9.5.6 Monitored dataTable 182:STEFPHIZ (59THD) Monitored dataName Type Values (Range) Unit DescriptionVT3 REAL - kV Mag. of 3rd harm. voltage atgenerator terminal sideVN3 REAL - kV Mag. of 3rd harm. voltage atgenerator neutral sideE3 REAL - kV Total induced stator 3rdharmonic voltageANGLE REAL - deg Angle between 3rd harmonicvotage phasorsDV3 REAL - kV Diff. between 3rd harm. volt. atboth sides of gen.BV3 REAL - kV Bias voltage, a part of UN3V_N REAL - kV Fund. frequency voltage atgenerator neutral9.5.7 Operation principleThe protection is a combination of the 95% fundamental frequency ground faultprotection and the100% Stator ground fault protection, 3 rd harmonic based, (STEFPHIZ,59THD). The 3rd harmonic based 100% stator ground fault protection is using the 3rdharmonic voltage generated by the generator itself. To assure reliable function of theprotection it is necessary that the 3rd harmonic voltage generation is at least 0.8 V RMS onVT secondary side.The 3rd harmonic voltage generated by the generator has the same phase angle in the threephases. It has the characteristic of a zero sequence component. If the generator isconnected to the power system via a block transformer that cannot transform zerosequence voltages between the voltage levels, the 3rd harmonic voltage, that is V3N andV3T in fig 144, in the generator system is not influenced by the external power system. Atnormal operation the generator third harmonic voltage characteristic can be described asin figure 144.Note that angle between V3N and V3T is typically close to 180°.1MRK 502 048-UUS A Section 9Voltage protection313Technical manual
