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I.L. 41-496.52D
Sin 75°
Z 1
=
Z 1θ
------------------ - and Z 0
Sin θ
θ
=
Angle to which relay is recalibrated
Then, for factory calibration
θ
=
75°, Z 1
=
2.
Now refer to Table II or IV giving preferred
Zone1 settings for the SDGT relays. If the
desired reach exceeds the relay range for S =
1, use S = 2 and Table III or V (set for 75% of
line).
2a.
Locate a table value for relay reach nearest to
the desired Z
value (it will always be within
1
1.5% of the desired value.)
2b.
From this table read off the "S", "T" and "M" set-
tings. The "M" column includes additional infor-
mation for the "L" and "R" lead setting for the
specified "M" value. If the desired settings can-
not be found on this table proceed to Table III
or V to find the desired setting in this case. The
relay reach must now be reduced from 85 to 75
percent to avoid overreach on two phase-to-
ground faults on high fault resistance faults.
2c.
Recheck relay settings for Z
equation:
(
)
Z
=
TS 1
+
M
For example, assume the desired reach,
Z
is 7 ohms at 60° (step 1a) and Z
1θ
60°.
Next step is (1b). Making correction of maximum
sensitivity angle of the relay to match the characteris-
tic angle of the line (60°) that is different from factory
setting of 75°, we find the relay tap setting.
Z
=7 x 1.11 = 7.77 ohms
1
Z
=21 x 1.11 = 23.31 ohms
0
This procedure is followed when R
tings are changed, otherwise follow alternative pro-
cedure below.
Step (2a). In Table IV we find 7.65 to be the nearest
value to 7.77 ohms.
8
Sin 75°
=
Z 0
------------------ -
Sin θ°
Z 1θ and Z 0
=
Z 0θ
and Z
using
1
0
is 21 ohms at
0θ
, R
, R
, R
1
2
3
4
7.65
100
¥
---------- -
=
98.8%
7.77
value.
For the Z
selection find the nearest value to 23.31
θ
ohms using the same S setting as above. 23.0 ohms
is the nearest value.
23.0
100
¥
-------------- -
=
23.31
value.
Step (2b). From Table IV read off:
S = 1
T = 8.7
T
M = -.12
M
The "R" lead should be connected over "L" lead, with
"L" lead connected to "0" tap and "R" lead to tap "09".
(The sum of the values between L and R is 0.12).
Step (2c). Recheck Settings.
Z
=TS (1±M) = 1 x 8.7 (.88) = 7.65 and
1
Z
=1 x 26.1 (.88) = 23.0
0
sin
60°
Z
=
Z 1θ
----------------- - 7.65
1
sin
75°
sin
60°
Z
=
Z 0
----------------- -
0
sin
75°
Where balance point product of 3I
(where Z
is positive sequence relay setting and 3I
1
is zero sequence current in the relay for L-G fault)
disconnect the phase-to-phase desensitizer and set
Zone1 as calculated.
4.1. Alternative Calculations and Settings
If it is desired to avoid recalibration of the relay maxi-
mum sensitivity setting, the following procedure
should be followed.
Follow Step (1a) as above.
set-
Change Step (1b) to compute desired reach accord-
ing with equation:
Z
Z
=
------------------------------- -
Cos 75 θ
(
–
o r 1 . 2 % f r o m t h e d e s i r e d
98.8%
or 1.2% within the desired
= 26.1
0
= -.12
0
¥
900
=
6.90 ohms at 60°
=
23.0
¥
900
=
20.7 ohms at 60°
0
)
Z
<40 volts
1
0
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