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The device at End A samples its current signals at times tA1, tA2, etc. The device at End B samples its current
signals at time tB1, tB2, etc. The sampling of the signals at the two ends are not synchronised, but both operate in
the same way. The filtering and processing of the current inputs produces current vectors together with timing
information, which are sent between devices as shown in the figure.
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. The follow section describes time alignment.
The same ping pong technique is used for all remote terminals to derive the propagation time delay from all
terminals. The ping pong technique measures the propagation time delay from all the terminals continuously.
4.2 REMOTE TERMINAL TIME ALIGNMENT
In a system with multiple terminals, in order to align all the data to the same reference time, it is crucial to select a
reference time for which data of all the terminals are available. Therefore, the terminal with maximum time delay is
identified and the rest of the terminals are aligned with the time delay difference between them and the terminal
with maximum time delay. For example the following figure explains how the time alignment is carried out where
there are 3 remote terminals.
P54A/B/C/E Chapter 6 - Current Differential Protection
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