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2 CALCULATION OF SETTINGS
100
80
60
40
20
Figure 2–2: ALLOWABLE ZONE 1 REACH (
106: Z1GRDCHAR - Zone 1 Ground Characteristic
Z1GRDCHAR can be set to either MHO, REACT or QUAD. This setting determines the type of measuring unit
used for the Zone 1 ground-distance functions, either a Mho function, a reactance function, or a quadrilateral
function. Except for very short lines, it is recommended that the Mho function be used, since its operating time
is slightly faster than that of the reactance or quadrilateral functions. A "very short line" is one where the posi-
tive sequence source impedance (equivalent source impedance behind the relay location) divided by the posi-
tive sequence impedance of the protected line is greater than 5. Note that the value of 5 is a suggested
boundary value, not an absolute cutoff, and that a reactance function can be selected for a long line if desired.
107: Z1SUREACH - Mho Supervision of Ground Reactance Function
This setting is relevant only when the Zone 1 ground-distance functions have been set as reactance functions
(see Protection Setting 106: Z1GRDCHAR). Since the reactance function is nondirectional, it must be super-
vised by a Mho function as shown in Figure 2–4: MAXIMUM ALLOWABLE REACH on page 2–13. This super-
vision is automatically implemented whenever the reactance function is selected via the Z1GRDCHAR setting.
Z1SUREACH sets the minimum reach that can be made on the supervising Mho function. This setting can be
easily calculated if the maximum load flow across the line is known. The criterion used for establishing the min-
imum reach is based on maintaining a 40° angular margin between angles A and B as shown in Figure 2–4:
MAXIMUM ALLOWABLE REACH. Note that B is the "constant chord" angle of the characteristic where the
minimum reach is the chord. Since the supervising Mho unit has a circular characteristic, angle B is 90°.
The LPS-D uses an adaptive feature to adjust the reach of the supervising Mho function as the load flow
changes. The reach can never be less than Z1SUREACH, but it can be larger. As the load flow decreases, the
load impedance becomes larger, and the reach is increased while maintaining the 40° difference between
angles A and B. If the load now increases, the reach will be decreased but will never be made less than
Z1SUREACH. This adaptive feature allows the reactance function to provide the maximum possible coverage
for ground fault impedance.
GE Power Management
0
0
5
10
Positive Sequence Source to Line Impedance Ratio, ZS/ZL
LPS-D Line Protection System
2.3 PROTECTION SETTINGS
15
20
25
CVT
WHEN USED WITH
2
30
)
S
2-
11
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