Question

In per-unit sequence models of three-phase three-winding transformers, for the general zero-sequence network, the connection between terminals H and $\mathit{H}\mathbf{\text{'}}$depends on how the high-voltage windings are connected:

(i) For solidly grounded Y, ___________ H to $\mathit{H}\mathbf{\text{'}}$.

(ii) For grounded Y through ${\mathit{Z}}_{\mathbf{n}}$connect ___________ from H to $\mathit{H}\mathbf{\text{'}}$.

(iii) For ungrounded Y, leave role="math" localid="1656061422648" ${\mathit{H}}^{\mathbf{-}}\mathbf{-}\mathit{H}\mathbf{\text{'}}$___________.

(iv) For $\mathbf{∆}$, ___________ ${\mathit{H}}^{\mathbf{\text{'}}}$to the reference bus.

Short answer

Given

(i) The connection between H and $H\text{'}$should be shorted for solidly grounded Y.

(ii) The H and $H\text{'}$should be connected through $3Z_n$for grounded Y through $Z_n$.

(iii) For ungrounded Y leave$H-H\text{'}$open.

(iv) The $H\text{'}$should be shorted to reference bus for $∆$.

Step-by-step solution

Determine the connection between terminal H and H' for solidly grounded Y.

(i)

Draw the general per unit zero sequence network of three winding transformer.

Here, $H,M$and $X$represents the high, medium and low values of the current voltage and impedances.

For the solidly grounded Y, the connection between the H and $H\text{'}$for general zero sequence network should be shorted at high voltage side.

Hence the connection between H and $H\text{'}$should be shorted for solidly grounded Y

Determine the connection between terminal H and H' for grounded Y through Zn.

(ii)

For general per unit zero sequence network, the connection between H and $H\text{'}$on high voltage side should be done through $3Z_n$neutral impedance.

Hence the H and $H\text{'}$should be connected through $3Z_n$for grounded through $Z_n$.

Determine the connection between terminal H and H' for ungrounded Y.

(iii)

The connection between H and $H\text{'}$should be open at high voltage side of three phase winding for zero sequence network.

Hence, for ungrounded Y leave $H-H\text{'}$open.

Determine the connection H'to reference bus.

(iv)

For per unit zero sequence network of three-phase three-winding transformer the $H\text{'}$should be shorted to the reference bus.

Hence, the $H\text{'}$should be shorted to reference bus for $∆$.