Siemens SIPROTEC 7SS60 Manual page 53

7SS60 Manual
E50417-G1176-C132-A2
Figure 5-2 Differential protection of a busbar with n feeders, 1-phase, without restraining
devices
For simplicity's sake, it is assumed that the current transformers of all feeders have
the same transformation ratio. 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 feeder bays, 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 feeder bay 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 Figure 5-2 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 countered by
employment of the so-called stabilization (restraining) devices.
If the short-circuit does not occur at the voltage peak of the cycle, a dc component is
initially superimposed on the short-circuit current which decays with a time constant τ
= L / R of the impedance from source to fault. With the growing output ratings of the
generator units, these time constants in the supply system tend to grow longer. A
superimposed dc component speeds up the magnetic saturation in the transformer
cores, thus considerably affecting the transformation task.
Several measures have been introduced into the 7SS601 measuring system of the
7SS60 busbar protection to cope with these problems. It was thus possible to give the
7SS60 busbar protection system a maximum degree of security against spurious
operation for external short-circuits while ensuring, in the event of internal short-
circuits, that a tripping signal is initiated within a very short time.
The measuring circuit of the 7SS60 busbar protection system is characterized by the
following features:
Basic principle
Measures taken to guard
against the disturbing
influences due to current-
transformer saturation
Measures taken to obtain
very short operating times
• Monitoring the sum of the currents as the tripping
quantity
• Restraint
• Separate evaluation of the current transformer
currents during the first milliseconds after the
occurrence of a fault (anticipating the current
transformer saturation)
Functions
5-3