Formation Of Measuring Currents From The Transformer Currents - Siemens SIPROTEC 7SS60 Manual

5.2

Formation of measuring currents from the transformer currents

7SS60 Manual
E50417-G1176-C132-A2
The output currents used by the busbar protection for the current comparison are the
secondary currents supplied by the feeder transformers. They are, however, not fed
directly into the measuring circuit but first transformed by means of matching CTs. In
the following the reasons are explained:
• For the measuring circuit it is necessary to "reproduce" the corresponding switch
position of the power system. Therefore the rated currents must be changed over.
The secondary currents of the transformers (rated current 1 A or 5 A) are not suited
for this purpose because the changeover contacts therefore needed to be designed
for currents (appearing when a fault occurs) that are much more higher. If the
currents are transformed, the changeover contact is less strained. This means that
the contact need not be designed for very high currents.
• Usually the current transformers in the outputs do not have the same transformation
ratio. However, the measuring circuit requires a consistent transformation of the
currents. For the protection differences resulting from the transformation must be
balanced by matching CTs.
• The currents of the 3 phases can be summated to a single-phase alternating current
by means of a matching CT. In this way, only one measuring system is necessary
to protect the 3 phases of the power system. As a consequence, the protection each
time picks up differently in its sensitivity, depending on which phase is affected by
the fault. In most cases this level of protection is sufficient.
The 7SS60 busbar protection is designed for a rated input current of 100 mA. With the
current transformer carrying rated current at the primary side, the current for this
feeder that is fed into the protection for comparison will be 100 mA. This applies if 3
phase currents are summated to a single-phase alternating current for a common
measuring circuit and if each phase has its own measuring circuit. The latter is
preferentially used for protection equipment for busbars in systems of very high rated
insulation voltages (380 or 220 kV). However, the consistency of the pickup values for
all types of faults costs much more effort. Another reason for using this method
preferentially is that the safety is twice or three times higher for tripping in case of two-
phase or three-phase faults.
Depending on whether differential protection is applied on all 3 phases or only as a
single phase, one has to distinguish between 2 types of input circuits. Each method
requires different matching transformers.
There are 2 basic models:
• phase-selective protection (3 measuring systems in each busbar section)
• protection with summation current transformers (one measuring system in each
busbar section)
If summation current transformers are used, a smaller number of modules is sufficient
to form a protection system. On the other hand, the asymmetric analog summation of
the currents entails different sensitivity of the protection system for different fault types.
You can influence the sensitivity by performing the connection mode which is optimally
matched to the system conditions. Refer also to the following explanations concerning
the normal and increased earth fault sensitivity.
In the version with summation current transformers, increased restraint may be
produced by load currents in the phases not involved in the fault current path. But this
is of less importance since the fault currents exceed normally clearly the tripping
characteristic.
Functions
5-7