The center of the electromechanical oscillation can be in the generator unit (or generator-
transformer unit) or outside, somewhere in the power system. When the center of the
electromechanical oscillation occurs within the generator it is essential to trip the generator
immediately. If the center of the electromechanical oscillation is outside any of the generators
in the power system, the power system should be split into two different parts; so each part
may have the ability to restore stable operating conditions. This is sometimes called
“islanding”. The objective of islanding is to prevent an out-of-step condition from spreading to
the healthy parts of the power system. For this purpose, uncontrolled tripping of
interconnections or generators must be prevented. It is evident that a reasonable strategy for
out-of-step relaying as well as, appropriate choice of other protection relays, their locations
and settings require detailed stability studies for each particular power system and/or
subsystem. On the other hand, if severe swings occur, from which a fast recovery is
improbable, an attempt should be made to isolate the affected area from the rest of the
system by opening connections at predetermined points. The electrical system parts swinging
to each other can be separated with the lines closest to the center of the power swing allowing
the two systems to be stable as separated islands. The main problem involved with systemic
islanding of the power system is the difficulty, in some cases, of predicting the optimum
splitting points, because they depend on the fault location and the pattern of generation and
load at the respective time. It is hardly possible to state general rules for out-of-step relaying,
because they shall be defined according to the particular design and needs of each electrical
network. The reason for the existence of two zones of operation is selectivity, required for
successful islanding. If there are several out-of-step relays in the power system, then
selectivity between separate relays is obtained by the relay reach (for example zone 1) rather
then by time grading.
The out-of-step condition of a generator can be caused by different reasons. Sudden events in
an electrical power system such as large changes in load, fault occurrence or slow fault
clearance, can cause power oscillations, that are called power swings. In a non-recoverable
situation, the power swings become so severe that the synchronism is lost: this condition is
called pole slipping.
Undamped oscillations occur in power systems, where generator groups at different locations
are not strongly electrically connected and can oscillate against each other. If the connection
between the generators is too weak the magnitude of the oscillations may increase until the
angular stability is lost. More often, a three-phase short circuit (unsymmetrical faults are much
less dangerous in this respect) may occur in the external power grid, electrically close to the
generator. If the fault clearing time is too long, the generator accelerates so much, that the
synchronism cannot be maintained even if the power system is restored to the pre-fault
.
Section 7
1MRK 505 343-UEN B
Impedance protection
390
Application manual
Summary of Contents for Relion 670 series
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