ABB Relion 670 series, Technical Manual

Page: 357 / 1432

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 GUID-46762EB9-50CA-4045-9D39-80B604C66E0F v1
The plain Mho circle has the characteristic as in figure 178 . The condition for deriving the angle β is
according to equation 99 .
( ) ( )
arg 1 arg
set p AB AB ol
I Z V V
b
- × = -
ANSIEQUATION15027 V1 EN-US
(Equation 99)
where
AB
V
EQUATION1790- ANSI V1 EN-US
is the voltage vector difference between phases A and B
AB
I
EQUATION1791- ANSI V1 EN-US
is the current vector difference between phases A and B
Z
set
1
is the positive sequence impedance setting for phase-to-phase fault in zone direction
V
pol
is the polarizing voltage
Z R FwPPZx j X FwPPZx
set
1 1 1
= + ⋅
IECEQUATION15010 V1 EN-US
(Equation 100)
where
R1FwPPZ
x
is the positive sequence resistive reach for phase-to-phase fault in zone direction for zone x
(x=1-5 and RV)
X1FwPPZ
x
is the positive sequence reactance reach for phase-to-phase fault in zone direction for zone
x (x=1-5 and RV)
The polarized voltage consists of 100% memorized positive sequence voltage (V
AB
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°.
1MRK 502 066-UUS B Section 7
Impedance protection
351
Technical manual