Name Type Values (Range) Unit DescriptionANGLE REAL - deg Angle between 3rdharmonic votagephasors, radiansDU3 REAL - kV Diff. between 3rd harm.volt. at both sides of gen.,VoltsBU3 REAL - kV Bias voltage, a part ofvoltN3rdHarmonic,primary VoltsUN REAL - kV Fund. frequency voltageat generator neutral,primary Volts9.6.7 Operation principleThe protection is a combination of the 95% fundamental frequency earth faultprotection and the 3 rd harmonic based stator earth fault protection, (STEFPHIZ). The3rd harmonic based 100% stator earth fault protection is using the 3rd harmonic voltagegenerated by the generator itself. To assure reliable function of the protection it isnecessary that the 3rd harmonic voltage generation is at least 0.8 V RMS on VTsecondary side.The 3rd harmonic voltage generated by the generator has the same phase angle in thethree phases. It has the characteristic of a zero sequence component. If the generatoris connected to the power system via a block transformer that cannot transform zerosequence voltages between the voltage levels, the 3rd harmonic voltage, that is U3Nand U3T in fig 258, in the generator system is not influenced by the external powersystem. At normal operation the generator third harmonic voltage characteristic canbe described as in figure 258.Note that angle between U3N and U3T is typically close to 180°.Section 9 1MRK502052-UEN BVoltage protection536Technical manual
