Carsten Leu

Breakdown of polymer dielectrics at high direct and alternating voltages superimposed by high frequency high voltages

This paper deals with the measuring of the breakdown voltage of polymeric insulation materials stressed by high direct voltages and power-frequency high-voltages superimposed by high-frequency voltages. High-frequency highvoltage stress results in an intense heating caused by higher dielectric losses commensurate to the frequency, the permittivity and the dielectric loss factor of the materials. The raised thermal stress leads to an influence on the breakdown voltage. The aim of the paper is to make a statement regarding to the influence of superimposed high frequency voltages on dielectrics in steady state operation. The test voltages up to several 10 kV are realized by a test system consisting of an innovative high-frequency high-voltage generator combined with conventional AC- and DC-test equipment. A constant sinusoidal high-frequency voltage up to 50 kV in a frequency range from 1 kHz to 50 kHz is superimposed on a high 50 Hz alternating voltage or a high direct voltage in different ratios. The analysis of the measured breakdown voltage leads to a mathematical approach to calculate the breakdown voltages of different superimposed voltage forms.

Applying of surge arresters in power electronic network components

The increased use of renewable energy sources and connection of energy storage systems to energy distribution networks of the future on the basis of DC voltage demands new power electronic modules with a higher degree of efficiency, reliability and with materials saving. In this article, the overvoltage protection concept on the basis of metal oxide varistors (MOV) for such network components as pulse-width modulation (PWM) modules and DC-to-DC converters as well as a new patented, innovative power-electronic topology concept [1, 2], which allows the regenerative energy sources connect direct to the medium-voltage distribution system is introduced. The distorted voltages from converters and from other power electronic network components are characterized by high-frequency pulsating voltages which generate leakage currents because of MOV capacitance. This leakage current in its turn can overheat the MOV and accelerate its thermal ageing. The main focus of this work is to study the distorted voltages in already mentioned network concepts and its influence on installed MOV and vice versa. The concept of overvoltage protection for some network components is presented. Additionally, an effort to combine the overvoltage protection system with some other useful functions for converter, such as definition of reference potential of the internal ground, was fulfilled.

Loss factor and permittivity measurement at medium-frequency high voltage with an HVDC offset

Medium frequency (MF) transformers are key components of power electronic topologies of modern energy supply systems. The insulation of MF transformers is exposed to the low frequency AC or DC grid voltage and at the same time to rectangular medium frequency converter switches featuring high amplitude and steepness. The high harmonic content of such voltage can cause increased dielectric losses in the insulation, leading to local hotspots [1]. Studying dielectric properties of insulating paper materials to be utilized in MF transformers by applying mixed voltages is of great interest. Such mixed voltage variations of MF and superimposed DC or low frequency AC components are shown in Figure 1. Combination of MF AC and DC voltage components is considered in this work. Both components are in kV-range, so that the dielectric parameters are derived from measured u-i-signals. By using mixed MF AC voltage with a DC offset following questions are treated. Firstly, the loss factor and relative permittivity are measured at high MF voltage amplitudes that yields values for realistic field strengths. Secondly, the impact of DC voltage on tand and εr can be studied. Thirdly, other effects, e.g. change of material size due to high DC field can be observed by means of tand and εr.

Surface discharges at superimposed medium frequency high voltage

Due to new technological possibilities decentral structures will characterize the future electrical energy grid. To ensure the energy transport and to increase the efficiency Medium-Frequency High-Voltage (MF-HV) transformers are used. With these inductive couplers the electrical isolation between loads, sources and the AC-distribution system is possible. In this process a superposition of the MF-HV generated by power electronic circuits and the power frequency voltage occurs. Figure 1 shows the calculated voltage waveforms of these Multi-Cell Solid-State Transformers for two different power electronic topologies (3LL and 2LL), which are described in [1]. The voltage consists of a 50 Hz component and medium frequency harmonics.

Development of a head-phantom and measurement setup for lightning effects

Direct lightning strikes to human heads lead to various effects ranging from Lichtenberg figures, over loss of consciousness to death. The evolution of the induced current distribution in the head is of great interest to understand the effect mechanisms. This work describes a technique to model a simplified head-phantom to investigate effects during direct lightning strike. The head-phantom geometry, conductive and dielectric parameters were chosen similar to that of a human head. Three layers (brain, skull, and scalp) were created for the phantom using agarose hydrogel doped with sodium chloride and carbon. The head-phantom was tested on two different impulse generators, which reproduce approximate lightning impulses. The effective current and the current distribution in each layer were analyzed. The biggest part of the current flowed through the brain layer, approx. 70 % in cases without external flashover. Approx. 23 % of the current flowed through skull layer and 6 % through the scalp layer. However, the current decreased within the head-phantom to almost zero after a complete flashover on the phantom occurred. The flashover formed faster with a higher impulse current level. Exposition time of current through the head decreases with a higher current level of the lightning impulse. This mechanism might explain the fact that people can survive a lightning strike. The experiments help to understand lightning effects on humans.