Fast clock synchronization modeAt start-up and after interruptions in the IRIG B time signals, the deviation betweenthe external time system and the internal differential time system can besubstantial. A new start-up is also required, for example, after maintenance of theauxiliary voltage system.When the time difference is > 16 μs, the line differential protection function isblocked and the time regulator for the hardware clock automatically uses a fastmode to synchronize the clock systems. Time adjustment is made with anexponential function, that is, with big time adjustment steps in the beginning, andthen smaller steps until a time deviation of < 16 μs between the external timesystem and the internal differential time system has been reached. The protectionfunction is then enabled and the synchronization remains in fast mode or switchesto slow mode depending on the setting.Slow clock synchronization modeDuring normal service, a setting with slow synchronization mode is used. Thisprevents the hardware clock to make too big a time steps (> 16 μs) emanating fromthe requirement of correct timing in the line differential protection function.Synchronization principle M11346-83 v4From a general point of view synchronization can be seen as a hierarchicalstructure. A function is synchronized from a higher level and providessynchronization to lower levels.FunctionSynchronization froma higher levelOptional synchronization ofmodules at a lower levelIEC09000342-1-en.vsdIEC09000342 V1 EN-USFigure 362: Synchronization principleA function is said to be synchronized when it periodically receives synchronizationmessages from a higher level. As the level decreases, the accuracy of thesynchronization decreases as well. A function can have several potential sources ofsynchronization, with different maximum errors. This gives the function thepossibility to choose the source with the best quality, and to adjust its internal clockbased on this source. The maximum error of a clock can be defined as:1MRK 505 394-UEN A Section 21Basic IED functionsLine differential protection RED650 2.2 IEC 781Technical manual
