Line distance (21) protection is primarily used to protect transmission lines or other electrical equipment where the equipment’s impedance characteristic can be calculated or modeled with software applications. The protective relay calculates the ratio of measured voltage and current and uses Ohm’s Law to continually monitor the measured impedance while energized. The measured impedance is compared to the equipment impedance characteristic created by the relay settings and the relay will trip if the measured impedance falls within the setting characteristic. This method of protection creates a Zone of Protection which allows the relay to selectively trip when internal faults occur and ignore external faults. The relay impedance characteristic can also be applied with a larger characteristic and a time delay to provide backup protection for external equipment.
This is another video from the previously mentioned series, but it is an excellent introduction to distance or impedance protection and I learned something new while watching it!
I know that is seems we are just re-posting other information we find on the web right now. Don’t worry, we will be posting original content in June as we get all the procedures in place.