05 overvoltages and insulation coordination

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5.1.3.2. Load switching
de-energizing loads
inductive load
single-phase circuit
Let us consider the equivalent single-phase diagram in figure 5-25 with an ideal circuit-breaker
CB which has a zero arc resistance the instant the contacts separate and which carries out
interruption when the current crosses zero. Before operation of the circuit-breaker, between
pointsA and B, there is a voltage drop due to the load current flowing through L s .
At the instant of interruption, the voltage at B suddenly reaches the voltage at A and the
capacitance C s is charged through L s . The energy exchanges between C s and L s make
voltage oscillations at frequencies of 5 to 10 kHz occur.
The voltage at
C suddenly decreases to zero and the capacitance C p is then discharged
through L . The energy exchanges between C p and L create voltage oscillations at
frequencies going from 1 to 100 KHz.
A
I s
B
I D
I 0
CB
C
I L
L s
V A
C s
C p
L
L p
L s : network inductance upstream of the circuit-breaker
C s : network capacitance upstream of the circuit-breaker
L : load inductance
L p : stray inductance
C p : network capacitance downstream of the circuit-breaker
CB : circuit-breaker
Figure 5-25: interruption in an inductive load network
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Industrial electrical network design guide T & D 6 883 427/AE