Ef frV Hz >£IECEQUATION2297 V2 EN (Equation 87)where:V/Hz> is the maximum continuously allowed voltage at no load, and rated frequency.V/Hz> is a setting parameter. The setting range is 100% to 180%. If the user does notknow exactly what to set, then the default value for V/Hz> = 110 % given by the IEC60076-1 standard shall be used.In OEXPVPH, the relative excitation M is expressed according to equation 88.( ) E fM p.u. = Ur frIECEQUATION2299 V1 EN (Equation 88)It is clear from the above formula that, for an unloaded power transformer, M = 1 forany E and f, where the ratio E/f is equal to Ur/fr. A power transformer is notoverexcited as long as the relative excitation is M ≤ V/Hz>, V/Hz> expressed in % ofUr/fr.It is assumed that overexcitation is a symmetrical phenomenon, caused by events suchas loss-of-load, etc. A high phase-to-earth voltage does not mean overexcitation. Forexample, in an unearthed power system, a single phase-to-earth fault means highvoltages of the “healthy” two phases-to-earth, but no overexcitation on any winding.The phase-to-phase voltages will remain essentially unchanged. The importantvoltage is the voltage between the two ends of each winding.9.4.7.1 Measured voltageA check is made if the Selected voltage signal is higher than 70% of the rated phase-to-earth voltage. When below this value, OEXPVPH exits immediately and no excitationis calculated.The frequency value is received from the pre-processing block. The function operatesfor frequencies within the range of 33-60 Hz and of 42-75 Hz for 50 Hz and 60 Hzrespectively.• OEXPVPH can be connected to any power transformer side, independent fromthe power flow.• The side with a load tap changer must not be used, since the tap changer canchange the relative excitation (M)1MRK 502 048-UEN A Section 9Voltage protection279Technical manual
