Question

In order to avoid difficulties with third-harmonic exciting current, which three-phase transformer connection is seldom used for step-up transformers between a generator and a transmission line in power systems.

$$\begin{gathered} \left(a\right)\mathbf{}\mathbf{Y}\mathbf{-}\mathbf{∆} \\ \left(b\right)\mathbf{∆}\mathbf{-}\mathbf{Y} \\ \left(c\right)\mathbf{Y}\mathbf{-}\mathbf{Y} \end{gathered}$$

Short answer

Therefore, the correct option is (c) $Y-Y$.

Step-by-step solution

Define the concept of circulating current.

The third-harmonic current is generated in a power system because of the non-linear nature of the B-H magnetization curve.

The advantage of using$∆$connection is that, the harmonic current cannot enter or leave the $∆$ connection, instead it keep rotating in circular motion.

Therefore, often$∆-Y$ transformer is used after a generator in power system.

Explane about using Y-Y connection.

To completely avoid the third-harmonic exciting current, the Y-Y connection is seldom used.

Therefore, the correct option is (c) Y-Y.