Overall Function - Siemens SIPROTEC 7SS60 Manual

Introduction
1.1

Overall function

Measuring
principle
Restraint
Detection of
measured values
Pick-up
characteristic
Selective tripping,
isolator replica
1-2
The main function of the 7SS60 protection system is a busbar protection that operates
with the differential current measuring principle. The algorithm of the 7SS60 relies on
Kirchhoff's current law, which states that in fault-free condition the vectorial sum ∆I
flowing into a closed busbar section must be zero. This summated current will be re-
ferred to in this chapter as differential current I
Some slight deviations from this law may be caused by current transformer errors, in-
accuracies in the matching of the transformation ratios and measuring inaccuracies.
Further errors, which may be due, for instance, to transformer saturation in case of
high-current external short-circuits, are counteracted by a load-dependent supple-
mentary restraint. From the load condition the restraint current I
straint current is formed as the summated magnitudes of all currents in the peripheral
module 7TM700.
The differential and the restraint current are fed into the 7SS601 measuring system.
In systems with multiple busbars or sectionalized busbars, each selective section uses
one 7SS601 measuring system (variant with summation current transformer) or three
measuring systems (phase-selective measurement). The correct allocation of the
feeder currents to the appropriate 7SS601 measuring system is ensured by the pe-
ripheral module 7TR710 (preferential treatment/isolator replica).
The characteristic can be set in the parameters for Id> (pick-up value) and for the
k factor, which considers the linear and non-linear current transformer errors. The set-
ting for the pick-up value must be chosen according to the smallest fault current to be
expected. Differential currents above the set characteristic lead to tripping. The
threshold for the differential current supervision is set in the parameter ID thr.
Differential current I
d
Id>
ID thr
0
Figure 1-1 Characteristic of the 7SS601 measuring system
If a measuring system 7SS601 recognizes a tripping condition, the circuit breakers of
the involved feeders must be tripped. This requires an isolator replica which is gener-
ated on the module 7TR710 (isolator replica/preferential treatment) from the position
of the isolators. Basing on this isolator replica a TRIP command of a measuring sys-
tem is distributed to the circuit breakers.
.
d
Tripping zone
Restraint
zone
is derived. This re-
R
k = 0.80
.
.
.
.
Pick-up
.
charact-
.
eristics
.
.
k = 0,25
Normal load line
Restraint current I
7SS60 Manual
E50417-G1176-C132-A2
R