GE C30 Instruction Manual page 162

5.2 PRODUCT SETUP
PTP is a protocol that allows multiple clocks in a network to synchronize with one another. It permits synchronization accu-
racies better than 1 ns, but this requires each and every component in the network achieve very high levels of accuracy and
a very high baud rate, faster than normally used for relay communications. When operating over a generic Ethernet net-
work, time error may amount to 1 ms or more. PP is a profile of PTP which specifies a limited subset of PTP suitable for use
in power system protection, control, automation and data communication applications, and thereby facilitates interoperabil-
ity between different vendor's clocks and switches. PP specifies a worst-case delivered time error of less than 1 µs over a
16-hop network.
In a PTP system and in a PP system, the clocks automatically organize themselves into a master-slave synchronization
hierarchy with the "best" clock available making itself the "grandmaster" at the top of the hierarchy; all others make them-
selves "slaves" and track the grandmaster. Typically the grandmaster clock receives its time from GPS satellites or some
other link to the international time standard. If the grandmaster fails, the next "best" clock available in the domain assumes
the grandmaster role. Should a clock on starting up discover it is "better" that the present grandmaster, it assumes the
grandmaster role and the previous grandmaster reverts to slave.
Time messages issued by the grandmaster are delayed as they pass through the network both due to the finite speed of
the signal in the interconnecting fiber or wire. Each clock and switch implementing PP measures the propagation delay to
each of its PP neighbors, and compensates for these delays in the time received. Each network device implementing PP
measures the processing delay it introduces in each time message and compensates for this delay in the time it transmits.
As a result, the time delivered to end-devices such as the UR are virtually identical to the grandmaster time. Should one of
the network devices in the hierarchy not fully implement PP, the associated propagation delay and/or latency may not be
compensated for, and the time received at the end-device could be in error by more than 100 µs.
See the Settings > Product Setup > Real Time Clock section of this manual for a description of when time values
received via PTP are used to update the relay's real time clock.
The following settings are available for configuring the relay for PTP.
5
PORT 1 ... 3 FUNCTION
• While this port setting is selected to disabled, PTP is disabled on this port. The relay does not generate or listen to
PTP messages on this port.
PORT 1 ... 3 PATH DELAY ADDER
• The time delivered by PTP is advanced by the time value in this setting prior to the time being used to synchronize the
relay's real time clock. This is to compensate to the extent practical for time delivery delays not compensated for in the
network. In a fully compliant PP network, the peer delay and the processing delay mechanisms compensate for all the
delays between the grandmaster and the relay. In such networks, this setting should be zero.
• In networks containing one or more switches and/or clocks that do not implement both of these mechanisms, not all
delays are compensated, so the time of message arrival at the relay will be later than the time indicated in the mes-
sage. This setting can be used to approximately compensate for this delay. However, as the relay is not aware of net-
work switching that dynamically changes the amount of uncompensated delay, there is no setting that will always
completely correct for uncompensated delay. A setting can be chosen that will reduce worst-case error to half of the
range between minimum and maximum uncompensated delay, if these values are known.
PORT 1 ... 3 PATH DELAY ASYMMETRY
• This setting corresponds to "delayAsymmetry" in PTP, which is used by the peer delay mechanism to compensate for
any difference in the propagation delay between the two directions of a link. Except in unusual cases, the two fibers are
of essentially identical length and composition, so this setting should be set to zero.
• In unusual cases where the length of the link is different in different directions, this setting should be set to the number
of nanoseconds the Ethernet propagation delay to the relay is longer than the mean of path propagation delays to and
from the relay. For instance, if it is known say from the physical length of the fibers and the propagation speed in the
fibers that the delay from the relay to the Ethernet switch it is connected to is 9 000 ns and the that the delay from the
switch to the relay is 11 000 ns, then the mean delay is 10 000 ns, and the path delay asymmetry is 11000 - 10000 =
+1000 ns.
STRICT POWER PROFILE
• Power profile (IEEE Std C37.238 2011) requires that the relay only select as a grandmaster power profile compliant
clocks, that the delivered time have worst-case error of ±1 µs, and that the peer delay mechanism be implemented.
With the strict power profile setting enabled, the relay will only select as master clocks displaying the IEEE_C37_238
identification codes. It will use a port only when the peer delay mechanism is operational. With the strict power profile
5-46 C30 Controller System
5 SETTINGS
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