Weak-Infeed Tripping; Method Of Operation - Siemens siprotec 7SA6 User Manual

6.7

Weak-Infeed Tripping

6.7.1

Method of Operation

7SA6 Manual
C53000-G1176-C133-1
In cases, where there is no or only weak infeed present at one line end, the distance
protection does not pick up there during a short-circuit on the line.
If there is no or only a very small zero sequence current at one line end during an earth
fault, the earth fault protection can also not function.
By coordinating the weak infeed function with the teleprotection in conjunction with
distance protection (refer to Section 6.4) and/or the teleprotection in conjunction with
earth fault protection (refer to Section 6.6), fast tripping can also be achieved at both
line ends in the above cases.
At the strong infeed line end, the distance protection can always trip instantaneously
for faults inside zone Z1. With permissive teleprotection schemes, fast tripping for
faults on 100 % of the line length is achieved by activation of the echo function (refer
to Sub-section 6.4.1.11). This provides the permissive release of the trip signal at the
strong infeed line end.
The permissive teleprotection scheme in conjunction with the earth fault protection
can also achieve release of the trip signal at the strong infeed line end by means of
the echo function (refer to Sub-section 6.6.1.5).
In many cases tripping of the circuit breaker at the weak infeeding line end is also
desired. For this purpose the device 7SA6 has a dedicated protection function with
dedicated trip command.
In Figure 6-81 the logic diagram of the weak-infeed tripping is shown. It may be
activated in address  )&7 :HDN ,QIHHG (Weak Infeed MODE) ((&+2 DQG
75,3) or deactivated (2))). If this "switch" is set to (&+2 RQO\, the tripping is also
disabled; however the echo function to release the infeeding line end is activated
(refer also to Sub-section 6.4.1.11 and 6.6.1.5). The tripping function can be blocked
at any time via the binary input "!%/2&. :HDN ,QI".
The logic for the detection of a weak-infeed condition is built up per phase in
conjunction with the distance protection and additionally once for the earth fault
protection. This allows single-pole tripping in conjunction with the distance protection
assuming the device version has the single-pole tripping option.
In the event of a fault, it may be assumed that only a small voltage appears at the line
end with the weak-infeed condition, as the small fault current only produces a small
voltage drop in the short-circuit loop. In the event of zero-infeed, the loop voltage is
approximately zero. The weak-infeed tripping is therefore dependent on the measured
undervoltage which is also used for the selection of the faulty phase.
If a signal is received from the opposite line end without fault detection by the local
protection, this indicates that there is a fault on the protected feeder. In the case of
three terminal lines, a receive signal from neither of the two opposite ends may be
present.
After a security margin time of 40 ms following the start of the receive signal, the weak-
infeed tripping is released if the remaining conditions are satisfied: undervoltage,
circuit breaker closed and no fault detection.
Functions
6-135