GE C70 Instruction Manual page 596

OVERVIEW
This can be substituted into equation 9.33 to obtain:
For our purposes, C
Under normal system conditions (non-fault), V
V , which by convention is on the bus voltage base, converted to the base of nominal value of the neutral-point VT used
Spg
in these equations, is:
This gives:
This can be written as:
In these equations, ΔC(pu) is the capacitance change as a per-unit of the leg capacitance, and V
signal resulting from the failure in per-unit of the nominal value of the neutral-point VT. V
voltage in per-unit of the nominal system phase-to-ground voltage, so can be taken as 1 when the system is normal (not
faulted). Note however that under external fault conditions sensitivity can be much different from the non-fault sensitivity.
n and n
bus_phs ntrl_aux
source, respectively.
Note that under external fault conditions sensitivity can be much different form the non-fault sensitivity. To prevent mis-
operation, use a restraint and/or coordinating time delay.
9.1.4.4 Restraint
Severe system voltage unbalance, such as can occur during near-by bolted ground faults, can exacerbate measurement
error in V or V , resulting in spurious operating signal. In addition, as discussed, the sensitivity can be affected. To prevent
X 0
operation under these conditions, percent restraint supervision is provided using a restraint signal that is the magnitude of
9
the vector sum of V
During external ground faults, these two voltages are approximately in phase generating a large restraining signal being
twice the zero-sequence voltage at the bus. A slope of few percent is typically sufficient to provide good security under
large system unbalances.
9-10
≅ C ≅ C , so k ≅ 1 and k
A B C AB AC
>> V
C
are the VT ratios of the phase VT bank of the bus source and of the auxiliary VT channel of the neutral
and V , defined as follows.
X 0
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
≅ 1. This results in:
, and V is the system phase-to-ground voltage. The system voltage
0 C
V = |V + V |
REST X 0
CHAPTER 9: THEORY OF OPERATION
Eq. 9-37
Eq. 9-38
Eq. 9-39
Eq. 9-40
Eq. 9-41
(pu) is the operating
OP
is the system phase-to-ground
Spg
Eq. 9-42