Question

Sequence networks for three-phase symmetrical series impedances are (a) coupled or (b) uncoupled; positive-sequence currents produce only ___________ voltage drops.

Short answer

Sequence networks for three-phase symmetrical series impedances are coupled positive-sequence currents produce only positive sequence voltage drops.

Step-by-step solution

Determine the equation to calculate the voltage drop produce by the positive sequence current.

The sequence impedance diagonal matrix for the series impedance is given as follows.

${\mathit{z}}_{\mathbf{s}}\mathbf{=}\mathbf{\left[}\begin{array}{ccc}{\mathbf{Z}}_{\mathbf{aa}}\mathbf{+}\mathbf{2}{\mathbf{Z}}_{\mathbf{ab}}& \mathbf{0}& \mathbf{0}\\ \mathbf{0}& {\mathbf{Z}}_{\mathbf{aa}}\mathbf{-}{\mathbf{Z}}_{\mathbf{ab}}& \mathbf{0}\\ \mathbf{0}& \mathbf{0}& {\mathbf{Z}}_{\mathbf{aa}}\mathbf{-}{\mathbf{Z}}_{\mathbf{ab}}\end{array}\mathbf{\right]}$

Here Z_aa is the series impedance and same for all phases and Z_abis the mutual impedance between the phases.

Calculate the voltage drop produce by the positive sequence current.

Draw the zero-sequence network for the three-phase symmetrical series impedance

Draw the positive-sequence network for the three-phase symmetrical series impedance.

Draw the negative-sequence network for the three-phase symmetrical series impedance.

Since, the current depends on the particular sequence network, therefore networks are uncoupled and positive current only produce the positive sequence voltage drop.