-1.5
-1
-0.5
0
0.5
1
1.5
-1
-0.5
0
0.5
1
1.5
---------
-------
------
------
------
------
-------
-----
---
---
--
---
---
---
---
----
----
----
----
----
----
----
----
-----
-----
------
Real part (R) of Z in Ohms
Im
ag
ina
ry
pa
rt
(X
) o
f Z
in
O
hm
s
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
--
--
--
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
--
--
--
-
--
--
--
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
--
--
--
-
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
StartAngle = 120°
limit of reach
→
IEC10000109-1-en.vsd
IEC10000109 V1 EN
Figure 111:
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
. 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
, 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
, that is, Z(R, X) under normal load, with the points SE and RE,
respectively. These two lines are not shown in Figure
. Normal values of the power
angle, that is, under stable, steady-state, load conditions, are from 30 to 60 electrical
1MRK502052-UEN B
Section 7
Impedance protection
263
Technical manual
Summary of Contents for Relion REG670
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