ABB RELION 670 SERIES Applications Manual page 474

Section 8
Impedance protection
468
Synchronous Synchronous
machine 1 machine 1
SM1 SM1
E1 E1
V, I
E1 E1
Center of oscillation
ANSI10000107 V3 EN
Figure 241: The center of electromechanical oscillation
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.
Synchronous Synchronous
machine 2 machine 2
SM2 SM2
Voltages of all phases
E2 E2
to ground are zero in
the center of
oscillation
ANSI10000107_3_en.vsd
Line distance protection REL670 2.2 ANSI
1MRK 506 369-UUS -
E2 E2
Application manual