Model-based accuracy enhancements for guarded conductivity measurements:determination of effective electrode areas utilising numerical field simulation

Methods utilising current measurements for conductivity and permittivity determination require precise knowledge of the effective electrode area in order to obtain accurate results.Owing to field distortions(e.g.caused by fringing)in guarded electrode setups,the effective electrode area differs significantly from the geometrical calculated.Focusing on guarded electrode setups for conductivity determination,a generic method based on numerical field simulation is presented allowing a convenient determination of the relevant effective electrode area.For this purpose,a brief overview of yet existing normative guidelines and related research work is provided.State-of-the-art conductivity measurement setups are presented in order to identify parameters which affect the field distribution within the measurement arrangements.The description of the implemented method and its realisation in COMSOL multiphysics is followed by its validation using analytical fringing calculations.Furthermore,presented method is used for the evaluation of fringing effects and additional field distortion caused by design aspects of the measurement cell itself and potential imbalances related to the measurement setup.Moreover,dependencies on conductivity of the surrounding environment are considered.Achieved model-based accuracy enhancements are calculated and are leading to a gain in precision for conductivity determination of up to 25%compared to yet existing approaches.

Transient voltage stresses in MMC-HVDC links-impulse analysis and novel proposals for synthetic laboratory generation

To evaluate and optimise insulation coordination concepts for state of the art high-voltage direct current(HVDC)transmission systems,appropriate test voltage shapes are required for laboratory imitation of occurring stresses.While especially transient voltages in the monopolar modular multilevel converter(MMC)-HVDC links show an extensive deviation from commonly applied switching impulse shapes,this study focusses on the analysis of over-voltages subsequent to direct current pole to ground faults.Additionally,novel methods for synthetic laboratory test voltage generation are proposed.Based on simulated transients occurring during fault scenarios in different symmetrical monopolar±320 kV MMC-HVDC schemes,curve fitting,and related analysis techniques are used in order to compare simulated over-voltages with standard test voltage shapes.Moreover,these techniques further allow the identification of novel relevant impulse characteristics.Subsequently,design considerations for the generation of non-standard impulses based on single-stage circuits are derived and discussed.Those synthetically generated voltages may,later on,provide the basis for future investigations on related dielectric effects caused by those non-normative over-voltages.