ABB RELION RET670 Applications Manual page 488

Section 8
Impedance protection
482
Synchronous Synchronous
machine 1 machine 1
SM1 SM1
E1 E1
V, I
E1 E1
Center of oscillation
ANSI10000107 V3 EN-US
Figure 243: 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
Transformer protection RET670 2.2 ANSI
1MRK 504 163-UUS A
E2 E2
Application manual