ABB RES670 Technical Manual page 165

1MRK 511 408-UUS A
Phasor measurement unit RES670 2.2 ANSI
Technical manual
1.5
The 2nd
1
pole slip
occurred
0.5
Zone 2
Zone 1
0
limit of reach ®
lens determined ®
-0.5
by the setting
Pickup Angle = 120°
-1
-1.5 -1
ANSI10000109 V1 EN-US
Figure 42: 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 42. 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 42, 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.
¬
X in Ohms
The 1st
pole slip
occurred
RE
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3
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1
-
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2
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^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
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^ ^ ^ ^ ^ ^ - - -
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relay
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0 ® pre-disturbance Z(R, X)
- - - - -
- -
- -
- -
- -
- -
1 ® Z(R, X) under 3-phase fault
SE
2 ® Z(R, X) when fault cleared
3 ® Z when pole-slip declared
-0.5 0 0.5
Real part (R) of Z in Ohms
Section 7
Impedance protection
trajectory
of Z(R, X)
to the 3rd
pole-slip
Pre-disturbance
normal load
Z(R, X)
0
R in Ohms
1 1.5
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