SIOV Metal Oxide Varistors - DEMAR

Application and Design Examples
FRAME4/home/SMC-Archiv-DB/GG-KB-engl/DB-SIOV_97_BNR-B462-P6214-X-X-7600/R-Beispiele
Selection example
How to determine suitable varistors for each case covered by tables 2a and b is demonstrated by
the example of the S20K275.
Operating voltage
According to IEC 38, from the year 2003 onwards the operating voltage tolerance in Europe will be
230 V ± 10%, i.e. a maximum operating voltage of 253 V can be expected. This means that only
varistors of the voltage classes “K275” (or higher) may be selected.
Surge current
The surge current caused by the hybrid generator depends to a large extent on the load, i.e. on the
varistor, in this case. It is easy to determine amplitude and current duration by simulation. Here, one
should take into account that the greatest current flows across the varistor whose V/I characteristic
is at the lower limit of the tolerance band.
In this case, the resulting current amplitude for S20K275 is approx. i* ˆ = 1560 A, the current duration
of the equivalent rectangular wave is < 20 µs. For 10 repetitions, the value of (t r ≤ 20 µs) i max =
2500 A can be deduced from the derating curve for S20K275.
Since i max > i* ˆ the selection criterion for the surge current is fulfilled.
If no simulation is available, then the surge current has to be determined by a test circuit or by mathematical
approximation.
The approximation solution (equation 12) results in:
V s – V SIOV 4000 – 750
i* = -------------------------- = ----------------------------- = 1625 A < i
Z source 2
max
A maximum value of 940 V is obtained from the V/I characteristic for the S20K275 at 2000 A; the
lower tolerance limit will then be approx. 940 V – 20 %.
The current duration can be estimated on the basis of the fact that the hybrid generator produces a
short-circuit surge current wave of 8/20 µs. Under load with a low-resistance varistor one can expect
a waveform similar to that of the 8/20 µs wave, whose transformation into an equivalent rectangular
wave has a t* r < 20 µs. Again this leads to a reading of i max = 2500 A for 10 times in the
derating curve. With i* = 1625 A < i max the selection criterion of equ. 9 is fulfilled. The difference
between the results of this calculation and the simulation results is negligible.
Energy absorption
The energy absorption of the S20K275 varistor can be calculated directly by simulation.
It is possible to estimate the value by means of equation 15 (lower tolerance field limit):
W* = ˆv* i* ˆ t* r = 750 · 1625 · 20 · 10 -6 = 24 J
62 Siemens Matsushita Components