Siemens SIPROTEC 4 7VK61 Manual page 51

This no-voltage check on the line is of advantage if a small generator (e.g. wind generator) is connected along
the line.
Reduced Dead Time (RDT)
If automatic reclosure function is performed in connection with time-graded protection, non-selective tripping
before reclosure is often unavoidable in order to achieve fast, simultaneous tripping at all line ends. 7VK61
has a "reduced dead time (RDT" procedure which reduces the effect of the short-circuit on healthy line sections
to a minimum. All phase-to-phase and phase-to-earth voltages are measured for the reduced dead time proce-
dure. These voltages must rise above the threshold U-live> (address 3440) for the voltage measuring time T
U-stable (address 3438). The value set for U-live> is appropriately converted for the phase-to-phase
voltages. 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
mounting location III). If the network is meshed and there is at least one other infeed on busbar B, the voltage
there returns immediately after clearance of the fault. For 1-pole tripping it is sufficient if there is an earthed
transformer with delta winding connected at busbar B which ensures symmetry 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.
[beispiel-verkuerzte-we-wlk-310702, 1, en_GB]
Figure 2-8
A, B, C Busbars
I, II, III Relay locations
X Tripped circuit breakers
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 requires the voltage
transformers to be connected to the device and to be located on the line side of the circuit breaker. This func-
tionality is only available if the device is connected across 3 voltage transformers.
Figure 2-9 shows an example with voltage measurement. It is assumed that device I operates with defined
dead times whereas the adaptive dead time is configured at position II. It is important that the line is at least
fed from busbar A, i.e. the side with the defined dead times.
When using the adaptive dead time, the automatic reclosing function at line end II decides autonomously
whether to allow reclosing or not. Its decision is based on the line voltage at end II, which was reapplied from
end I following reclosure. Device II will thus initiate reclosing as soon as it is evident that the line has been
reenergized from end I. All phase-to-phase and phase-to-earth voltages are monitored.
SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018
Example of a reduced dead time (RDT)
2.2 Automatic reclosure function (optional)
(Figure 2-8,
Functions
51