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

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

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

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

Analysis of Cable Overvoltages in Symmetrical Monopolar and Rigid Bipolar HVDC Configuration

The symmetrical monopolar configuration is the prevailing scheme configuration for high voltage direct current (HVDC) interconnectors utilizing themodular multilevel converter (MMC) topology. However, the rigid bipolar configuration is gaining significance, as first projects are currently in planning stage. The purpose of this paper is to analyse bothHVDCscheme configurations with regard to cable voltage stresses originating from shortcircuit faults. The study focuses on HVDC cables with extruded insulation connected to MMCs equipped with half-bridge (HB) submodules. Basic design aspects in rigid bipolar configuration such as the station grounding concept are discussed. Moreover, various station internal as well as dc faults are examined by means of electromagnetic transient (EMT) simulations. Occurring cable voltage stresses are analyzed with respect to voltage polarity, peak value and wave front-parameters and compared in symmetrical monopolar and rigid bipolar configuration. It is demonstrated that cable overvoltages with same polarity as the dc operating voltage are significantly reduced in schemes operating in rigid bipolar configuration. Voltage reversals caused by the discharge process during a cable fault are nearly independent of the selected scheme configuration. Obtained results are relevant with regard to upcoming projects in rigid bipolar configuration and provide insights to further refine insulation co-ordination aspects related to dc cable systems

Modeling of German Low Voltage Cables with Ground Return Path

In this contribution, measurement data of phase, neutral, and ground currents from real low voltage (LV) feeders in Germany is presented and analyzed. The data obtained is used to review and evaluate common modeling approaches for LV systems. An alternative modeling approach for detailed cable and ground modeling, which allows for the consideration of typical German LV earthing conditions and asymmetrical cable design, is proposed. Further, analytical calculation methods for model parameters are described and compared to laboratory measurement results of real LV cables. The models are then evaluated in terms of parameter sensitivity and parameter relevance, focusing on the influence of conventionally performed simplifications, such as neglecting house junction cables, shunt admittances, or temperature dependencies. By comparing measurement data from a real LV feeder to simulation results, the proposed modeling approach is validated

Machine-learning-based Bayesian state estimation in electrical energy systems

In many algorithmic applications in electrical power grids, state estimation (SE) represents the first step of a process chain. In SE, sensor measurements are processed to infer the most probable grid state. Classical methods such as weighted least squares (WLSs) based approaches use statistical methods that can be based on sensor noise and erroneous measurements. With these methods, only point estimates are made, which results in a lack of knowledge about prediction uncertainties. In this study, machine-learning-based methods for determining the actual state of the grid are proposed. Bayesian optimisation is applied to find the optimal hyperparameter configurations for neural networks (NNs) for SE tasks. The application of Bayesian inference using Bayesian NNs is proposed, which allows the prediction of point estimates as well as uncertainty intervals for the system states. The advantages of using Bayesian approaches in comparison to classical SE methods like WLS are shown