Question

In sequence networks, a Y-connected synchronous generator is represented by its source per-unit voltage only in ___________ network, while (a) synchronous, (b) transient or (c) sub-transient impedance is used in positive-sequence network for short-circuit studies.

Short answer

The source voltage is only presented in the positive sequence and sub transient impedance is used in positive sequence network for short circuit studies.

Step-by-step solution

Sequence diagram of the network.

The balance three phase system has three sequence networks: zero sequence network, positive sequence network and negative sequence network.

This sequence networks are independent, decoupled and contains only their specific quantitates. The zero sequence network contain only zero sequence quantities and it is also true for positive and negative sequence network.

Draw the positive sequence network.

In the sequence network for the generator only positive sequence network consist the voltage source. The negative and zero sequence network is represented by the equivalent impedance of their networks only.

The positive sequence network is shown below.

Here ${\mathit{E}}_{\mathbf{g}\mathbf{1}\mathbf{,}}{\mathit{Z}}_{\mathbf{g}\mathbf{1}}$and ${\mathit{I}}_{\mathbf{1}}$are voltage, impedance and current of the positive sequence network respectively. The sub transient impedance is used in positive sequence network for short circuit studies.

Therefore, the source voltage is only presented in the positive sequence and sub transient impedance is used in positive sequence network for short circuit studies.