High-Impedance Differential Protection - Siemens SIPROTEC 7UT613 series Manual

2 Functions
2.7.2

High-impedance Differential Protection

Application
Example
High-impedance
Principle
198
With the high-impedance scheme all current transformers at the limits of the protection
zone operate parallel to a common relatively high-ohmic resistance R whose voltage
is measured. With 7UT613/63x the voltage is registered by measuring the current
through the external resistor R at the high-sensitivity single-phase current measuring
input.
The current transformers have to be of equal design and provide at least a separate
core for high-impedance differential protection. They also must have the same trans-
formation ratio and approximately the same knee-point voltage.
With 7UT613/63x, the high-impedance principle is very well suited for detection of
earth faults in transformers, generators, motors and shunt reactors in earthed sys-
tems. High-impedance differential protection can be used instead of or in addition to
the restricted earth fault protection (refer also to Section 2.3).
Figure 2-86 (left side) illustrates an application example for an earthed transformer
winding or an earthed motor/generator. The example on the right side shows a non-
earthed transformer winding or an non-earthed motor/generator where the earthing of
the system is assumed to be somewhere else.
Figure 2-86 Earth fault protection according to the high-impedance principle
|The high-impedance principle is explained on the basis of an earthed transformer
winding.
No zero sequence will flow during normal operation, i.e. the starpoint is I
line currents 3 I = I + I
0 L1
With an external earth fault (left in figure 2-87), which fault current is supplied via the
earthed starpoint, the same current is flowing through the transformer starpoint and
the phases. The corresponding secondary currents (all current transformers having
the same transformation ratio) compensate each other, they are connected in series.
Across resistance R only a small voltage is generated. It originates from the inner re-
sistance of the transformers and the connecting cables of the transformers. Even if
any current transformer experiences a partial saturation, it will become low-ohmic for
the period of saturation and creates a low-ohmic shunt to the high-ohmic resistor R.
Thus, the high resistance of the resistor also has a stabilising effect (the so-called re-
sistance stabilisation).
+ I
= 0.
L2 L3
= 0 and the
St
7UT613/63x Manual
C53000-G1176-C160-2