Siemens SIPROTEC 7SS52, Operation Manual

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4 - 10
Siemens AG
⋅
May 1998
For this reason, the busbar protection 7SS52 has softĆ
ware logic which detects the inception of a short-cirĆ
cuit from the currents. Evaluating the current condiĆ
tions during the next 3 ms determines whether an
external or internal short-circuit is present. In case of
an internal short-circuit, the measurement initiates
the issue of trip signals to the trip coils. For external
short-circuits the measurement initiates the change-
over between the "2-out-of-2-mode" and the
"1-out-of-1-mode". The measurement is usually
performed in the 2-out-of-2-mode, under no-fault
conditions.The module for weighted evaluation of the
initial values, in the following referred to as "accelerĆ
ated measurement" controls the logic in order to
change the operating mode to 1-out-of-1 as soon as
it detects ashort-circuit in the protected zone. If the
measured result indicates an external fault, the
2-out-of-2 mode is maintained for 150 ms by blockĆ
ing of this changeover.
The accelerated measurement is performed with the
stabilizing quantity |āā |. The rate of change
d [āā |āā |āā] / dt is compared with a set limit in order to
detect ashort-circuit.
If the set limit value is exceeded, then the criterion of
the characteristic Fig 4.5 is checked. If the criterion is
met, the protection trips after one measurement
("1-out-of-1"). Otherwise two measurements
("2-out-of-2") are performed and required for a trip.
4.3.1.4 Response threshold, pick-up
The working conditions for the busbar protection may
differ widely from one station to another. Great differĆ
ences exist regarding the design and the switching
possibilities of the various substations. The span beĆ
tween the highest and lowest currents liable to occur
in the event of ashort-circuit may differ from sub-
station to substation. The performance data and burĆ
den of the current transformers used for connection
to the protection also play an essential part. Finally,
the starpoint conditions in the high voltage system
are of importance with respect to design and setting
of the protection. It is for these reasons that the busĆ
bar protection system needs a high degree of versatilĆ
ity.
The tripping characteristic is determined by the two
settable parameters "stabilizing factor k" and "diffeĆ
rential current threshold >DIFF".
4.3.1.5 Summary of the measuring method
The measuring method of the busbar protection can
be summarized as follows:
Tripping occurs, when
1.
āā > set limit and
2. āā > k x | | a nd
3. release from "1-out-of-1" or "2-out-of-2".
The measuring method is illustrated in Figs 4.7 and
4.8.
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