Analytical calculation of the frequency-dependent litz wire resistance considering the wire connectors

Eddy-current effects have an impact on the resistance of litz wire. They reduce the utilization of the wire cross section. In a certain parameter range, the dominant influencing factor is the current distribution in the wire connectors. The connector itself does not significantly contribute to the wire resistance but it results in an inhomogeneous current distribution among the strands. We quantify the resistance increase by introducing a connector skin-effect factor and give a mathematical derivation. Taking the twisting of the strands into account requires a probabilistic model of the radial strand position. Resistance measurement results validate the calculation approach. We discuss the assumptions and derive the parameter range where the model is valid

Emulation of grid-forming inverters using real-time PC and 4-quadrant voltage amplifier = Emulation netzbildender Wechselrichter mittels Echtzeit-PC und 4-Quadranten-Spannungsverstärker

Vorgestellt wird ein Versuchsstand zur Evaluierung neuartiger Regelungsverfahren für Wechselrichter zur regenerativen Stromerzeugung. Das Verhalten von netzgeführter und netzbildender Regelung wird in einem Testszenario untersucht und verglichen. Unter Verwendung einer echtzeitfähigen Regelungsplattform mit einer Zykluszeit von 50 μs können mit Matlab/Simulink entwickelte Regelungsverfahren implementiert werden. Der Einfachheit halber wird anstelle eines Wechselrichters ein dreiphasiger 4-Quadranten-Spannungsverstärker verwendet. Somit kann auf den Einsatz von Modulation und geschalteten Leistungshalbleitern verzichtet werden. Um ein realistisches Verhalten eines netzseitigen Filters zu zeigen, können passive Komponenten automatisiert als L-, LC- oder LCL-Filter zugeschaltet werden. Der Versuchsstand hat eine Nennwirkleistung von 43,6 kW und eine Nennspannung von 400 V. Als modernes netzfolgendes Regelverfahren ist im Versuchsstand eine Stromregelung im d/q-System implementiert. Als netzbildendes Regelverfahren wird eine virtuelle Synchronmaschine, der Synchronverter, eingesetzt. In Kombination mit einer frequenzvariablen Netznachbildung wird das Verhalten beider Regelverfahren im Falle einer Lastzuschaltung in einer Inselnetzumgebung untersucht

Partial discharge diagnostics on inverter-fed drives of electric vehicles = Teilentladungsdiagnostik an umrichtergespeisten Antrieben der Elektromobilität

In this paper, the possibilities of partial discharge measurement at square-wave voltages are discussed. The focus is on low-voltage machines, such as those used in electric vehicles. An insight into common measurement methods is given. In this work, high-frequency current transformers and near-field probes are used as sensors. The generation of a realistic steep-edged test voltage according to IEC/TS 61934 is also discussed and demonstrated on an inverter with silicon carbide semiconductors. Experiments are conducted to compare the sensors used. A direct comparison between cable-based sensors (HFCT) and antennas (near-field probes) is provided. Resonance points to open motor windings according to IEC/TS 60034-27‑5 and their significance for partial discharge measurement are also investigated. The results show that the high-frequency current transformers and the ball probe are able to detect PD even at steep voltage edges. Excitation with a fast voltage edge results in non-negligible overshoots in the overall system due to resonances. It can be shown that the partial discharges occur during these overvoltages

Possibilities of Conventional PD Measurements with Non-Sinusoidal Waveforms for Electric Vehicles

Electrical traction machines in electric vehicles are normally fed by converters with DC link voltages up to 800 V. The resulting voltage pulses place particular stress on the insulating system of the drivetrain. In order to be able to investigate insulating material samples, e.g. twisted pair enameled wire, with voltages of different shapes and high frequency, a shielded experimental setup for the investigation of partial discharges (PD) at low voltages and high frequencies is presented. A medium frequency transformer with a frequency range up to 2500 Hz is used for this purpose, fed by a linear power amplifier on the primary side. The amplifier has a slew rate of 52 V/μs and is capable of sourcing various voltage waveforms such as sinusiodal, triangular or rectangular, with a maximum frequency of 30 kHz. Electrical PD measuring methods according to IEC 60270 as well as acoustic and optical measurement methods are applied for PD diagnosis. The experimental setup is intended to demonstrate the possibilities and limitations of conventional PD diagnostics for nonsinusoidal voltages. Focus is placed on the application of filters, the damping behavior of the step-up transformer and the rise times of the voltages

Optimized meter placement in low voltage grids based on asymmetric state estimation

Alongside the ongoing energy system transition towards sustainability new challenges for low voltage grids arise. New technologies connected to those subordinate grids are less predictable, especially decentralized solar plants. Larger loads and a possible reversed power flow lead to increasingly unknown states and can evoke violations of power quality. This paper presents a method to determine an optimized meter placement in low voltage grids using an asymmetric state estimation in order to achieve a cost-efficient monitoring. First, the utilized state estimation method is introduced as well as the usage and parameterization of pseudo measurement values are discussed. Furthermore, a new approach for an optimized meter placement is presented and simulation results for exemplary grids and corresponding power flow data are shown. Subsequent discussions focus on the quality of results subject to the amount as well as the specific positioning of meters placed

Operating Power Grids with Few Flow Control Buses

Future power grids will offer enhanced controllability due to the increased availability of power flow control units (FACTS). As the installation of control units in the grid is an expensive investment, we are interested in using few controllers to achieve high controllability. In particular, two questions arise: How many flow control buses are necessary to obtain globally optimal power flows? And if fewer flow control buses are available, what can we achieve with them? Using steady state IEEE benchmark data sets, we explore experimentally that already a small number of controllers placed at certain grid buses suffices to achieve globally optimal power flows. We present a graph-theoretic explanation for this behavior. To answer the second question we perform a set of experiments that explore the existence and costs of feasible power flow solutions at increased loads with respect to the number of flow control buses in the grid. We observe that adding a small number of flow control buses reduces the flow costs and extends the existence of feasible solutions at increased load.Comment: extended version of an ACM e-Energy 2015 poster/workshop pape

Interference of composite bosons

We investigate multi-boson interference. A Hamiltonian is presented that treats pairs of bosons as a single composite boson. This Hamiltonian allows two pairs of bosons to interact as if they were two single composite bosons. We show that this leads to the composite bosons exhibiting novel interference effects such as Hong-Ou-Mandel interference. We then investigate generalizations of the formalism to the case of interference between two general composite bosons. Finally, we show how one can realize interference between composite bosons in the two atom Dicke model

Fault discrimination and protection coordination for a bipolar full-bridge MMC-HVDC scheme

Fault discrimination and protection design for bipolar high-voltage direct current transmission solutions based on modular multilevel converters (MMC-HVDC) links are of significant importance for a reliable and resilient power transmission. If full-bridge submodules are utilised, fault-dependent handling concepts considering the location of an event are enabled. This study presents a comprehensive approach to differentiate and deal with internal converter and dc side faults. While a multitude of measurements inside and at the clamps of each converter is usually only used for simple threshold-based hardware-related protection, additional differential and derivative criteria may further improve selectivity. However, this requires careful configuration to avoid improper reactions. To highlight the coordinated manner of the proposed concept, various faults are analysed and selected examples are explicitly investigated and visualised using the PSCAD EMTDC software

An universal high-voltage source based on a static frequency converter

Abstract: For testing electrical equipment the test voltage has to achieve special requirements, which are noted in the international standards. Tests showed that it's not possible to fulfill those requirements with a passive pulse pattern. This paper deals with the development of an electronic control unit for a static frequency converter. The focus is on the synthesis of the control and the obtained results after realizing in a static frequency converter with semiconductor valves