GE C70 Instruction Manual page 464

OVERVIEW
The voltage differential protection uses the following operating signal for grounded banks:
The voltage differential protection uses the following operating signal for ungrounded banks:
In these equations, k
figure and are in per-unit values on the nominal bus phase-to-ground voltage base. The first equation is implemented
using primary voltage magnitudes, while the second equation is implemented using primary voltage phasors. The
protection operates when the operate signal is greater than the set pickup level for the set pickup delay.
Identical relations apply to phases B and C.
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.
9.1.3.2 Balanced case
To understand how the voltage differential protection works, consider for simplicity the following grounded bank operating
equation:
The ungrounded case is similar except that all voltages have V
an initial capacitance of C
with system phase-to-ground voltage V
9
9-4
Figure 9-2: Voltage differential application to grounded and ungrounded banks
C
1A
C
A
C
2A
V
2A
Grounded bank
V
OP A ( )
V =
OP A ( )
is a match factor setting for the A-leg of the bank. The voltages are as defined in the preceding
A
V
OP A ( )
, divided into sub-strings with initial capacitances of C
A
, then the string forms a voltage divider and the initial measured voltages are:
Spg
V
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
C
B
A
V
1A
= V k V
–
1A A 2A
( ) k ( )
V V V V
– – –
1A X A 2A X
= V k V
–
1A A 2A
subtracted. If the initial factory balanced A-leg string has
X
V = V
1A Spg
C
A
= V × ------- -
2A Spg
C
2A
CHAPTER 9: THEORY OF OPERATION
C
1A
C
A
V
C
2A
V
2A
V
X
Ungrounded bank
834750A1.CDR
and C , and the bank is energized
1A 2A
C
B
A
1A
Eq. 9-7
Eq. 9-8
Eq. 9-9
Eq. 9-10
Eq. 9-11