SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Appendix B
Page 1 of 2
SKETCHING RELAY CHARACTERISTICS
Cross polarized mho, characteristics
The mho impedance characteristics for unbalanced faults are unique due to the
addition of square wave cross polarizing signals to the sinusoidal comparator
voltages (see Chapter 2).
It can be seen from figures quoted that the boundary of comparator operation
when progressing in a clockwise direction from the vertical axis on an impedance
diagram, initially follows a fully cross polarized “mho” circle. Then at the capture
point, the characteristic “pulls in” to meet a 16% partially cross polarized mho
circle on the source impedance vector.
In order to sketch the unique cross polarized mho impedance characteristic it is
necessary to be able to plot the following:
i) the 16% cross polarized mho circle (offset differs slightly from that of a 16%
sine wave cross polarized mho circle).
ii) the 100% fully cross polarized mho circle.
iii) the impedance vector to the capture point (Z
C
).
To plot the 16% and 100% cross polarized circles, their offset points along the
source impedance vector need to be established. Table 1 gives the offset point
factors K
1
and K
2
for phase or ground fault impedance characteristics.
Table 1 also gives details of how to find the capture point impedance vector.
For phase fault elements the capture point impedance vector can be plotted by
calculating the angle a and the magnitude [Z
C
] from the formulae given.
The portion of the impedance characteristic between the captive impedance
vector[Z
C
] and the 16% offset point must be sketched by hand.
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