Alstom MiCOM P546 - 1.1.1 Time alignment of current vectors; 1.1.1.1 Time alignment of current vectors without GPS input (traditional technique)

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-15
OP
1.1.1 Time alignment of current vectors
1.1.1.1 Time alignment of current vectors withou
t GPS input (traditional technique)
This section relates to P54x relays when the GPS synchronization is not used.
To calculate differential current between line ends it is necessary that the current samples
from each end are taken at the same moment in time. This can be achieved by time
synchronizing the sampling, or alternatively, by the continuous calculation of the propagation
delay between line ends. The P54x range of relays has adopted this second technique.
Consider a two-ended system as shown in Figure 3.
Two
identical relays, A and B are placed at the two ends of the line. Relay A samples its
current signals at time tA1, tA2 etc., and relay B at time tB1, tB2 etc.
Note: The sampling instants at the two ends will not, in general, be
coincidental or of a fixed relationship, due to slight drifts in sampling
frequencies.
A B
tA1
tA2
tA3
tA4
tA5
tB3*
tA*_ _
tB1
tB2
tB3
tB4
tB5
tB*
td
tp2
tp1
tdtA1tB3
tA1
P1002ENa
End A End B
Digital communications link
Measure sampling time
tB3* = (tA - tp2)
Propagation delay time
tp1 = tp2 = 1/2 (tA* - tA1 - td)
Current vectors
tA1, tA2 - sampling instants of relay A
tB1, tB2 - sampling instants of relay B
tp1 - propagation delay time from relay A to B
tp2 - propagation delay time from relay B to A
td - time between the arrival
of message tA1
at relay B and despatch of message tB3
tA* - arrival time of message tB3 at relay A
tB* - arrival time of message tA1 at relay B
tB3*-the measured sampling time of tB3 by relay
A
Current vectors
Figure 3 Propagation delay measurement
Assume that at time tA1, relay A sends a data message to relay B. The message contains a
time tag, tA1, together with other timing and status information and the current vector values
calculated at tA1. The message arrives at end B after a channel propagation delay time, tp1.
Relay B registers the arrival time of the message as tB*.