2 Functions
236
7SA522 Manual
C53000-G1176-C155-3
send a three pole trip command so that the circuit-breaker does not remain open with
one pole (forced three-pole coupling).
Dead Line Check
(DLC)
If the voltage of a disconnected phase does not disappear following a trip, reclosure
can be prevented. A prerequisite for this function is that the voltage transformers are
connected on the line side of the circuit breaker. To select this function the dead line
check must be activated. The automatic reclosure function then checks the discon-
nected line for no-voltage: The line must have been without voltage for at least an ad-
equate measuring time during the dead time. If this was not the case the reclosure is
blocked dynamically.
This no-voltage check on the line is of advantage if a small generator (e.g. wind gen-
erator) is connected along the line.
Reduced Dead
Time (RDT)
If automatic reclosure is performed in connection with time-graded protection, non-
selective tripping before reclosure is often unavoidable in order to achieve fast, simul-
taneous tripping at all line ends. The 7SA522 has a “reduced dead time (RDT)” proce-
dure which reduces the effect of the short-circuit on healthy line sections to a mini-
mum. The three phase voltages are measured for the reduced dead time. The voltage
transformers must be located on the line side of the circuit breaker.
In the event of a short-circuit close to one of the line ends, the surrounding lines can
initially be tripped because, for example, a distance protection detects the fault in its
overreaching zone Z1B (2-100Figure , relay location III). If the network is meshed and
there is at least one other infeed on the busbar B, the voltage there returns immedi-
ately after clearance of the fault. For single-pole tripping it is sufficient if there is an
earthed transformer with delta winding connected at busbar B which ensures symme-
try of the voltages and thus induces a return voltage in the open phase. This allows a
distinction between the faulty line and the unfaulted line to be made as follows:
Since line B - C is only tripped singled-ended at C, it receives a return voltage from the
end B which is not tripped so that at C the open phase(s) also has(have) voltage. If
the device detects this at position III, reclosure can take place immediately or in a
shorter time (to ensure sufficient voltage measuring time). The healthy line B - C is
then back in operation.
Line A–B is tripped at both ends. No voltage is therefore present identifying the line as
the faulted one at both ends. The normal dead time comes into service here.
Figure 2-100 Example of a reduced dead time (RDT)
Adaptive Dead
Time (ADT)
In all the previous alternatives it was assumed that defined and equal dead times were
set at both line ends, if necessary for different fault types and/or reclose cycles.
It is also possible to set the dead times (if necessary different for various fault types
and/or reclose cycles) at one line end only and to configure the adaptive dead time at
the other end (or ends). This can be done provided that the voltage transformers are
To Next Page
Loading...
