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Siemens AG ⋅ May 1998
Although such a busbar protection would certainly
detect any short-circuit inside the protection zone,
the transformation errors of the current transformers,
which are unavoidable to some degree, are also liable
to cause spurious tripping as a result of an external
short-circuit. In that case, for instance with a close-
up fault on one of the outgoing feeders, the current
flowing into the short-circuit is shared on the infeed
side by several bays. The current transformers in the
infeeding bays carry only a fraction of the total fault
current while the current transformer in the faulted
outgoing feeder carries the full current in its primary
winding. If the fault current is very high, this set of
current transformers may therefore be saturated, so
tending to deliver only a fraction of the actual current
on the secondary side while the rest of the current
transformers, due to the distribution of currents
among several bays, perform properly. Although the
sum of the currents is zero on the primary side, the
sum of the currents in equation (2) is now no longer
zero.
In conventional differential protection systems where
the sum of the currents is zero on the primary side,
for busbars and similar objects, this difficulty is countĆ
ered by employment of the so-called stabilization (reĆ
straining) devices.
If the short-circuit does not occur at the voltage peak
of the cycle, a DC component is initially superimĆ
posed on the short-circuit current which decays with
a time constant governed by the ratio L/R of the imĆ
pedance from source to fault. With the growing outĆ
put ratings of the generator units, these time conĆ
stants in the supply system tend to grow longer. A
superimposed DC component speeds up the magnetĆ
ic saturation in the transformer cores, thus considerĆ
ably affecting the transformation task.
Several measures - some of which are already
known from the conventional protection - have been
introduced into the measuring system of the busbar
protection 7SS52 to cope with these problems. They
supplement the basic principle of monitoring the sumĆ
mation (differential) current. It was thus possible to
give the busbar protection system 7SS52 a maximum
degree of security against spurious operation for exĆ
ternal short-circuits while ensuring, in the event of
internal short-circuits, that a tripping signal is initiated
within the very short time of less than a half-cycle.
The measuring circuit of the busbar protection sysĆ
tem 7SS52 is characterized by the following features:
D Basic principle:
Monitoring the sum of the currents as the tripping
quantity
D Measures taken to guard against the disturbing
influences due to current-transformer saturation:
1. Stabilization (against large through currents)
2. Separate evaluation of each half-wave (particu-
larly effective against DC components)
Measures taken to obtain short tripping times:
Separate evaluation of the current transformer curĆ
rents during the first milliseconds after the occurĆ
rence of a fault (anticipating the current transformĆ
er saturation).
The stabilization has the function of reducing the influĆ
ence on the measurement of transformation inaccuraĆ
cies in the various feeders to such a degree that spuĆ
rious behaviour of the protection system is
prevented. The busbar protection 7SS52 solves this
problem by forming both the vectorial sum of the CT
seondary currents which acts in the operating sense
as well as the arithmetic sum of those quantities
which has a restraining effect.
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