GE CEY51A Instructions Manual page 7

GEK-1275
Zprim
=
50(0.14
+
jO.BO)
=
7
+
j40 ohms total
PT Ratio
=
230,000/115
=
2000/1
CT Ratio
=
600/5
=
120/1
Assume that the mi nimum three- phase faul t current fl owi ng in the rel ay for a
three-phase fault at the remote end of the 1 ine is about B secondary amperes, and
for a close-in fault, it is 11 secondary amperes.
Zsec.
=
Zprim CT Ratio
PT Ratio
Zsec.
=
(7.0
~
j40.0) 120
=
0.42 + j2.4 ohms
-zuuu--
Zsec.
=
2.43 /BO.50 ohms
Assume that the CEY51A is to be used to provide first-zone protection and it is
desired to set the relay to protect 90% of the line
0.9 (Zsec)
=
0.9(2.43)
/BO.5
=
2.19 /BO.5
For this application, the highest suitable basic minimum tap is 1.5 ohms.
The ohmic-reach equation given in the section under OPERATING CHARACTERISTICS -
TAPPED AUTOTRANSFORMER is used to obtain the required tap setting.
Percent Tap Setting
Minimum Ohms
=
1.5
Z
=
Zsec.
=
2.19 ohms
~
=
BO.50
(100) (Min. Ohms) Cos
(0-9)
Z
The selection of the maximum-torque angle for a first-zone relay should depend
primarily on accommodation of arc resistance. Smaller maximum-torque angles will
accommodate more arc resistance than larger maximum-torque angles. Since arc
resistance is independent of line length, the shorter lines will provide the
greatest problem on this score because of the shorter reach setting on the relay.
Thus, on short lines it is desirable to use smaller angles of maximum torque. On
longer lines, where the mho unit must be set with a longer reach, it would have more
significant arc-resistance capability even with a larger angle of maximum torque.
Another factor to consider is immunity to system swings. In general, an angle
of maximum torque approaching the angle of the line will provide the greatest
immunity to system swi ngs. Thus, for most 1 i nes, the 75
0
setti ng woul d provi de the
greatest immunity.
5