Page 38 - Application Guide Semiconductor Fuse Link

Fuse selection examples

2
2
Total clearing I t at 700V 72000 A s
Peak let-through current at 10kA 6.705 kA
2
I t correction factor at 299V 0.449

So the total clearing I t in the application will be 0.449 x 72000 = 32 328 A s.
2
2
Assuming a triangular short-circuit current waveshape, the fault duration can be found from

2
Ipeak TFAULT
2
i t =--
3

i.e. 32328 = 6705 x 6705 x TFAULT / 3

which gives
TFAULT = 2.16 ms

Assuming a constant I3t for the thyristor, the thyristor withstand at 2.16 ms is (see p19)

t = 68,000 ( - ) 0,333
2,16
0,333
I t
2
2
2

= I0 t0 x ( - ) 8,33 = 43356 A s
t
0
2
This is greater than the fuse total clearing I t so this fuse will protect the thyristor.
Next we check the occasional overload requirement. Referring to the 350A fuse’s time-current
characteristic, we fi nd that IMELT = 2482A at the 0.1s point. The maximum allowable overload current
is Cf3 x 2482 = 0.75 x 2482 = 1861A for 0.1s. This is acceptable as it is greater than the actual
overload current, which is 500% of 144.25A = 721A for 0.1s.
Finally, the product datasheet gives the peak arc voltage for this fuse as 1.179kV at the full line
voltage of 460V. This is well within the 1.5kV p.i.v. capability of the device.

Hence, the A70QS 350A fuse is suitable for this application.

(Note that if the equipement designer had specifi ed that a 3-phase fault must be considered, the fuse
2
I t would need to be evaluated at 0.866 x 660 = 572V)

For the fuse selected in this example the voltage factors KAC and KDC (see section 14) are

700 635
KAC = - = 1,2 and KDC = - = 1,27
460 500

When considering the commutation fault (non-diametric fault) some designers may wish to increase
the fuse a.c. voltage rating to 1.7 x 460 = 782V, as discussed in section 14. In this case the A70QS
would not meet the requirement and a higher a.c. voltage rated fuse would have to be used. (One
solution is to check the PSC square bodied fuse rated 1250V/1300V in sizes 70 or 71.The assessment
of the severity of the commutation fault and the need to increase the fuse a.c. voltage rating is one
that should be considered carefully even though higher voltage rated fuses add cost to the system.
The relatively small added cost in comparison to the possible damage to the system can easily be
justifi ed.

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