Center for High Voltage Engeneering and Insulating Systems

Management Prof. Zink, Prof. Dr. Kobus

Research partners Siemens Energy Global GmbH & Co. KG, Weidmann Electrical Technology AG

Duration 2018-2025

As a result of the energy transition and the restructuring of energy networks, high-voltage direct current transmission (HVDC) is becoming increasingly important and brings with it increasing demands on operating resources. The insulation systems of HVDC equipment, especially transformers, are of particular interest. The main components of such insulation systems are insulating fluids and oil-impregnated pressboard made of cellulose, which exhibit complex electrical conduction and polarization mechanisms under direct voltage stress that have not yet been studied in depth. Conventional RC circuit models are only partially suitable for the dielectric description of these materials and the mechanisms that occur. Rather, multiphysical modeling approaches are required that take different physical-chemical effects into account. These mainly include the Poisson-Nernst-Planck (PNP) system of equations. However, there has so far been no clear consensus regarding the parameterization when using it and the parameters on which the simulation is based are often only meaningfully estimated or varied empirically.

As part of the EFI-DC project, the layered insulation system will therefore be examined with different DC loads and configurations of the insulation system in order to gain a deeper understanding of the dominant charge carrier phenomena. Additional environmental parameters (temperature, pressure, etc.) can be varied, which can be used to test various hypotheses. The primary measurement methods for verification are the simultaneous measurement of the transient polarization current (PDC measurement) and the stationary and transient field strength in existing transparent areas of the insulating medium. Field and current curves resulting from these measurements can be used to parameterize or verify the existing models. With an accurate model and the understanding gained from it, the design of the insulation system of various equipment under DC load can be designed more effectively in terms of weak points and a potential reduction in the installation space of the transformers, which at the same time significantly increases the competitiveness of HVDC technology.

Management Prof. Zink

Internal research partner

Duration 2023-2026

The development of energy transmission with high-voltage direct current (HVDC) is a central element of the energy transition. A particular challenge is the safe design of high-voltage insulation systems, whose task is to safely control the high voltages and field strengths within the equipment. While in applications for alternating voltage the electric field (displacement field) can be controlled by the geometry of the electrodes and interfaces, this is not possible for direct voltage (flow field). The conductivities of the insulating materials determine the field distribution in the insulation system. However, the conductivities of the insulating materials are not only very different from one another, but are also highly dependent on the parameters field strength and temperature, whereas this dependence does not apply to the permittivity, which makes the design in the displacement field easier than in the flow field. During operation of the insulation systems, especially when, for example, the temperature distribution changes or temperature gradients develop, the above-mentioned dependencies can lead to the formation of space or surface charge zones, which can drastically change the field strength distribution in the insulation system, so-called field migration or inversion, see . Figure 1. Under certain circumstances, this can lead to the formation of (partial) discharges, which can then erode the materials and damage the insulation system to the point of total failure. Such problems occur not only in the insulation systems of HVDC equipment, but also, for example, in high-tech applications with high direct voltage, such as fundamental physics, semiconductor technology or microscopy. An innovative approach to improve the problems described lies in the use of so-called field grading materials (FGM) with a specifically adjusted or even field strength-dependent electrical conductivity. With these materials, areas with higher field strength can be automatically relieved and the field distribution in the insulation system can be evened out. Such materials are based on a carrier material, e.g. varnish or epoxy resin, in which filling materials (e.g. silicon carbides or metal oxides) are embedded, which show a field strength-dependent, varistor-like conductivity behavior.

Management Prof. Zink, Prof. Rahimpour

Research partner NN

Duration 2023-2024

The transformation of the automotive industry towards fully electric vehicles brings with it various challenges in the design and development of individual components. Due to ever-increasing electrical consumers, the need for high voltages in the on-board electrical system is increasing in order to keep currents low and thus enable economical design of the machines. This results in a high electrical load on the insulating materials in the drive train, which is why proven insulation systems are often no longer sufficient. In order to ensure reliable operation over the service life in the future, it is important to examine the aging behavior of the insulation systems.

In the MEMO research project, the aging mechanisms under different types of stress are to be investigated in order to gain knowledge about the dominant aging factors. The accelerated aging tests take place on complete stators and motor formats with various insulation systems. In order to evaluate the aging condition of the test specimens in the individual aging stages, various non-destructive dielectric measurements from high-voltage technology are used. Among other things, the measurement of the insulation resistance (PDC measurement), the loss factor, the partial discharge activity at surge and alternating voltage as well as the frequency domain measurement (FDS) are used to determine the aging condition. If necessary, additional breakdown tests (HiPot) are used to determine the degradation of the insulation systems.

The characteristic values ​​resulting from the tests should be used to parameterize a service life model. Particular attention is also paid to the partial discharge behavior of the test specimens in different aging states, which will be investigated using phase-resolved partial discharge analysis (PRPDA) and pulse sequence analysis (PSA). This serves to further understand the aging mechanisms of the insulation systems and offers the cooperation partner the opportunity to continue to guarantee the usual and required reliability. The results of the investigations show the limits of the insulation systems and offer a comparison with each other. In addition, the tests used can be used in the qualification and manufacturing process of a new product.