Siemens Siprotec 7VK61 Manual page 50

Functions
2.2 Automatic reclosure function (optional)
There are also various selectable possibilities for the response of the internal auto- reclose function to a de-
tected evolving fault.
• EV. FLT. MODE blocks AR:
The reclosure is blocked as soon as a sequential fault is detected. The tripping by the sequential fault is
always three-pole. This applies irrespective of whether three-pole cycles have been permitted or not. There
are no further reclosure attempts; the automatic reclosure is blocked dynamically (see also margin heading
„Blocking reclosure", above).
• EV. FLT. MODE starts 3p AR:
As soon as a sequential fault is detected, the recloser switches to a three-pole cycle. Each trip command is
three-pole. The separately settable dead time for sequential faults starts with the clearance of the sequential
fault; after the dead time the circuit breaker receives a close command. The further sequence is the same
as for single- and three-pole cycles.
The complete dead time in this case consists of the part of the dead time for the single-pole reclosure up to
the elimination of the sequential fault plus the dead time for the sequential fault. This makes sense because
the duration of the three-pole dead time is most important for the stability of the network.
If reclosure is blocked due to a sequential fault without the protection issuing a three-pole trip command (e.g.
for sequential fault detection with starting), the device can send a three-pole trip command so that the circuit
breaker does not remain open with one pole (forced three-pole trip).
Dead Line Check (DLC)
If the voltage of a disconnected phase does not disappear following a trip, reclosure can be prevented. A pre-
requisite 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
disconnected line for no-voltage: the line must have been without voltage for at least an adequate 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 generator) is connected along
the line.
Reduced Dead Time (RDT)
If automatic reclosure is performed in connection with time-graded protection, non-selective tripping before re-
closure is often unavoidable in order to achieve fast, simultaneous tripping at all line ends. The 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 procedure.
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 voltag-
es. 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 (Figure 2-6, mounting location
III). If the network is meshed and there is at least one other infeed on busbar B, the voltage there returns im-
mediately 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 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.
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SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009