GE C70 Instruction Manual

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CHAPTER 5: SETTINGS GROUPED ELEMENTSC70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 5-1775Figure 5-84: Neutral directional voltage-polarized characteristicsNEUTRAL DIR OC1 POLARIZING — This setting selects the polarizing mode for the directional unit.• If “Voltage” polarizing is selected, the element uses the zero-sequence voltage angle for polarization. The user can useeither the zero-sequence voltage V_0 calculated from the phase voltages, or the zero-sequence voltage suppliedexternally as the auxiliary voltage V_X, both from the NEUTRAL DIR OC1 SOURCE.The calculated V_0 can be used as polarizing voltage only if the voltage transformers are connected in Wye. Theauxiliary voltage can be used as the polarizing voltage provided SYSTEM SETUP  AC INPUTS  VOLTAGE BANK AUXILIARY VT CONNECTION is set to “Vn” and the auxiliary voltage is connected to a zero-sequence voltage source(such as broken delta connected secondary of VTs).The zero-sequence (V_0) or auxiliary voltage (V_X), accordingly, must be greater than the VOLTAGE CUTOFF LEVELsetting specified in the PRODUCT SETUP  DISPLAY PROPERTIES menu to be validated for use as a polarizing signal. Ifthe polarizing signal is invalid, neither forward nor reverse indication is given.• If “Current” polarizing is selected, the element uses the ground current angle connected externally and configuredunder NEUTRAL OC1 SOURCE for polarization. The ground CT must be connected between the ground and neutral pointof an adequate local source of ground current. The ground current must be greater than 0.05 pu to be validated as apolarizing signal. If the polarizing signal is not valid, neither forward nor reverse indication is given. In addition, thezero-sequence current (I_0) must be greater than the PRODUCT SETUP  DISPLAY PROPERTIES  CURRENT CUT-OFFLEVEL setting value.For a choice of current polarizing, it is recommended that the polarizing signal be analyzed to ensure that a knowndirection is maintained irrespective of the fault location. For example, if using an autotransformer neutral current as apolarizing source, ensure that a reversal of the ground current does not occur for a high-side fault. Assume that thelow-side system impedance is minimal when checking for this condition. A similar situation arises for a wye/delta/wyetransformer, where current in one transformer winding neutral can reverse when faults on both sides of thetransformer are considered.• If “Dual” polarizing is selected, the element performs both directional comparisons as described. A given direction isconfirmed if either voltage or current comparators indicate so. If a conflicting (simultaneous forward and reverse)indication occurs, the forward direction overrides the reverse direction.• If "Dual-V" polarizing is selected, "Voltage" polarizing is performed and "Current" polarizing is ignored if the voltagepolarizing signal is valid; otherwise "Current" polarizing is performed if the current polarizing signal is valid. If neither ofthem is valid, neither forward nor reverse indication is given.827805A1.CDRVAG(reference)VBGVCG–3I_0 line3I_0 lineECA line–ECA lineLALALALAECAFWD LAlineFWD OperatingRegionREV OperatingRegionFWD LAlineREV LAlineREV LAline–3V_0 line3V_0 line