Hitachi Relion 670 Series Applications Manual page 110

Section 7
Impedance protection
the turbine is under this condition used to accelerate the moving parts, that is, the rotor and the turbine. If
the fault is not cleared quickly, the generator may not remain in synchronism after the fault has been
cleared. If the generator loses synchronism (Out-of-step) with the rest of the system, pole slipping
occurs. This is characterized by a wild flow of synchronizing power, which reverses in direction twice for
every slip cycle.
The out-of-step phenomenon occurs when a phase opposition occurs periodically between different parts
of a power system. This is often shown in a simplified way as two equivalent generators connected to
each other via an equivalent transmission line and the phase difference between the equivalent
generators is 180 electrical degrees.
Synchronous Synchronous
machine 1 machine 1
SM1 SM1
E1 E1
E1 E1
ANSI10000107 V3 EN-US
Figure 51:
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
104
V, I
Center of oscillation
The center of electromechanical oscillation
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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
1MRK511407-UUS Rev. N
E2 E2
Phasor measurement unit RES670
Application manual