ABB Relion 670 series, Техническое руководство

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-1.5 -1 -0.5 0 0.5 1 1.5
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Real part (R) of Z in Ohms
Im
a
g
in
a
ry
p
a
rt
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X
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o
f Z
in
O
h
m
s
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0
1
Zone 1
Zone 2
The 1st
pole slip
occurred
relay
X in Ohms
The 2nd
pole slip
occurred
R in Ohms
RE
SE
2
3
to the 3rd
pole-slip
¬
trajectory
of Z(R, X)
Pre-disturbance
normal load
Z(R, X)
0 ® pre-disturbance Z(R, X)
1 ® Z(R, X) under 3-phase fault
2 ® Z(R, X) when fault cleared
3 ® Z when pole-slip declared
lens determined ®
by the setting
Pickup Angle = 120°
limit of reach ®
ANSI10000109-1-en.vsd
ANSI10000109 V1 EN-US
Figure 200: Loci of the complex impedance Z(R, X) for a typical case of generator losing step
after a short circuit that was not cleared fast enough
Under typical, normal load conditions, when the protected generator supplies the active and the
reactive power to the power system, the complex impedance Z(R, X) is in the 1st quadrant, point 0
in Figure 200 . One can see that under a three-phase fault conditions, the centre of oscillation is at
the point of fault, point 1, which is logical, as all three voltages are zero or near zero at that point.
Under the fault conditions the generator accelerated and when the fault was finally cleared, the
complex impedance Z(R, X) jumped to the point 2. By that time, the generator has already lost its
step, Z(R, X) continues its way from 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 will continue pole-slipping
— see Figure 200 , 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-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 200 , that is, Z(R, X) under normal load, with the points SE and RE, respectively. These two
lines are not shown in Figure 200 . Normal values of the power angle, that is, under stable, steady-
state, load conditions, are from 30 to 60 electrical degrees. It can be observed in Figure 201 that
the angle reaches 180 degrees when the 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. Figure
201 shows the rotor (power) angle and the magnitude of Z(R, X) against time for the case from
Figure 200 .
1MRK 502 066-UUS B Section 7
Impedance protection
379
Technical manual