GE C70 Instruction Manual page 289

5 SETTINGS
• CUR UNBALCE 1 DIF SOURCE — Indicate signal source that signifies the differential currents between the two par-
allel banks. In order to improve accuracy, a relatively low-ratio window CT can be used for instrumentation. CT primary
and secondary ratings are entered under a separate menu. Directionality of wiring these current signals is not impor-
tant for proper functioning of the feature as long as the sign of the factor compensating for the inherent unbalance is
appropriate. For ease of analysis and consistency of relay records, wire all three currents with the same polarity.
• CUR UNBALCE 1 BANK SOURCE — Indicate signal source that signifies the total current of the two parallel banks.
Either the main CTs are placed in a way ensuring measurement of the total current, or the currents of individual banks
are summed externally to the relay by paralleling the main CTs, or summed up internally to the relay using the configu-
ration mechanism of sources. The total bank current is used for compensation for the inherent bank unbalance. CT pri-
mary and secondary ratings are entered under a separate menu. Directionality of wiring these current signals is not
important for proper functioning of the feature as long as the sign of the factor compensating for the inherent unbal-
ance is appropriate. For ease of analysis and consistency of relay records, wire all three currents with the same polar-
ity. If any of the compensating factors is non-zero, the BANK SOURCE must have phase CT current configured for this
function to run.
• CUR UNBALCE 1 INHNT FACTOR A(B,C) — This setting specifies the compensating factor for the inherent unbal-
ance between the two banks. Normally, the compensating factors are small numbers being a ratio of the difference
between the two impedances, and the sum of the two impedances. For example, with 1% difference between the
impedances, the factor is (1-0.99)/(1+0.99) = 0.005. The sign of the compensating factor depends on the polarity of
wiring the differential signals, and on which of the two banks has the lower impedance. Quality of balancing the bank
with a given value of this setting can be viewed under the ACTUAL VALUES menu. An auto-setting procedure is avail-
able under the COMMANDS menu to calculate the compensating factors automatically. Setting the compensating fac-
tors to zero effectively disables the compensation and turns this feature into a plain overcurrent element responding to
the differential (split phase) currents.
• CUR UNBALCE 1 STG 1A PICKUP to CUR UNBALCE 1 STG 4C PICKUP: These settings specify pickup thresholds
for stages 1 through 4 in phases A through C in per-units (pu) of the nominal current of the differential source. A value
of 1 pu is the nominal secondary current, or the nominal primary current, respectively, as configured under the differen-
tial source of this function.
• CUR UNBALCE 1 STG 1 PKP DELAY to CUR UNBALCE 1 STG 4 PKP DELAY: This setting specifies the pickup
time delay, individually per each stage of alarming/tripping. Note that each of the twelve comparators (four stages in
three phases) uses an individual timer, but settings for these timers are controlled individually per each stage, not per
phase. The timers use a common drop out time delay specified by the next setting.
• 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.
• CUR UNBL 1 BLK STG1 to CUR UNBL 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.
GE Multilin C70 Capacitor Bank Protection and Control System
5.6 GROUPED ELEMENTS
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