P.3.23
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Theory of Operation
through the averaging filters and result in alignment errors. This is
advantageous, because many cases of channel asymmetry are short lived and
result from the SONET/SDH systems switching paths.
Data alignment through use of the channel-based method is often considered
superior because it requires no use of explicit time sources (traditionally
including Global Positioning Systems (GPS) clocks) as part of the line
protection scheme. The Alpha Plane operating characteristic of the relay
handles synchronization errors on the order of a few milliseconds. However, if
channel asymmetry exceeds permissible limits given targeted sensitivity and
settings of the Alpha Plane, the relay provides an option to align the data
according to explicit time sources (the external-time-based mode).
In the external-time-based mode, the two relays connected with an 87L
channel must connect via the IRIG-B inputs to high-precision clocks that
provide an absolute time. Historically, these clocks have been GPS
synchronized. In the near future, however, these clocks may be configured to
use terrestrial, network-based time distribution schemes such as the SEL
ICON™ system or the IEEE 1588 method.
The connected clocks must report time quality via the time-quality bits
embedded in the IRIG-B signal according to the C37.118 standard
specification, so that the relay can respond to situations when supplied time is
insufficiently accurate for the 87L application.
In the external-time-based mode, the free-running clocks of the relays are each
phase locked to the external time and are therefore mutually aligned. There is
no need to calculate the time offset, because it is known to be zero.
Equation 3.32
The remainder of the data alignment algorithm, beginning with Equation 3.31,
works identically to the channel-based mode after determination of time offset
per Equation 3.32.
The relays monitor the presence and quality of connected time sources. The
87L data packet contains a bit that informs the remote relays if the local relay
loses absolute time. In this way, the 87L scheme is guaranteed fail safe if
configured to use external time and this time becomes unavailable or degraded
beyond the point of secure application.
Note that the 87L scheme allows configuration of the data alignment method
on a per-channel basis. Some channels (symmetrical) may be configured to
use the channel-based method. Alignment of data over these channels is not
dependent on the presence and quality of connected time sources. Other
channels (asymmetrical) may be configured to use the external-time-based
method. Data alignment over those channels is dependent on the presence and
quality of the connected time.
This flexibility in configuration can provide a way to limit exposure of the
entire scheme to availability of time sources. Consider, for example, a two
terminal application with a redundant channel. Assume further that the
primary channel is a direct point-to-point fiber connection and is, therefore,
guaranteed to be symmetrical. This channel should be configured to use the
channel-based synchronization mode. Assume that the secondary channel is a
multiplexed channel for which there can be no guarantee of symmetry because
of limitations in the communications network. This channel should be
configured to use the external-time-based method. In this way, the availability
and presence of time sources come into play only if the secondary channel is
in use. When the primary channel is available and in use, the scheme is never
exposed to time sources.
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