This paper addresses the accelerated aging of medium-voltage (MV) cable terminations with resistive stress-grading due to supraharmonics. The paper introduces a simple and quick way to relate the risk of cable termination failure to the characteristics of supraharmonic distortion in the system. The motivation is to give practical recommendations and guidelines to evaluate the risk of failure of cable terminations under the presence of supraharmonics in MV networks. The underlying model relates the heating in the cable termination linearly with the frequency of the voltage applied and proportionally with the square of the magnitude of the voltage. The indicator can be used to decide whether given levels and frequencies of supraharmonics in the MV network represent a risk to cable terminations. The parameters of the cable termination design are not needed for that decision. However, the decision criterion is based on one sample data (Eagle Pass) and more field information is crucial to improve the approach.

Railway power systems operating at a nominal frequency below the frequency of the public grid (50 or 60 Hz) are special in many senses. One is that they exist in a just few countries around the world. However, for these countries such low frequency railways are a critical part of their infrastructure.

The number of published dynamic models as well as stability studies regarding low frequency railways is small, compared to corresponding publications regarding 50 Hz/60 Hz public grids. Since there are two main type of low frequency railways; synchronous and asynchronous, it makes the number of available useful publications even smaller. One important reason for this is the small share of such grids on a global scale, resulting in less research and development man hours spent on low frequency grids.

This work presents an open model of a (synchronous-synchronous) rotary frequency converter for electromechanical stability studies in the phasor domain, based on established synchronous machine models. The proposed model is designed such that it can be used with the available data for a rotary frequency converter.

The behaviour of the model is shown through numerical electromechanical transient stability simulations of two example cases, where a fault is cleared, and the subsequent oscillations are shown. The first example is a single-fed catenary section and the second is doubly-fed catenary section.

This paper continues the pursuit of getting a deeper understanding regarding the transient stability of low-frequency AC railway power systems operated at 16 2/3 Hz synchronously to the public grid. The focus is set on the impact of different load models. A simple constant-current load model is proposed and compared to a previously proposed and studied load model in which the train’s active power is regulated.

The study and comparison is made on exactly the same cases as and grid as with the already proposed and more advanced load model. The railway grid is equipped with a low-frequency AC high-voltage transmission line which is subjected to a fault. The study is limited to railways being fed by different distributions of RFC (Rotary Frequency Converter) types. Both AT (auto transformer) and BT (booster transformer) catenaries are considered.

The RFC dynamic models are essentially Anderson-Fouad models of two synchronous machines coupled mechanically by their rotors being connected to the same shaft.

The differences in load behaviour between the proposed constant-current load model and the previously proposed and studied voltage-dependent active power load model are analyzed and described in the paper.

Most low-frequency AC single-phase railway grids have both power-electronic based Static Frequency Converters (SFCs) and electrical-machine based Rotary Frequency Converters (RFCs) connecting them to the three-phase public grid.

Already today, in some such grids, a majority of the power conversion is from SFCs. As railway traffic (and thus power demand) increases, more SFCs are installed for capacity increase, while the number of RFCs remains (almost) constant. Thus, the share of SFCs is expected to increase, and the ratio of installed rotational inertia over installed power to decrease.

This paper investigates how different shares of SFCs affect the transient stability of low-frequency AC railway grids when having a mix of RFCs and SFCs converting three-phase AC power to single-phase AC power. Results from numerical simulations of the interactions that occur between converters when and after the grid is subject to a fault are presented.

The numerical studies show that with an increased share of SFCs there is an increased oscillatory behavior, for example in the voltage magnitude and active power after fault clearance.

This paper continues the pursuit of getting a deeper understanding regarding the transient stability of low-frequency AC railway power systems operated at 16 2/3 Hz that are synchronously connected to the public grid. Here, the focus is set on such grids with a low-frequency AC high-voltage transmission line subject to a fault. The study here is limited to railways being fed by different distributions of Rotary Frequency Converter (RFC) types. Both auto transformer (AT) and booster transformer (BT) catenaries are considered. No mixed model configurations in the converter stations (CSs) are considered in this study. Therefore, only interactions between RFCs in different CSs and between RFCs, the fault, and the load can take place in this study. The RFC dynamic models are essentially two Anderson-Fouad models of synchronous machines coupled mechanically by their rotors being connected to the same mechani- cal shaft. Besides the new cases studied, also a new voltage-dependent active power load model is presented and used in this study.

Using low frequency High Voltage Transmission systems (HV-T) in parallel with the catenary systemstrengthens the railway system by reducing the total impedance of the railway grid. A consequence ofthe reduced impedance is that converter stations are electrically closer to each other. Inside a converter station, different types of Rotary Frequency Converters (RFCs) are used. It is not wellexplored how different RFCs behaves and interacts with each other during and after a large disturbance, like a short circuit. The dynamics of an RFC are modelled by using the Andersson-Fouad model of a synchronous machine. The study presented in this paper investigates interactions inside and between converter stations, withdifferent types of RFC, for an HV-T system in parallel with a Booster Transformer catenary system. The numerical simulation results show, for instance, that the main power oscillation take place inside aconverter station with mixed configuration of RFC type after fault clearance.

One method of strengthening low frequency AC railway grids is to upgrade Booster Transformer (BT) catenary systems, to Auto Transformer (AT) catenary systems. An AT catenary system has lower equivalent impedance compared to a BT system. Thus, an upgrade makes the existing converter stations electrically closer.

Converter stations may have different types of Rotary Frequency Converters (RFCs) installed in them, and it is not well explored how different RFCs behaves and interact during and after a large disturbance.

Using the Anderson-Fouad model of synchronous machines to describe the dynamics of RFCs, several case studies have been performed through numerical simulations. The studies investigate the interactions within and between converter stations constituted with different RFC types, for BT as well AT catenary systems.

The numerical studies reveal that replacing BT with AT catenary systems, results in a more oscillatory system behaviour. This is seen for example in the power oscillations between and inside converter stations, after fault clearance.

This paper presents some methods to extract additional information from voltage dip recordings, beyond residual voltage and duration. Additionally it discusses some issues related to the massive amount of data obtained from modern measurements that, is referred to as Big Data. The paper proposes some Deep Learning based algorithms as good candidates to extract complex features from big data as a step towards additional information. The applications of the information include predicting individual equipment performance, fault type and location, protection operation, and overall load behavior. Individual equipment and overall load include production as well as consumption

This paper presents a review of literature on voltage dips, from several points of view, throughout the last decade. It also summarizes the results related to voltage dip mitigation in both AC and DC power systems whereas it shows the remaining challenges that requires further research on voltage dips.