Siemens siprotec 7SD5 User Manual page 505

Measuring Differen- The test for two ends is terminated with the reading of the differential, restraint and
tial and Restraint load currents which simultaneously check that the current transformer connections
Currents have been restored correctly after the I
• Read out the differential, restraint and load currents. They are available for every
with
I
I
I
I
In the „WEB-Monitor", the differential and restraint currents are displayed as a graph
in a characteristics diagram. One example is illustrated in Figure 3-33.
• If there is a differential current in the size of twice the through-flowing current, you
• Finally, open the circuit breaker again.
• If parameter settings have been changed for the tests, reset them to the values nec-
7SD5 Manual
C53000-G1176-C169-1
phase at the device display or in DIGSI
– The differential currents must be low, at least one scale less than the currents
flowing through. If high charging currents are to be expected in long overhead
lines or cables, these are additionally included in the differential currents.
– The highest of the measured values of the load current (3 values) is converted to
Ampere and entered in I-DIFF>. The recommended setting for the pickup
threshold is 1 · I .
cN
– The restraint currents result from the pickup value I-DIFF> (address 1210, see
Section 2.3.2) in addition to the sum of the current errors to be tolerated: such as
the locally permissible current transformer error according to address 253 E%
ALF/ALF_N (see Section 2.1.2) the permissible current transformer errors at the
other ends according to that setting there, as well as the internal estimation of the
system errors (frequency, synchronization and delay time difference errors). With
the default values for I-DIFF> (0.3I
is
the actual current flowing,
the nominal operating current (as configured),
NB
the primary nominal current of the local current transformers,
N1
the primary nominal current of the current transformers of the remote end.
N2
may assume a polarity reversal of the current transformer(s) at one line end. Check
the polarity again and set it right after short-circuiting all the three current transform-
ers. If you have modified these current transformers, also perform a power and
angle test.
essary for operation.
test (if performed).
4
®
amongst the measured values.
) and E% ALF/ALF_N (5.0% = 0.05) there
N
3.3 Commissioning
505