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3. 3. 3. 3.
3.1.
BASIC PRINCIPLE
The measurement method relies on Kirchhoff´s current law.
This law states that the vectorial sum of all currents flowing into a closed area must be zero. This law
applies, 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.
Assuming that the currents I
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 ) :
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 protection, to any other known way of measurement. A single
quantity, the sum of currents, characterises 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. Above law is always valid, whereas with a comparison of only the zero crossing
points of the currents or of the current directions may involve phase displacements that would have to be
considered accordingly. For instance, in a fault-free three-phase load, the instants of zero current are
displaced by 50º or 120º with respect to e. Unbalanced load may produce other displacements. The sum of
the currents, on the other hand, remains constantly zero as long as no currents flow through some other
path due to a fault.
The above considerations apply strictly to the primary 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 grounding
measures, can keep dangerous high voltages away from the protection system.
The current transformers are an integral 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 system.
GEK-98514B
I
I
1
2
Figure 3.1. Busbar with "n" feeders
, I
,I
... I
flow in the feeders ( Fig 3.1) connected to the busbar, the following
1
2
3
n
I
+ I
+ I
1
2
3
BUS1000 Busbar Protection
OPERATION PRINCIPLES
OPERATION PRINCIPLES
OPERATION PRINCIPLES
OPERATION PRINCIPLES
I
. . . .
I
3
n
... + I
= 0
n
3. OPERATION PRINCIPLES
3-1
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