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This paper presents i) an equivalent model of the half-bridge modular multilevel converter (HB-MMC) which is suitable for real-time applications, ii) a hybrid central-processing unit/field-programmable gate array (CPU/FPGA)-based architecture for real-time simulation of electromagnetic transients of systems which include HB-MMC, and iii) a novel arrangement for sorting results referred to as the “sub-module (SM) rank list”, which tackles the bottleneck for parallel implementation of the MMC arm model solver on the FPGA. The Adam–Bashforth (AB) method is used for numerical integration of the HB-SM capacitor model. The second-order AB method provides a constant admittance matrix of the HB-MMC and, thus, reduces computational burden while offering the same accuracy as that of the widely used Trapezoidal method. The CPU/FPGA-based architecture is optimized to obtain maximum parallelism of the HB-MMC model implementation, adopting a standard, single-precision, floating-point computational engine. The proposed sorting arrangement is independent of the utilized sorting algorithm and its application to the odd–even bubble sorting scheme is presented in this paper. The proposed architecture offers a simulation time-step of 825 ns while including the sorting module as the SM capacitor voltage-balancing control unit. This enables accurate analysis of MMC controls based on either software-in-the-loop or hardware-in-the-loop approaches. Performance and accuracy of the MMC model and the hybrid CPU/FPGA-based architecture are evaluated based on a set of case studies on a 401-level HB-MMC-based HVDC station and verified based on offline simulation results in the PSCAD/EMTDC environment.

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CB's design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved.

Power-electronic sources, such as Type 3 wind turbine generators and static synchronous compensators (STATCOM), interface to the grid through, partial- or full-scale, and power converters that have inherently fast switching capability to control their output current during short circuits. The short-circuit current is a function of the specific converter control algorithm and differs significantly from the conventional rotating machine sources without converter interfaces. Therefore, if a transmission-line protection scheme is designed for conventional sources, (not taking into account these differences in short-circuit current characteristics), reliability can be at risk. Using real-life short-circuit currents on lines supplied by sources having a power converter interface, this paper illustrates the reliability risk to conventional line protection schemes, in particular, to those which use negative-sequence quantities for the detection of unbalanced faults. This paper discusses the protection schemes, adopted by BC Hydro–a large Canadian Electric Utility, for transmission lines interconnecting Type 3 wind turbine and STATCOM sources. Their application for ground faults is independent of the converter control algorithm as long as the source is interconnected to the grid via a transformer which is a source of zero-sequence current.

Locating a fault in a series-capacitor-compensated transmission line (SCCTL) is a challenging task due to the action of a metal-oxide varistor a nonlinear element present as a part of the protection system of the series capacitor. In this paper, an accurate fault-location algorithm that uses synchronized measurements from both ends of the SCCTL is presented which provides fault-location results without using the model of MOV or natural fault loop for single-phase-to-ground, and double-phase-to-ground faults. Another salient feature of the proposed technique is that the subroutines for locating faults in different sections transmission line yield almost identical fault-location results regardless of which section the transmission line is faulted. The proposed technique could also be extended to the double-circuit transmission lines. First, the proposed technique is introduced and its features are elaborated through detailed mathematical analysis. Thereafter, a 500- $\text{kV}$ system with an SCCTL is designed in PSCAD, while the fault-location algorithm is modeled in MATLAB. The proposed algorithm is tested through simulations covering various fault scenarios in an SCCTL. For performance evaluation, the comparative analysis of the proposed technique with a well-known existing technique is performed in this paper.

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.

Series compensated transmission lines challenge legacy protection schemes. In this paper, a dynamic state estimation based protection (EBP) method is proposed to address these challenges. The method requires GPS synchronized measurements at both ends of the line and a high-fidelity model of the protection zone. The paper presents the dynamic model of the protection zone and its impact on the performance of the protection scheme. Numerical simulations show that the method can correctly identify faults, independently of position and type. The paper also compares the proposed method versus legacy protection functions such as distance protection and line differential protection. The comparison shows faster detection of internal faults, immunity to current inversion caused by series capacitors (SCs) and improved detection sensitivity for high-impedance faults.

Condition monitoring of circuit-breaker (CB) operating mechanisms is one the most important aspects of predictive maintenance. This paper proposes a contact force monitoring scheme for 10 $\sim$ 35-kV vacuum CB. With specially designed force sensors and a signal-processing method, we can accurately find the instants of contact touch and separation. With the indirectly measured contact touch-and-separation instants, we can measure the opening speed, closing speed, and open gap over the travel of each phase. The accuracy of our online monitoring scheme is compared with the conventional offline methods that can directly obtain the instants of contact touch and separation by measuring the connectivity of the contacts. The results show that the accuracy of the scheme proposed in this paper is satisfactory and it can meet the engineering requirements of online condition monitoring of CBs. By introducing a set of new force sensors, some additional information that is useful for overall condition assessment of CB operating mechanism can also be extracted; this will help us to find new ways to improve our CB condition monitoring systems.

Several methods have been proposed and investigated so far on mitigation of very fast transient overvoltages (VFTO) in gas-insulated switchgear (GIS). The state-of-the-art methods are primarily based on dissipation of the energy associated with electromagnetic waves that the VFTO originate from and are composed of. Present paper reports on an alternative concept of VFTO mitigation based on the principle of controlling voltage conditions preceding voltage breakdown in SF6 gas that leads to VFTO generation. The paper introduces different control algorithms and shows how the algorithms can limit VFTO maximum value and total number of voltage breakdowns during operation of the GIS disconnector. The concept is applied for mitigation of VFTO in ultra-high voltage GIS. As the study case, an 1100 kV test set-up is used as recently reported for Wuhan (China) GIS station, with the disconnector characteristics obtained from 1100 kV development tests.

This paper focuses on estimating the substation grounding resistance to improve the modeling of geomagnetically induced currents (GICs). Grounding resistances are not included in the standard power flow models, and their approximate values are often used for performing GIC studies. This paper provides an algorithm to estimate the resistances from the GIC measurements. This algorithm calculates the linear sensitivity factors of the GICs around the local grounding resistive components and uses linear regression to solve for the resistances. The effectiveness of the proposed algorithm is demonstrated using both a small test case as well as a 62 500-bus model of the North American Eastern Interconnection.

For the fast transient study of power transformers, several types of models are available that can be categorized into three groups: white box models, black box models, and gray box models. The identification of their parameters and their usages make the models different. Since white and black box models cannot cover certain cases, such as the failure analysis of a transformer with unknown geometrical data, a new type of model has recently become important for industry and academic researchers. This model, called the gray box model, is intensely analyzed in the current paper, both theoretically and experimentally. The gray box model conceptually lies between the black and white box models. The outcomes of this work have been verified using a suitable test object. The parameter identification of the model is performed by artificial methods. Some useful new ideas, including applying the Weibull Distribution function in inductance estimation and employing an exponential function for the series resistance of windings, are developed in this paper. The number of unknown parameters to be identified in the gray box model is reduced by applying these ideas, which makes the model more simple and feasible.

This paper presents a system which predicts the grounding topologies of grounding systems (GS) with one, two, three, or four ground rods in straight line, when no prior information about the grounding topology, soil or rod properties is given. This is achieved through the exploration of the information contained into the impulse response of the GS together with the pattern recognition framework. The proposed system is composed by four main elements, an excitation system, a high-speed data acquisition system, a feature extraction, and a pattern recognition model. Experimental setup comprises the evaluation of the accuracy of the proposed system in discriminating among four grounding configurations, composed by the topologies of one, two, three, and four ground rods in straight line. This paper also proposes a new data driven approach to improve the classification performance, in pattern classification models, when dealing with undersampling problem in the acquired signal used to extract the features.

During monopolar operation or commissioning of high-voltage direct-current (HVdc) links, ground-return current (GRC) leads to power transformer dc bias issues. A GRC model is proposed in this paper, and the penetration depth, penetration ratio, and earth surface potential of various earth models are simulated. The result indicates that the influence of deep earth resistivity increases rapidly by the increasing distance from substations to HVdc electrodes. The simulation results are compared with measured data of GRCs distribution in the Hubei power grid of China. The result demonstrates that the simulation considering deep earth resistivity is credible and the shallow earth model could lead to poor accuracy. Furthermore, earth resistivity measurements are carried out at different geological regions and the deep earth characteristics and the GRCs distribution are quite different. Thus, it can be concluded that to calculate GRCs distribution using uniform earth structure without measurement is not appropriate. Finally, the relationship between accurately measured earth depth and the accuracy ratio of GRCs calculation is analyzed. The result of this paper contributes to determine the depth of earth model in GRCs simulation modeling and earth resistivity measurements.

This paper proposes a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC). Under normal conditions, the system's neutral point grounds through a reactor of high reactance. When a single line-to-earth fault occurs, the EHPC can be controlled flexibly to suppress the recovery voltage of faulty phase and to reduce the ground-fault current to almost zero. This paper expounds the two operating modes and corresponding characteristics, the measuring method of insulation parameters of the system (mainly including capacitance to earth and leakage resistance), and the full compensation principle for arc suppression based on the equivalent negative impedance of EHPC. This paper also proposes a compound control method of neutral-to-earth impedance based on neutral-point displacement voltage and a compensating method for the harmonic component of the ground-fault current. Compared to traditional grounding methods, this method can be fast and reliably make the residual current approach almost zero without arc reignition while greatly reducing the output capacity of APC and, thus, decreasing the cost of this device. Simulations and experiments verify the correctness of this measuring method and feasibility of this flexible grounding control strategy.

This paper presents an improved solution of the implicitly coupled electromechanical and electromagnetic transient analysis problem using a frequency-dependent network equivalent (FDNE). In previous work, the fundamental frequency equivalent was used as the network equivalent of the external system. While simple, this fundamental frequency equivalent is only accurate for a single frequency, the fundamental frequency. This may lead to an inaccurate equivalent of the transient stability (TS) network. This paper extends previous work by using an FDNE derived from the vector-fitting technique. The FDNE is able to resolve the transient behavior over a wider frequency spectrum leading to a more accurate representation of the TS network. Results demonstrating the accuracy of the proposed scheme are presented on a 3-bus test system and the IEEE 118-bus system.

Resonance and harmonic phenomena can occur when HVDC and Flexible Alternating Current Transmission System devices are inserted into ac grids. Frequency response analysis and electromagnetic transient (EMT) type simulations of converter stations are essential for analyzing and preventing negative effects. This paper presents two approaches for deriving the high frequency response of a converter station: using simplified analytical representation and accurate EMT-type model. Validations with control parameter impacts are presented. This paper also presents a test case, based on a recent HVdc project, where potential resonances can occur between the converter station and its ac network. It is demonstrated analytically that these resonances are mainly due to control system parameters and ac network configurations.

This paper proposes a novel bipolar modular multilevel converter (MMC) topology with dc fault ride-through capability. The proposed bipolar MMC topology is a hybrid of two types of arms: the half-bridge submodule (HBSM)-based arms (HBSM-arms) which are connected to the ground pole, and the unipolar-voltage full-bridge submodule (UFBSM)-based arms (UFBSM-arms) which are connected to the positive or negative pole. During a pole-to-ground or pole-to-pole dc fault, the hybrid-arm design offers the proposed UFBSM-based hybrid-arm bipolar MMC (UHA-BMMC) the capability to provide additional reactive power to support the ac grid while blocking the dc fault current. The topology, operation principle, dc fault mechanisms, and dc fault ride-through scheme of the UHAB-MMC are presented in this paper. Compared with other possible bipolar MMC topologies, the UHA-BMMC provides good dc fault ride-through capability with a small cost of power-electronic devices. The validity and features of the proposed UHA-BMMC are verified by simulation results conducted in PSCAD/EMTDC.

In this paper, a new adaptive single-phase reclosing method for high-voltage transmission lines is proposed. This method employs local voltage magnitude and angle of the faulted phase. It is shown in this paper that the proposed method is able to quickly identify the fault type and the arc extinction if the fault is transient. This method is effective for various system configurations, including ideally and partially transposed transmission lines, untransposed lines, as well as transmission lines with or without a shunt reactor. Superior performance of the proposed method has been verified using 550 case studies simulated in PSCAD and Matlab and a real case related to a 765-kV transmission line.

This paper describes a systematic method for determining an area of vulnerability (AOV) to fault induced voltage sags. The concept of an AOV is very useful for evaluating vulnerability and the relationship between sensitive loads and system voltage sag performance. In general, a system performance assessment pertaining to voltage sags requires a determination of AOVs for system buses. Because voltage sags can be caused by remote faults in a transmission system, to completely identify an AOV, a large part of the power system including adjacent circuits and remote supply systems should be considered. Therefore, accurately calculating critical points and identifying AOVs are challenging tasks. In this paper, we present an effective numerical method to determine an accurate AOV in a large scale power system and to overcome limitations of other well-known methods.

An effective computational tool for the simulation and design of voltage dividers has been realized. It takes advantage of a finite-element procedure for the accurate computation of the divider stray capacitances. These latter are then the input of a circuit solver procedure based on the modified node potential technique, which provides the divider frequency response. Furthermore, a test divider has been set up for the purpose of the tool validation. A very good agreement has been found between measured and computed results. This paper highlights that a correct representation of the resistors is necessary in the finite-element method (FEM) model, during the computation of the stray parameters. This issue has been clarified for the first time in this paper. In addition, a comparison between the FEM and boundary-element method has been carried out before the implementation of the numerical code, stating the superior effectiveness of FEM. The computational tool has been finally tested on a real voltage divider. Good results have been found in terms of computational accuracy. Limited discrepancies between measured and computed results have been found up to 10 kHz, with 5.7% for the scale factor and less than 24 mrad for the phase error.

As the key equipment of ultra-high-voltage direct-current (UHVDC) transmission systems, the converter valve would bear different forms of overvoltage. Consequently, a wideband model for the $\pm $ 1100 kV UHVDC converter valve is essential for the converter system to make overvoltage analysis, insulation design, and electromagnetic disturbance prediction. This paper proposed a wideband modeling method for the $\pm $ 1100 kV converter valve. First, the physics-based wideband model for the key component in the converter valve module is built, of which the parameters could be obtained through genetic algorithm. Second, the parasitic capacitances are obtained through a simplified boundary-element method according to the given structure of the $\pm $ 1100 kV converter valve. By combining wideband models for key components in the converter valve module and the parasitic capacitances of the valve tower, the integrated wideband model for the converter valve could be obtained. The systematic modelling method proposed in this paper could also be of reference value to the modelling of other large-scale complex high-voltage power-electronic equipment. Finally, the experiments for converter valve module and converter tower are carried out, and the corresponding results justified the effectiveness and correctness of the proposed wideband model for the converter valve tower.

This paper proposes a frequency-based synchronization control (FSC) approach for the voltage-sourced converter in rectification and inversion modes of an HVDC station that is connected to a weak ac grid. Similar to the conventional vector current control (VCC) approach, FSC includes a conventional phase-locked loop (PLL), and its outer loop controller generates the current reference for the inner loop. Different from VCC, the inner loop of FSC utilizes a supplementary frequency-based control loop to provide damping for the oscillations due to the PLL and generates the angle/magnitude for the converter ac-side voltage. The inner loop of FSC switches to VCC to limit over-current during fault scenario. This paper develops a linearized dynamic model of VSC-HVDC station, analytically evaluates behavior of the FSC dynamics, and validates the results based on time-domain simulation studies in the PSCAD platform. The eigenanalysis and simulation results show that FSC can effectively mitigate the instability issues of the PLL even under very weak ac grid condition.

The underlying substation communication network (SCN) is of paramount importance in facilitating the advanced functionalities of substation automation. To design and maintain an efficient SCN to guarantee high transmission availability and information integrity, an accurate traffic model is required and the traffic characteristics need to be understood. This paper exploits the challenge of modeling and analyzing SCN data traffic from the data generating mechanism to the transmission and retransmission mechanisms, and reveals the symbiosis of short-range dependence and self-similarity. Recognizing that there is little research effort available, this paper proposes a viable approach in modeling the SCN traffic and mathematically analyzes the associated parameters aiming to accurately model the self-similarity behavior of SCN traffic, describes the symbiotic characteristics, and effectively predicts the traffic pattern. Further, the proposed modeling approach is validated and assessed through a case study by using a realistic 24-h dataset collected from a 110 kV substation's supervisory-control-and-data-acquisition system. The result clearly demonstrates that the model can accurately describe and forecast the SCN traffic with the autocorrelation function mean square error as small as $\mathrm{MSE}\left(\mathrm{m}\right)=2.077\times {10}^{-3}$ . The insights obtained from this work can significantly promote the design and operation of SCNs.

A saturated core fault current limiter (SCFCL) is a promising device for limiting the fault currents effectively, and improving the safety and reliability of power systems. Due to the core structure and the coupling effect between the ac and dc windings, traditional SCFCLs suffer the drawbacks of insufficient clipping performance and large size. Moreover, theoretical analysis of fault clipping performance has received little attention. Therefore, this paper proposes a novel bridge-type SCFCL (BSFCL). Compared with the traditional type, the BSFCL effectively reduces size and cost by using a bridge-type structure. In addition, the proposed additional limiting inductor improves the fault clipping performance of SCFCL. This paper first presents the principle and characteristics of BSFCL. Then, a theoretical analysis of clipping performance and parameter design method of BSFCL are presented in detail. Finally, the principle and performance of BSFCL are validated through electromagnetic simulation and experiments. The simulation and experimental results demonstrate the effectiveness of the theoretical analysis and the structure. The influence of different BSFCL parameters on fault clipping performance is also discussed.

In Australia and many other countries, distribution network service providers (DNSPs) have an obligation to their customers to provide electrical power that is reliable and of high quality. Failure to do so may have significant implications ranging from financial penalties theoretically through to the loss of a license to distribute electricity. In order to ensure the reliability and quality of supply are met, DNSPs engage in monitoring and reporting practice. This paper provides an overview of a large long-running power-quality monitoring project that has involved most of Australia's DNSPs at one time or another. This paper describes the challenges associated with conducting the project as well as some of the important outcomes and lessons learned. A number of novel reporting techniques that have been developed as part of the monitoring project are also presented. A discussion about large-volume data management, and issues related to reporting requirements in future distribution networks is included.

This paper presents and discusses the concepts of a gapless-power-quality disturbance recorder (G-PQDR) and a novelty detector. In the G-PQDR, power signals are divided into continuous frames and the energy difference between an actual frame and a reference frame is used for comparison. If the current frame is different from the reference frame, it will be recorded in the equipment and it becomes the new reference frame; otherwise, the frame is discarded and the frequency of the fundamental component is stored. The G-PQDR is able to detect and record the waveforms of the previously known PQ disturbances as well as those of the PQ disturbances not yet observed in power systems, thanks to the novelty detection concept. This characteristic is important in the context of smart grids where hidden disturbances can be detected by this methodology. For efficient data storage by minimizing the volume of the data stored, the G-PQDR uses lossy and lossless compressors. This paper describes the entire process of detection and compression, and compares the performance of the proposed method with that of a commercial-power-quality data recorder. In addition, some of the prototype implementation details are presented.

Dependable power quality (PQ) monitoring is crucial for evaluating the impact of smart grid developments. Monitoring schemes may need to cover a relatively large network area, yet must be conducted in a cost-effective manner. Real-time communications may not be available to observe the status of a monitoring scheme or to provide time synchronization, and therefore undetected errors may be present in the data collected. This paper describes a process for automatically detecting and correcting errors in PQ monitoring data, which has been applied in an actual smart grid project. It is demonstrated how to: unambiguously recover from various device installation errors; enforce time synchronization between multiple monitoring devices and other events by correlation of measured frequency trends; and efficiently visualize PQ data without causing visual distortion, even when some data values are missing. This process is designed to be applied retrospectively to maximize the useful data obtained from a network PQ monitoring scheme, before quantitative analysis is performed. This work therefore ensures that insights gained from the analysis of the data - and subsequent network operation or planning decisions - are also valid. A case study of a UK smart grid project, involving wide-scale distribution system PQ monitoring, demonstrates the effectiveness of these contributions. All source code used for the paper is available for reuse.

In conventional HVDC systems where the harmonic filters and shunt capacitors are installed at the ac grid side, the converter transformers inevitably suffer the detrimental impacts caused by harmonic penetration. In this paper, combined with an inductive filtering method, a parallel delta-connected filtering-winding scheme for 12-pulse converter transformer units is proposed to reduce the harmonic currents flowing through the transformers and, thus, to attenuate the harmonic effects on them. This paper describes the presented transformer configuration in terms of the wiring scheme of the 12-pulse converter transformer unit and corresponding flowing paths of different harmonic currents with the orders of $3,6n\pm 1\left(\mathrm{n}=1,3,5,\ldots \right)$ , 11 and 13 in the filtering winding circuit. Then, the mechanism of the proposed scheme on harmonic suppression to the converter transformers is explained theoretically. In an attempt to validate the theoretical analysis, simulations based on PSCAD/EMTDC and experiments in a laboratory prototype system are performed to evaluate the harmonic eliminating performance of the proposed scheme.

This paper proposes a probabilistic method aimed at estimating the voltage sag frequency of nonmonitored sites given the number of sags recorded by a limited set of voltage sag monitors. The proposed method is based on simulating a representative database of feasible pseudomeasurements by using probabilistic values of fault coefficient, fault impedance, fault type, and fault location. This information is processed through Bayesian filtering based on the Bayes' probabilistic theorem. This enables obtaining the most probable voltage sag index at nonmonitored busbars given the index measured at monitored busbars. This statistical estimation of sag indices at locations where there are no monitors constitutes the main contribution of this paper. This study is developed in three power systems: the IEEE 24-busbar reliability test system, the IEEE 118-busbar test system, and the transmission network of Ecuador with 357 busbars.

In this paper, a new dc switchyard structure for a multiterminal dc (MTDC) grid is proposed. Fast fault current breaking capability, which is essential for a dc grid, can be achieved with available HVDC breaker solutions (e.g., ABB's hybrid HVDC breaker). However, the hybrid HVDC breaker concept also opens up possibilities for unconventional (relative to ac) switchyard solutions which may be more cost effective. The main aim of this paper is to expound how the proposed dc switchyard can be built with hybrid HVDC breakers in a configuration where the number of components is much less compared to a conventional switchyard structure. After that, the switchyard operation, fault current handling flexibility, and actions against component failure are described conceptually. Finally, the performance of the proposed switchyard is validated and compared with that of a conventional double breaker double busbar switchyard structure for selected cases through time-domain simulations

Harmonic pollution of the traction power-supply system (TPSS) has become a vital concern for its adverse impacts on the tractive drive system of the train and power-quality issues of TPSS and connected power system. Harmonic modeling is the precondition to evaluate the harmonic pollution for TPSS. This paper systematically discusses the main harmonic sources in TPSS as well as their characteristics based on accurate field measurements. Moreover, the background harmonic voltage of the connected utility system and the harmonic of the train are compartmentalized and modeled with consideration of the stochastic and statistical features. This paper studies the background harmonic voltage with time slicing and probability statistics technique. Meanwhile, the harmonic of the train is described with the Norton equivalent model, which considers the interaction between harmonic voltage and harmonic current. Moreover, the influence of the working conditions and power output on the harmonic distribution is mentioned in the low-order harmonic modeling of the train. The results suggests the proposed methods can effectively restore the distribution and characteristics of the harmonics in TPSS.

In this paper, typical strengths, fault levels, and source impedances are thoroughly analyzed and calculated for the study of quality of supply in 230/400 V 50 Hz distribution systems. Considering all the disparity in distribution network design, this study is based on a comprehensive database containing typical arrangements and equipment in U.K./European systems, as well as on fully documented generic network models supplying four residential load subsectors in the U.K., i.e., from metropolitan to rural areas. Thus, this paper proposes an alternative method for determining reference values of network supply impedances and short-circuit fault levels at different points and locations of the medium-to-low voltage distribution system. The aim of this study is to provide a wider range of benchmark values than those stipulated in the IEC 60725 Standard, which only defines a single-reference threshold of public supply impedances for all types of distribution systems and residential customers. In order to assist network operators in the planning and design of their distribution systems, these values are further disaggregated and classified in this paper according to network/demand type.

This paper presents results of experimental and analytical evaluation of power-dependent harmonic emission of three common types of modern low-voltage (LV) power-electronic (PE) devices. After a detailed analysis of comprehensive test results, based on both existing and new waveform distortion indices, the development of component-based models of PE devices is discussed. This paper demonstrates the importance of including PE devices' controls for accurate modelling of their characteristics over the entire range of operating powers. Most of the analyzed PE devices exhibit strong power-dependent changes of characteristics, additionally influenced by supply-voltage conditions, which are important for the analysis of both existing networks and future 'smart grids'.

A method of calculating LC parameters of a reactive balancing compensator for AC arc furnaces at the supply voltage asymmetry is the subject of this paper. The developed method is illustrated numerically with the calculation of LC parameters of a balancing compensator for an AC arc furnace approximated by a linear model with fixed parameters. The presented method can be regarded as an initial step toward developing balancing compensation for ultra-high-power AC arc furnaces. The method of calculation of LC parameters of balancing compensators is based in this paper on the Currents' Physical Component (CPC) power theory.

Electronic-smart meters are increasingly installed in electricity networks of many countries. In several cases, their operation in parallel with photovoltaic inverters and other power-electronic devices can result in measurement errors reaching up to 45%. In practice, this problem cannot be tackled by current standards due to the gap in standardization of electromagnetic immunity and emissions in the 2–150 kHz range. This paper provides a comprehensive review of international and national standards, guidelines, technical reports, and papers on this challenging electromagnetic-compatibility issue highlighting the gap in the 2–150 kHz range. The ongoing standardization activity to establish both emission and immunity levels and suitable testing procedures is described in detail. Laboratory setups for testing the immunity of smart meters and the emissions of grid-tie inverters are described and experimental results to validate the suitability of the proposed approaches are presented.

Vibration measurement has been adopted in some utilities in Australia, Canada, and several European countries for online condition monitoring of the power transformer's onload tap changer (OLTC). By comparing measured vibration signals at different stages, it might be possible to assess changes of OLTC's condition. However, there are still considerable difficulties in correlating vibration signals to the events of OLTC operation, which may impair the capability of vibration measurement for OLTC condition monitoring. Therefore, arcing measurement is proposed in this paper to complement the vibration measurement. Arcing is provoked when the OLTC switching contact closes at a fixed tap position and can lead to electromagnetic signals flowing through transformer windings, finally reaching earth. The arcing measurement is achieved by using a high-frequency current transducer (HFCT) clamping on the transformer's grounding cable. The joint vibration and arcing measurement can provide a better means for interpreting events involved in OLTC operation and facilitating improved OLTC condition monitoring. Since HFCT measured arcing signals can be coupled with noise, a probabilistic wavelet transform is thus employed in this paper to extract arcing signals from noise. Field measurements on two different types of OLTCs are performed using the joint vibration and arcing measurement system to validate the proposed methodology.

This paper proposes a nonunit protection scheme for DC microgrids (DC MGs) utilizing only local measurements. This scheme is developed based on the natural characteristics of DC current and its first and second derivatives under fault transients. Since it is based on local measurements, the problems associated with the communication delay are avoided. The selected protection scheme detects and discriminates the faults within a few microseconds of its inception. In this paper, a method to calculate the thresholds, used for the protection scheme, is also discussed. The proposed scheme is validated on a ring-type DC MG architecture under different fault scenarios and tested through MATLAB/Simulink simulations.

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

This paper presents analytical and simulation studies of wind farms (WFs) connected to power system through a unified interphase power controller (UIPC), which should control power flow and the short-circuit contribution of WF. The UIPC is based on an interphase power controller (IPC) in which the phase-shifting transforms are replaced by two series converters (SECs). In this paper, the UIPC model is developed based on phase angles of injected SEC voltage and a control scheme is proposed to control power flow and limit the short-circuit contribution of WF. The WF model is based on an equivalent aggregated variable-speed double-fed induction generator. The PSCAD/EMTDC simulation results show that the UIPC is an effective and novel solution for power-flow control and limitation of short-circuit contribution of WF in power systems.

This paper presents challenges and potential impacts of photovoltaic (PV) integration in the low-voltage downtown secondary networks (downtown networks). In the conventional secondary networks, substation feeders are the sole source of electric power and establish unidirectional power to the downtown network. The network protectors prevent the flow of power from inside the network to the upstream feeder by disconnecting the circuit to protect the feeder transformer against upstream faults. The assumption of unidirectional power flow can be violated by PV generation due to the possibility of excess power inside the network. It is shown in this paper that a large number of network protector trips can occur and, thus, voltage collapse may follow even in low PV penetration levels. In addition, it is demonstrated that the reclose action of the network protector relays is adversely affected by the PV power. Other adverse effects of such distributed-generation units, such as voltage profile, line overloads, and flicker, are also briefly discussed. Finally, a solution is proposed, based on differential currents, to upgrade the network protector relays in order to avoid false trips due to excessive PV power. Part of the New Orleans downtown network is modeled and the study is performed through simulations.

This paper highlights the importance of chronology on voltage protection and control apparatuses in studies of voltage security in active distribution networks with high generation capacity. This paper presents simulation results of a real-life example of a long rural distribution feeder having three voltage regulators in cascade, and a customer whose demand accounts for a large percentage of the total feeder load. The customer is also an independent power producer and its location at the far end of the feeder yields an intricate voltage protection coordination when the customer ramps up and down its own generation at the peak-demand period.

Proliferation of distributed generations (DGs) and power-electronics-based loads is bringing about more harmonic-polluted power signals. While some failures may occur as a consequence of nonsinusoidal current or voltage waveforms, existing reliability models of protection relays have not taken the harmonic-related failures into consideration. This paper develops a comprehensive Markov reliability model to categorize the possible functional states of a component, say a transmission line, protected by a relay, operating in a harmonic-polluted environment. In order to make the proposed model practically tractable, it is further simplified in two ways, through merging the states of the same consequence. The first simplified model focuses on the power component reliability assessment; while the other is built for reliability analysis of a protection system. Calculation of transition rates and how the resultant model is mathematically evaluated are then discussed. Numerical analyses and outcomes are based on the real-world data taken from Canadian substations. Accordingly, the results and conclusions drawn in this paper would be interesting to both academia and industry. Moreover, the proposed reliability assessment methodology is readily applicable in scrutinizing the impacts of harmonic pollution on the protection devices of other power components.

This paper proposes an effective scheme for real-time operation and protection of microgrids based on the distributed dynamic state estimation (DDSE) that is applied to a single renewable distributed energy resource (DER) or other components. First, the DDSE can be used for setting-less component protection that applies dynamic state estimation on a component under protection with real-time measurements and dynamic models of the component. Based on the results, the well-known chi-square test yields the confidence level that quantifies the goodness of fit of models to measurements, indicating the health status of the component. With this approach, renewable DERs in microgrids can be protected on an autonomous and adaptive basis. Meanwhile, the estimated state variables of each component are converted to phasor data with time tags, and then collected to the distributed energy resources management (DERMS) of microgrids. These aggregated phasor data that are once filtered by the DDSE are input to the static state estimation in the DERMS along with unfiltered data sent from conventional meters, relays, and digital fault recorders, ultimately generating real-time operating conditions of microgrids. This paper also provides numerical simulations for comparing the DDSE-based approach with the conventional centralized state estimation in terms of data accuracy and computational speeds.

Several methods of attenuation of very-fast transient overvoltages (VFTOs) in gas-insulated switchgear (GIS) are under development. An approach that is currently gaining high attention among researchers is based on the application of magnetic cores dissipating the energy associated with VFTO due to eddy current losses and magnetic hysteresis losses. This paper presents an advancement in this approach by introducing a new material type (denoted in this paper as “nanoflakes”). This new material comprises macro pieces of nanocrystalline ribbons (with the size ranging in square millimeters), acquired from crumbled nanocrystalline cores of a standard design. The material is characterized by relatively high magnetic permeability, poor electrical conductivity, and high saturation flux level. The material is suitable for application as a coating layer applied to the surfaces of an inner and/or an outer GIS conductor, as a fill-in material in the GIS shielding elements, or as a standard shaped core. This paper presents the material major features, outlines its development process, and reports on validation of the material effectiveness in real VFTO conditions. The validation is based on the measurement of VFTO attenuation in a full-scale 550 kV GIS test setup.

This paper proposes and evaluates an alternative statistical methodology to analyze a large number of voltage dips. For a given voltage dip, a set of lengths is first identified to characterize the root mean square (rms) voltage evolution along the disturbance, deduced from partial linearized time intervals and trajectories. Principal component analysis and K-means clustering processes are then applied to identify rms-voltage patterns and propose a reduced number of representative rms-voltage profiles from the linearized trajectories. This reduced group of averaged rms-voltage profiles enables the representation of a large amount of disturbances, which offers a visual and graphical representation of their evolution along the events, aspects that were not previously considered in other contributions. The complete process is evaluated on real voltage dips collected in intense field-measurement campaigns carried out in a wind farm in Spain among different years. The results are included in this paper.

In this paper, the behavior of three-phase transformers in the presence of geomagnetically induced current (GIC) is investigated. The associated phenomena involve very slow transients, in the millihertz range, while representing significant magnitudes of even and odd harmonics of power frequency. As a result, the transformer response involves nonlinear and frequency-dependent effects. This paper considers the impact of variations in the transformer core construction and design parameters on its saturated behavior and the resulting reactive power absorption, when subjected to GIC. A detailed transformer model is described including air-gap and tank effects. The study results highlight significant parameters that can affect transformer reactive power and promote them as being important parameters for GIC studies. Furthermore, the transformer dynamics during the slow transient of the GIC variations and polarity reversal are discussed.

Harmonic resonance has become an important concern for the application of shunt capacitors in recent years. A potential solution to address this challenge is to convert a shunt capacitor into a passive filter. This paper presents design methods to configure a shunt capacitor as a C-type filter or a third-order high-pass filter with guaranteed resonance-free performance. The concept of resonance-free condition is first introduced in this paper. It is then used to develop filter design methods that always meet the resonance-free condition. The two filter configurations are also compared. It was found that the third-order high-pass filter has more advantages than the C-type filter. Another useful finding of this work is that the filter parameters, as determined using the proposed design methods, are independent of the system conditions. As a result, a lookup table for the filter parameters has been created to facilitate immediate use by the industry.

This two-part paper is intended to clarify definitions in dual transformer modeling that are vague, provide accurate modeling guidelines, clarify misconceptions about numerical instability, provide a unified dual model for a specific type of transformer, and introduce new paths of research to the power systems/electrical machinery community for low-frequency transients. Part I discussed the topology of duality-based models and some important issues, such as the most common approaches to derive dual models and their variety, the equivalence of negative inductance and mutual couplings to represent the leakage inductance of three-winding transformers, and the numerical oscillations caused by the use of nontopological models. This part of the paper discusses and compares white-, gray-, and black-box models. The paper also reviews hysteresis models (static and dynamic) and highlights the differences between the air-core inductance and saturation inductance. The available dual models for the representation of the transformer tank are then presented. A unified and accurate model of a three-phase core-type transformer adequate for all low-frequency transients is presented. Finally, concrete guidelines are presented for the appropriate selection of the model topology and parameters for different low-frequency transient studies.

Modular multilevel converters (MMCs) have become the converter topology of choice for voltage-source converter-high-voltage direct-current systems. Excellent work has previously been conducted to develop much needed average value models (AVM) for these complex converters; however, there a number of limitations as highlighted in this paper. This paper builds on the existing models, proposing numerous modifications and resulting in an enhanced MMC-AVM, which is significantly more accurate and which can be used for a wider range of studies, including DC faults.

The dc–dc modular multilevel converter (MMC), which originated from the ac–dc MMC, is an attractive converter topology for the interconnection of medium-/high-voltage dc grids. This paper presents design considerations for the dc–dc MMC to achieve high efficiency with reduced component sizes. A steady-state mathematical model of the dc–dc MMC in the phasor domain is developed. Based on the developed model, a design approach is proposed to size the components and to select the operating frequency of the converter to satisfy a set of design constraints while achieving high efficiency. The design approach includes sizing the arm inductor, submodule capacitor, and phase filtering inductor along with selection of the ac operating frequency of the converter. The accuracy of the developed model and the effectiveness of the design approach are validated based on the simulation studies in the PSCAD/EMTDC software environment. The analysis and developments of this paper can be used as a guideline for designing the dc–dc MMC.

In China, two modular multilevel converter (MMC) projects were recently commissioned, and another two are currently under construction. The real-time hardware-in-the-loop (HIL) test bench played an important role in validating the manufacturer's controllers. The full detail MMC model does not fit with the HIL test bench, which has to solve the circuit containing numerous switches and handles a large amount of inputs and outputs at a small time step for real-time simulation. The main challenge is to find a method to model and simulate the MMC systems with sufficient detail, accuracy, and speed. This paper presents an equivalent circuit method for the HIL test bench. The circuit inside the submodule is represented by mathematical equations, implemented in CPU or a field-programmable gate array, and solved in parallel to achieve real-time performance. This method-based test bench is used in those MMC projects in China and connected to the manufacturer's controllers for HIL tests. The model accuracy and simulation speed achieved by this method met the requirements of the HIL tests. In this paper, various scenarios are tested in an MMC HVDC study system. The results achieved by the proposed method have high agreement with those of a reference model in EMTP-RV.