Modularer Isolationstester

Translated title of the contribution: Modular Insulation Tester

Research output: ThesisMaster's Thesis


The insulation system of electrical machines in contrast to conventional grid operation is especially stressed when fed by inverters due to the occurrence of steep voltage slopes. In order to qualify existing or new insulation systems for this type of stress, test voltage sources are used which generate a trapezoidal shaped test voltage whose amplitude, frequency and rise and fall time can be varied.

In practice, these test voltage sources usually consist of a switching element that alternately switches the voltages from multiple DC voltage sources to the insulation of the electrical machine via suitable charging or discharging resistors. Depending on the size of the insulation capacitance and the resistances, a rectangular voltage is generated whose slopes have the form of exponential functions. In this case, the energy stored in the insulation capacitance is periodically converted into heat in the resistors and in the switching element. This leads to high power losses within the whole test system and therefore demands a high amount energy from the supplying DC voltage sources.

In this thesis a concept is presented in which the energy stored in the insulation capacitance can be fed back into the test voltage source so that only the losses of the test system have to be covered by the energy supply. This is achieved by a circuit, similar to modular multilevel converters, with full bridge modules. These full bridge modules are built up from a DC bus capacitor and semiconductor switches and can output zero voltage with a low or a high-impedance state or positive or negative DC bus voltage at their output terminals. Through the series connection of several submodules, the desired rectangular test voltage can then be reproduced.

If this test voltage curve is applied to the insulation capacitance via an additional serially connected inductance, oscillacions occur when the stages are switched. These oscillacions can then be actively damped when the next voltage step occurs. This causes a current flow, which can be used to shift energy between the DC buses of the individual submodules so that the losses of the individual modules can be covered by this effect and only one submodule has to be actively supplied with energy. This further lowers the effort on the power supply. In order to prove this concept, it was simulated and a prototype was developed, implemented and tested in several experiments.
Translated title of the contributionModular Insulation Tester
Original languageGerman
QualificationMaster of Science
Awarding Institution
  • Graz University of Technology (90000)
  • Krischan, Klaus, Supervisor
Award date14 Oct 2019
Publication statusPublished - 1 Oct 2019


  • voltage test
  • electric drive
  • modular multilevel converter


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