IEC 60255-121:2014

FOREWORD
1 Scope
2 Normative references
3 Terms and definitions
4 Specification of the function
5 Performance specifications
6 Functional tests
7 Documentation requirements
Annex A (informative) - Impedance characteristics
Annex B (informative) - Informative guide for the behaviour
        of timers in distance protection zones for
        evolving faults
Annex C (normative) - Setting example
Annex D (normative) - Calculation of mean, median
        and mode
Annex E (informative) - CT saturation and influence on
        the performance of distance relays
Annex F (informative) - Informative guide for testing
        distance relays based on CT requirements
        specification
Annex G (informative) - Informative guide for dimensioning
        of CTs for distance protection
Annex H (normative) - Calculation of relay settings based
        on generic point P expressed in terms of voltage
        and current
Annex I (normative) - Ramping methods for testing the basic
        characteristic accuracy
Annex J (normative) - Definition of fault inception angle
Annex K (normative) - Capacitive voltage instrument
        transformer model

IEC 60255-121:2014 specifies minimum requirements for functional and performance evaluation of distance protection function typically used in, but not limited to, line applications for effectively earthed, three-phase power systems. This standard also defines how to document and publish performance tests. This standard covers distance protection function whose operating characteristic can be defined on an impedance plane and includes specification of the protection function, measurement characteristics, phase selection, directionality, starting and time delay characteristics. The test methodologies for verifying performance characteristics and accuracy are included in this standard. The standard defines the influencing factors that affect the accuracy under steady state conditions and performance characteristics during dynamic conditions. It also includes the instrument transformer requirements for the protection function. The general requirements for measuring relays and protection equipment are defined in IEC 60255-1.