ABB Relion 670 series - Theory of Operation

• 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 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.
7.3.6.4 Theory of operation
SEMOD154224-46 v6
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
SEMOD154224-240 v2
Mho
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The plain Mho circle has the characteristic as in Figure
99. The condition for deriving the angle β is
according to equation 55.
( )
( )
AB AB pol
ang V I ZPP ang V
b = - × -
EQUATION1789-ANSI V1 EN-US
(Equation 55)
where
AB
V
EQUATION1790-ANSIV1 EN-US
is the voltage vector difference between phases A and B
AB
I
EQUATION1791-ANSIV1 EN-US
is the current vector difference between phases A and B
ZPP is the positive sequence impedance setting for phase-to-phase fault
V
pol
is the polarizing voltage
The polarized voltage consists of 100% memorized positive sequence voltage (VAB for phase A to
B fault). The memorized voltage will prevent collapse of the Mho circle for close in faults.
Operation occurs if 90≤β≤270
Section 7 1MRK 502 066-UUS B
Impedance protection
222
Technical manual