Directional Earth-Fault Principles - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.2.2.6
420

Directional earth-fault principles

In many cases it is difficult to achieve selective earth-fault protection based on the
magnitude of residual current only. To obtain a selective earth-fault protection
scheme, it is necessary to take the phase angle of Io into account. This is done by
comparing the phase angle of the operating and polarizing quantity.
Relay characteristic angle
The Characteristic angle setting, also known as Relay Characteristic Angle (RCA),
Relay Base Angle or Maximum Torque Angle (MTA), is used in the "Phase angle"
mode to turn the directional characteristic if the expected fault current angle does not
coincide with the polarizing quantity to produce the maximum torque. That is, RCA is
the angle between the maximum torque line and polarizing quantity. If the polarizing
quantity is in phase with the maximum torque line, RCA is 0 degrees. The angle is
positive if the operating current lags the polarizing quantity and negative if it leads the
polarizing quantity.
Example 1
The "Phase angle" mode is selected, compensated network (φRCA = 0 deg)
=> Characteristic angle = 0 deg
1MRS759142 C
REX640
Technical Manual