Adjustment of Protection Zones in an Impedance Relay for Two Adjacent Transmission Lines

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This Demonstration shows how to calculate adjustments for the protection zones in an impedance relay between two adjacent transmission lines.


The "AB transmission line parameters" are the resistance and reactance of the first line. The adjacent BC line is treated analogously.

The "properties of the adjustment" controls are the parameters needed to adjust in the impedance relay. Zone 1 gives the percentage of the AB transmission line that needs protection. Zones 2 and 3 protect the AB and BC lines. Zone 3 is backup protection for zone 2, and zone 2 is backup protection for zone 1.

The "maximum torque angle" control is a parameter for electromechanical relays and represents the angle of adjustment. Every protection adjustment is referenced to this angle.


Contributed by: Daniel Motter (February 2012)
Open content licensed under CC BY-NC-SA




[1] W. D. Stevenson, Jr., Elements of Power System Analysis, 3rd ed., New York: McGraw–Hill, New York, 1975.

[2] C. R. Mason, "Line Protection with Distance Relays," Art & Science of Protective Relaying, Hoboken, NJ: Wiley, 1956 pp. 295–321.

[3] "Distance Protection," Network Protection & Automation Guide, Paris, France: AREVA, 2005 pp. 171–191.

[4] "Impedance Relay and Protection Assemblies," Zurich, Switzerland: ABB, 1999.

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