I.L. 41-496.52D
8
Then, for factory calibration
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
1
value (it will always be within
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
1
and Z
0
using
equation:
For example, assume the desired reach,
Z
1
θ
is 7 ohms at 60
°
(step 1a) and Z
0θ
is 21 ohms at
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
1
=7 x 1.11 = 7.77 ohms
Z
0
=21 x 1.11 = 23.31 ohms
This procedure is followed when R
1
, R
2
, R
3
, R
4
set-
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.
o r 1 . 2 %
f r o m t h e d e s i r e d
value.
For the Z
θ
selection find the nearest value to 23.31
ohms using the same S setting as above. 23.0 ohms
is the nearest value.
or 1.2% within the desired
value.
Step (2b). From Table
IV
read off:
S = 1
T = 8.7
T
0
= 26.1
M = -.12
M
0
= -.12
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
1
=TS (1
±
M) = 1 x 8.7 (.88) = 7.65 and
Z
0
=1 x 26.1 (.88) = 23.0
Z
1
=
Z
0
=
Where balance point product of 3I
0
Z
1
<40 volts
(where Z
1
is positive sequence relay setting and 3I
0
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.
Change Step (1b) to compute desired reach accord-
ing with equation:
Z
1
Z
1
θ
Sin75
°
Sin
θ
------------------- and
Z
0
Z
0
Sin75
°
Sin
θ°
-------------------
=
=
θ
Angle to which relay is recalibrated
=
θ
75
°
,
Z
1
Z
1
θ
and
Z
0
Z
0
θ
=
=
=
Z
TS 1
M
+
(
)
=
100
7.65
7.77
-----------
¥
98.8%
=
100
23.0
23.31
---------------
¥
98.8%
=
Z
1
θ
60
°
sin
75
°
sin
------------------ 7.65
900
¥
6.90 ohms at 60
°
=
Z
0
60
°
sin
75
°
sin
------------------
23.0
900
¥
20.7 ohms at 60
°
=
=
Z
Z
Cos 75
θ
–
(
)
--------------------------------
=
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