Page 24 - Application Guide Semiconductor Fuse Link

3-phase controlled bridge rectifi er

VA VB VC

Voltage

0

VB VC VA
Id
1 3 5

4 6 2 4

line current, phase A

Fig.17 Three-phase bridge rectifi er waveforms

Location of fuses

In Fig.16 three possible locations for fuses are shown. For rectifi ers the principal choice is between
F1 and F2, as follows

F in the converter legs, in series with the device. This gives the best possible protection of the
1
devices, with the smallest fuses.

F in the a.c. supply lines. This requires only 3 fuses rather than 6 and may be a more
2
economical choice. However the line current is 2 times higher than the device current,
so higher ampere rated fuses are needed, compared with fuses in location F1. The higher
2
ampere rated fuses have a higher clearing I t, which makes it somewhat more diffi cult
to protect the semiconductor devices. In critical cases it may not be possible to give proper
protection using fuses in location F .
2

F are on the d.c. side. These fuses are normally used only when the converter is regenerative
3
and in conjunction with F fuses. This will be discussed further in section 11.
2

External fault

If there is a sustained fault external to the bridge (short circuit on the d.c. side) this fault is cleared
by either a high speed d.c. circuit breaker or in most cases by the fuses when there is only 1 semi-
conductor per leg.

(a) In the case of fuses in the F1 position, the fuses clear the fault and protect the semi-conductors.
2
Their total clearing I t is less than the withstand I t of the device (adjusted for the fault duration as
2
previously discussed).

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