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 algorithmfor 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 outputsThe activation of input signal BLKZ can be made by external fuse failure function orfrom the loss of voltage check in the Mho supervision logic (ZSMGAPC). In bothcases the output BLKZ in the Mho supervision logic shall be connected to the inputBLKZ in the Mho distance function block (ZMHPDIS)The input signal BLKZMTD is activated during some ms after fault has been detectedby ZSMGAPC to avoid unwanted operations due to transients. It shall be connectedto 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. Theinput BLKHSIR is connected to the output signal HSIR on ZSMGAPC for increasingof the filtering and high SIR values. This is valid only when permissive underreachscheme is selected by setting ReachMode=Underreach.7.1.6.4 Theory of operationThe mho algorithm is based on the phase comparison of an operating phasor and apolarizing phasor. When the operating phasor leads the reference polarizing phasor by90 degrees or more, the function operates and gives a trip output.Phase-to-phase faultMhoThe plain Mho circle has the characteristic as in Figure 66. The condition for derivingthe angle β is according to equation 37.EQUATION1789 V2 EN (Equation 37)Section 7 1MRK502052-UEN BImpedance protection184Technical manual
