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It is essential to protect control, regulation and data storage units used in automated robotic manufacturing from the consequences of harmful electrical grid harmonics. In this paper, the effectiveness of active/passive harmonic filters built into LED lamps, test benches, and industrial robots was investigated (in the latter two cases, light and heavy load states were applied). Based on network analysis, it was found that the built-in passive harmonic filters of the luminaries were ineffective because the THD of the current was approximately two or three times the permissible value according to the IEEE 519-2022 standard. It was proved that the built-in active harmonic filter of the test bench at a heavy load worked properly, but at a light load it was not effective, while the magnitude of harmonic distortion of the industrial robot exceeded the allowed level (three times the limit at a light load, seven times the limit at a heavy load). Further measurements were also performed at three locations: an engineering room, a gearbox house machine tool, and an office room (where there were no built-in filters). The results proved that in each case, there was a significant exceedance of the limit value. Therefore, protection against harmonics needs to be installed, and then the measurements must be repeated. [ABSTRACT FROM AUTHOR]

The widespread use of static converters for controlling electrical machines and the concern for electrical power quality in industrial environments provide an opportunity for utilizing these devices to enhance the power quality. In this context, this work presents a back-to-back converter model for driving induction machines. The converter is designed to correct the power factor of the point common coupling (PCC), compensate for harmonic currents (acting as an active filter), and withstand voltage sags. The necessary control system models were developed, and an alternative implementation for these functions in the converter was proposed. The results demonstrate the technical feasibility of this solution, as the converter operated within its nominal limits by compensating for harmonics and reactive power. Moreover, the equipment showed resilience to severe voltage sags. The contribution of this paper focuses on the multifunctionality of the frequency converter for driving induction machines. It emphasizes the advantage of the inverter in improving power quality in industrial environments through reactive power compensation and harmonic current compensation, thus functioning as an active power filter. Additionally, it is worth highlighting its ability to handle voltage dips. In this regard, this paper contributes by providing an operational strategy for driving the induction machine during such transients. [ABSTRACT FROM AUTHOR]

Power quality is a major issue in the power transfer process. This is caused by disturbances such as voltage sags, voltage spikes, and harmonics. Voltage sag is the most common disturbance in the electric power system. However, the dynamic voltage restorer (DVR) is the most effective device for voltage sags. This research uses the DVR to overcome voltage sags using fuzzy logic controller (FLC) and battery energy storage system (BESS) to improve the performance of the DVR. The results showed that DVR using FLC improved the quality of voltage recovery compared to BESS because FLC injected a greater voltage of 0.0991 pu than BESS. [ABSTRACT FROM AUTHOR]

This research paper addresses the issue of enhancing the operational availability of NPC three-level converter-based high-voltage direct current (HVDC) transport systems during alternating current (AC) grid fault conditions. During short-circuit faults in power transmission lines, voltage sags can occur, causing fluctuations in the DC link voltage of converter systems. These voltage sags have the potential to induce a reversed power flow and trip the VSC-HVDC transmission system. The objective of this paper is to develop a nonlinear control technique that investigates the fault ride-through (FRT) capability of VSC-HVDC transmission system characteristics during voltage sag events. To achieve this, we conduct semi-experimental investigations using Processor-in-the-Loop (PIL) simulations and analyze the results. Symmetrical and asymmetrical voltage sag events with different remaining voltages are applied to an AC grid, and their effects are observed for varying durations. The proposed nonlinear control technique aims to mitigate the impact of voltage sags on the operational availability of HVDC transport systems. By analyzing the semi-experimental results, we aim to gain insights into the FRT capability of the VSC-HVDC transmission system. [ABSTRACT FROM AUTHOR]

Power quality is one of the problems in power systems, caused by increased nonlinear loads and short circuit faults. Short circuits often occur in power systems and generally cause voltage sags that can damage sensitive loads. Dynamic voltage restorer (DVR) is an efficient and flexible solution for overcoming voltage sag problems. The control system on the DVR plays an important role in improving the quality of voltage injection applied to the network. DVR control systems based on particle swarm optimization (PSO) and artificial neural networks (ANN) were proposed in this study to assess better controllers applied to DVRs. In this study, a simulation of voltage sag due to a 3-phase short-circuit fault was carried out based on a load of 70% of the total load and a fault location point of 75% of the feeder’s length. The simulation was carried out on the SB 02 Sibolga feeder. Modeling and simulation results are carried out with MATLAB-Simulink. The simulation results show that DVR-PSO and DVR-ANN successfully recover voltage sag by supplying voltage at each phase. Based on the results of the analysis shows that DVR-ANN outperforms DVR-PSO in quality and voltage injection into the network. [ABSTRACT FROM AUTHOR]

This paper presents the development of advanced adaptive controller for dynamic voltage restorer (DVR) to regulate the power quality connected to a distribution system. Various voltage quality issues have been generated as distorted grid conditions are responsible for abnormal operation of the consumer electronics and domestic household appliances. Therefore dynamic voltage restorer (DVR) is employed for elimination of voltage quality issues in the distribution lines. To improve the performance and efficiency of DVR, an improved robust SRF (IR-SRF) controller has been developed in this paper for the mitigation of voltage quality issues under weak grid conditions. In the developed IR-SRF controller, moving average filter (MAF) has been utilized for better extraction of ripples from the sensed voltage. For better grid voltage signal synchronization with the proposed controller under highly dynamic condition, an enhanced Phase Locked Loop (PLL) scheme has been utilized instead of conventional PLL. For the regulation of DC link voltage under dynamic environmental conditions, MAF based fuzzy tuned PI controller has been utilized with the ability to adapt the situation. The developed controller has been simulated and results for various grid voltage conditions have been analyzed. The obtained simulation results are compared with conventional control algorithm to validate the efficacy. The efficiency of the proposed controller has been validated using the experimental results. [ABSTRACT FROM AUTHOR]

A single voltage sag source may trigger multiple power quality monitors to record waveforms resulting in the redundancy of voltage sag monitoring data. To address this problem, a homology detection method for voltage sags based on a space vector and maximum mean discrepancy was proposed, which is conducive for locating accurate voltage sag sources and inverting sag event propagation paths. First, a waveform transformation method based on a space vector was proposed to eliminate the influence of the transformer on voltage sag propagation. Second, a two-sample test method for voltage sags based on the maximum mean discrepancy was proposed to detect whether any two voltage sag waveforms are caused by the same voltage sag source. The connected graph method was used to identify the number of sag sources; this avoids the difficulty in accurately selecting the number of clusters. The measured and simulation data of the IEEE 30-bus system were used for verification, and the results verified the correctness and effectiveness of the proposed method. The proposed method can be implemented in a power quality monitoring system as a pre-processing step to support related research activities.

In an industry, analysis of load disconnection indices is essential to determine the annual economic loss because of the high cost of load interruptions. With the interconnection of PVs in modern industrial networks, this analysis gets more complicated due to controls associated with PV inverters for reactive power support. Hence, the paper proposes a unique method for such analysis that relies on dynamic simulation to calculate load disconnections. The dynamic simulation allows the integration of the voltage tolerance curve of loads and controls associated with PVs making the scenarios more realistic. The indices are determined by employing statistical experiments which account for annual variability of daily solar and load profiles with the least possible runs of dynamic simulation. The proposed method is applied to calculate load disconnection indices for an IEEE 399-1997 industrial distribution system. A comparative analysis of the cost of disconnection under a variety of solar penetration and operation scenarios is presented. Network planners and reliability engineers can potentially utilize the proposed method to identify PV inverter controls, placement, and size effects on the annual load disconnection indices due to voltage sags. • Method for calculation of annual Load loss indices for Industrial distribution network consisting distributed PVs. • Simulation case study on IEEE 399-1997 industrial distribution system. • Analysis showing economic benefit of having distributed PV w.r.t. annual load loss due to voltage sags. [ABSTRACT FROM AUTHOR]

The levels of renewable power, particularly from photovoltaic power plants (PVPPs), injected into different electrical systems bring with them a series of fundamental technical changes that need to be addressed. Power quality (PQ) and voltage stability are the ones of great concern. In this sense, voltage sags commonly affect many users. For this reason, the need to implement dynamic voltage support or fault ride through (FRT) schemes and to inject reactive current into PV generators is evident taking into account the experience of network operators. However, this strategy presents important limitations to comply with the additional requirements established in the IEEE 1547 standard. The standard establishes the need in the continuity of the PV inverter contribution opposite successive voltage sags, delay in voltage recovery and island operation, among others. Therefore, any FRT strategy must consider these additional aspects in order to effectively mitigate voltage events. In the present work, a novel methodology for dynamic grid support based on adaptive FRT schemes is described. The proposed dynamic control will be the result of a detailed voltage support strategy. The methodology implemented will consider all the fault conditions that can lead to the occurrence of voltage sags and similar events. The results of those tests carried out for a PV generator linked to the IEEE 33 buses system show that the implemented strategy operates according to the requirements of current standards for different voltage events. [ABSTRACT FROM AUTHOR]

Information about the interruption probability of sensitive industrial processes is important for devising mitigation strategies against voltage sags. However, the existing method for assessing interruption probability is less practical, owing to the difficulty in collecting technical information and data from industrial users. This study proposes a low-data-dependence assessment method for the interruption probability of an industrial process. First, it establishes a general assessment model for the tripping probability of sensitive equipment. Second, it proposes a parameter estimation method to determine the probability density function of the voltage tolerance curve of sensitive equipment in the uncertainty region, according to the equipment trip information recorded by the user. This improves the accuracy of the assessment. Third, this study establishes the logic function expression form of the industrial process structure to describe the relationship between the process interruption probability and the equipment tripping probability. Finally, it proposes a solution method for the process interruption probability based on a heuristic search algorithm, which overcomes the problem of technological process information acquisition, such as the logical relationship among various equipment in the industrial process. The rationality and applicability of the proposed method are verified through a case study. [ABSTRACT FROM AUTHOR]

With the developing technology, the need for electrical energy is increasing day by day. Many power quality problems such as voltage spikes, voltage sags, harmonics can occur during the distribution of electrical energy. Systems such as STATCOM and DVR are used to correct these problems. Thus, the quality of the energy is tried to be increased. In this study; By connecting a load to a distribution line where voltage sag occurs as an error, the condition and sensitivity of this load against voltage sagging, and then the condition and sensitivity when using DVR as a corrector, were investigated. The situation of DVR against the loads has been analyzed in depth. Relevant data have been transferred to MATLAB / Fuzzy Logic application and analyzed.

This research paper addresses the issue of enhancing the operational availability of NPC three-level converter-based high-voltage direct current (HVDC) transport systems during alternating current (AC) grid fault conditions. During short-circuit faults in power transmission lines, voltage sags can occur, causing fluctuations in the DC link voltage of converter systems. These voltage sags have the potential to induce a reversed power flow and trip the VSC-HVDC transmission system. The objective of this paper is to develop a nonlinear control technique that investigates the fault ride-through (FRT) capability of VSC-HVDC transmission system characteristics during voltage sag events. To achieve this, we conduct semi-experimental investigations using Processor-in-the-Loop (PIL) simulations and analyze the results. Symmetrical and asymmetrical voltage sag events with different remaining voltages are applied to an AC grid, and their effects are observed for varying durations. The proposed nonlinear control technique aims to mitigate the impact of voltage sags on the operational availability of HVDC transport systems. By analyzing the semi-experimental results, we aim to gain insights into the FRT capability of the VSC-HVDC transmission system.

Voltage sags and swells are significant issues of power quality. In order to alleviate voltage sags and swells on sensitive loads, a method is proposed to evaluate the bus and grid structure based on voltage sags/swells using voltage ellipse parameters. The voltage matrix is established for voltage sags/swells caused by four types of short-circuit faults. The voltage matrix can be converted into a minor axis matrix and a major axis matrix using the Clarke transformation, and accordingly a voltage sag matrix and a voltage swell matrix can be obtained with the interaction between voltage sags/swells and voltage ellipse parameters. The key indicator and the vulnerability indicator of bus as well as the indicator of grid structure are defined according to the voltage sag matrix and swell matrix. The buses are sorted using the indicators proposed, and the result of bus sorting provides a basis for selecting the buses that need to be managed to alleviate voltage sags/swells. The experiment results in IEEE 30-bus system and IEEE 57-bus system show that these indicators facilitate the selection of the access point for newly-added sensitive loads, and provide a theoretical guidance to the mitigation of voltage sags and swells, and help to accomplish the planning, operation and transformation of power grid.

The power systems are transitioning to renewables. The power system infrastructure digitalization is the next stage in this evolution. A digitalized future distribution network (DN) has an opportunity to evolve into real-time analysis based on a huge volume of data. Such real-time analysis will be feasible through the integration of the theoretical background of fault analysis and machine learning techniques. Fault location is one of the major issues to improve reliability indices. The voltage sags characterization is used to locate faults in the DN. This article presents a methodology to characterize voltage sags using fault analysis and deep convolutional neural networks. The voltage divider model allows the characterization and the discrete wavelet transform is used in signal processing. The machine learning and deep learning models used allow estimating the sag magnitude, fault location, phases involved, duration, and impact of distributed generation (DG) in each event. The IEEE 13-node test feeder including DG was used to validate the effectiveness of the proposed methodology. This paper provides a way to handle a Big Data stream in the DN control center and to efficiently locate faults in several operational scenarios. [ABSTRACT FROM AUTHOR]

The doubly fed induction generator (DFIG) and back-to-back converter are very sensitive to power quality disturbances in grid-connected wind energy conversion systems (WECSs). Special attention has been given to protect the system from voltage sags, considering the introduction of several low-voltage ride-through (LVRT) techniques in the literature. However, only few works have really analyzed the behavior of power semiconductors during such phenomena in terms of the thermal stresses, whereas the existing studies are focused on balanced voltage sags only. In this context, this work presents a thermal profile analysis of power semiconductors in the grid-side converter (GSC) and rotor-side converter (RSC) considering a DFIG-based WECS submitted to symmetrical and asymmetrical voltage sags. The system is modeled using PLECS software and results on a 2.0 MW system are presented and thoroughly discussed. The results show that it is possible to meet the ride-through requirements during both balanced and unbalanced sags in terms of acceptable thermal stresses on the semiconductors as long as the back-to-back converter and its respective control system are properly designed. [ABSTRACT FROM AUTHOR]

Voltages sags have become the major issue that prevents the customers from getting high-quality power supply, and the dynamic voltage restorer (DVR) is considered an effective way to solve this issue. In this work, a method for voltage sag detection implemented in the time-domain is firstly addressed, which features highly accurate and fast response. Then, the dual-loop voltage–current control for the DVR is intensively investigated. Specifically, the optimal tuning of the inner current loop to achieve the maximum active damping is approached, and the voltage controller implemented in the discrete-time is developed. Tuning of the voltage loop based on critical damping is also approached, which features reduced settling time and avoidance of overshoot. The simulation and experimental results have verified the effectiveness of the proposed method for detection and management of voltage sags.

Summary: This study aims at evaluating the impact of the current changes in power generation on voltage sags (dips) in HV networks. This changing process started about 15 to 20 years ago and concerns the increasing diffusion of inverter‐based generators (IBGs) and the contemporary reduction of synchronous generators (SGs). A major effect of this ongoing transition is the reduction of short‐circuit currents/powers in the electrical grid, with consequent changes of the voltage sags caused by faults in the grid. This study is the first step of a larger research project involving the whole Italian HV network. The goal is a quantitative evaluation of the effects of the transition from an "old" power park (with only SGs) to a present power park (with less SGs and IBGs) on voltage sags in a HV network. In this paper, the analysis concerns the real HV network of Sardinia, which comprises 262 nodes altogether. Despite some simplifying assumptions, the results—consistent with the sag classification of the Standard EN 50160—give a clear and original quantitative evaluation of the negative effects of the considered change of production units on voltage sags. [ABSTRACT FROM AUTHOR]

The power electronic transformer (PET) is an emerging technology that is quickly becoming a key component of the next-to-come power distribution networks (PDNs), due to its versatility on energy management, as well as, the improvement on the quality of the energy. PDNs are characterized by their unbalanced conditions, causing that PETs driven by conventional dq0 controls introduce current distortions on the primary winding of the transformer. Such distortion is evidenced in the 2 $\boldsymbol{\omega }$ oscillations of ${V_d}$ and ${V_q}$ acting as harmonic sources. In this sense, this article proposes a novel control approach for PETs. The key idea behind of this proposal consists of operating each phase independently, which is achieved through the enclosed rectification and the mitigation of the 2 $\boldsymbol{\omega }$ oscillations in a dual active bridge topology. The attained advantages by this control scheme are: a) balancing of the primary winding currents; b) unitary power factor; c) negligible harmonic distortion; and d) 2 $\boldsymbol{\omega }$ oscillation mitigation on the dc bus. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering & Architectural Faculty of Eskisehir Osmangazi University / Eskişehir Osmangazi Üniversitesi Mühendislik ve Mimarlık Fakültesi Dergisi is the property of Eskisehir Osmangazi University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Estimating the amplitude of the grid voltage is important for low-voltage ride-through (LVRT) operation in single-phase systems. It requires fast dynamics and is typically achieved through a phase-locked loop (PLL). This paper proposes a fast amplitude estimation (FAE) method without a PLL structure. The grid voltage is formulated as a vector sum of sine and cosine functions, resulting in an algebraic model with two unknown parameters. The proposed FAE identifies the two parameters, and thus achieving a fast fault detection (i.e., the amplitude estimation). Experimental results verify the performance of the proposed method in terms of fast dynamics.

The paper presents dynamic and transient behavior of the doubly fed induction generator (DFIG) in the wind farms in the normal and faulted grid respectively. When Voltage sag or any fault occurs in the network, the variables in the Doubly Fed Induction Generators are varying severely. If a voltage sag occurs, active and reactive power generated by the DFIG start to oscillate. The DC-link voltage will be bigger and will have fluctuation, and the rotor current will increase. In this paper, proportional integral (PI) controllers are used to control the DFIG in the wind farms for driving of the electronic devices including Rotor Side Converter (RSC) and Grid Side Converter (GSC) and controlling the active and reactive power of DFIG. PI parameters are tuned by particle swarm optimization algorithm (PSO). Whereas the model of DFIGs and electronic device in the paper are nonlinear so PI controllers cannot protect and control the DFIG as well. Hence, effect of PI parameters is investigated on the DFIG with simulating in MATLAB software. Also, low voltage ride through (LVRT) feature for DFIG is explored in presence of PI controllers. The results of the simulation present DC-link over voltage and rotor and stator over current in the DFIG. In addition, it will explore the effect of proportional integral controllers when three-phase short circuit fault occurs.

In several countries, during the grid faults, the large distributed generation (DG) is expected to support the voltage at the point of common coupling, as per the prevailing grid code. Large DGs are connected to a high voltage transmission line, where the voltage support is achieved by supplying the reactive power. The recent research trend, however, is toward voltage support by the medium size DGs that are connected to the resistive-inductive grid. The resistive-inductive grid requires both active and reactive power for voltage support. The existing voltage support schemes (VSS) for the resistive-inductive grid impose certain constraints on the proportion of active and reactive components of currents that adversely affect the performance. This article proposes a novel VSS that does not impose such constraints and provides enough compensation when available active power is not sufficient. It also provides the required compensation with much less active power curtailment when available active power is high. That is, the proposed scheme, in both cases, helps in achieving the same voltage compensation in spite of the nature of the injected current being significantly different. This feature is clearly demonstrated graphically using the vector diagram. The reference current expressions for the proposed VSS are analytically derived. The simulation and hardware results demonstrate that the proposed VSS achieves its objectives. [ABSTRACT FROM AUTHOR]

The financial loss assessment of industrial users due to voltage sag and short interruptions is important for distribution network performance evaluation and power quality enhancement scheme design. Existing assessment models are highly dependent on complete industrial process information, which is difficult to obtain. To solve this problem, a new sensitive industrial process model needs limited industrial processes information is proposed. In the proposed model, the sensitive equipment is modeled by voltage tolerance curves (VTC), and the truth table is used to characterize the logical relationship. As the industrial process information is limited, the boundary value of VTC and partial logical function values of the truth table are uncertain. To determine the model's uncertain parameters objectively, an optimization model with the goal of minimizing the deviation of the assessment loss is constructed, and the details of the algorithm are also given out. An example based on some investigation data is given to verify the effectiveness of the model and show how to use it. [ABSTRACT FROM AUTHOR]

Droop-controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: 1) guarantee current limitation with maximum power injection during grid faults and 2) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD and then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to the existing control schemes that require adjustment of both the real and the reactive powers. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief. [ABSTRACT FROM AUTHOR]

This article proposes a voltage regulation scheme for distributed energy resource-connected distribution networks based on static synchronous compensator to compensate for both short-term and long-term voltage sags. The proposed voltage regulation scheme provides a fast and precise control on the nodal voltage, with no need to load shedding, capacitor switching, transformers, or power management schemes. The principles supporting the proposed voltage regulation scheme are discussed and the effectiveness of the proposed control is demonstrated through time-domain simulation of a two-unit test distribution network in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

Summary: Constant power (Scte) loads such as large induction motors or power converters in customer facilities of distribution systems have a significant effect on the fault currents and thus on the relative location of voltage sag sources. This paper evaluates the methods for determining the relative location of voltage sag sources at the point of common coupling (PCC) of Scte loads. The results of simulating 800 voltage sag events on IEEE 13 bus unbalance system (modelled by ATP) show that many of the methods fail at classifying upstream (US) faults at the PCC of Scte loads. Therefore, a new method is introduced using the ratio of current magnitudes and changes of power angle in positive sequence components, during and before the voltage sag, with a determined threshold for the currents and a specified tolerance for the power angle obtained from voltage sags generated by different fault types. The results obtained in the proposed index for similar simulation cases show that the method applied to monitors installed at the PCC of the Scte loads is very accurate. Also, the proposed index is effective for higher resistance faults and changes in the short circuit capacity (SCC) of the main grid. [ABSTRACT FROM AUTHOR]

This article presents a new approach for classification and localization of voltage sags triggered by symmetrical and unsymmetrical faults in electricity networks. The proposed method utilizes five unique parameters of three-dimensional (3-D) polarization ellipse (PE) obtained from the mapping of three-phase voltage signals in 3-D coordinates. The decision boundaries, which are set by the parametric equations exploiting the five ellipse parameters, namely, elevation, azimuth, tilt angles, semi-major axis, and semi-minor axis, are used to classify the fault-initiated sags. The localization of faults is conducted from the parametric equations demonstrating the variation of ellipse parameters with respect to distance from the fault-location. These parametric equations would be helpful to make decision faster and thereby, eliminate the uncertainty in decision-making process. The applicability of the proposed method is tested and validated using real data measured by DOE/EPRI, the recorded waveforms of IEEE 1159.2 Working Group, and the data extracted from the simulation of faults at several feeders of IEEE 14-bus test system using RSCAD and Real Time Digital Simulator (RTDS) platform. [ABSTRACT FROM AUTHOR]

Voltage sags mitigation has become a significant concern for utilities due to the pressure from both customers and regulators recently. This paper proposes a novel multi-objective optimization formulation for STATCOM allocation problem to mitigate voltage sags. The allocation of STATCOM is formulated as a multi-objective optimization problem (MOP) with two conflicting objectives including the voltage sag performance and annual investment costs of STATCOM. To analytically evaluate the voltage sag performance in the optimization process, a mathematical voltage sag profile calculation model of power system with multiple STATCOMs is proposed and solved by nonlinear programming method. System Average RMS Variation Frequency index-X index is chosen to quantify the voltage sag performance for optimization. Sensitivity-based candidate bus selection is also performed as an essential step to reduce problem size. An improved multi-objective evolutionary algorithm (MOEA) called SPEA2+SDE (Strength Pareto Evolutionary Algorithm with shifting density estimation) is utilized to solve the proposed MOP model, so a set of Pareto optimal STATCOM allocations for voltage sag mitigation can be derived demonstrating the trade-off between two objectives and better supporting the decision-making. IEEE 30-bus system and IEEE 118-bus system are tested to demonstrate the effectiveness and feasibility of proposed method. [ABSTRACT FROM AUTHOR]

—Since voltage interruptions, sags and swells are Power Quality (PQ) disturbances with great economic impact, studies that seek alternatives to mitigate its effects have been conducted extensively. The PQ Brazilian standard names these disturbances as Short-Duration Voltage Variations (SDVVs), classified them in terms of magnitude, duration, and frequency of occurrence. These parameters are used to calculate the Impact Factor (IF) of SDVVs, a severity-characterization index of their incidence. In this context, this work assesses the influence of three-phase transformer winding connection and neutral grounding on the quantities of SDVVs and the IF observed by an industrial consumer in a distribution system. Fault simulations at ATP-EMTP perform the study, considering two different winding connections for the secondary transformer and applying a grounding resistance to the neutral of the main transformer of the distribution system. The SDVVs are verified in two different ways: phase-to-ground and phase-to-phase voltages. It is observed that there are differences in these quantities and for the value of the IF due to the winding connection of the secondary transformer, the value of the neutral grounding resistance of the main transformer and the ways of voltage verification. [ABSTRACT FROM AUTHOR]

Asymmetrical faults in the grid cause oscillations in dc‐link capacitor voltage or overcurrent in the converters. In this study, an optimal controller with a fixed structure is introduced based on the command generator tracker (CGT) to restrict the fault currents and increase stability performance under asymmetrical grid faults. For this purpose, first, the integrated dynamical model including grid‐connected converters and dc‐link voltage are introduced. Then, using the optimal control techniques, a CGT is designed to enhance the fault ride through performances. Next, the proposed controller is applied to the system including converter and dc‐link at the source side and the dynamic performances of the closed‐loop system under different fault conditions are determined. Finally, the simulation results are compared with the other conventional controllers and the dynamic performances of the proposed controller are evaluated. The results indicate that the proposed controller provides stability for the system and satisfies allowable constraints of converter current for different grid fault types such as voltage sags, asymmetrical and symmetrical faults. [ABSTRACT FROM AUTHOR]

The levels of renewable power, particularly PV, injected into different electrical systems in Latin America, bring with them a series of fundamental technical changes that need to be addressed. Among them, power quality is one of the main concerns. In this matter, the voltage sags are usually that most affect users. For this reason, the need for the implementation of dynamic voltage support (FRT) schemes and the injection of reactive current for PV generators is evident from the expertise of network operators. In the present work, the main technical/conceptual characteristics for the implementation of dynamic voltage support schemes with reactive injection were described. Likewise, the similarities and differences of the several grid codes implemented in Latin American countries were analyzed, as well as its main points to improve. [ABSTRACT FROM AUTHOR]

The paper presents a vector control with two cascaded loops to improve the properties of Dynamic Voltage Restorer (DVR) to minimize Voltage Sags on the grid. Thereby, a vector controlled structure was built on the rotating dq-coordinate system with the combination of voltage control and the current control. The proposed DVR control method is modelled using MATLAB-Simulink. It is tested using balanced/unbalanced voltage sags as well as fluctuant and distorted voltages. As a result, by using this controlling method, the dynamic characteristics of the system have been improved significantly. The system performed with higher accuracy, faster response and lower distortion in the voltage sags compensation. The paper presents real time experimental results to verify the performance of the proposed method in real environments.

This study focuses on sensitivity of electronic circuit in low‐voltage release to voltage sags based on a large‐scale test results. Although studies about ride‐through capability of some electronic devices during voltage sags have been carried out, there is few research available on sensitivity of electronic circuit in low‐voltage release to voltage sags. Operation principle and working states of electromagnetic structure are discussed. Subsequently, a detailed test scheme is proposed based on latest standards and several kinds of 220 V low‐voltage releases have been tested. Test results indicate that output waveform of electronic circuit under voltage sags can be classified into two types, which shows a clear correspondence with working state of electromagnetic structure and tripping condition of low‐voltage release. Finally, six working modes are presented to analyse the relationship between output waveform of electronic circuit and the magnitude and duration of voltage sags in details.

Abstract Virtual synchronous generator (VSG) is grid-friendly for integrating distributed generations (DGs) since it can emulate the operation mechanism of traditional synchronous generator (SG). However, the traditional VSG control strategy, which is mainly suitable for balanced voltage conditions, may lead to power oscillations, current unbalance and even overcurrent under unbalanced voltage sags. To overcome this difficulty, a flexible unbalanced control with peak current limitation for VSG under unbalanced operating conditions is proposed. Based on the basic VSG control algorithm, the control strategy integrates two novel control modules, which are current reference generator (CRG) and power reference generator (PRG). The proposed control strategy can flexibly meet different operation demands, which includes current balancing, constant active or reactive power. And the injected currents are kept within safety values for a better utilization of the VSG power capacity. Furthermore, the experimental platform is built. Experimental results demonstrate the validness and effectiveness of the proposed control strategy.

The large current generated by a direct start of the super-large-capacity high-voltage induction motor would have a huge impact on the power grid as well as the motor itself. Traditional soft starters have the shortcomings of discontinuous adjustment, voltage sags, sudden torque mutation, secondary current impact and high cost. To resolve this issue, the authors propose a novel coupled autotransformer and magnetic-control (CATMC) soft-start method. The structure of the new CATMC soft starter combines the functions of the autotransformer and magnetic control reactor via an innovative electric and magnetic circuit design. In this study, the authors analyse the magnetic circuit structure and working principles of the CATMC soft starter. Then, to validate its principle and performance, the authors conduct a simulation study using ANSYS software and design and test an 18 MW/10 kV CATMC soft starter prototype. The simulation results demonstrate that the CATMC soft starter effectively avoids secondary current impact and constrains the motor starting current to

Due to a limitation in the magnitude of the three-phase output inverter currents, the output active power of the photovoltaic (PV) unit has been de-rated during low voltage ride through, which brings great instability risk to the power system. With the increase in the penetration rate of new energy, the impact of the power shortage on the system transient stability increases. It is of great significance to analyze the impact of this transient power shortage on system stability. This article explores methods to improve the active power output capability of photovoltaic units during low-breakthrough periods. A transient simulation model of a grid-connected PV generator with low-voltage ride-through (LVRT) capability is presented, under the condition of meeting the overcurrent capacity of the PV inverter and the requirement of dynamic reactive power support supplied by the PV generator specified in the China grid codes (GB/T 19964-2012) during grid fault. An example system with high PV penetration is built. The change principle and influencing factors of PV transient active power output are analyzed. The simulation model is designed in PowerFactory/DIgSILENT, and several types of three-phase voltage sags are performed in simulation to assess the impact of the active current reference calculation method and the maximum inverter output current (Imax) limit value on the PV active power output. According to the three indexes, namely the maximum active power of PV unit during the fault, the power improvement gradient and the power surge after the fault is cleared. Simulation results showed that using the orthogonal decomposition method to calculate the active current reference can make full use of the current capacity of the converter. Setting Imax to 1.1 rated current of photovoltaic inverter (IN) can reduce the cost-effectiveness ratio of the transient active power output of the PV unit. Therefore, we aim to improve the unit’s transient active power output capacity and realize the optimal effect of improving the transient active power shortage of the system.

This study presents a new technique for power quality (PQ) disturbance detection. The technique focuses on voltage sags and interruptions that are related to various faults, i.e. transmission line, feeder, and transformer faults. A maximal overlap discrete wavelet transform‐based PQ detection algorithm is proposed to provide accurate points of disturbance initiation and recovery. The proposed PQ detection algorithm is robust even without a detection threshold and independent of the sampling frequency of PQ recording. In consideration of the presence of noise conditions, the preprocessed PQ waveforms are converted into 2D binary vectors using space vector transformation. Then, an improved stacked sparse denoising autoencoder combined with supervised backpropagation training is proposed as a robust classifier. Results show that the proposed method is suitable for detecting various types of PQ disturbances and possesses high recognition accuracy despite insufficient training samples. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive S transform (AST) to extract the feature vectors of voltage sags. With the effective window width matches the Fourier spectrum of sag signals, the standard deviation σ of Gaussian window may be determined as well. The narrowest and the widest window width of AST are obtained without additional parameters and iterative computing. Then, the optimal frequency resolution and time resolution are got respectively. Compared with ST, AST provides better time–frequency resolution to extract more precise feature vectors of eight types of voltage sags. Based on the time–frequencyrepresentation of AST, five disturbance features are extracted to construct the feature vector in this paper. In addition, four machine learning classifiers and two fuzzy clustering classifiers are used to analyze the validity and redundancy of these features. Through analyzing the classification accuracies and time costs of these classifiers with different training sets and different level of noise, it can be concluded that the machine learning classifiers perform better in classification accuracy and stability than fuzzy clustering classifiers. • A adaptive S-transform (AST) is proposed to extract the feature vectors of voltage sags. • The window width is optimized depend on signals without iterative computing. • Only five discriminative disturbance features are extracted to construct the feature vector. • Three features combinations are input to six classifiers to analyze the validity and redundancy. [ABSTRACT FROM AUTHOR]

The impact of photovoltaic distributed generation (PVDG) in the voltage sag indices of low-voltage distribution systems is assessed in this paper. A sensitivity study of these indices in relation to the penetration level of PVDG is performed considering the effect of the feeder sections loading on their failure rates. It is showed that, when this dependence is neglected, the indices are predicted with errors, especially for scenarios with large integration of PVDG. This study was performed using clustering techniques to model the uncertainty of solar irradiation, and the state enumeration method (SEM) is used to evaluate voltage sags indices. The results obtained by the clustering method and SEM were compared with those obtained with Monte Carlo Simulation based on the beta probability density function (pdf) model for solar irradiation. This comparison showed that the clustering model can obtain indices with precision comparable to those obtained with the beta pdf model, but with a significant reduction in CPU time. [ABSTRACT FROM AUTHOR]