At internal fault, the secondary circuit voltage can easily exceed the isolationvoltage of the CTs, connection wires and IED. To limit this voltage, a voltage-dependent resistor VDR is used as shown in Figure 234.The whole scheme, that is, the stabilizing resistor, voltage-dependent resistor andwiring, must be adequately maintained (operation- and insulation-tested regularly)to be able to withstand the high-voltage pulses which appear during an internalfault throughout the lifetime of the equipment. Otherwise, during a fault within thezone of protection, any flashover in the CT secondary circuits or in any other partof the scheme may prevent a correct operation of the high-impedance differentialfunction.4.3.4.6 The measuring configurationThe external measuring configuration is composed of four current transformersmeasuring the currents and a stabilizing resistor. A varistor is needed if highovervoltages are expected.The value of the stabilizing resistor is calculated with the formula:R UIs srs=GUID-00FCABE9-93E2-4BDD-83C6-EB1BE7FFE986 V1 EN (Equation 58)Rs the resistance of the stabilizing resistorUs the stabilizing voltage of the IEDIrs the value of theLow operate value settingThe stabilizing voltage is calculated with the formula:U In R Rs k in m= +max ( )GUID-6A4C58E7-3D26-40C9-A070-0D99BA209B1A V1 EN (Equation 59)Ikmax the highest through-fault currentn the turns ratio of the CTRin the secondary internal resistance of the CTRm the resistance of the longest loop of secondary circuitAdditionally, it is required that the current transformers' knee-point voltages Uk areat least twice the stabilizing voltage value Us.Section 4 1YHT530004D05 DProtection functions436 615 seriesTechnical Manual
