GE UR Series C70 Instruction Manual page 284

5.6 GROUPED ELEMENTS
The terms used in the above equation are defined as follows.
– V0 is the bus primary zero-sequence voltage during system fault.
BUS
– 3I0 is the primary current in the bank neutral during system fault.
NTRL
– n is the bus VT ratio set in the source selected by the
BUS_VT
– n is a neutral point VT ratio set in the source selected by the
NTRL_VT
can be chosen precisely, then n
n is used to match zero-sequence voltage as per the equation above.
NTRL_VT
– n is the CT ratio in the bank neutral.
NTRL_CT
• NTRL VOL UNBAL 1 AB RATIO: This setting compensates for the bank inherent unbalance. Physically this value is a
ratio of the reactance in phase A divided by the reactance in phase B. Auto-setting procedure is available under the
Commands menu to calculate this value automatically. When tuning this setting manually, after bank repairs, this ratio
shall be increased or decreased from the nominal value of 1.00 depending on whether the A or B phase got affected by
failures or repairs and whether a given reactance increased or decreased as a result of the failure or repair. This set-
ting should only be used when
• NTRL VOL UNBAL 1 AC RATIO: This setting compensates for the bank inherent unbalance. Physically this value is a
ratio of the reactance in phase A divided by the reactance in phase C. Auto-setting procedure is available under the
Commands menu to calculate this value automatically. When tuning this setting manually, after bank repairs, this ratio
shall be increased or decreased from the nominal value of 1.00 depending on whether the A or C phase got affected
by failures or repairs and whether a given reactance increased or decreased as a result of the failure or repair. This
5
setting should only be used when
• NTRL VOL UNBAL 1 STG 1 PKP to NTRL VOL UNBAL 1 STG 1 PKP: These settings specify pickup thresholds for
stages 1 through 4 of the function in per-units (pu) of the nominal value of the neutral-point VT (the auxiliary channel of
the neutral source).
• NTRL VOL UNBAL 1 STG 1 SLOPE to NTRL VOL UNBAL 1 STG 4 SLOPE: These settings specify the slopes of the
operating characteristic. Neglecting compensation for the inherent bank unbalance, the operating signal is a vectorial
difference between the neutral point voltage and the zero-sequence bus voltage. Therefore, the optimum restraining
signal is a vectorial sum of the two signals. During external ground faults, the 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 typi-
cally sufficient to provide good security under large system unbalances. If a given stage is used with a time delay lon-
ger than the expected external fault duration, the slope may be reduced or eliminated. In general, the slope can be
understood as a adaptive threshold, and the user is provided with three ways to control sensitivity and security: pickup,
time delay and slope. Refer to the Application of settings chapter for more information.
• NTRL VOL UNBAL 1 STG 1 PKP DELAY to NTRL VOL UNBAL 1 STG 4 PKP DELAY: These settings specify the
pickup time delays for each stage of alarming or tripping. Note that each of the four comparators (four stages) uses an
individual pickup timer, but a common drop out time delay as specified by the next setting.
• NTRL VOL UNBAL 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 VOL 1 BLK STG1 to NTRL VOL 1 BLK STG4: These settings are used to block the function when required on
a per-stage basis. At minimum this function shall be blocked when the bank is off-line, or else it will operate unneces-
sarily during external ground faults and/or natural system unbalance if set sensitive. Either breaker position or under-
current indication via any available instantaneous overcurrent function, or a combination, may be used for blocking.
Note that when unblocked, the function becomes operational after five cycles of intentional delay. Optionally only the
sensitive stages are blocked, and one stage set high enough to ride through possible problems is left operational. This
stage can be used for abnormal conditions such as a failure of one pole of the breaker to close. The function becomes
blocked if all four blocking inputs are asserted. The individual blocking inputs inhibit the associated stages of the func-
tion.
5-160
× ×
3V0 3I0 R
BUS NTRL
------------------ - – ------------------------------------------------------------ -
×
n n
BUS_VT NTRL_CT
can be set same as n
NTRL_VT
setting is set as "Calculated".
NTRL VOL UNBAL 1 BUS 3V0
NTRL VOL UNBAL 1 BUS 3V0
C70 Capacitor Bank Protection and Control System
n
NTRL_VT
= 0
n
BUS_VT
NTRL VOL UNBAL 1 BUS SOURCE
NTRL VOL UNBAL 1 NTRL SOURCE
. If R cannot be chosen precisely, then
BUS_VT
setting is set as "Calculated".
5 SETTINGS
(EQ 5.18)
setting.
setting. If R
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