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(*) Current values shown in the parenthesis are in the case of a 1 A rating. Other current values are in
the case of a 5 A rating.
To determine the dead time, it is essential to find an optimal value while taking factors,
de-ionization time and power system stability, into consideration which normally contradict each
other.
Normally, a longer de-ionization time is required as for a higher line voltage or larger fault current.
For three-phase autoreclose, the dead time is generally 15 to 30 cycles. In single-phase
autoreclose, the secondary arc current induced from the healthy phases may affect the
de-ionization time. Therefore, it is necessary to set a longer dead time for single-phase autoreclose
compared to that for three-phase autoreclose.
In three-phase autoreclosing, if the voltage and synchronism check does not operate within the
period of time set on the delayed pickup timer T∗S∗R which is started at the same time as the dead
time counter T∗S∗ is started, reclosing is not performed and three-phase autoreclose is reset to its
initial state. Therefore, for example, TRR is set to the time setting of the T∗S∗ plus 100 ms.
The TEVLV determines the possibility of three-phase reclosing for an evolving fault.
When the TEVLV is set to the same setting as the T∗S∗S, three-phase reclosing is performed for
all evolving faults. As the setting for the TEVLV is made shorter, the possibility of three-phase
reclosing for an evolving fault becomes small and that of three-phase final tripping becomes large.
2.5.4
Characteristics of Measuring Elements
Voltage and Synchronism Check Elements OVI, UVI, OVR, UVR, and SYN
The voltage check and synchronism check elements are used for autoreclose.
The output of the voltage check element is used to check whether the line and busbar are dead or
live. The voltage check element has undervoltage detectors UVI and UVR, and overvoltage
detectors OVI and OVR for the line (incoming) voltage and busbar (running) voltage check. The
under voltage detector checks that the line or busbar is dead while the overvoltage detector checks
that it is live. These detectors function in the same manner as other level detectors described later.
Figure 2.5.5 shows the characteristics of the synchronism check element used for the autoreclose
if the line and busbar are live.
The synchronism check element operates if both the voltage difference and phase angle difference
are within their setting values.
Incoming voltage(V
)
I
θ
ΔV
θ
SYNOV
Synchronism check area
(a) Synchronism check zone
Running voltage(V
)
R
SYNUV
Figure 2.5.5 Voltage and Synchronism Check Element
⎯
103
V
I
Incoming
voltage
A
OVI
UVI
C
0V
UVR
A, C, D: Voltage check
B: Synchronism check
(b) Voltage and synchronism check zone
⎯
6 F 2 S 0 8 4 2
B
D
V
R
Running
voltage
OVR
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