GE C70 Instruction Manual

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Contents

5-216 C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS5When the “CTxR (grnd)” value is selected, the measured bank neutral point voltage is shifted by 90° lagging to compensatefor the angular difference between the bus neutral voltage and the voltage derived from the bank neutral point current.Choose the value of resistor R so the voltage measured by the auxiliary input of the relay matches bus V0 during systemfaults (for example, an SLG fault at the bus). This is accomplished by the following condition:Eq. 5-20whereV0 BUS is the bus primary zero-sequence voltage during system fault3I0 NTRL is the primary current in the bank neutral during system faultn BUS_VT is the bus VT ratio set in the source selected by the NTRL VOL UNBAL 1 BUS SOURCE settingn NTRL_VT is a neutral point VT ratio set in the source selected by the NTRL VOL UNBAL 1 NTRL SOURCE setting. If R can bechosen precisely, then nNTRL_VT can be set same as nBUS_VT. If R cannot be chosen precisely, then n NTRL_VT is used to matchzero-sequence voltage as per the equation above.n NTRL_CT is the CT ratio in the bank neutralNTRL VOL UNBAL 1 AB RATIO — This setting compensates for the bank inherent unbalance. Physically this value is a ratio ofthe reactance in phase A divided by the reactance in phase B. Auto-setting procedure is available under the Commandsmenu to calculate this value automatically. When tuning this setting manually, after bank repairs, this ratio shall beincreased or decreased from the nominal value of 1.00 depending on whether the A or B phase got affected by failures orrepairs and whether a given reactance increased or decreased as a result of the failure or repair. Use this setting only whenthe NTRL VOL UNBAL 1 BUS 3V0 setting is set as “Calculated.”NTRL VOL UNBAL 1 AC RATIO — Compensates for the bank inherent unbalance. Physically this value is a ratio of thereactance in phase A divided by the reactance in phase C. Auto-setting procedure is available under the Commands menuto calculate this value automatically. When tuning this setting manually, after bank repairs, this ratio shall be increased ordecreased from the nominal value of 1.00 depending on whether the A or C phase got affected by failures or repairs andwhether a given reactance increased or decreased as a result of the failure or repair. Use this setting only when the NTRLVOL UNBAL 1 BUS 3V0 setting is set as “Calculated”.NTRL VOL UNBAL 1 STG 1 PKP to NTRL VOL UNBAL 1 STG 1 PKP — Specify pickup thresholds for stages 1 through 4 of thefunction 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 — Specify the slopes of the operating characteristic.Neglecting compensation for the inherent bank unbalance, the operating signal is a vectorial difference between theneutral point voltage and the zero-sequence bus voltage. Therefore, the optimum restraining signal is a vectorial sum ofthe two signals. During external ground faults, the two voltages are approximately in phase generating a large restrainingsignal being twice the zero-sequence voltage at the bus. A slope of few percent is typically sufficient to provide goodsecurity under large system unbalances. If a given stage is used with a time delay longer than the expected external faultduration, the slope may be reduced or eliminated. In general, the slope can be understood as a adaptive threshold, and theuser is provided with three ways to control sensitivity and security: pickup, time delay and slope. See the Application ofSettings chapter for information.NTRL VOL UNBAL 1 STG 1 PKP DELAY to NTRL VOL UNBAL 1 STG 4 PKP DELAY — Specify the pickup time delays for each stage ofalarming or tripping. Each of the four comparators (four stages) uses an individual pickup timer, but a common drop outtime delay as specified by the next setting.NTRL VOL UNBAL 1 DPO DELAY — Controls a drop out delay for the operate FlexLogic operands. Note that individual stagesapply 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-stagebasis. At minimum this function shall be blocked when the bank is off-line, or else it will operate unnecessarily duringexternal ground faults and/or natural system unbalance if set sensitive. Either breaker position or undercurrent indicationvia any available instantaneous overcurrent function, or a combination, may be used for blocking. Note that whenunblocked, the function becomes operational after five cycles of intentional delay. Optionally only the sensitive stages areblocked, and one stage set high enough to ride through possible problems is left operational. This stage can be used forabnormal conditions such as a failure of one pole of the breaker to close. The function becomes blocked if all four blockinginputs are asserted. The individual blocking inputs inhibit the associated stages of the function.3V0 BUSn BUS_VT----------------- 3I0 NTRL R× n NTRL_VT×n NTRL_CT n BUS_VT×--------------------------------------------------– 0=