2.10 Defrosting Protection
111
7ST6 Manual
E50417-G1176-C251-A3
2.10
Defrosting Protection
In some models of the 7ST6 device, the basic variant is equipped with the defrosting
protection (3xI, 3xU).
In order to remove icing from overhead contact line systems, unused lines are series-
connected (by the operator) and heated with a low current, if necessary with an inter-
posed resistor.
For sensitive detection of short-circuits during the defrosting, the current flowing into
the line and back are measured in the protection device and processed by a differen-
tial protection function. The differential protection protects 100% of the measuring
range.
The characteristic curve of the differential protection can be parameterized. As a
backup protection of the differential protection, a two-stage definite-time overcurrent
protection is provided that monitors the defrosting current.
2.10.1 Functional Description
General
Note
During defrosting, the overhead contact line is in a special switching state. All other
protection functions remain active when the defrosting protection is switched on. The
defrosting current can cause spurious tripping by one or more protection functions
(e.g. by the overvoltage protection or a back-up stage of the distance protection). In
such cases, these protection functions must be deactivated prior to activating the de-
frosting protection, or to switching on the defrosting current. Use for this purpose e.g.
the settings group change option via a binary input or via the system interface.
In defrosting mode, the overhead contact line must be put into a special switching
state. An example of such a switching state is shown in Figure 2-31. The breakers Q0
(overhead contact line 1), Q11 and Q9 are closed, breaker Q0 (overhead contact line
2) is open.
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