GE C70 Instruction Manual page 300

5.6 GROUPED ELEMENTS
To improve accuracy, a relatively low-ratio window CT can be used for instrumentation of the split-phase neutral cur-
rent, and be wired to a 1 A or even sensitive ground relay input for extra sensitivity. CT primary and secondary ratings
are entered under a separate menu. Directionality of wiring of the split-phase current is not important for proper func-
tioning of the feature as long as the angle of the factor compensating for the inherent unbalance is set appropriately.
• NTRL CUR UNBALCE 1 K MAG: This setting defines magnitude of the factor compensating for the inherent unbal-
ance of the bank. Under balanced conditions, the amount of the spill current measured in the neutral connection
between the banks is proportional to the positive-sequence component of the total current of the two banks. This set-
ting defines the magnitude of the proportionality factor. Quality of balancing the bank with a given value of this setting
can be viewed under
calculate the compensating factor automatically. Setting the compensating factor to zero effectively disables the com-
pensation. If further the slope is set to zero as well, this feature becomes a plain overcurrent element responding to the
neutral split-phase current.
• NTRL CUR UNBALCE 1 K ANG: This setting defines angle of the factor compensating for the inherent unbalance of
the bank. Under balanced conditions, the amount of the spill current measured in the neutral connection between the
banks is proportional to the positive-sequence component of the total current of the two banks. This setting defines the
angle of the proportionality factor. Quality of balancing the bank with a given value of this setting can be viewed under
menu. An automatic setting procedure is available under the
ACTUAL VALUES
pensating factor automatically. See the Theory of operation chapter for more information regarding the meaning of the
inherent unbalance compensating factor of this protection method.
• NTRL CUR UNBALCE 1 STG1 PKP to NTRL CUR UNBALCE 1 STG4 PKP: These settings specify pickup thresh-
olds for stages 1 through 4 in per-units (pu) of the nominal current of the ground current channel of the bank source. A
value of 1 pu is the nominal secondary current, or the nominal primary current, respectively, as configured under the
bank source of this function.
• NTRL CUR UNBALCE 1 STG1 SLOPE to NTRL CUR UNBALCE 1 STG4 SLOPE: These settings specify the slopes
5
for stages 1 through 4. The slope setting is applied if the phase CTs are configured under the bank source. The magni-
tude of the zero-sequence current is applied as a restraining signal. Typically 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. See the Theory of operation chapter
for more information on this protection method.
• NTRL CUR UNBALCE 1 STG1 DEL to NTRL CUR UNBALCE 1 STG4 DEL: This setting specifies the pickup time
delay, individually per each stage of alarming/tripping. The timers use a common drop out time delay specified by the
next setting.
• NTRL CUR UNBALCE 1 DPO DELAY: This setting controls a drop out delay for the operate FlexLogic operands. Note
that individual stages apply individually configurable pickup delays, but use this common drop out delay.
• NTRL CUR 1 BLK STG1 to NTRL CUR 1 BLK STG4: These settings are used to block the function on a per stage
basis when required. At minimum this function shall be blocked when the two banks are not paralleled. Note that when
unblocked, the function becomes operational after five cycles of intentional delay. High-set instantaneous protection on
energization can be provided by a separate instantaneous overcurrent function.
5-176
menu. An automatic setting procedure is available via the
ACTUAL VALUES
C70 Capacitor Bank Protection and Control System
5 SETTINGS
COMMANDS
menu to calculate the com-
COMMANDS
menu to
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