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Center for High Voltage Engeneering and Insulating Systems

The research field of high-voltage engeneering and insulation systems is of central importance in modern industrial society. High voltages or high field strengths in the insulation systems are required for the low-loss transmission of electrical energy as well as for a variety of industrial applications from medical to manufacturing to automotive technology.

High-voltage technology includes the control of high electric field strengths in all types of electrical insulation. The simple basic principle is:

Under all conditions, the electrical stress (i.e. the electrical field strength) must always be smaller than the electrical strength of the insulating media.

The task of high-voltage technology is therefore not to generate electrical discharges, even if these are always impressive for visitors to our laboratory, but rather to prevent them in order to ensure the safe operation of devices and systems.

It is now necessary to further exploit insulation systems for technical and economic reasons. That's why a deep understanding of the materials is essential, which is why research in the area of ​​high-voltage insulation materials and systems has now become very much focused on materials science. The direct cooperation of several laboratories involved in the IEHT is a great advantage. Modern analysis methods such as FTIR or Raman spectroscopy are available in the materials science working group and in the chemical-physical laboratory.

The THWS high-voltage laboratory offers for student training, research and cooperation with industrial partners a partially worldwide unique infrasructure. A more detailed description of the equipment can be found on the laboratory's website.

Insulating materials and systems for high-voltage direct current transmission (HVDC)

[Translate to Englisch:] Messung von Raumladungen an einer PTFE-Probe mittels der Pulsed-Electro-Acoustik-Methode (PEA)

... because in the future three-phase networks worldwide will reach their performance limits and will have to be networked with high-voltage direct current transmission (HVDC) ...

Research fields:

  • Investigation of the electrical conductivity of insulating materials under defined boundary conditions
  • Investigation of charge carrier generation and recombination as well as charge carrier transport in insulating liquids
  • Measuring space charges with the electro-optical Kerr effect
  • Measurement of space charges with the Pulsed Electro Acoustic Method (PEA)
  • Multiscale modeling of charge transport in liquid and solid insulating materials
    • Quantum chemistry
    • Molecular dynamics
    • Bipolar Charge Transport Models
    • FEM simulation of charge transport in insulating materials and systems

Diagnosis, monitoring and condition assessment

... because the networks of industrialized nations were expanded decades ago, so that the safe operation of systems and devices requires reliable diagnostics of the aging condition ...

Research fields:

  • Measurement and analysis of partial discharges, e.g. for HVDC applications
  • Use of modern PD measurement technology and fault location detection for online and offline diagnosis
  • Spectroscopic material analysis

Insulation systems for electrical machines and power electronic applicationsInsulation systems for electrical machines and power electronic applications

...because the new requirements in the area of ​​electrical machines, such as an increase in efficiency, the use of modern, fast-switching power semiconductors and new insulating materials, are leading to increasing loads on the insulating systems...

Research fields:

  • Investigation of the TE activity of engine test specimens
  • Investigation of the aging behavior of engine insulation systems
  • Investigation of the (aging) behavior of insulating materials at high frequencies

Transient behavior of power transformers

… since modern power transmission systems are becoming increasingly complex, and the integration of power-electronic components more frequently leads to fast, high-frequency overvoltages that stress transformer insulation systems in new ways, making a deep understanding of transient phenomena essential for safe and reliable operation …

Research fields:

  • Modelling & Validation: Development of validated white-box, black-box, and grey-box models for ransformer simulation
  • Assessment Methods: Application of severity indices (FDSF/TDSF) and investigation of internationally discussed approaches for the assessment of non-standard overvoltages
  • Analysis of Recorded Transients: Evaluation of transient events measured on in-service transformers and comparison with EMTP models

Modeling, simulation and measurement of electric field strength distributions in insulating fluids under transient and steady-state DC loading of layered insulation systems (EFI-DC)

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.

Kerr-Effekt-Prüfstand zur kombinierten Feldstärke- und Strommessung
Comparison between measurement and simulation Field strength curve in the oil
Comparison between measurement and simulation leakage current in the oil

Elastomers with specific conductivity and their aging behavior (ELSA)

Management Prof. Zink, Prof. Kobus

Research partner Pfisterer contact systems

Duration 2022-2025

Elastomers, among other things, are used as insulating materials in complex insulation systems for cable applications (cable sleeves) in high-voltage direct current (HVDC) transmission. The selection and qualification of suitable materials for specific applications is challenging. In addition to the electrical parameters that are important for the design, knowledge about the aging behavior of these materials also plays a central role. In particular, construction and assembly-related situations in a cable sleeve, such as the "noble joint", or additives (lubricants) required for assembly can significantly influence the service life behavior. The combination or layering of various insulating materials such as silicones or EPDM (ethylene-propylene-diene rubbers) with the XLPE (Cross-linked Polyethylene) used in the cable as part of HVDC also brings with it new challenges for the insulation systems. The electrical conductivity has a decisive influence on the field distribution within the layer insulation system. However, this is significantly influenced by its dependence on various influencing factors such as the ambient temperature, the applied electrical field or production-related defects.

As part of the ELSA project (elastomers with specific conductivity and their aging behavior), different aging mechanisms of different elastomers are being investigated in collaboration with the project partner Pfisterer. By using dielectric diagnostic methods, such as polarization and depolarization measurements or space charge measurements using the PEA method, the condition of the insulating material is assessed over its service life. By additionally interpreting the dielectric properties using spectral investigation methods such as infrared and Raman spectroscopy, which allow information to be drawn about the bonding situation in the polymer, a uniform picture of the aging stage can be obtained.

This newly gained knowledge should make a significant contribution to the better qualification and testing of insulating materials, which should lead to advanced and more reliable fittings.

Conductivity investigation of insulating materials with field strength-dependent behavior (LUISE)

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.


Field inversion in the insulating material of an XLPE cable between the inner and outer conductors.

Measurement of Motorettes (MEMO)

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.

Partial discharge test bench for motor formats.

Studying the effect of biodegradable molecule on dielectric failure of high voltage liquid insulation system: environmentally friendly dielectric liquids based on natural esters (BioLiq)

Management Prof. Zink, Prof. Kobus

Research partner Alexander von Humboldt Foundation

Duration 2023-2025

The remarkable development in high-voltage direct current and high-voltage alternating current transmission systems calls for a renewed assessment of dielectric liquids for insulation systems of transformers. The function of liquid insulation used in high voltage equipment is cooling and insulation. It should have several features like high dielectric strength, low viscosity, high flash point, very low moisture or water content, high specific resistance and many more. Petroleum dependent synthetic and mineral oil has been conventionally applied as dielectric fluids in transformers during previous some decades that disturbs the environment on account of their low biodegradability and low fire point which have persuaded the exploration of substitutes. The application of alternate insulating fluids is increasing gradually, with safety and environmental apprehensions at the lead of the grounds for shifting from mineral oil.

Dielectric failure phenomenon in high voltage (HV) liquid dielectric insulation is still not well understood and it poses major scientific and technological complications. The understanding of dielectric failure is required to get insight about breakdown process mechanisms and theoretical basis for molecular modification hence application of dielectric insulation at appropriate applications. Hydrocarbon based liquids extracted from finite resources have been used as insulation in HV applications for more than a century. They have been long tested with long history and set design rules for applications in HV equipment. The non-renewable nature of these hydrocarbon-based liquids presents much burden on the energy security and environmental protection. Renewable oils (natural esters/vegetable oils) mainly composed of triacylglycerol molecules extracted from plants are increasingly being adopted for use in electrical insulation, lubricants, and biodiesel. Natural esters, as the renewable resources, present excellent physiochemical and dielectric features, e.g., fire resistance, high biodegradability, and satisfactory dielectric breakdown performance. Their environmental performance makes these materials extremely popular, and they are being anticipated as potential dielectric liquid insulation. Until now, despite all these mentioned advantages, they could only find applications in medium voltage applications. The main reason for their limited applications at high voltage levels is non-availability of fundamental data about dielectric parameters and the knowledge about failure phenomena, which is significant for design rules to achieve a long-term reliable performance. The absence of fundamental data about natural esters makes the equipment manufacturers, utilities, regulators and especially insulation community demotivated for their application. Hence the electrical performance of natural esters with different structures needs further evaluations, which is in the focus of the project BioLiq.

Contact

Prof. Dr. Tanja Mühling

Phone
+49 931 3511-8025
E-Mail
tanja.muehling[at]thws.de
Visiting Address
Tiepolostraße 6
97070 Würzburg
Room
T.2.11
Profile picture Prof. Dr. Tanja Mühling