Gps Synchronisation - GE MiCOM P40 Agile Technical Manual

P543i/P545i
At time tA1, End A sends a data message to End B. The message contains a time tag, tA1, plus other timing,
control, and status information as well as the calculated current values. The message arrives at End B after a
channel after a propogation delay time tp1. End B registers the arrival time of the message as tB*.
Since the devices at both terminals operate in the same way, End B also sends messages to End A. In the figure,
End B sends a message at tB3. The message contains the time tag tB3. It also returns the last received time tag
from End A (tA1) and the delay time, td, between the time of the the message was received, tB*, and the sampling
time, tB3, where td = (tB3 - tB*).
The message arrives at End A after a channel propagation delay time, tp2. The arrival time is registered by End A
as tA*. From the returned time tag, tA1, End A can measure an elapsed time as (tA* - tA1). This equals the sum of
the propagation delay times, tp1, and tp2, as well as the time between End B receiving the message and returning
it. So:
(tA* - tA1) = (td + tp1 + tp2)
The device assumes that the time to communicate data between two terminals is the same in each direction, and
on this basis tp1 and tp2 can be calculated as:
tp1 = tp2 = ½(tA* - tA1 - td)
The propagation delay time is measured for each received message. This is used to monitor changes on the
communication link and to manage the response of the protection. When the propagation delay time has been
calculated, the sampling instant of the received data from End B (tB3*) can also be calculated. As shown in the
figure, the sampling time tB3* is measured by End A as:
tB3* = (tA* - tp2)
In the figure, tB3* is between tA3 and tA4. To calculate the differential and bias currents, the values at each
terminal must correspond to the same point in time. So the values received at tB3* must be aligned with values
taken at sampling instants tA3 and tA4. This is achieved by rotating the received current vector by an angle
corresponding to the time difference between tB3* and tA3 (and tA4).
After this time-alignment process, the respective differential and bias currents can be calculated.
3.2

GPS SYNCHRONISATION

In schemes where protection communications operate over switched communication channels, there is a
possibility of asymmetry of communications channels between connected terminals. The term "asymmetry" is
used to describe an application in which the time taken to communicate information between a pair of connected
terminals is different in one direction to the other.
If the communication between devices is asymmetric, the ping-pong method of synchronisation may not work
correctly, so the products are equipped with features designed to provide stability for asymmetric
communications.
To overcome the shortcomings of the ping-pong technique, each device provides a GPS input, which can be used
together with an appropriate synchronising device (e.g. P594). GPS synchronisation works with asymmetric
communications paths.
The GPS synchronisation device produces a synchronisation signal that can be connected to the protection device
via optical fibre. If you have an application that needs GPS synchronisation, you must connect the GPS
synchronisation input on the protection, and you must enable the function using the GPS Sync setting in the PROT
COMMS/ IM64 column.
The figure below shows the different propagation delays and how compensation is applied. The GPS
synchronisation compensates for the different delays associated with the transmit and receive communication
paths.
P54x1i-TM-EN-1
Chapter 6 - Current Differential Protection
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