Einsatz von Metalloxid-Varistoren zum Überspannungsschutz ...

Abstract Seite XIX
Abstract
During operation of electrical machines at inverters with high frequency switching PWM
(pulse width modulation) voltage travelling wave effects cause transients of
unacceptably high amplitudes (i.e. up to two times the d.c. link voltage or even higher)
at the machine winding insulation which may lead to partial discharges and, in the long
term, to insulation failures. Furthermore, the very fast switching IGBT-modules in
today’s inverters cause a high voltage steepness, leading to a non linear voltage
distribution in the stator coils of the machine. Over the years these phenomena have
been well understood and mitigation methods have been developed to effectively
reduce the dielectric stress of the insulation material. Progress has been made
especially in the design of filters, but such technologies also have their limits. For cable
lengths of several hundred meters dv/dt filters do not affect the voltage at the machine
terminals anymore because they are effective up to a specified cable length only.
Sinusoidal filters always have a resonance frequency in the range of one kilohertz, and
thus the inverter pulse frequency cannot be chosen below a value of e.g.
5 kHz. But for large drives pulse frequencies are often below this value, and
consequently sinusoidal filters cannot be used for high power applications. Another
criterion for the choice of the filter type is finally the costs which are made up on one
hand from the initial investment and on the other hand from the operating costs which
also include total losses. Filters always have a self inductance in the main current path
of the drive, leading to additional losses. Moreover, the self inductance has to be
dimensioned for the total current and power of the drive, making it very large and
expensive in many cases.
Metal-oxide (MO) varistors in this application are just connected line to ground in all
three phases in order to reduce the line to ground overvoltage. They are not arranged
in the main current path and are therefore independent from the power of the drive.
The dimensioning of the varistors only depends on the surge impedance of the
connecting cable and the accepted maximum overvoltage at the machine terminal.
Power losses in the varistors additionally depend on the pulse frequency of the applied
PWM voltage; they decrease with lower pulse frequency. These facts make the use of
varistors in inverter-fed drives extremely attractive for large drive applications.
However, this new kind of stress for metal-oxide varistors under permanently occurring
transients sets new requirements on dimensioning rules, as well as on the specification
of operating duty and accelerated aging tests. Dimensioning of a metal-oxide surge