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GEK-26437
TABLE X
CIRCUIT
TERM
WATTS
+
J VARS
WATTS
+
J VARS
-
H 1 Restraint
a
+
Jb
3.12
+
J3.62
M 1 Polarizing
C (+) Jd
7.53
-
J4.2l
H 2 Restraint
e
+
Jf
3.46
+
J5.52
H 2 Polarizing
g
(±)
Jh
7.34
-
J3.76
The total burden can be obtained by adding the H 1 and N 2 watts and vars for each unit as listed In
Table X and converting to volt—amperes as indicated in the equations for transformer settings less than 100
percent.
The maximum burden imposed on the potential transformer by the offset transactor in addition to the
unit's burden in Table IX is shown In XI.
TABLE XI
R
JX
P.F.
WATTS
V.A.
VARS
7.66
11.59
0.66
7.66
11.60
8.72
If offset is used the volt-amperes of the offset transactor must be added to the M 1 and N 2 units volt
amperes to determine the maximum volt amperes of the relay's potential circuit with offset.
CALCULATION OF SETTINGS
The following is a typical worked example of how to determine the reach and offset tap settings for the
relay to provide the proper line protection.
Assume a 230kV line 100 miles long with a positive sequence primary impedance of
Z(pri)
=
8O
With a CT ratio of 1200/5 and a PT ratio of 2000/1 the secondary impedance becomes
CT Ratio
Z(sec)
=
Z(pri)
X
PT Ratio
Z(sec)
ohms
Set the M 1 first zone unit to 90 percent of the protected line section taking into account the dif
ference between the line angle
0
=
85 degrees and the H 1 angle of maximum torque 9
=
75 degrees by the func
tion of cos
(0—9).
The N 1 reach at its angle of maximum torque should be
-
0.90 x 9.6
—
8.64
—
—
—
8.77 ohms
Using the three ohm basic minimum reach tap, the N 1 tap setting will be
TAP (M
1
)
=
—-
=
34
Assume that we are using a relay where the N 2 unit has a 0-4 ohm forward offset range.
It is obvious
that with a full four ohm offset, the overlap with the H 1 unit reach set at 8.83 will be ample.
Set the M,
rit to reach approximately 150 percent of the protected line section with zero infeed at the remote bus
again taking into account the difference in line ano
1
e with maximum torque angle.
The M 2 reach at its angle
c airnur torque will then be
Reach (N
)
1.5(9.6)
14.62 /75°
2
cus 10°
8
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