Negative Sequence Impedance for Directional Over Current Protection

In the previous post, we learned how to use the negative sequence components to calculate the torque and then to define the fault direction. In this post, a similar quantitates are used but to calculate the impedance and then to identify the fault direction.

The negative sequence impedance can be easily calculated using the ratio between negative sequence voltage and current:

Z2=V2/I2

For a forward fault, the negative sequence current lags the driving voltage (the faulted phase voltage) by the MTA (Maximum Torque Angle) of the line and is considered positive.

For a reverse fault, the negative-sequence current is 180 degrees out-of-phase, or in the reverse direction of the negative sequence current for a forward fault. This direction is considered reverse or negative.