24.1.7.3 Current transformers according to ANSI/IEEECurrent transformers according to ANSI/IEEE are partly specified in different ways.A rated secondary terminal voltage UANSI is specified for a CT of class C. UANSI is thesecondary terminal voltage the CT will deliver to a standard burden at 20 times ratedsecondary current without exceeding 10 % ratio correction. There are a number ofstandardized UANSI values for example, UANSI is 400 V for a C400 CT. Acorresponding rated equivalent limiting secondary e.m.f. EalANSI can be estimated asfollows:alANSI sr ct ANSI sr ct sr bANSIE 20 I R U 20 I R 20 I Z= × × + = × × + × ×EQUATION971 V2 EN (Equation 314)where:ZbANSI The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific Cclass (W)UANSI The secondary terminal voltage for the specific C class (V)The CTs according to class C must have a calculated rated equivalent limitingsecondary e.m.f. EalANSI that fulfils the following:alANSI alreqE maximum of E>EQUATION1384 V2 EN (Equation 315)A CT according to ANSI/IEEE is also specified by the knee point voltage UkneeANSIthat is graphically defined from an excitation curve. The knee point voltage UkneeANSInormally has a lower value than the knee-point e.m.f. according to IEC and BS.UkneeANSI can approximately be estimated to 75 % of the corresponding Eal accordingto IEC 61869-2. Therefore, the CTs according to ANSI/IEEE must have a knee pointvoltage UkneeANSI that fulfills the following:kneeANSI alreqV 0.75 (max imum of E )> ×EQUATION2101 V2 EN (Equation 316)24.2 Voltage transformer requirementsThe performance of a protection function will depend on the quality of the measuredinput signal. Transients caused by capacitive voltage transformers (CVTs) can affectsome protection functions.Magnetic or capacitive voltage transformers can be used.Section 24 1MRK 502 071-UEN -Requirements758 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
