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Restraint
Check zone
7SS60 Manual
E50417-G1176-C132-A2
The restraint (stabilization) has the function of reducing the influence on the
measurement of transformation inaccuracies (e.g. for transformer saturation) in the
various feeders to such a degree that spurious behaviour of the protection system is
prevented.
The vectorial sum I
of all feeder currents acts as tripping quantity.
d
I
= | I
+ I
+ ... + I
|
d
1
2
n
The value sum of the all feeders is counterbalanced by the restraint quantity I
I
= | I
| + | I
| + ... + | I
R
1
2
The restraint R is determined by setting the restraint factor.
.
R = k
I
R
Figure 5-4
Differential current measuring circuit with restraint in the 7SS60 busbar protection
system
For an explanation of the circuit shown in Figure 5-4, let us consider first the conditions
at a given point in time. The n feeders of the busbars carry the currents J
These are defined as positive as they flow towards the busbar. The secondary
currents I
, I
... I
from the current transformers, which are proportional to the primary
1
2
n
currents if we assume (for the moment) an ideal transmission behaviour, flow
individually through the primary windings of the measuring transducers MS and
summated through the primary winding of the measuring transducer MD. On the
secondary side, each transducer MS of the restraint module feeds a measuring
rectifier GS. Regardless of the direction of the individual currents in the measuring
transducers, the current direction at the output of the measuring rectifiers is fixed and
the same at the considered and any other point in time.
The input I
thus detects the vectorial sum of the currents, and the input I
d
summated magnitudes of the currents.
The safety of operation of the busbar protection system 7SS60 can be further in-
creased by an additional so-called check zone.
This requires an additional measuring system which - regardless of the position of the
isolator - monitors the current sums of all busbar feeders. Possible tripping failures
originating from faulty information sent to the real state of the isolator are thus exclud-
ed.
When a busbar fault occurs, a TRIP-command decision is only given if both the meas-
uring system belonging to the faulty busbar section and the check zone measuring
system gives the decision for tripping (2 out of 2 decision).
|
n
R
Functions
R
Measuring
System
"k-factor"
=
, J
... J
1
2
the
R
.
.
n
5-5
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