is set to the nominal voltage level (phase-to-phase) of the power system or the high voltageequipment under consideration.The time delay for the OV2PTOV can sometimes be critical and related to the size of theovervoltage - a power system or a high voltage component can withstand smaller overvoltagesfor some time, but in case of large overvoltages the related equipment should be disconnectedmore rapidly.Some applications and related setting guidelines for the voltage level are given below:The hysteresis is for overvoltage functions very important to prevent that a transient voltageover set level is not “sealed-in” due to a high hysteresis. Typical values should be ≤ 0.5%.7.2.3.1 Equipment protection, such as for motors, generators, reactors andtransformers M13852-10 v2High voltage will cause overexcitation of the core and deteriorate the winding insulation. Thesetting has to be well above the highest occurring "normal" voltage and well below the highestacceptable voltage for the equipment.7.2.3.2 Equipment protection, capacitors M13852-13 v1High voltage will deteriorate the dielectricum and the insulation. The setting has to be wellabove the highest occurring "normal" voltage and well below the highest acceptable voltagefor the capacitor.7.2.3.3 Power supply quality M13852-16 v1The setting has to be well above the highest occurring "normal" voltage and below the highestacceptable voltage, due to regulation, good practice or other agreements.7.2.3.4 High impedance earthed systems M13852-19 v5In high impedance earthed systems, earth-faults cause a voltage increase in the non-faultyphases. Two step overvoltage protection (OV2PTOV) is used to detect such faults. The settingmust be above the highest occurring "normal" voltage and below the lowest occurring voltageduring faults. A metallic single-phase earth-fault causes the non-faulted phase voltages toincrease a factor of √3.7.2.3.5 The following settings can be done for the two step overvoltageprotection M13852-22 v9ConnType: Sets whether the measurement shall be phase-to-earth fundamental value, phase-to-phase fundamental value, phase-to-earth RMS value or phase-to-phase RMS value.Operation:Off/On.UBase (given inGlobalBaseSel): Base voltage phase to phase in primary kV. This voltage is usedas reference for voltage setting. OV2PTOV measures selectively phase-to-earth voltages, orphase-to-phase voltage chosen by the settingConnType. The function will operate if thevoltage gets lower than the set percentage ofUBase. WhenConnType is set toPhN DFT orPhNRMS then the IED automatically divides set value forUBase by √3. WhenConnType is set toPhPh DFT orPhPh RMS then set value forUBase is used. Therefore, always setUBase as ratedprimary phase-to-phase voltage of the protected object. If phase to neutral (PhN)measurement is selected as setting, the operation of phase-to-earth over voltage isautomatically divided by sqrt3. This means operation for phase-to-earth voltage over:1MRK 505 355-UEN A Section 7Voltage protection121Application manual
