ABB Relion 670 Series Applications Manual page 118

Section 7
Impedance protection
power from 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.
112
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
1MRK 511 407-UUS Rev. J
E2 E2
Phasor measurement unit RES670
Application manual