1 Scope
This part of IEC 60255 specifies the minimum requirements for functional and performance evaluation of (longitudinal)
differential protection designed for the detection of faults in ac motors, generators
and transformers. This document also defines how to document and publish performance
test results.
This document covers the differential protection function whose operating characteristic
can be defined on a bias-differential plane. It includes specification of the protection
function, measurement characteristics, compensation of energizing quantities, additional
restraint or blocking methods (for overexcitation and magnetizing inrush), starting
and time delay characteristics. This document also covers unrestrained differential
protection functions traditionally combined with the restrained (biased) differential
element to form a single differential relay.
This document defines the influencing factors that affect the accuracy under steady
state conditions and performance characteristics during dynamic conditions. The test
methodologies for verifying performance characteristics and accuracy are also included
in this document.
This document also includes current transformer requirements for the protection functions.
The differential protection functions covered by this document are as follows:
| IEEE/ANSI C37.2 function numbers | IEC 61850‑7‑4 logical nodes | |
|---|---|---|
| Transformer differential | 87T | PDIF |
| Motor differential | 87M | PDIF |
| Generator differential | 87G | PDIF |
| Restricted earth fault (ground differential) | 87N | PDIF |
| Inrush restraint or inrush blocking | PHAR | |
| Overexcitation restraint or overexcitation blocking | PHAR |
This document does not specify the functional description of additional features often
associated with biased differential relays such as current transformer (CT) supervision
(CTS), switch onto fault (SOTF) and detection of geo-magnetically induced currents
(GIC).
This document does not cover differential relays designed for bus bar protection (including
high impedance differential protection and low impedance differential protection) or line protection. Additionally, this document does not explicitly cover generator
incomplete longitudinal differential protection, generator split-phase transverse differential protection, self-balancing or magnetic balanced protection scheme, differential protection of
phase-shifting transformers, directional restricted earth fault protection, railway
transformers, convertor transformers and reactors. However, the principles covered
by this document can be extended to provide guidance on these applications.
The general requirements for measuring relays and protection equipment are defined
in IEC 60255‑1.