GE C70 Instruction Manual page 476

OVERVIEW
Alternately, we can say:
where ΔC(pu) represents the capacitance change as a per-unit value of the string capacitance, and I
operating signal resulting from the failure in per-unit of the nominal current of the differential source. I
phase A terminal current on the same base. When the system is normal (no fault), I
rated primary per-phase current I
is again the differential CT primary current rating, or with system normal:
Note that under external fault conditions, sensitivity can be much different from the non-fault sensitivity.
9.1.6.4 Restraint
Severe system voltage unbalance, such as can occur during near-by bolted ground faults, can exacerbate measurement
error, resulting in spurious operating signal. To prevent operation under these conditions, percent restraint supervision is
applied using a restraint signal that is the magnitude of the zero-sequence current.
Typically, a few percent of slope is enough to ensure security of the function. The factor compensating for the inherent
bank unbalance zeroes out the operating signal under relatively balanced bank currents. If the said currents contain a
significant zero sequence component, the quality of compensation is lower, hence the need for the restraint, and this slope
setting.
9.1.6.5 Auto-setting
As a convenient alternative to manually determining the unbalance k-factor 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 that the relay uses is to set the operate signal variable to zero in equation 9.54 and solve for the unbalance
factor k using the average of several successive measurements of the currents.
1
However, the assumption made here is that when the auto-set command is executed, the capacitor is in an acceptably
balanced state, wherein the operating signal ought to be zero. Following the auto-set command, the protection measures
changes from the state that existed at the time the auto-set command executed.
9.1.6.6 Application to ungrounded capacitor banks
The forgoing discussion of the neutral current balance protection applied to the ground configuration. When applied to
9
ungrounded configurations, all of it applies, with the following exceptions:
• The phase voltages in equations 9.54 through 9.59 must have the neutral voltage V
voltages are eventually eliminated from the calculations, the outcome is unaffected.
• A low ratio CT can be used for ungrounded-wye configurations in place of a window type CT as there is no
requirement to form the difference of the primary neutral currents
9-16
( )
I pu
OP
converted to the differential source base. For example, I
rated
I ( pu ) =
OP
Figure 9-7: Neutral current unbalance restraint
I
OP
NTRL CUR PKP
Pickup
setting
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
I
A
--- × Δ ( )
= C pu
2
A
I
rated
------------- - × Δ C pu ( )
2I
base
NTRL CUR DPO
CHAPTER 9: THEORY OF OPERATION
(pu) represents the
OP
represents the
A
can be taken as the capacitor bank's
= I / I , where I
A rated base
I
0
834749A1.CDR
subtracted. However, as these
X
Eq. 9-67
base
Eq. 9-68