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2.7 Out-of-step Protection
The GRL100 out-of-step protection (OST) operates only when the out-of-step loci cross the
protected line and provides optimal power system separation in case of power system step out.
The OST compares the phase of the local and remote positive sequence voltages and detects the
out-of-step when the difference in the phase angle exceeds 180°. The OST can detect any of the
out-of-steps with slow or fast slip cycles.
Figure 2.7.1 show the loci of the voltage vectors measured at terminals A and B when an
out-of-step occurs on the power system. P and Q are equivalent power source locations. Loci 1
and 2 are the cases when the locus crosses the protected line, and passes outside the protected
line, respectively.
Voltage phase angle differs by θ between terminals A and B. In case of Locus 1, θ gets larger as
the voltage locus approaches the protected line and becomes 180° when the locus crosses the
line. In case of Locus 2, θ becomes 0° when the locus crosses the power system impedance
outside the protected line.
X
Q
B
V B3
V B1
V B2
3
×
2
θ
×
V A2
V A3
V A1
A
P
(a) Internal
X
3'
×
2' ×
V B2'
V B3'
B
V B1'
V A3'
V A2'
A
P
(b) External
Figure 2.7.1 Out-of-step Loci
88
1
×
Locus 1
R
Q
1'
×
Locus 2
θ
V A1'
R
6 F 2 S 0 8 5 0
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