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Compared to conventional ac cables, the current rating of HVDC cable circuits is more complicated because the permissible current is not only limited by the operating temperature, but also by the electrical stress within the insulation. Therefore the IEC60287 current rating standard is considered insufficient for some HVDC applications because it fails to include the stress limitation. This paper presents an analytical methodology to calculate an electrical stress-limited rating for HVDC cables, which considers the maximum dielectric stress as the current limiting factor. This paper represents the first part of a project which has comprehensively reviewed cable rating calculations for mass impregnated HVDC cable systems. [ABSTRACT FROM PUBLISHER]

An extensive revision of the IEEE C37.011 ‘Application Guide for TRV for AC High-Voltage circuit Breakers’ has been in process for the last four years. The main aim was to introduce the two- and the four- parameter description of the transient recovery voltage (TRV) as used in IEC Switchgear standards that have been introduced in the revisions of Standards C37.04 and C37.06. These two standards are being concurrently revised, but IEEE C37.011 provides the necessary explanations on TRV characteristic in advance. Non-specialist engineer users of the guide will better understand these standards. [ABSTRACT FROM AUTHOR]

Negative-sequence current is required to be intentionally injected to support the power grid or compensate the imbalanced load in asymmetrical faults utilizing STATCOM. However, the extreme compensation range of negative-sequence current injection becomes complicated when grid voltage becomes asymmetrical. This article presents a novel off-line strategy for negative-sequence current compensation under two typical kinds of asymmetrical grid faults — single-phase voltage drop and double-phase voltage drop, with the following two benefits simultaneously: maximizing negative current compensation capacity; avoiding iterations and reducing computation burdens. In this paper, the mechanism of DC voltage balancing among three phases in STATCOM is analyzed and a new identification method of negative-sequence current compensation range is explored out. To simplify the calculation, this paper takes the ratio of negative-sequence reactive (Iq-) and active (Id-) current as a variable (s). In this way, the relationship between the extreme compensation range, asymmetrical voltage sag depth of power grid h is drawn with MATLAB. Different from the previous iterative method, the off-line identification method is applied to avoid huge iterations. By fitting the curve, the precise ranges of negative-sequence current compensation under different asymmetrical grid faults are approximated as simple polynomials, thus significantly reducing the computation burdens. Simulation and experimental results verify the practicability of the proposed compensation method. [ABSTRACT FROM AUTHOR]

This paper presents an online method to monitor the conditions of the substation grounding grid. The proposed method utilizes phase-to-ground faults occurring in downstream feeders as test current sources and the neutral of a specially selected feeder as the electrode for GPR (ground potential rise) measurement. The measured fault currents and GPRs are then used to estimate substation grounding grid impedance. The method, once established, can monitor the grounding grid impedance continuously and automatically, and the trend of the impedance change can be observed. As a result, the proposed method can yield much more reliable information on the substation grounding conditions. The method also overcomes many disadvantages of the offline methods. The proposed idea, algorithm, and hardware setup are described in the paper. Factors contributing to measurement errors are investigated in detail. [ABSTRACT FROM AUTHOR]

The surface voltage of an insulator assumes importance for understanding tracking and flashover characteristics. Until now, determination of surface voltage of insulating surfaces is left to simulation software and such simulations are left unverified experimentally, leading to uncertainty of the results, which may sometimes be erratic with improper choice of boundary or initial conditions or inaccurate material properties, nonlinearities etc. The paper presents a novel method, using which the surface voltage of any insulating surface such as insulator strings or transformer bushings can be measured. Unlike a conducting surface, the measurement of insulating surface voltage using voltage dividers would be erratic as the measuring arrangement itself will vary the voltage division due to the local unknown impedance between the insulated surface and hv conductor. Recently a method for measurement of surface voltage of an insulated conductor using cylindrical strips was reported. In this paper, based on analytical derivations, a more generalized method of measurement, applicable to any point on an insulating surface of arbitrary geometry is proposed using circular-disc strips. The proposed experimental method is applied to insulator strings and transformer bushings and validated by simulation of the entire systems. The simulation and experimental results are in excellent agreement. [ABSTRACT FROM AUTHOR]

Electrical equipment of the floating nuclear power plant operates in a long-term vibration environment. Ground faults occur frequently and are often accompanied by arc, a reliable arc suppression algorithm for the generator stator winding is necessary. In this paper, an active arc suppression method is proposed by connecting a controllable voltage source to the generator neutral point. Through regulating the neutral point voltage to be equal in amplitude and opposite in phase to the fault potential, the fault point voltage can be clamped to a low level to achieve arc suppression. Since the generator's phase potential contains certain harmonic components, the controllable voltage source includes fundamental and third-order harmonic components. To accurately calculate the output voltage, online calculation methods for the fault location, fault potential and grounding capacitance are proposed. The accuracy of the fault location method is verified by simulation and dynamic test. And simulation results based on PSCAD/EMTDC software show that the proposed method can achieve effective arc suppression at different ground fault conditions. [ABSTRACT FROM AUTHOR]

Direct lightning poses a serious threat to the safety of power systems. In recent years, the lightning fractal model, which can reflect the fractal characteristics and randomness of lightning leaders, has been gradually applied to direct lightning protection for substations, transmission lines, etc. However, since few studies have linked simulations with experiments, the reliability of the lightning fractal model in terms of attachment probability prediction lacks experimental verification. In this paper, a series of attachment tests are carried out under a rod-rod gap. The equivalence between simulation tests and natural lightning is discussed, and the influencing factors of the attachment results are investigated. The attachment probability is simulated and compared to the test value under each test configuration. The test results show that the fractal dimension of discharge channel is consistent with the observation results of natural lightning. Under different configurations, the test value of attachment probability basically conforms to the simulation value. In general, the dispersion of the test results is slightly greater than the simulation results, but the average probability differences are all within 10%. The test results strongly indicate that the lightning fractal model is of good reliability in the prediction of strike probability. [ABSTRACT FROM AUTHOR]

Phase–locked loop (PLL) is commonly used for the grid-connected voltage-source converter (VSC) to synchronize with the grid. Under weak grid conditions, it has been found that the frequency-dependent characteristic of the network impedance inherently introduces a positive feedback loop into PLL. Previous studies focused on the PLL analysis intuitively believe that the positive feedback loop will deteriorate the dynamic performance of the PLL. However, this conclusion is not rigorous since only the dynamic of the PLL is considered and the theoretical analysis is insufficiency to reveal the influence of the positive feedback loop on system stability. In this paper, the impact of the frequency-dependent network on VSC stability is quantified in terms of the overall system based on the damping torque analysis. It is found that the influence of the positive feedback loop on stability depends on the parameters of the PLL and the damping of the dominant eigenvalues. Besides, an angular frequency feedback scheme is proposed to eliminate the adverse effect of the frequency-dependent network and enhance the stability of the VSC under weak grid conditions. Time-domain numerical simulations and reduced-scale experiments are performed to validate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC), owing to the presence of a large number of submodules (SMs) and energy storing elements, requires a large footprint and initial investment. To address these issues, this paper proposes the switched modular multilevel converter (SMMC). It consists of two major elements; the chain-links (CLs), consisting of cascade connection of SMs as in MMC, and the single-pole-triple-throw (SPTT) switches, which can be realized using either the T-type or NPC-type switch strings. The SMMC works on the principle of connecting all the phase-legs in series and their respective positions are mutually interchanged. It offers a compact structure as the SMs count is reduced to 33% of MMC, which results in a much lower energy storage requirement. Furthermore, it does not have inter-phase circulating current problem and hence the CL inductors can also be avoided. The SMMC structure, operating principles, control, and modulation schemes are explained in detail. A detailed comparison of SMMC with the other major existing converters is carried out to highlight the important features of SMMC. The efficacy of SMMC is validated using PSCAD simulation studies with a 19-level grid-connected SMMC model. A 3-phase 19-level SMMC laboratory prototype is developed to further validate its feasibility. [ABSTRACT FROM AUTHOR]

This paper proposes a time-domain method of characteristics to simulate the impulse corona with background space charge, and derives the ordinary differential equations of the space charge generated by the impulse corona along the electric field line. The corresponding boundary conditions, the selection of time step and space step, and the update strategy of space charge are given. The calculation result of the impulse corona onset voltage is consistent with the test result, proving that the background space charge has almost no effect on the impulse corona onset electric field which is obtained by Kaptzov's assumption. The q-u curve obtained by simulation is compared with the test results to verify the effectiveness of the simulation method. The simulation method is used to obtain the development process of space charge during the application of the opposite polarity impulse voltage. By analyzing the space charge distribution during the process of impulse corona, and the corresponding microscopic charge distribution and macroscopic charge amount at different corona states, the characteristics of impulse corona are explained. [ABSTRACT FROM AUTHOR]

This paper presents the induced voltage measured at the terminal of a 10 kV distribution line when triggered lightning taking place at a distance of 40 m. It is shown that the induced voltage waveforms exhibit three components including a slow increase preceding the initial negative peak, following a rapid polarity reversal and the attenuated oscillations. The waveform parameters are defined in this paper. The observed negative peak value ranges from −47.1 kV to −7.5 kV. The positive peak value ranges from 5.3 kV to 20.9 kV, and the peak to peak value ranges from 12.8 kV to 63.2 kV. The test results show that the negative peak value of the induced voltage at the terminal of distribution generated by nearby negative triggered lightning generally exceeds the positive peak value, and in some cases the negative peak value even reaches twice the positive peak value. The negative 10%-90% rise time ranges from 2.3 μs to 88.5 μs, and the average oscillation period is about 18 μs. Both negative and positive peaks of induced voltages show good linear relationship with the peaks of lightning return stroke current. [ABSTRACT FROM AUTHOR]

An ion-flow field is very important for the design of an HVDC transmission line. Due to the space ions resulting from corona discharge, there are few practical methods to measure the space electric fields around HVDC transmission lines. In this paper, a compensation method is presented to measure the space potential in the ion-flow field. The space potential can be obtained by applying an adjustable voltage source to a suspended conductor, increasing the voltage step-by-step and measuring the current flowing through the voltage source or the ground-level ion current density under the suspended conductor. When the current is equal to zero or the ground-level ion current density is equal to that without the suspended conductor, the output voltage of the voltage source is the space potential to be measured. The paper first introduces the principle of the measurement method. Then, the feasibility of the method is testified on unipolar and bipolar reduced-scale models. Finally, the measured results are discussed and are compared with the results by the numerical method. [ABSTRACT FROM AUTHOR]

The impulse grounding performance of distribution lines tower is the key factor to lightning protection. Few field high-current impulse test researches are conducted on distribution line tower. In this paper, we used a mobile high-current impulse generator to conduct test on a 10 kV distribution line tower. The peak value of the current injection is 6 kA to 16 kA. The impulse impedance and the surface potential distribution are measured and analyzed. Based on the CDEGS software, an equivalent modeling of the experiment was carried out. The influence of soil ionization was taken into account by the method of changing the conductor radius through two rounds of calculation. The simulation and measured results were basically consistent. Experiments have shown that the soil ionization effect has an important impact on lightning performance of distribution line tower. The graphite earthing modules (GEMs) and pulling wires can significantly reduce the impulse impedance of the tower. The potential rise of the earth surface at a distance of 30 m from the tower can reach 1 kV at peak, under an injection of 7.15 kA. [ABSTRACT FROM AUTHOR]

High-frequency overvoltages are most dangerous for the insulation of transformer coils. It is because high-frequency pulse voltage creates an uneven distribution of voltage across the windings. The coil's first turns receive a disproportionately high voltage drop, leading to the inter-turn circuit. The paper analyzes the types of lightning pulses and proposes a new suppression method based on the anomalous skin effect in a multilayer material and the design in the form of a coil. The developed device is simulated, and the results of overvoltage monitoring at the substation with the installed high-frequency overvoltage suppression device are analyzed. [ABSTRACT FROM AUTHOR]

This paper proposes a novel layout for a grid-connected DC wind farm (DCWF) and investigates the performance under continuous operation and direct lightning strikes particularly for high soil resistivity. The DCWF employs internal dc grid that supplies generated power to the host AC utility through high voltage direct current (HVDC) transmission link. The proposed layout ensures sustainability of power supply against potential interruptions of DCWFs either during normal conditions or lightning incidents to comply with grid codes regulations. During continuous operation, the proposed layout safeguards the entire DCWF cables against flow of return currents to avert derating of conductor ampacity. Besides, protects WT nacelle switchgear and entire interface converters against hazards of surge currents. Furthermore, an effective grounding system design is proposed to introduce a low impedance path for system return currents, minimize voltage drop across grounding impedance and thus maximize DCWF delivered power to the grid. A detailed system that sufficiently represents models of system cables, grounding network and interface converters is built to verify the proposed scheme significance. The obtained results assure the capability of proposed layout to cease return currents via cable sheath during continuous operation and provide superior mitigation of lightning-associated transient voltages and currents. [ABSTRACT FROM AUTHOR]

An electric potential appears at floating wires located within a strong electric field. This phenomenon can be used to supply small isolated communities situated close to Extra-High Voltage transmission lines. This non-conventional distributed generation system has been studied for the last years. However, to improve the power generation system the optimal position of the floating wires, number of wires used, bundle radius, and type of wires must be properly identified. This paper presents a mathematical model solved with genetic algorithm to design a collector system with optimized induced voltage and minimized project cost, identifying the optimal reactor used for voltage regulation. The algorithm was applied to design a collector system that can tap-off 100 kW from a 230 kV transmission line. The trade-off between objectives and the Pareto optimal solutions were evidenced via several simulations using the weighting sum approach. These results allowed to identify a final geometry according to a-posteriori designer preferences. [ABSTRACT FROM AUTHOR]

This paper analyzes the effects of shunt capacitors installed on the low voltage sides of 10/0.4 kV distribution transformers on the operation of these transformers. Using the results of an extensive measurement campaign, this paper compares: real and reactive power losses, secondary-side current, and primary-side apparent power before and after the installation of transformer capacitors. The following four cases of reactive power compensation are considered and discussed: adequate, conditionally adequate, and total compensation, as well as overcompensation. The presented analysis helps to identify transformer substations where the initial compensation should be changed or adjusted, in order to improve transformers’ operational performance. Voltage dependencies of real and reactive power demands of the transformer-supplied loads are taken into account, based on the previously identified load characteristics in the considered networks as well as voltage dependence of reactive power of shunt capacitors. The result with and without including these voltage dependencies in the analysis is compared and discussed. [ABSTRACT FROM AUTHOR]

A method is proposed in this paper to determine the harmonic impedances in low-voltage networks in a stochastic way. The consequences of resonances for harmonic propagation and stability of power converters are summarized. By using Monte Carlo simulation, the method includes the uncertainties in customer impedances, specifically due to electronic loads and local generation. The uncertainty in customer impedance is included by considering probability distribution for the resistive, inductive, and capacitive parts of the impedance. The concept of transfer impedance is used for phase-to-neutral connections. A method is developed and applied to two existing low-voltage networks in Sweden. Results show that, for these two networks, the resonant frequencies decrease around 28% once photovoltaic panels are installed. This paper includes a discussion of some of the practical aspects of applying the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses the resonance problem in a parallel-inverter-based grid-connected system. Harmonic interactions between inverters and the grid exhibit various number of complicated characteristics that threatens the system stability and power quality. In this paper, a method of harmonic resonance analysis is proposed to investigate the resonance problem in a multi-inverter grid-connected system based on resonance modal analysis (RMA) and participation factor (PF) calculations. Initially, an impedance-based analytical approach is employed and expanded to an electricity distribution network that is dominated by multiple inverters with LCL filters and proportional resonant controllers. Then, the RMA is introduced to investigate the resonant interactions among inverters and the grid. Specifically, harmonic resonance characteristics are addressed through transfer functions and the proposed method considering variation of inverter number, different inverter combinations, and critical system components. Finally, time domain simulations on the PSCAD/EMTDC platform are presented. The case study results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper introduces an averaging control strategy of the phase-unit losses of modular multilevel converters (MMCs). The quantitative analysis of the MMC phase-unit loss distribution under unbalanced ac conditions is carried out by investigating the individual component of the MMC arm currents. It shows that the unbalanced arm currents will result in unbalanced phase-unit losses. To address this issue, this paper proposes a predictive closed-loop-control-based zero-sequence voltage injection method that results in much averaged loss distribution under unbalanced ac conditions. The essence of the new method is that it adjusts the three-phase sinusoidal references so that the unbalanced arm currents are fully utilized to average phase-unit losses. This paper shows the effectiveness of the new approach by electromagnetic transient simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Harmonic distortions at a point of common coupling (PCC) are caused by the harmonic sources in both supply system and customer facilities. In order to verify compliance with harmonic limits for an operating facility, it is essential to determine the contribution of a facility to the harmonic distortions measured at the PCC. Over the past many years, a significant amount of research work has been conducted in developing different harmonic analysis methods and especially harmonic contribution assessment methods. A common need of this line of research is consistent test systems that can be used to verify/validate a proposed method or to compare different approaches. To address this gap, a benchmark test system representing a typical medium-voltage/low-voltage network supplying industrial loads is proposed in this paper. This paper also includes examples on how to use the system to assess the performance of harmonic contribution determination methods. [ABSTRACT FROM AUTHOR]

In the event of a single line to ground (SLG) fault in a non-effectively grounded distribution network, the faulted current is weak (only a few amperes or less) and the existing devices cannot accurately judge the faulted feeder. In this paper, we proposed algorithms that combine complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and Hilbert transform to construct a multi-criteria comprehensive voting method. First, CEEMDAN algorithm is used to decompose the zero-sequence current to obtain the IMF1(the first intrinsic mode function) component and the Hilbert transform is used to calculate the instantaneous amplitude and instantaneous phase. Then, according to the three largest instantaneous amplitudes information, we constructed the characteristic instantaneous phase, characteristic instantaneous energy relative entropy and characteristic instantaneous zero sequence current polarity criteria from the phase, energy and polarity, respectively. Finally, we proposed a comprehensive voting method, which is specifically shown as follows: when two or more criteria show that one feeder or the bus has an SLG fault, it is voted that the feeder or the bus has an SLG fault. In contrast, if the judgment results of the three criteria are inconsistent, then we would return to recalculation and then vote. Compared with existing method, simulation tests and field experiments show that the method proposed in this paper has higher accuracy and a faster calculation speed. [ABSTRACT FROM AUTHOR]

In high-voltage direct current (HVDC) transmission systems comprising overhead line (OHL) sections as well as cable sections, the dc cable system is exposed indirectly to lightning strokes. In case of HVDC cable systems with extruded insulation the present test recommendations do not specify lightning impulse test levels related to rated system voltage. Instead, preliminary electromagnetic transient studies are required to determine the absolute maximum lightning impulse voltage the cable might experience. Within this paper, the general lightning behavior of OHL-cable mixed systems subsequent to the occurrence of a backflashover as well as a shielding failure is investigated for a 525 kV HVDC transmission scheme. This paper presents a modeling method to calculate occurring lightning overvoltages along a dc cable embedded between OHL sections. Based on a detailed parametric study using time-domain simulation, maximum lightning impulse voltages along the cable system are determined, and evaluated with regard to impulse level and polarity. The impact of project related parameter such as cable length, tower design, and tower grounding conditions on lightning overvoltage affecting the cable is demonstrated. [ABSTRACT FROM AUTHOR]

Foil-winding transformers with high currents in low voltage winding normally have copper/aluminum bars as leads of the coils. A practical solution for locating bar leads without increasing the size of the active part is to flatten the coil in the leads zone causing a deformation. Abnormalities in the leakage reactance have been identified in this type of transformers. In order to take this effect into account, this paper proposes a correction to the mathematical formulation for calculating the leakage reactance in transformers with deformations in the low voltage winding. Three methods for calculating the leakage reactance have been compared for four real case studies: the proposed formulation, the commonly used formulation, and detailed simulations with the finite element method in three-dimension (FEM3D). The leakage reactance calculated with the proposed method has been found to have an improved accuracy compared with the one calculated using the formulation without correction in all case studies. The main contribution of this paper is that the proposed correction in the formulation makes it possible to increase the accuracy in the calculation of leakage reactance, avoiding the need to resort to detailed simulations with FEM3D. [ABSTRACT FROM AUTHOR]

A novel traveling-wave-based fault-location algorithm for multiterminal transmission lines is introduced in this paper. Existing traveling-wave fault-location methods that based on the data measured at line terminals lack reliability and accuracy because time synchronization is needed, which brings error. To solve this problem, the complex structure of multiterminal power system is divided into several fault identification branches, current at the midpoint of each branched is measured, a novel fault-location algorithm is proposed, and the time synchronization error is eliminated. A line-based device is designed to measure the current at the midpoint of each branch. An accurate value of the traveling wave speed can be obtained by merely applying the data record in one measuring device. Thus, the fault-location accuracy is improved. The system model applied in this paper comprises one transmission line and two tapped lines, and the proposed method applies to networks with more terminals. Performance of this method is tested by different cases by PSCAD. The hardware platform has been tested in Sichuan Province, China. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method for voltage dip classification using deep convolutional neural networks. The main contributions of this paper include: 1) to propose a new effective deep convolutional neural network architecture for automatically learning voltage dip features, rather than extracting hand-crafted features; 2) to employ the deep learning in an effective two-dimensional (2-D) transform domain, under space-phasor model (SPM), for efficient learning of dip features; 3) to characterize voltage dips by 2-D SPM-based deep learning, which leads to voltage dip features independent of the duration and sampling frequency of dip recordings; and 4) to develop robust automatically-extracted features that are insensitive to training and test datasets measured from different countries/regions. Experiments were conducted on datasets containing about 6000 measured voltage dips spread over seven classes measured from several different countries. Results have shown good performance of the proposed method: average classification rate is about 97% and false alarm rate is about 0.50%. The test results from the proposed method are compared with the results from two existing dip classification methods. The proposed method is shown to outperform these existing methods. [ABSTRACT FROM AUTHOR]

Circuit models of horizontal and vertical grounding electrodes are traditionally used in high-frequency (HF) analysis, although underlying approximations limit their accuracy in some low-frequency ranges. Recently, a number of new circuit models have been introduced, but the improvement of the accuracy at HF has not been systematically evaluated. In this paper, we show that new circuit models can be directly derived from method of moments solutions of the integral forms of Maxwell equations by introducing different assumptions. This approach helps to categorize different models into only a few categories based on underlying approximations. To determine the applicable range, we analyze the error for impedance to ground computed by different circuit models in comparison to that calculated by a rigorous full-wave model over wide ranges of parameters, such as electrode length, soil resistivity, and frequency. This paper: 1) provides a parametric analysis that enables the estimation of the applicability of the particular circuit model for application to a practical case based on its accuracy; 2) presents a unified systematic approach to derive circuit models with different capabilities; and 3) reveals that the modeling of the mutual coupling between different parts of the grounding electrode is the key factor for radically improving the accuracy of circuit models at HF. [ABSTRACT FROM AUTHOR]

A comprehensive time series power flow (TSPF) framework is proposed for the analysis of fault-induced delayed voltage recovery (FIDVR). TSPF bridges the gap between static power flow simulations and time-domain simulations for FIDVR analysis. FIDVR events can be simulated faster with TSPF, while transient simulations normally require much longer time. In the TSPF framework, a random model for the disconnection of the induction motors is proposed to determine the load for different “snapshots” during FIDVR events. Regression analysis is used to predict the parameters needed in the simulations. There is no need to simplify the network topology or aggregate loads into clusters as in measurement-based load modeling approaches. The techniques presented in this paper successfully reproduced two FIDVR events recorded in heavily meshed distribution networks in New York City in 2010 and 2015. The paper uncovers that the proper modeling of motor protections (thermal and under-voltage) is key to properly predict FIDVR events. [ABSTRACT FROM AUTHOR]

One of the most widely used installation types of power cables is inside nonmetallic ducts. Ampacity calculations for this type of installations were summarized back in 1957 by Neher and McGrath. The method has been standardized in the IEC 60287 family of standards. It is shown in this paper that there is a noticeable mismatch between the IEC standard calculations for steady-state conditions with experimental tests and finite-element method simulations. After an in-depth investigation, it was found that the source of the discrepancy between reality and the standard is an inaccuracy in the calculation of the heat transfer of the air between the cable surface and the internal duct surface. The standard calculations simplify the problem too much, resulting in relative large errors. A more precise formulation, yet simple enough for computer implementation, is proposed in this paper. The accurate calculation of the steady-state temperature is obtained from first heat transfer principles. Numerous laboratory experiments and finite-element simulations are used to evaluate the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a fast dc short-circuit fault detection method for HVdc grids, which relies on the voltage across the dc fault-current-limiting reactor. In the proposed approach, primary detection uses only the single-terminal dc reactor voltage information and is shown to be able to identify the faulted line as well as the faulted pole within 1 ms. The backup detection is primarily required for high resistance faults and uses communication between the two sides of the line. Its speed is dependent on the travel time of signals between the two ends, but will operate, as shown in the paper, typically in 2 ms or less. The approach is justified using analytical calculations and exhaustively validated via electromagnetic transient simulation for pole-to-pole and pole-to-ground faults, different fault resistance, fault locations, and dc reactor values. The validation shows that the method is able to identify all genuine dc line faults, but is not prone to misoperation with ac line faults, different loading levels, and so on. [ABSTRACT FROM AUTHOR]

This paper presents a decentralized fault detection technique for power distribution systems with resonant grounding. The aim of this paper is to detect single phase to ground faults and identify the faulty feeder within three cycles of the fault occurrence. In the proposed technique, faults are first detected based on the neutral voltage displacement. The pre-fault and post-fault voltages (phase to ground) are then used to identify the faulty phase. Finally, the faulty feeder (as well as the faulty area of a long feeder) is identified from the relationship between the initial transient of the zero-sequence current and the faulty phase voltage just after the occurrence of faults. A signal processing tool called mathematical morphology is utilized to identify the faulty feeder. To identify the faulty feeder, it is also required to know the fault occurrence time that is estimated using the slope of the neutral voltage. The main feature of the proposed scheme is that this technique only uses voltage and current signals from the corresponding voltage transformers and current transformers. Therefore, it does not require communication among protection devices in the same or different feeders to identify the faulty feeder. The proposed technique also has the ability to distinguish the nature of faults, i.e., whether the faults are permanent or temporary. The effectiveness of the proposed scheme is tested, on an IEEE test system as well as on a practical test system, using MATLAB/SimPowerSystems. Simulation results show that the proposed technique can quickly detect the single phase to ground faults in a resonant grounding power distribution system and identify the faulty feeder. It is also capable of distinguishing faults from other disturbances. Moreover, it works under different compensation levels as well as for different fault inception angles. [ABSTRACT FROM AUTHOR]

This paper proposes a novel islanding detection method (IDM) for grid-connected photovoltaic systems (GCPVSs) through a disturbance injection in the maximum power point tracking (MPPT) algorithm. When an absolute deviation of the output voltage exceeds a threshold, the applied disturbance shifts system operating point from its maximum power point (MPP) condition. This leads to a sharp active power output reduction and consequently, a significant voltage drop in islanded mode beyond the standard voltage limit. The proposed algorithm is defined in a way that the distributed generator (DG) can be restored to MPP after islanding classification. It is thereby effective in microgrid in where the power injection at maximum level to cater the critical loads and maintain the stability of the isolated area are pursued. An intentional time delay has also been considered to avoid nuisance tripping in short-circuit faults which do not require tripping. The assessment of the proposed technique has been conducted for a sample network containing two GCPVSs in a real-time platform including actual relays in hardware-in-the-loop (HiL). The provided results under extensive islanding scenarios defined in islanding standards endorse timely and accurately detection with negligible non-detection zone (NDZ) as well as no false tripping in non-islanding disturbances. The comparative analysis of the presented scheme with a few recent IDMs for GCPVS highlights its overall superiorities, including very small NDZ, fast detection, thresholds self-standing determination, no adverse effect on power quality, and simple and inexpensive integration. [ABSTRACT FROM AUTHOR]

The medium-voltage grid is often planned in loops, but operated in a radial structure to allow a robust protection scheme. This allows the reconfiguration of the grid. This paper focuses on reconfiguration after a fault to re-establish the service, also referred to as self-healing. It includes the localization of faults and finds new grid configurations based on the actual status of the grid. The solving of the reconfiguration problem is nonpolynomial difficult. Finding an acceptable solution to resupply nonfaulted grid parts under tight time limits is difficult. The innovative contribution of this paper is an approach that finds new configurations as local as possible to the faulty area, evolvingly increasing the search space. New configurations are generated based on a heuristic method relying on graph theory. An aggregated fitness function is used to evaluate the solutions. For the realization, a distributed control architecture in the form of an advanced substation automation is proposed. [ABSTRACT FROM PUBLISHER]

Analyzing the harmonic distortions at the point of common coupling is thoroughly addressed in this two-part paper. The main aim of such an analysis is to identify which of the utility and customer sides is significantly responsible for the harmonic distortions at the point of common coupling. In Part I, the meaning of the dominant side is challenged fundamentally. Here, two different definitions for the dominant side are discussed: The side with the larger equivalent harmonic “voltage” or “current” source. It is shown that these definitions are not identical. Subsequently, common methods are categorized in three general concepts based on: the direction of active power flow, the reactive power, and the voltage-current ratio. These three categories as well as their validity are analytically reviewed for both the definitions. The concept based on the voltage–current ratio has been explicitly presented in the original work for identifying the dominant equivalent harmonic voltage source. This concept is developed in this paper for identifying the dominant equivalent harmonic current source as well. In Part II, the application of the methods is illustrated using some common calculation examples. In addition, these methods are described verbally to provide a physical interpretation. Some practical aspects regarding the implementation of the methods are discussed as well. [ABSTRACT FROM AUTHOR]

This two-part paper presents the review, application, and interpretation of the methods for analyzing the harmonic distortions at the point of common coupling thoroughly. In Part I, two different definitions for the dominant side are discussed regarding the equivalent harmonic “voltage” or “current” source. In addition, common methods are categorized in three general concepts based on the direction of active power flow, the reactive power, and the voltage–current ratio. These three categories as well as their validity are analytically reviewed for both the definitions. In Part II, the application of the methods is illustrated using two common calculation examples. The magnitude and the phase angle of the equivalent harmonic sources are separately varied in the first and second calculation example, respectively. The validity of the methods is investigated using these calculation examples. Afterward, the physical interpretation of the methods is addressed. The difference between the dominant sides regarding the equivalent harmonic “voltage” and “current” source is discussed using the calculation examples. It is shown that if one of the utility and customer equivalent harmonic voltage sources is “much” larger than the other, the dominant sides regarding the equivalent harmonic “voltage” and “current” source will be the same. Some practical aspects such as the tolerance of the methods to errors in input data are discussed. In this regard, nonresonance and resonance conditions are taken into account. This two-part paper provides a comprehensive but clear review of methods for identifying the dominant side. [ABSTRACT FROM AUTHOR]

Diversification of control schemes adopted by inverter-interfaced distributed generators (IIDGs) leads to difficulties in fault current estimation in a microgrid, which might make preexisting protection systems invalid and threaten the safety of power electronic devices. It is therefore important to study fault characteristics of IIDGs. This paper investigates characteristics of fault current of IIDGs caused by both symmetrical and asymmetrical faults. Two kinds of widely used control modes, current control (constant current control and PQ control) and voltage control (V/F control and droop control), are under investigation to provide an intuitive comparison on fault current. In particular, a novel algorithm is proposed to calculate fault current of droop-controlled IIDGs. It is found that different limiters have great impacts on fault response of IIDGs and detailed research works are carried out to identify the effects in this paper. Simulation results based on PSCAD/EMTDC and calculation results based on MATLAB/Simulink verify the correctness of the proposed fault models. [ABSTRACT FROM AUTHOR]

Electrical parameter estimation of the soil is an essential step for the design of grounding systems, and is based on optimization techniques that minimize the error between a theoretical function and the field measurements. The determining factor for reaching the most adequate solution depends on a method for solving the indefinite integral of the theoretical function, as well as the accuracy and performance of the parameter estimation. This paper presents a precise and tested technique for achieving these goals known as Chebyshev polynomials. This paper also introduces an error calculation procedure for the truncation of the Chebyshev series. Besides this, this paper points toward an optimization algorithm for estimating soil parameters by use of measurements associated with differential evolution. The effectiveness of this approach has been proven by applying cases extracted from the literature. [ABSTRACT FROM AUTHOR]

Low-voltage (LV) feeder voltage magnitude and unbalance are often the constraining factors on a feeder's capacity to absorb rooftop photovoltaic (PV) generation. This paper presents a new probabilistic method for voltage management in distribution networks through the placement of distribution static compensators (dSTATCOMs) and on-load tap changers (OLTCs) considering the reactive capability of PV inverters in multiple LV and medium-voltage distribution networks. The method uses a modified particle swarm optimization. In this paper, several scenarios for the placements of multiple dSTATCOMs with and without embedded energy storage systems using both reactive and real power compensation are investigated in combination with an OLTC equipped with independent per-phase tap-changing control. The voltage constraints in the proposed method are statistically defined using three duration curves. These are the voltage unbalance, maximum voltage, and minimum voltage duration curves. The method is comprehensively tested for varying load and PV generation based on data from a real Australian distribution network with considerable unbalance and distributed PV generation. The results show that PV hosting capacity increases where the proposed approach is applied. [ABSTRACT FROM PUBLISHER]

An online approach based on application of search coil for transformer winding fault detection is presented in this paper. Earlier attempts report use of search coil for the detection of transformer interturn fault. This paper extends application of search coil for detection of winding displacement in addition to interturn fault identification. Experimental results based on specially designed 10-kVA transformer are presented to analyze search coil voltages during healthy and faulty conditions. The algorithm detects phase ($U$ , $V$, $W$), side (LV, HV), and precise location of the fault. Discrimination between interturn fault and winding radial displacement is also achieved by search coil voltage analysis. The results are experimentally verified based on impedance analyzer plots. Experimental analysis indicates that the proposed method is independent of load on the transformer, vector group, nature of winding, balanced or unbalanced source voltage, source voltage harmonics, inrush, and overexcitation conditions. [ABSTRACT FROM AUTHOR]

In this paper, the finite-elements method was used to analyze the approximation being typically used in the international standards for the calculation of the touch voltage, appearing at earth faults in power networks. The calculations in this paper were performed on the series of earthing systems applied in practice and the results are compared with the results of the widely accepted approximate method from standards. The origin of discrepancy in the results is analyzed and evaluated. The outcome of this paper is the justification of the application of Thevenin's theorem and the development of the higher accuracy equations for the resistance between feet and remote earth in case of the application of high resistivity additional surface material. [ABSTRACT FROM PUBLISHER]

The synchronization of the voltage and current from different terminals is generally unachievable in the power grids at mountainous area, which leads to the fault location method with synchronized data to be impossibly utilized. This paper proposes a voltage distribution based fault location scheme with asynchronous data for three-terminal transmission lines. At first, the faulty section is identified by the voltage computed from each terminal due to the boundaries formed by fault point and tee-point. Since the fault current is stable after the transients decay, the asynchronous calculated voltage and current from the terminals without faults can be synchronized by their angle differences to form the voltage and current at faulty section near the tee-point. Then, the voltage distribution of the faulty section can be obtained from the tee-point and the terminal to find the intersection point of them. In order to fully utilize the fault characteristics in the voltage and current, the earth mode and the minimum of the voltage distribution are applied to optimize the fault location results for grounding faults and multi-phase faults, respectively. Moreover, the robustness of the proposed fault location scheme is thoroughly validated by the simulation of PSCAD/EMTDC. At last, a set of the field data acquired by wave recorders is applied to test the feasibility of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper presents a design-oriented analysis on the voltage stability and the transient fault current limitation of voltage-controlled modular multilevel converter (MMC) with L-filters. First, a dual-loop voltage control based on the L-filter plant is systematically designed, where the upper limits of controller parameters to ensure the small-signal stability are identified. Then, considering the transient current limitation during faults, the lower boundaries of inner current controller parameters are derived. Within the region of allowed controller parameters, the optimization is further made to minimize the closed-loop output impedance of MMC, which enhances the output voltage stiffness against the load current disturbance. Lastly, time-domain simulations corroborate the theoretical analysis. [ABSTRACT FROM AUTHOR]

Hybrid ac–dc distribution systems with multiple parallel operating ac and dc links are being developed to maximize system capacity during (n-1) contingencies. This paper shows that offline reconfigurability between such ac and dc links post-fault clearance allows an additional design flexibility of reducing the required substation converter rating to maintain the same required capacity by maximizing the utilization of healthy infrastructure. The associated variation of system capacity with power rating and number of substation converters is determined for different types of component faults. It is discussed that while converter downsizing is possible, as a consequence the parallel ac–dc link system is forced to operate at a sub-optimal efficiency during normal conditions. Using case studies, the economics of this trade-off is quantified for determining the optimal de-rating factor for varying grid voltage, link length, and conductor area. [ABSTRACT FROM AUTHOR]

An improved version of power flow-controller (PFC) which is based on multi-winding transformer (MWT) is presented in this paper. The objective of this article is to control the power flow in a power-line to alleviate overload in a heavily loaded system and increase power flow of an under loaded line. The novelty of multi-winding transformer based power-flow controller is the use of unequal voltage ratio in the tap-changer windings of MWT with bi-directional switches to provide a compensating voltage required to achieve the desired power flow. This new model drastically increases the number of compensating points to make it more compact with higher flexibility. It aims to utilize the full potential of a line (bound by its thermal limits) to contribute in power flow by providing several times higher resolution with limited number of switches and reduced amount of copper and iron volume compared to existing state-of-the-art technology. The proposed MWT model has been included in Newton Raphson Load Flow (NRLF) method for power flow analysis. The feasibility of the model has been realized using a standard 5 bus test system and a modified IEEE 57 bus system. Operation of the PFC has been demonstrated using a hardware laboratory prototype for generating nearest compensating points (voltage vectors) corresponding to the obtained compensating voltages after load flow solution. The results obtained are verified by simulating the controller in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

For droop control in voltage source converter based multi-terminal HVDC systems, the determination of droop coefficients is a key issue, which directly affects both power distribution and DC control performances. This paper proposes a novel design of droop coefficients considering the requirements of power distribution, DC voltage control and system stability. Considering the power margins of different converters, the ratio relationship among droop coefficients is established. Converters with larger power margins take bigger portion of power mismatch to avoid overload problem. Furthermore, the integral square error of converters DC voltage is adopted as the DC voltage control performance index, and optimization of droop coefficients to achieve coordinated DC voltage control of steady-state deviation and transient variation, is derived. Finally, the constraint of droop coefficients is established to guarantee the DC system stability after power disturbance. Case studies are conducted on the Nordic 32 system with an embedded 4-terminal DC grid to demonstrate the feasibility and effectiveness of the proposed droop control scheme. [ABSTRACT FROM AUTHOR]

Point-on-wave (POW) is a single-event characteristic of voltage dips, influencing the performance of sensitive equipment, such as AC contactor and wind turbines. However, definition and calculation of POW are unsolved issues. The existing definition of POW is for single-phase dips; it does not consider three-phase dips. Moreover, existing POW calculation methods are inaccurate. This paper proposes a definition of POW as a single-event characteristic for three-phase voltage dips. Then, it proposes a POW calculation method based on space vector, including POW detection and angle calculation. The proposed method is based on the ellipse characteristics during the dip-stage and non-dip stage. The proposed method does not depend on zero crossing for angle reference. To verify the performance of the proposed method, a power-system simulation model is built to generate voltage dips with different origin. Furthermore, the accuracy of the proposed method is verified by 425 sets measured voltage dips, and compared with the published methods. [ABSTRACT FROM AUTHOR]

The multi infeed interaction factor (MIIF) index was introduced by CIGRE as a measure of inter-inverter interactions in multi-infeed HVDC systems, which include only line-commutated converter based HVDC (LCC–HVDC) links. The MIIF is although valid among LCCs in hybrid multi-infeed HVDC systems, but not among voltage-source converters (VSCs) as well as between LCC and VSC when the VSC operates under the constant ac voltage control mode. In this paper, a multi-infeed voltage interaction factor (MVIF) index is firstly proposed as the quantitative measure of inter-inverter interactions in hybrid multi-infeed HVDC systems. Compared to the MIIF, the MVIF is unified since it is applicable regardless of inverter types and control modes. Moreover, the quasi-steady-state power-flow model of hybrid multi-infeed HVDC systems is utilized as the mathematical framework to develop the analytical calculation method of the MVIF. This lends a theoretical basis for the efficient evaluation of inter-inverter interactions. Furthermore, the parametric dependence of the MVIF is analyzed to provide an exhaustive perspective of inter-inverter interactions. This offers a useful tool to aid planning and operation of hybrid multi-infeed HVDC systems. Finally, dynamic simulations conducted on the PSCAD/EMTDC program validate the MVIF and its analytical calculation method. [ABSTRACT FROM AUTHOR]

DC–DC converters are expected to interconnect High Voltage Direct Current (HVDC) grids with different configurations or voltage levels, which will increase reliability and operational flexibility. The dc Modular Multilevel Converter (DC-MMC) is suggested as a potential non-isolated topology for HVDC applications. This paper presents an energy-based closed-loop control for the DC-MMC. Such control structure provides a full control of the current components, while ensuring energy balancing in normal and fault operation. First, the converter configuration is described and the fundamental equations are presented. Then, the normal operation of the converter is explained from the steady state conditions and the energy and current components. A closed-loop control is designed to ensure a proper dynamic response, balance the internal energies of the converter, and minimize the ac circulating current. Also, the control structure is modified to ensure dc fault ride through capability. Simulation results in MATLAB/Simulink are used to validate the converter control under normal operation and DC faults. [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]

Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one. [ABSTRACT FROM AUTHOR]