Busbar Protection
99
7SS52 V4 Manual
C53000-G1176-C182-3
A sophisticated combination of the above two distinguishing features stabilizes the op-
erating characteristics of the SIPROTEC 7SS52 V4 distributed busbar and breaker
failure protection even under extremely difficult conditions, namely:
•
high degree of stability even during current transformer saturation
•
short tripping times for internal short-circuits
•
correct behaviour and proper response to evolving faults
At first glance, the stability in the event of external short-circuits appears to be jeop-
ardized by the current conditions shown in Figure 5-5, page 98 b). The tripping quan-
tity markedly exceeds the restraining quantity on two occasions.
However, the logic included in the 7SS52 V4 ensures stability even in this case and
reliably prevents overfunctioning.
5.1.1.4
Evaluation of the Initial Values
For normal load currents, the magnetic flux in the current transformer's iron circuit is
relatively small. It is of the order of, at most, a few percent of that value at which the
iron saturation begins. The current transformers must have an overcurrent factor
which is sufficiently high to ensure the proper behaviour of the associated relay equip-
ment during large short-circuits currents.
Since the magnetic flux under normal conditions is low, a certain time will elapse after
short-circuit inception before the magnetic flux in the iron core reaches the saturation
level. This is true even under extreme conditions such as a very large short-circuit cur-
rent or a large DC component with a long time constant.
This process will typically last from a quarter-cycle to a half-cycle in both conventional
current transformers with a closed iron core and in linearized current transformers with
an air gap in their cores.
The current transformers transform accurately according to ratio before saturation.
Therefore the secondary currents delivered during the first milliseconds after inception
of a short-circuit has the most conclusive information.
For this reason, the SIPROTEC 7SS52 V4 distributed busbar and breaker failure pro-
tection has a software logic which detects the inception of a short-circuit from the cur-
rents and determines whether an external or internal short-circuit is present.
To do so, it determines the differential quotient dI
s
/ dt of the stabilizing magnitude and
compares it to a set threshold.
When that threshold is exceeded, the characteristic condition (Figure 5-3, page 95) is
checked. If the tripping condition is fulfilled, the protection trips after one measurement
(”1-out-of-1”) within 3 ms. If it is not, two measurements are performed (”2-out-of-2”)
and a trip initiated in the presence of an internal fault.
If the measured result indicates an external fault, the "1-out-of-1" mode remains
blocked for 150 ms.
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