Martin Carlen

Theory of Ragone plots

The general theory of Ragone plots for energy storage devices (ESD) is discussed. Ragone plots provide the available energy of an ESD for constant active power request. The qualitative form of Ragone plots strongly depends on the type of storage (battery, capacitor, SMES, flywheel, etc.). For example, the energy decreases as a function of power for capacitive ESD, but increases for inductive ESD. Analytical results for a representative set of ideal ESD (battery, capacitor, and SMES) are compared. Furthermore, the effect of leakage and of the specific loss type (Coulomb, Stokes, and Newton friction) is discussed for inductive ESD. Finally, we address the problem of how composite ESD should be treated, and illustrate it for a battery with inductance.

Simulation and Measurement of Lightning-Impulse Voltage Distributions Over Transformer Windings

This paper presents: 1) a novel method for accurate high-frequency modeling of dry-type transformer windings based on magnetic and electric field simulations for parameter extraction of the detailed equivalent circuit of the entire winding system; 2) a method for fast and accurate transient solution of the circuit differential equations that describe the voltage distribution over the winding system; 3) an efficient, cheap, and nondestructive low-voltage measurement system based on a self-developed lightning-impulse generator; and 4) verification of the simulation results by comparison with measurement.

Numerical Computation of Ohmic and Eddy-Current Winding Losses of Converter Transformers Including Higher Harmonics of Load Current

Two different numerical algorithms for computing the Ohmic and eddy-current winding losses of converter transformers are presented. The higher harmonic components of the nonharmonic electric current, generated by the rectifying part of the power electronic converter, are fully taken into account. The calculated numerical results are verified by comparison with measured results.

Numerical Study of the Core Saturation Influence on the Winding Losses of Traction Transformers

The winding system of traction transformers is subjected to non-sinusoidal voltages and currents of the adjacent traction converter. Unlike its fundamental component, the higher harmonics of the load current have a completely non-uniform distribution over the conductors cross section causing hazardous hot spots. Due to dynamic operating conditions in railway grids, the railway voltage can temporarily increase as much as 20% saturating magnetic cores of traction transformers. This paper presents the analysis of the core saturation influence on the distribution of the transformers winding losses along with suitable numerical methodologies for their simulation. Thus, a fast 2-D and 3-D frequency-domain methodology for computing winding losses of traction transformers by considering all the relevant higher current harmonics and core saturation effects caused by the fundamental voltage component is described in detail. The obtained results are verified by comparison against the available corresponding measurements and against the results of a rigorous and from the CPU time perspective much more demanding time-domain simulation approach.

Long-term conductivity decrease of polyethylene and polypropylene insulation materials

This work summarizes the results of a number of DC conductivity measurement studies on polypropylene (PP), low density polyethylene (LDPE), and cross linked polyethylene (XLPE). The main observation is that under apparently time-constant external conditions (voltage, temperature, etc.) no steady-state direct current (DC) was established even after very long measurement times. Nevertheless, this behavior seems to exhibit some common systematic features, and since the experiments were performed with different equipment at different R&D labs in different years by different teams, simple measurement artefacts can be excluded. One observation is that there are two electric field regimes with slightly different behavior, separated by crossover field of about 10–15 kV/mm. In this work we focus on the high-field region, where the main observation is that the conductance slowly decays sub-linearly with time, I∼ t−n, with 0.3 < n < 1, n mostly around 0.5. We provide experimental indications that this behavior is rather independent of the presence of 1) the voltage and 2) the peroxide decomposition products (in PE). The observations are in favor of an underlying thermally driven relaxation process related to structural changes (morphology, free volume) of the polymer. A main implication of the results is that the use of steady-state conductivity values for the characterization of certain polymer insulation is not appropriate and instead the decaying behavior of the conduction current must be considered.

Novel measurement methods for in-depth analysis of AC metallized film capacitors

Metallized film capacitors with self-healing properties become increasingly relevant for high voltage power applications due to their high capacitance density, high power density, and inherent safety. However, because of a lack of generic design rules, introduction of new capacitor designs requires time-consuming aging tests. We present a novel toolkit of measurement methods with the purpose of gaining a deeper knowledge on aging mechanisms and decreasing the development time for new capacitor designs.