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Real part (R) of Z in Ohms
→
Imaginary part (X) of Z in Ohms
→
^
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G
relay
relay
fault
RE
X [Ohm]
SE
X-line
→
limit of
reach
zone 2
zone 1
Z(R,X) 20 ms
after line out
this circle forms
the right-hand side
edge of the lens
0
6
pre-fault
Z(R,X)
5
1
2
3
4
0
→
pre-fault Z(R, X)
3
→
Z(R, X) under fault
5
→
Z 20 ms after line out
6
→
pow er line reclosed
R
SE RE
lens
→
110°
Z-line
→
fault
→
IEC10000111-1-en.vsd
IEC10000111 V1 EN
Figure 113: A stable case where the disturbance does not make the generator to
go out-of-step
It shall be observed that for a stable case, as shown in Figure 113, where the
disturbance does not cause the generator to lose step, the complex impedance Z(R, X)
exits the lens characteristic on the same side (point 4) it entered it (point 2), and never
re-enters the lens. In a stable case, where the protected generator remains in
synchronism, the complex impedance returns to quadrant 1, and, after the oscillations
fade, it returns to the initial normal load position (point 0), or near.
7.6.7.1 Lens characteristic
A precondition in order to be able to construct a suitable lens characteristic is that the
power system in which OOSPPAM is installed, is modeled as a two-machine
equivalent system, or as a single machine – infinite bus equivalent power system.
Then the impedances from the position of OOSPPAM in the direction of the normal
load flow (that is from the measurement point to the remote system) can be taken as
forward. The lens characteristic, as shown in Figure
111 and Figure113, is obtained so
that two equal in size but differently offset Mho characteristics are set to overlap. The
resultant lens characteristic is the loci of complex impedance Z(R, X) for which the
rotor (power) angle is constant, for example 110 degrees or 120 degrees; if the rotor
(power) angle approaches this value, then there is a high risk to have an out of step
condition. The limit-of- reach circle is constructed automatically by the algorithm; it
is about 10% wider than the the circle that has the line SE-RE as diameter (that is the
out-of-step characteristic which corresponds to the rotor (power) angle of 90 degrees).
Figure
114 illustrates construction of the lens characteristic for a power system.
1MRK502052-UEN B Section 7
Impedance protection
265
Technical manual
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