ABB RED650, Техническое руководство, Страница: 107 / 926

Note that all small pre-fault differential currents are subtracted, regardless of their
origin. Besides the true charging currents, the following currents are eliminated:
• Small false differential currents due to small errors (inequalities) of current
transformers.
• Small false differential currents because of off-nominal load tap changer
positions when a power transformer is included in the protected zone.
• Load currents of tap power transformers included in the protected zone.
6.1.7.2 Time synchronization
SEMOD52396-4 v5
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
amplitude error approximately around 3% whilst an inaccuracy of 1 ms gives a
maximum amplitude 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.
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 37 , 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-US
Figure 37: 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.
1MRK 505 394-UEN A
Section 6
Differential protection
Line differential protection RED650 2.2 IEC 101
Technical manual