P12x/EN T/Eb6 - Schneider - error

Application GuideP12x/EN AP/Eb6MiCOM P120/P121/P122/P123 Page 21/78SoR ST = k I f (R CT +2R L )I e >with Vk ќ 4.Is.R ST (A typical value to ensure the high speed operation for an internal fault).6.3.1 CT requirements for High Impedance Restricted Earth Fault ProtectionThe High Impedance Restricted Earth Fault element shall remain stable for through faultsand operate in less than 40ms for internal faults provided that the following equations aremet in determining CT requirements and the value of the associated stabilising resistor:withRs = [k* (If) * (R CT + 2R L )] / I SV K ≥ 4 * Is * RsK= 1 for Vk/Vs less or equal to 16 andK= 1.2 for Vk/Vs > 16.6.4 Use of METROSIL non linear resistorsMetrosils are used to limit the peak voltage developed by the current transformers underinternal fault conditions, to a value below the insulation level of the current transformers,relay and interconnecting leads, which are normally able to withstand 3KV peak.The following formula should be used to estimate the peak transient voltage that could beinduced by an internal fault. This peak voltage depends on:− CT Knee point (V K )− Voltage that would be induced by an internal fault if CT doesn’t saturate (V f )This prospective voltage itself depends on:−−−−−−Maximum internal fault secondary currentCT ratioCT secondary winding resistanceCT lead resistance to the common pointRelay lead resistanceStability resistor value• Vp = 2√ {2.V K (V f - V k )}V f = I' f .(R ct + 2R L + R ST )Where−Vp: peak voltage developed by the CT under internal fault conditionsV f : maximum voltage that would be produced if CT saturation did not occurV k : current transformer Knee point voltageI’ f : is the maximum internal secondary fault currentR ct : current transformer secondary winding transformerR L : maximum lead burden from CT to relayR ST: Relay stabilising resistor.When the value given by the formula is greater than 3KV peak, it is necessary to useMetrosils. They are connected across the relay circuit and they allow to shunt the secondarycurrent output of the current transformer from the relay in order to prevent very highsecondary voltages.Metrosils are externally mounted and have annular discs shape.