CHAPTER 5: SETTINGS GROUPED ELEMENTSL90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 5-3135The compensated overvoltage function provides protection against an overvoltage due to the opening of the remoteterminal of a transmission line—the so called the Ferranti effect. This can be achieved using a transfer-tripping scheme.However, with high voltage, more corona can exist on the line and inhibit the proper reception of a carrier-transfer-tripsignal. Also, the presence of a line with an open terminal in weak systems can raise the voltage level of the local bus.Detecting and tripping a line with an open terminal can prevent tripping at the local bus in this case.Figure 5-177: Transmission line with remote terminal openFigure 5-178: Phasor diagramThe function approximates the voltage rise at the far end of the transmission line according to the following relationship:Eq. 5-40whereV_1 is the positive-sequence voltage (phasor quantity) in secondary volts measured at the local terminalI_1 is the positive-sequence current (phasor quantity) in secondary amps measured at the local terminalVnominal is the phase VT secondary setting in the case of wye VTs and the phase VT secondary setting divided by in thecase of delta VTsZC_mag and ZC_ang represent an impedance between the local and remote terminalsV_1 C is the calculated positive-sequence voltage magnitude at the remote terminalIf the magnitude of ZC is set to one-half the series impedance of the line (R + jXL ), the compensated voltage isapproximately equal to the positive-sequence voltage at the remote end of the line. A more accurate setting of ZC can bemade if the positive-sequence charging current and the voltages at the local and remote line ends resulting from an openbreaker are known. In this case, the required reach setting is:Eq. 5-41 COMPENSATED OV BLK:OffRange: FlexLogic operand COMPENSATED OVTARGET: Self-resetRange: Self-reset, Latched, Disabled COMPENSATED OVEVENTS: DisabledRange: Disabled, Enabled
