ABB REG670 - Theory of Operation

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 time domain algorithm
• activating of the input BLKHSIR blocks the instantaneous part of the algorithm
for high SIR values
• activating of the input BLKTRIP blocks all output signals
• activating the input BLKPE blocks the phase-to-earth 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)
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 is 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.
7.1.6.4 Theory of operation
The mho algorithm is based on the phase comparison of an operating phasor and a
polarizing phasor. When the operating phasor leads the reference polarizing phasor by
90 degrees or more, the function operates and gives a trip output.
Phase-to-phase fault
Mho
The plain Mho circle has the characteristic as in Figure
66. The condition for deriving
the angle β is according to equation 37.
EQUATION1789 V2 EN (Equation 37)
Section 7 1MRK502052-UEN B
Impedance protection
184
Technical manual