Busbar Protection; Mode Of Operation; Basic Principle - Siemens SIPROTEC 7SS522 V4.6 Manual

Functions
5.1

Busbar Protection

The busbar protection represents the main function of the 7SS52 V4. It is character-
ized by a high measurement 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 bus zone-selective
protection as well as for the check zone.
5.1.1

Mode of operation

5.1.1.1

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.
Figure 5-1 Busbar with n feeders
Assuming that the currents I
nected to the busbar, the following equation applies in the healthy condition. A uniform
orientation of the currents is assumed; the currents flowing towards the busbar are de-
fined as positive, and the currents flowing away from the busbar as negative.
I
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. A single quantity characterizes faulty conditions. This quantity is the
sum of currents. It can be formed at any time. The current sum used for evaluation is
available at any instant without interruption. The current sum stays at zero, unless
there − due to a fault − another path whose current is not measured.
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 are
connected through current transformers. The current transformers reproduce on their
secondary side the currents flowing in the station. The currents are scaled down ac-
cording to the transformation ratio of the CTs while retaining the same phase relation.
Furthermore, the current transformers can keep dangerous high voltages away from
92
+ I + I ... + I = 0
1 2 3 n
, I , I to I flow in the feeders (Figure 5-1, page 92) con-
1 2 3 n
(1)
7SS52 V4 Manual
C53000-G1176-C182-3