Direct Power and Torque Control of AC/DC/AC Converter-Fed ...

5. Passive Components Design
[A]
0 U Ret U Su U F
U
[V]
Fig. 5. 1. Current-voltage characteristic of an aluminum – Al electrolytic capacitor [30]
In Fig. 5. 1 U Ret (rated voltage) – is the DC-link voltage value for which the
capacitor has been designed; U Su (surge voltage) – is the maximum voltage which
can be applied to the capacitor for short periods of time, i.e. up to 5 times for 1
minute per hour. The norm IEC 60384-4 specifies the above voltages as follows:
• for: U Ret ≤ 315 V: U Su = 1,15U Ret,
• for U Ret > 315 V: U Su = 1,10U Ret,
and finally U F is a forming voltage.
When the forming voltage U F is exceeded, the forming process starts a new and
large amounts of gas and heat are generated. The same effect, yet on smaller scale,
can already be observed in the knee of the curve. In order to achieve a high degree of
operating safety of the capacitor, the rated voltage
U
Re t
is defined as being on the
quasi-linear part of the curve. As the capacitor is subjected to surge voltages
U
Su
for
short periods only, this range lies between the rated voltage and the forming voltage.
The difference between forming voltage and operating voltage, the so-called overanodization,
thus has a substantial effect on the operating reliability of the capacitor.
High overanodization offers the possibility of producing especially reliable
capacitors designed as long-life grade “LL” capacitors in accordance with IEC
60384-1 [29], [30].
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