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Functions
5.1
Measurement method
Basic principle
5-2
The measurement method by which the 7SS60 numerical busbar protection detects a
short-circuit in the protected zone relies on Kirchhoff's current law. This law states that
the vectorial sum of all currents flowing into a closed area must be zero.
Figure 5-1
Busbar with n feeders
Assuming that the currents J
the following equation applies in the fault-free condition (the currents flowing towards
the busbar are defined as positive, and the currents flowing away from the busbar as
negative):
J
+ J
+ J
... + J
= 0
1
2
3
n
If this equation is not fulfilled, 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 protection, to any other known way of
measurement. Its simplicity is unequalled: 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, even over a whole cycle, and if formed as such, using
instantaneous current values, full use of above law can be made.
The above considerations apply strictly to the primary-side conditions in a high-voltage
switching station. Protection systems, however, cannot carry out direct measurements
of currents in high-voltage systems. Protection equipment measurement systems,
performing the current comparisons, are connected through current transformers. The
secondary windings provide the currents scaled down according to the transformation
ratio while retaining the same phase relation. Furthermore, the current transformers,
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 characteristics of the current transformers 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 system.
Since the current transformers proportionally transform the primary currents flowing in
the system, a single-phase busbar protection that monitors the summated feeder
currents would have the basic circuit shown in Figure 5-2.
, J
, J
to J
flow in the feeders connected to the busbar,
1
2
3
n
7SS60 Manual
E50417-G1176-C132-A2
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Troubleshooting
