4 VOLTAGE DEPENDENT OVERCURRENT ELEMENTAn overcurrent protection scheme is co-ordinated throughout a system such that cascaded operation is achieved.This means that if for some reason a downstream circuit breaker fails to trip for a fault condition, the nextupstream circuit breaker should trip.However, where long feeders are protected by overcurrent protection, the detection of remote phase-to-phasefaults may prove difficult due to the fact that the current pick-up of phase overcurrent elements must be set abovethe maximum load current, thereby limiting the minimum sensitivity.If the current seen by a local device for a remote fault condition is below its overcurrent setting, a voltagedependent element may be used to increase the sensitivity to such faults. As a reduction in system voltage willoccur during overcurrent conditions, this may be used to enhance the sensitivity of the overcurrent protection byreducing the pick up level.Voltage dependent overcurrent devices are often applied in generator protection applications in order to giveadequate sensitivity for close up fault conditions. The fault characteristic of this protection must then co-ordinatewith any of the downstream overcurrent devices that are responsive to the current decrement condition. Ittherefore follows that if the device is to be applied to an outgoing feeder from a generator station, the use ofvoltage dependent overcurrent protection in the feeder device may allow better co-ordination with the VoltageDependent device on the generator.4.1 VOLTAGE DEPENDENT OVERCURRENT PROTECTION IMPLEMENTATIONVoltage Dependent Overcurrent Protection (VDep OC) is set in the OVERCURRENT column of the relevant settingsgroup, under the sub-heading V DEPENDANT O/C.The function is available for stages 1, 2 and 5 of the main overcurrent element. When VDep OC is enabled, theovercurrent threshold setting is modified when the voltage falls below a set threshold.If voltage dependant overcurrent operation is selected, the element can be set in one of two modes, voltagecontrolled overcurrent or voltage restrained overcurrent.4.1.1 VOLTAGE CONTROLLED OVERCURRENT PROTECTIONIn Voltage Controlled Operation (VCO) mode of operation, the under voltage detector is used to produce a stepchange in the current setting, when the voltage falls below the voltage setting V Dep OC V<1 Set. The operatingcharacteristic of the current setting when voltage controlled mode is selected is as follows:E00642Current pickup settingK x Current pickup settingVoltage threshold settingCurrentsettingMeasured voltageFigure 38: Modification of current pickup level for voltage controlled overcurrent protectionP14x Chapter 6 - Current Protection FunctionsP14xEd1-TM-EN-1 105
