9-4 C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
OVERVIEW CHAPTER 9: THEORY OF OPERATION
9
Figure 9-2: Voltage differential application to grounded and ungrounded banks
The voltage differential protection uses the following operating signal for grounded banks:
V
OP(A)
= |V
1A
- k
A
V
2A
| Eq. 9-7
The voltage differential protection uses the following operating signal for ungrounded banks:
V
OP(A)
= |(V
1A
- V
X
) - k
A
(V
2A
- V
X
)| Eq. 9-8
In these equations, k
A
is a match factor setting for the A-leg of the bank. The voltages are as defined in the preceding
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:
Eq. 9-9
The ungrounded case is similar except that all voltages have V
X
subtracted. If the initial factory balanced A-leg string has
an initial capacitance of C
A
, divided into sub-strings with initial capacitances of C
1A
and C
2A
, and the bank is energized
with system phase-to-ground voltage V
Spg
, then the string forms a voltage divider and the initial measured voltages are:
Eq. 9-10
Eq. 9-11
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