GUID-E424E267-CA5C-4FAE-AD20-A7472421E3A6 V1 EN
Figure 85: The equivalent circuit used in the calculation. U
1f
is the positive-
sequence component of the voltage during a fault at the farthest point
of a potential island near the substation A. X
kG
is the short circuit
reactance between the measuring point of the relay and the fault
reduced to the voltage level of the latter. X
d
’ is the transient reactance
of the generator of the power station. The positive-phase-sequence
component of the transient source voltage of the generator can be
approximated by 1.1 p.u.
Example 2.
Example 2 presents a case where the lines between the substations A, B and C are
protected by distance relays without communication schemes. Fast protection is
obtained for the whole line by making the first zone overreaching covering, for
example, 120 percent of the length of the line for the first trip. The possible second
trip after the high speed autoreclosure takes place according to the normal 1. zone
which covers, for example, 80-90 percent of the length of the line. The healthy line
will thus be tripped only once. An illustration can be made where the fault occurs in
the beginning of the line BC, and both the line AB and the line BC will be tripped at
first, which results in a loss-of-grid situation.
If all circuit breaker operations are three-phase ones for all faults, also a single phase-
to-earth fault results in an islanding condition and the setting has to be calculated
according to a single-phase fault.
1MRS756378 D Section 5
Protection functions
REF615 177
Application Manual
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