Omega)

(X_{2}= j 2.65 \times 10^{-3} ~\Omega)

• f) Repeat d-e
• g) Single-line diagram of a power system is given below

Obtain the single-phase equivalent circuit of the above system on the HV side (use per-phase-Wye basis)

a) (n=\frac{N1}{N_2}=\frac{V{1-phase}}{V_{2-phase}}=\frac{10000}{230}=43.5)

b) (\frac{V{1-line-to-line}}{V{2-line-to-line}}=\frac{10000}{230\sqrt{3}}=25)

c) Power Rating = 3 x 1 MVA = 3 MVA