Polarization/depolarization current (PDC) and dielectric frequency response (DFR) are two widely used dielectric response methods for evaluating insulation deterioration and aging. The insulation condition can be effectively characterized by DFR trace features, such as the loss peak frequency and voltage dependence. However, a low frequency range down to 10–4 Hz is often required for stator insulation systems, resulting in excessively long testing times. In addition, the influence of the end-corona protection (ECP) is often not excluded during condition assessment. The PDC method is generally faster, and depolarization current measurements are less influenced by high loss components within the stator insulation system, including moisture, contamination, and ECP. The conversion between time and frequency domains is therefore of particular interest. This paper investigates the dielectric properties of mica insulation and an ECP material. Corresponding mathematical models are then established. The accuracy of time–frequency domain conversion is verified through laboratory experiments and finite element method (FEM)-based simulations. The results demonstrate that the tan δ conversion error is 0.3 %–0.7 % for the coil without ECP and increases up to 4.4 % for the coil with ECP, mainly due to the nonlinear dielectric behavior of the ECP material. This indicates that the conversion method is reliable for linear insulation systems but becomes less accurate for nonlinear systems, particularly under higher test voltages. The research outcome provides a basis for the independent analysis of different material conditions in composite stator insulation systems, which is expected to improve the reliability of deterioration and aging assessment.

Distance protection is today one of the most common protection schemes and it traditionally applies phasor-based impedance estimation. However, as penetration of inverter-based resources (IBRs) grows, the phasor representation of voltages and currents is being challenged due to the fast control and nonlinear fault characteristics of IBRs, while the apparent impedance concept of Ohm's law in distance relays relies on a linear time-invariant (LTI) system. In contrast, differential equation algorithms (DEAs) are model-based approaches trying to estimate the distance to the fault and not data-driven approaches. Hence, DEAs make no assumptions about the signal waveforms but rather the governing system itself. However, DEAs are not source independent, due to inaccuracies in the assumed system model, numerical errors and a limited sampling frequency to capture the system dynamics. This paper investigates these issues by analyzing the performance of DEAs in the presence of a grid-following inverter and a grid-forming inverter. Performance comparisons are made with the classical discrete Fourier transform (DFT) impedance estimation approach. Results indicate that the DEAs can with proper filtering yield distance estimates faster and with enhanced precision compared to the DFT for distance relay applications with IBRs. A sampling frequency of 5 kHz was sufficient and neglecting the stray capacitance of the transmission line model was adequate.

Rapid expansion of solar photovoltaic (PV) installations worldwide has increased the importance of electromagnetic compatibility (EMC) of PV components and systems. This has been highlighted by interference reported from PV installations (PVI) in the Netherlands, the United States, Sweden, etc. Significant research and development efforts are seen in the domain of enhancing efficiency and economic viability of PVI, whereas the EMC aspects have received less attention and are mainly focused on the PV system acting as a victim of lightning and a victim of changing grid impedance. This article presents a review of the important EMC aspects of PVI as a disturbance source. It has the following main parts: (a) reported cases of emissions and interference from PV installations; (b) modeling and analysis of PV subcomponents from an EMC perspective; and (c) the main standards related to the topic. Mitigation techniques for improving EMC aspects of PVI are also described, along with suggested directions for future research. The compilation brings together wide-ranging sources, both for EMC engineers who want to understand the EMC context of PV systems and for PV system designers seeking to improve EMC performance.

In inverter-based resource (IBR)-dominated power systems, protection challenges have grown increasingly complex, especially for distance relays designed for traditional synchronous generator behavior. This paper explores the impact of grid-Forming(GFM) inverter settings on distance protection relay overreach, focusing on Zone I overreach triggered by Zone II faults. Then,a unified sequence network model is developed to analyze how GFM control parameters, such as active power reference and virtual impedance, influence relay-measured impedance during short-circuit (SC) faults. It is revealed that, under line-to-line(LL) fault, increasing the active power reference significantly reduces calculated impedance in line-to-ground (LG) and LL faults,heightening the risk of relay overreach. In contrast, changes to the virtual impedance cause only minor impedance fluctuations with limited impact on overreach. The theoretical analysis is validated through electromagnetic transient (EMT) simulations.

Grid-forming (GFM) inverters are anticipated to play an essential role in facilitating the integration of renewable energy in bulk power systems. The fault response of GFM inverters and its impact on traditional protection schemes are ongoing research topics. Distance protection is today one of the most commonly applied protection schemes and depends on multiple system preconditions for reliable operation—many of which may no longer hold in systems with a high penetration of inverters. This paper investigates the impacts of GFM inverters on distance protection, with the main objective of providing an improved understanding of the topic. Important interoperability issues are highlighted with simulation results and elaborated upon based on the theory behind the distance relay model and the behaviours of GFM inverters during faults. The simulations consider numerous fault types and two GFM inverters with different current-limiting control techniques in their fault-ride through strategies. Results indicate several challenges that state-of-the-art distance relays may face with GFM inverters.

One way forward in the development of protection schemes for power grids with a high penetration of renewable energy, is to assume very little about the sources and operate before the sources have had time to react to the fault. Instantaneous incremental quantities (IQs) are data signals that require very short time windows, while also being rich with information about the faulted system. Grid-forming (GFM) inverters are anticipated to take over the ancillary services currently provided by strong conventional sources to accommodate more renewable energy. They are controlled as voltage sources; hence the injected current changes near instantaneously upon a fault inception and activates the nonlinear current limiter. Consequently, the current limiter impacts not only the steady-state fault response of GFM inverters, but also their transient response, depending on the speed of the inner current control loop to realize the limited current reference. This paper investigates the performance of an instantaneous IQ-based relay with a GFM inverter, and explains from the fundamental theory of IQs, how GFM inverters may impact various IQ-based relay elements. A fault detection element, a phase-selection element, a directional element and a distance underreaching element are covered. Results indicate that, compared to traditional implementations of such relay elements, IQ-based relays can have improved dependability and security with GFM inverters. However, the current limiter still has a significant impact on the single-ended distance underreaching element.

This paper analyzes the interactions between conventional Line Distance Protection (LDP) relays and Grid-Forming (GFM) inverters. Various fault types, arc resistances and locations were investigated in a PSCAD model consisting of either a Synchronous Generator (SG) or a GFM inverter connected to the utility grid by a transformer and two transmission lines. A simple distance relay algorithm was coded in Matlab, tuned for the SG and then analyzed together with data obtained from the same system but using a GFM inverter instead. It is found that the inverter can make the relays able to trip correctly for many zone 1 faults, even if the impedance trajectory becomes unstable. However, the inverter causes several misoperations for zone 2 faults. The reasons are that the impedance trajectories either do not settle within zone 2 due to the inverter becoming unstable or they incorrectly enter zone 1. Moreover, the fault arc resistance and fault location can greatly impact the impedance trajectories obtained when the GFM inverter is connected. Additionally, operating the transmission line above its Surge Impedance Loading (SIL) also increases the risk of making the inverter unstable during faults, causing difficulties with zone 2 protection. When transferring power close to the SIL of the line, it can more reliably become stable but the fault current on the high voltage transmission line tends to be of capacitive nature and thereby making the impedance trajectories settle outside the trip zones.

Considering inverter as the source of electromagnetic emission signals in a photovoltaic (PV) plant, a comprehensive set of measurements of conducted emissions at the input and output of the inverter in a 10 kW PV plant are presented. These are particularly relevant on the backdrop of (a) ban of products in the EU market due to non-compliance and (b) the increased switching frequency in the inverters ( 100s of kHz) in near future. Specifically, the common mode (CM) and differential mode (DM) currents and voltages are measured, and their frequency domain behavior is studied. It is suggested that conducted emissions from PV can be classified into three zones: viz., extremely low frequency (ELF) zone, power frequency zone, and switching frequency zone. Important observations from this exercise are measurement of harmonic contents of current with total rated current distortion (TRD), imbalance in the output voltage and low frequency ripples in the DC voltage. Frequency domain behavior of the CM quantities is studied which throws light on important points like relation between input and output CM quantities, relation between CM voltage and CM current.

Detecting and locating local degradations at an incipient stage is very important for mission-critical high-voltage rotating machines. One particular challenge in the existing testing techniques is that the characteristic of a local incipient defect is not prominent due to various factors such as averaging with the healthy remainder, attenuation in signal propagation, interference, and varied operating conditions. This paper proposes and investigates the frequency domain reflectometry (FDR) technique based on the scattering parameter measurement. The FDR result presents the object length, wave impedance, and reflections due to impedance discontinuity along the measured windings. Experiments were performed on two commercial coils with artificially created defects. These defects include turn-to-turn short, surface creepage, loose coils, insufficient end-winding spacing, and local overheating, which are commonly seen in practice. Two practical water pumps in the field were also selected for investigation. The study outcome shows that FDR can identify and locate structural and insulation degradation in both shielded and unshielded objects with good sensitivity. This makes FDR a complementary technique for machine fault diagnosis and aging assessment.

Fault location methods are crucial for reducing fault restoration time, and thus improving a network's system average interruption duration index (SAIDI) and customer outage cost. Resonant-earthed systems pose problems for traditional fault location methods, leading to poor accuracy and a need for additional complexity. In this context, methods that detect fault direction (fault-passage indicators, FPI) at multiple points in the network may show advantages over a central distance-estimation method using fault locators (FL) of poor accuracy. This paper includes a comparative study of these two major fault location methods, comparing the reliability benefit from a varied number of FPIs or a central method. The optimal placement of the fault locating devices is found by formulating a mixed-integer linear programming (MILP) optimization approach that minimizes both outage and investment costs and assesses SAIDI. This approach has been tested on an example distribution system. However, to justify the universality of the algorithm, the RBTS reliability test system has also been analysed. The comparison of location methods and placement method of FPIs are useful for reliability centred planning of resonant-earthed distribution systems where fault location is to be used. Results show that a small number of FPIs that give accurate identification of direction may give more cost effective increase in reliability than a distance estimate by FL with typical levels of inaccuracy.