GE C70 Instruction Manual page 443

8 THEORY OF OPERATION
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 the above 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 may be much different from the non-fault sensitivity.
n and n are the VT ratios of the phase VT bank of the bus source and of the auxiliary VT channel of the neutral
bus_phs ntrl_aux
source, respectively.
Note that under external fault conditions sensitivity may be much different form the non-fault sensitivity. To prevent mis-
operation, a restraint and/or coordinating time delay should be used.
d) 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 above the sensitivity can be affected. To
X 0
prevent operation under these conditions, percent restraint supervision is provided using a restraint signal that is the mag-
nitude of the vector sum of V
X
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.
V
OP
Pickup
setting
e) AUTO-SETTING COMMAND
As a convenient alternative to manually determining the unbalance ratio settings, the relay can automatically calculate
these settings from its own measurements while the capacitor is in-service, as described in the Commands chapter. The
technique the relay uses is to set the operate signal variable in equation 8.25 to zero and solve for the unbalance ratio k-
values using the average of several successive measurements of the voltages.
GE Multilin
>> V , and V
C 0
n
bus_phs
------------------- -
n
ntrl_aux
d 1 1
---------- - V ≅ -- - × ------- ×
OP
dC 3 C
C C
n
1
bus_phs

≅ -- - × ------------------- -
dV
OP
3 n
ntrl_aux
n
1
bus_phs
( ) ≅ -- - × ------------------- -
V pu
OP
3 n
ntrl_aux
and V , as defined below.
0
V
=
REST
NTRL VOLT PKP
Figure 8–4: NEUTRAL OVERVOLTAGE RESTRAINT
C70 Capacitor Bank Protection and Control System
is the system phase-to-ground voltage. The system voltage
C
×
V
Spg
n
bus_phs
------------------- - ×
V
Spg
n
ntrl_aux
dC
C
× × ---------- -
V
Spg
C
C
× × Δ ( )
V C pu
Spg
Spg
V V
+
X 0
NTRL VOLT DPO
834746A1.CDR
8.1 OVERVIEW
(EQ 8.39)
(EQ 8.40)
(EQ 8.41)
(pu) is the operating
OP
is the system phase-to-ground
(EQ 8.42)
V
REST
8-9
8