GE C70 Instruction Manual page 474
Capacitor bank protection and control system
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OVERVIEW
These equations involve phasors, not magnitudes; that is, the vector sum of the currents is created by the protection
function implementing the method. The protection operates when the operate signal is greater than the set pickup level
for the set pickup delay.
Sensitivity is the key performance parameter. The applied comparator uses a simple integration method in addition to the
standard hysteresis approach, to deal with chattering of the operating signal at the boundary of operation.
9.1.6.2 Balanced case
The currents in the neutral current unbalance protection are driven by the individual admittances in each string:
In these equations, I
same per-unit base used by the relay. Voltages are in primary volts, capacitances in Farads, and frequency in radians per
second.
The two neutral currents can be derived from these equations, as follows:
The differential current is the vector difference between the two currents. By subtracting I
At the same time the total currents in each phase are driven by the total admittance of the two banks in each phase:
Inserting the equations above into equation 9.58 so as to eliminate the voltages:
9
Label k
, k
, and k
A
B
9-14
V
A
I
=
------------------------ -
A1
V
B
I
=
------------------------ -
B1
V
C
I
=
------------------------ -
C1
represents the differential CT primary current rating, inserted here to put the string currents on the
base
I
=
I
+
I
+
N1
A1
B1
I
=
I
+
I
+
N2
A2
B2
I
=
I
I
–
DIF
N1
N2
jω V
[
(
C
–
A
A1
=
-----------------------------------------------------------------------------------------------------------------------
I
=
I
A
A1
I
=
I
B
B1
I
=
I
C
C1
jω V
[
(
C
–
A
A1
I
=
-----------------------------------------------------------------------------------------------------------------------
DIF
C
C
–
A1
A2
=
I
×
--------------------- - I
A
C
+
C
A1
A2
as follows:
C
C
C
–
A1
A2
k
=
--------------------- -
,
A
C
+
C
A1
A2
×
jωC
V
×
jωC
A1
A
A2
;
I
=
------------------------ -
A2
I
I
base
base
×
jωC
V
×
jωC
B1
B
B2
;
I
=
------------------------ -
B2
I
I
base
base
×
×
jωC
V
jωC
C1
C
C2
;
I
=
------------------------ -
C2
I
I
base
base
jωC
V
+
jωC
V
+
jωC
A1
A
B1
B
I
=
---------------------------------------------------------------------- -
C1
I
base
jωC
V
+
jωC
V
+
jωC
A2
A
B2
B
I
=
---------------------------------------------------------------------- -
C2
I
base
C
) V
+
(
C
C
) V
+
(
C
–
A2
B
B1
B2
C
C1
I
base
(
)
jωV
C
+
C
A
A1
A2
+
I
=
------------------------------------- -
A2
I
base
jωV
(
C
+
C
)
B
B1
B2
+
I
=
------------------------------------- -
B2
I
base
jωV
(
C
+
C
)
C
C1
C2
+
I
=
------------------------------------- -
C2
I
base
C
) V
+
(
C
C
) V
+
(
C
–
A2
B
B1
B2
C
C1
I
base
C
C
C
–
–
B1
B2
C1
--------------------- -
--------------------- -
+
×
+
I
×
B
C
C
+
C
C
+
B1
B2
C1
C
C
C
–
B1
B2
C1
k
=
--------------------- -
,
k
=
--------------------- -
B
C
C
+
C
C
B1
B2
C1
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
CHAPTER 9: THEORY OF OPERATION
V
C1
C
V
C2
C
from I
, one obtains:
N2
N1
C
)
]
–
C2
C
)
]
–
C2
C
C2
C
C2
C
–
C2
+
C
C2
Eq. 9-56
Eq. 9-57
Eq. 9-58
Eq. 9-59
Eq. 9-60
Eq. 9-61
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