Page 25 - Application Guide Semiconductor Fuse Link

3-phase controlled bridge rectifi er

(b) In the case of fuses in the F2 position and a high speed circuit breaker on the d.c. side,
the fault is cleared by the breaker, and the fuses must be coordinated with the breaker so they are
not damaged by the current wave let through by the breaker.

(c) In the case when F2 and F3 fuses are used it is not possible to coordinate the fuses : several
F2 fuses and F3 fuses will melt.

Internal fault

This is the most common type of fault within a converter, and is illustrated in Fig.18, in which the bold
line shows the path of the normal load current at an instant when devices 3 and 6 are conducting. If
in this state device 1 fails to hold off the reverse voltage and breaks down, a line-to-line short circuit
across the a.c. supply lines is produced. The fault current from phase b to phase a fl ows through two
fuses in series (either 2xF1 or 2xF2) and these must clear the faulty leg before the healthy device 3
2
2
is damaged. In order to achieve this the total clearing I t must be less than the withstand I t of the
device (adjusted for the fault duration as previously discussed).

F3

Fault
1 3 5

F1
F2
a

b DC

c

AC
Supply
2 4 6

Fig.18 Internal fault

Since there are 2 fuses in series they assist each other to some extent under short circuit fault
2
conditions, and the total I t is lower than would be obtained with a single fuse. If the fuses were
2
identical, the total I t could be obtained by assuming equal sharing, and using the I t correction
2
curve (see Fig.11) with a voltage equal to 0.5 times the a.c. line voltage. However, there is also a
possible commutation from device 3 to device 5 during the fault, so that while fuse 1 is always in
the fault path, fuses 3 or 5 may be in the fault path for a lesser time. Fuse 1 may melt fi rst, followed
by fuse 3 or fuse 5 after a very short time. When this happens, fuse 1 will see the full source voltage
2
at the beginning of arcing. Therefore, the total I t of fuse cannot be calculated at a voltage of 0.5
times the a.c. line voltage. Also there are inevitable small differences between the fuses. Tests made
2
by Ferraz Shawmut have shown that the I t should be calculated at 0.65 times the a.c. line voltage.
However the fuses must still have an a.c. voltage rating at least equal to the a.c. line-to-line voltage.
If the fuses were to operate in situations other than a short-circuit fault, the melting times would be
much longer, and one fuse would melt fi rst and have to clear the circuit on its own against the full
line voltage. Satisfactory protection of the devices may not be possible under these circumstances,
but it is still essential that such faults should be cleared safely.

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