Schweitzer Engineering Laboratories SEL-587Z, Инструкция по эксплуатации

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3.7
Date Code 20020903
SEL-587Z Instruction Manual
Relay Elements
High-Impedance Protection
Figure 3.7 MOV Characteristic.
Determine
MOV Size
The metal oxide varistor plays an important role in clamping the voltage to an
acceptable level. It is important to note that two ordering options are available:
either a single 2500 J MOV or two MOVs to give a total of 5000 J.
Figure 3.8
shows the safe energy dissipation limits of the MOVs as a function of the
knee-point voltage and the secondary fault current.
CAUTION
: Do not expose the MOVs
to fault voltages for a period
exceeding 4.0 cycles.
Always connect external shorting contacts from the lock-out relay to limit
fault voltages applied at the relay input terminals to 4.0 cycles or fewer. For
example, wire an output contact from the SEL-587Z to operate an 86 lock-out
relay, which in turn shorts the inputs to the high-impedance elements. Do not
rely on the circuit breaker to clear the fault current, because the circuit breaker
operating time may exceed 4.0 cycles or may fail to open altogether.
Figure 3.8 Energy Dissipation Limits
of the MOVs (maximum four-cycle duration).
0.01 0.10 1.00 10.00 100.00
100
10000
M
O
V
Volt
age
(V
)
1000
MOV Current (A)
1•10-5 1•10-4 1•10-3 1•103 1•104
200 300 400 500 600 700 800 900 1000
0
100
200
300
400
500
600
700
800
900
1000
Safe Limits for One and Two MOVs
Knee-Point Voltage (V)
RMS Secondary Bus Fault Current (A)
2MOV
1MOV