Siemens SIPROTEC 7SS52, Operation Manual

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Mode of operation
4
4 - 4
Siemens AG
⋅
May 1998
4.3 Busbar protection
The busbar protection represents the main function of
the 7SS52. It is characterized by a high measureĆ
ment accuracy and flexible matching to the existing
station configurations. It is supplemented by a series
of ancillary functions.
The measurement methods described here below
apply for the check zone as well as for the bus-selecĆ
tive protection.
The measurement method relies on Kirchhoff's curĆ
rent law.
This law states that the vectorial sum of all currents
flowing into a closed area must be zero. This law apĆ
plies, in the first instance, to DC current. It applies to
AC current for instantaneous values. Thus, the sum
of the currents in all feeders of a busbar must be zero
at any instant in time.
1 2 3 n
. . . . . .
Assuming that the currents
1
,
2
,
3
...
n
flow in the
feeders (Fig 4.3) connected to the busbar, the followĆ
ing equation applies in the fault-free condition (the
currents flowing towards the busbar are defined as
positive, and the currents flowing away from the busĆ
bar as negative):
1
+
2
+
3
... +
n
= 0. (1)
If this equation is not fulfilled, then there must be
some other - impermissible - path through which a
current flows. This means that there is a fault in the
busbar region.
This law is superior, as the basis for busbar protecĆ
tion, to any other known way of measurement. A
single quantity, the sum of currents, characterizes
and can be used to detect faulty conditions. This sum
of all currents can be formed at any time and if
formed as such, using instantaneous current values,
full use of above law can be made. The current sum
used for evaluation is available at any instant without
interruption and stays at zero, unless there is another
path due to a fault whose current is not measured.
The above considerations apply strictly to the primary
conditions in a high-voltage switching station. ProtecĆ
tion systems, however, cannot carry out direct meaĆ
surements of currents in high-voltage systems. ProĆ
tection equipment measurement systems,
performing the current comparisons, are connected
through current transformers. The secondary windĆ
ings provide the currents scaled down according to
the transformation ratio while retaining the same
phase relation. Furthermore, the current transformĆ
ers, due to the isolation of their secondary circuits
from the high-voltage system and by appropriate
earthing measures, can keep dangerous high voltages
away from the protection system.
The current transformers are an essential part of the
whole protection system and their characteristics are
an important factor for the correct operation of the
protection. Their physical locations mark the limits of
the protection zone covered by the protection sysĆ
tem.
Since the current transformers transform in direct
proportion to the primary currents
p
in the station,
the following equation applies for the busbar protecĆ
tion in the fault-free condition:
1 s
n
1
+
2 s
n
2
+
3 s
n
3
... +
n s
n
n
= 0 (2)
n
1
, n
2
, n
3
... n
n
are the CT transfomation
ratios
p
/
s
and
1s
,
2s ... ns.
are the secondary currents.
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