25.1.7.3 Current transformers according to ANSI/IEEECurrent transformers according to ANSI/IEEE are partly specified in different ways. Arated secondary terminal voltage VANSI is specified for a CT of class C. VANSI 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 VANSI values for example, VANSI is 400 V for a C400 CT. A correspondingrated equivalent limiting secondary e.m.f. EalANSI can be estimated as follows:a lANS I S N CT ANS I S N CT S N bANS IE 20 I R V 20 I R 20 I Z= × × + = × × + × ×EQUATION1682 V1 EN (Equation 541)where:ZbANSI The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C class(W)VANSI The secondary terminal voltage for the specific C class (V)The CTs according to class C must have a calculated rated equivalent limiting secondarye.m.f. EalANSI that fulfils the following:alANSI alreqE maximum of E>EQUATION1384 V2 EN (Equation 542)A CT according to ANSI/IEEE is also specified by the knee point voltage VkneeANSI thatis graphically defined from an excitation curve. The knee point voltage VkneeANSInormally has a lower value than the knee-point e.m.f. according to IEC and BS. VkneeANSIcan approximately be estimated to 75 % of the corresponding Eal according to IEC61869-2. Therefore, the CTs according to ANSI/IEEE must have a knee point voltageVkneeANSI that fulfills the following:kneeANSI alreqV 0.75 (max imum of E )> ×EQUATION2101 V2 EN (Equation 543)The following guide may also be referred for some more application aspects of ANSI classCTs: IEEE C37.110 (2007), IEEE Guide for the Application of Current TransformersUsed for Protective Relaying Purposes.1MRK 506 369-UUS - Section 25RequirementsLine distance protection REL670 2.2 ANSI 923Application manual
