Siemens SIPROTEC 7SS523 V3.2 Manual page 109

the protection system, since their secondary circuits are isolated from the high-voltage
system and their shielding windings are earthed.
The current transformers are an essential part of the whole protection. Their charac-
teristics are an important factor for the correct operation of the protection. Their phys-
ical locations mark the limits of the protection zone covered by the protection system.
The current transformers transform the primary currents flowing in the station (I
proportionally into secondary currents (I
plies for the busbar protection in the fault-free condition:
I n + I n + I
1 sec. 1 2 sec. 2
n , n , n ... n are the CT transformation ratios and
1 2 3 n
I , I .. I are the secondary currents.
1 sec. 2 sec. n sec.
In order to be able to process currents from bays with different transformers on the
busbar level, all currents must refer to the same CT transformation ratio. For this rea-
son, a current standardisation is carried out in the bay unit.
In the plant configuration, the specification of the reference current of the system is
defined as the reference value as a basis by specification of the reference current of
the system. To calculate the standardisation factor for each current, all CT transforma-
tion ratios in the equation (2) have to be divided by this reference current.
Such a busbar protection would certainly detect any short-circuit inside the protection
zone. However, since transformation errors of the current transformers are unavoida-
ble to some degree, it would also be liable to cause spurious tripping as a result of an
external short-circuit. Such an error might be, for instance, 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 equation 2 is now
no longer zero.
In differential protection systems for busbars and similar objects, this difficulty is coun-
tered 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. This DC component 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 - some of which are already known from the conventional protec-
tion - have been introduced into the measuring system of the 7SS52 V4 busbar pro-
tection to cope with these problems. They supplement the basic principle of monitoring
the summation (differential) current. The 7SS52 V4 busbar protection has thus a max-
imum degree of security against spurious operation for external short-circuits. At the
same time, it ensures that in the event of internal short-circuits a tripping signal is ini-
tiated within the very short time of less than a half-cycle.
). As a result, the following equation ap-
sek.
n ... + I n = 0
3 sec. 3 n sec. n
C53000-G1176-C182-1
Busbar Protection
(2)
7SS52 V4 Manual
)
prim.
93