ABB Relion Technical Manual page 184

Section 7
Impedance protection
178
the right-hand side to the left-hand side, and the 1st pole-slip cannot be avoided. If the
generator is not immediately disconnected, it then continues pole-slipping — see
76, where two pole-slips (two pole-slip cycles) are shown. Under out-of-step conditions,
the centre of oscillation is where the locus of the complex impedance Z(R, X) crosses the
(impedance) line connecting the points SE (Sending End), and RE (Receiving End). The
point on the SE – RE line where the trajectory of Z(R, X) crosses the impedance line can
change with time and is mainly a function of the internal induced voltages at both ends of
the equivalent two-machine system, that is, at points SE and RE.
Measurement of the magnitude, direction and rate-of-change of load impedance relative
to a generator's terminals provides a convenient and generally reliable means of detecting
whether machines are out-of-phase step and pole-slipping is taking place. Measurement
of the rotor (power) angle δ is important as well.
Rotor (power) angle δ can be thought of as the angle between the two lines, connecting
point 0 in Figure 76, that is, Z(R, X) under normal load, with the points SE and RE,
respectively. These two lines are not shown in
angle, that is, under stable, steady-state, load conditions, are from 30 to 60 electrical
degrees. It can be observed in
complex impedance Z(R, X) crosses the impedance line SE – RE. It then changes the sign,
and continues from -180 degrees to 0 degrees, and so on.
(power) angle and the magnitude of Z(R, X) against time for the case from
Figure 76. Normal values of the power
Figure 77 that the angle reaches 180 degrees when the
1MRK 502 048-UUS A
Figure
Figure 77 shows the rotor
Figure
Technical manual
76.