91
6
F
2
S
0
8
3
4
2.4.13 Out-of-Step Protection
Application
For an out-of-step condition on a power system, power system separation is executed in order to
recover power system stability or prevent the failure from extending to the entire system. Power
system separation by the distance protection with several operating zones is not desirable because
it is not always carried out at the optimal points. For optimal power system separation, the
GRZ100 has an out-of-step tripping (OST) function. The OST function uses independent
impedance measuring elements to discriminate against transient power swings and reliably detects
out-of-steps and operates only when the out-of-step locus crosses the protected line.
Scheme logic
The out-of-step element has three operating areas A, B and C by combining two impedance
measuring elements ZM and ZN as shown in Figure 2.4.13.1.
If an out-of-step occurs, the impedance viewed from the impedance measuring element moves
through the areas A, B and C in the sequence of A
→
B
→
C or C
→
B
→
A. The out-of-step
tripping logic shown in Figure 2.4.13.2 outputs a three-phase tripping command M-TRIP to the
circuit breaker when the impedance viewed from the impedance measuring element passes
through those areas in the sequence above and enters the third area and it stays in area A and area
C for the time set with the timers TOST1 and TOST2. The tripping command continues for 100
ms. The output signal is blocked when the scheme switch [OST] is set to "OFF" or binary signal
OST_BLOCK is input. The tripping signal of the out-of-step protection can be separated from
other protection tripping signals by the switch [OST]. In this case, the switch [OST] is set to "BO"
and the tripping signal OST-BO is assigned to a desired binary output number (for details, see
Section 4.2.6.9). When the tripping signal of the out-of-step protection is not separated from other
protection tripping signals, the switch [OST] is set to "TRIP".
The tripping logic does not operate for cases other than out-of-steps, for example, a power swing
in which the impedance moves from areas A
→
B
→
A or C
→
B
→
C or a system fault in which
the impedance passes through area A or C instantaneously.
Out-of-step tripping can be disabled with the scheme switch [OST].
R
75
°
X
ZN
C B
A ZM
OSTR1
−
OSTR2
−
OSTXB
OSTXF
Figure 2.4.13.1 Out-of-Step Element
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Summary of Contents for GRZ100-211B
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