Question

Consider the per-unit sequence networks of $\mathit{Y}\mathbf{-}\mathit{Y}\mathbf{}\mathbf{,}\mathbf{}\mathit{Y}\mathbf{-}\mathbf{∆}$, and $\mathbf{∆}\mathbf{-}\mathbf{∆}$transformers with neutral impedances of ${\mathit{Z}}_{\mathbf{n}}$on the high-voltage -side and ${\mathit{Z}}_{\mathbf{n}}$on the low-voltage Y-side. Answer the following:

(i) Zero-sequence currents (a) can or (b) cannot flow in the Y winding with a neutral connection; corresponding zero-sequence currents (a) do or (b) do not flow within the delta winding; however, zero sequence current (a) does or (b) does not enter or leave the D winding. In a zero-sequence network, (a) 1, (b) 2, or (c) 3 times the neutral impedance comes into play in series.

(ii) In $\left(HW\right)\mathbf{-}\left(Y-∆\right)$transformers, if a phase shift is included as per the American-standard notation, the ratio ___________ is used in positive-sequence network, and ___________ the ratio ___________ is used in the negative-sequence network.

(iii) The base voltages depend on the winding connections; the per-unit impedances (a) do or (b) do not depend on the winding connections.

Short answer

Answer

(i) Therefore, the correct options are (a) can, (a) do, (b) does not and (c) 3.

(ii) The ratio for the positive sequence network is $e^{j{30}^{°}}:1$and for negative sequence network is $e^{-j{30}^{°}}:1$.

(iii) Therefore, the correct option is (b): do not.

Step-by-step solution

Determine the zero-sequence current can or can’t flow in star and delta connection.

(i)

Zero-sequence currents can flow in the Y winding with a neutral connection; corresponding zero-sequence currents do flow within the delta winding; however, zero sequence current does not enter or leave the D winding. In a zero-sequence network, 3 times the neutral impedance comes into play in series.

Determine the ratio for positive-sequence and negative sequence network.

(ii)

By referring the table of the figure 8.19, the ratio used for the positive sequence network is $e^{j{30}^{°}}:1$and ratio used for negative sequence network is $e^{-j{30}^{°}}:1$.

Therefore, if the phase shift included according to American standard, the ratio for the positive sequence network is $e^{j{30}^{°}}:1$and for negative sequence network is $e^{-j{30}^{°}}:1$.

Determine the ratio for positive-sequence and negative sequence network.

(iii)

The base voltage is calculated on the connection of the winding but the per unit value of impedance is calculated on the base value of the power and voltage.

Therefore, the base voltage is dependent on the winding connection but per unit impedance do not depend on winding connection.