Theory Of Operation - ABB Relion 670 Series Technical Reference Manual

Section 6
Impedance protection
6.4.2.4
244
status information related to the six measuring loops. Hence if any of the
measuring loop status is High, then the timers will be triggered.
• External start: Phase-to-ground and phase-to-phase timers are triggered by the
EXTNST input.
The function can be blocked in the following ways:
• activating of input BLOCK blocks the whole function
• activating of the input BLKZ (fuse failure) blocks all output signals
• activating of the input BLKZMTD blocks the delta based algorithm
• activating of the input BLKHSIR blocks the high speed part of the algorithm for
high SIR values
• activating of the input BLKTRIP blocks all output signals
• activating the input BLKPG blocks the phase-to-ground fault loop outputs
• activating the input BLKPP blocks the phase-to-phase fault loop outputs
The activation of input signal BLKZ can be made by external fuse failure function or
from the loss of voltage check in the Mho supervision logic (ZSMGAPC). In both
cases the output BLKZ in the Mho supervision logic shall be connected to the input
BLKZ in the Mho distance function block (ZMHPDIS, 21)
The input signal BLKZMTD is activated during some ms after fault has been detected
by ZSMGAPC to avoid unwanted operations due to transients. It shall be connected to
the BLKZMTD output signal of ZSMGAPC function.
At SIR values >10, the use of electronic CVT might cause overreach due to the built-in
resonance circuit in the CVT, which reduce the secondary voltage for a while. The
input BLKHSIR shall be connected to the output signal HSIR on ZSMGAPC for
increasing of the filtering and high SIR values. This is valid only when permissive
underreach scheme is selected by setting ReachMode=Underreach.

Theory of operation

The mho algorithm is based on the phase comparison of a operating phasor and a
polarizing phasor. When the operating phasor leads the reference polarizing phasor by
more than 90 degrees, the function operates and gives a trip output.
Phase-to-phase fault
Mho
The plain mho circle has the characteristic as in figure 118. The condition for deriving
the angle β is according to equation 34.
1MRK505222-UUS C
Technical reference manual