ABB Relion 670 series, Technical Reference Manual

Page: 140 / 1232

5.1.2.2 Time synchronization
In a numerical line differential protection, current samples from protections located
geographically apart from each other, must be time coordinated so that the currents
from the different line ends can be compared without introducing irrelevant errors.
Accuracy requirements on this time coordination are extremely high.
As an example, an inaccuracy of 0.1 ms in a 50 Hz system gives a maximum
magnitude error approximately around 3% whilst an inaccuracy of 1 ms gives a
maximum magnitude error of approximately 31%. The corresponding figures for a 60
Hz system are 4% and 38% respectively.
In Line differential protection, the time coordination is made with the so-called echo
method, which can be complemented with GPS synchronization as an option.
Each IED has an accurate local clock with a very small time drift. This clock makes
time tagging of telegrams, and the echo method is then used to find out the time
difference between the clocks in two ends of a power line.
Referring to figure 55 , it works such that the transmission time to send a message from
station B to station A (T
1
→ T
2
) and receive a message from A to B (T
3
→ T
4
) is
measured. The time instances T
2
and T
3
are taken with the local clock reference of
station A, and the time instances T
1
and T
4
are taken with the local clock reference of
station B.
T
1
T
2
T
3
T
4
B
A
en05000293.vsd
IEC05000293 V1 EN
Figure 55: Measuring time differences
Calculation of the delay time one-way T
d
and the time difference Δt between the
clocks in A and B is then possible to do with equation 2 and equation 3 , which are only
valid under the condition that the send and receive times are equal.
2 1 4 3
( ) ( )
2
d
T T T T
T
- + -
=
EQUATION1358 V1 EN
(Equation 2)
Section 5
1MRK505222-UUS C
Differential protection
134
Technical reference manual