ABB RELION 670 Series Applications Manual page 110

Section 7
Impedance protection
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 configuration, see
Figure 48.
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1MRK 511 364-UUS A
Application manual