ABB RER670 Applications Manual page 480

Section 21
Basic IED functions
474
via a PPS out from the GTM. If PTP is used in the IED and the merging unit
cannot be synchronized from the IED, then use GPS-based clocks to provide PTP
synch as well as sync to the merging unit.
If synchronization of the IED and the merging unit is based on GPS, set the
parameter LostSyncMode to BlockOnLostUTC in order to provide a block of
protection functions whenever the global common time is lost.
If PTP is not used, use the same synchronization method for the HwSyncSrc as the
merging unit provides. For instance, if the merging unit provides PPS as
synchronization, use PPS as HwSyncSrc. If either PMU or LDCM in GPS-mode is
used, that is, the hardware and software clocks are connected to each other,
HwSyncSrc is not used and other means to synchronize the merging unit to the IED
is required. Either FineSyncSource is set to the same source that the merging unit
uses, or the PPS output from the GTM module is used to synchronize the merging
unit. If the PPS output from the GTM module is used to synchronize the merging
unit and PTP is not used, the IED does not know how the merging unit is
synchronized and the parameter LostSyncMode must be set to NoBlock.
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Railway application RER670 2.2 IEC
Application manual