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As high-speed railways develop rapidly worldwide, the problems of neutral section and power quality are becoming increasingly prominent. These problems affect the normal operation of the power grid and the electric locomotives. The co-phase power supply schemes were proposed by researchers, which can solve these problems well. Among them, the MMC-based scheme (MMC-5 L) is a representative one. The existing scheme, when applied to the auto-transformer (AT)-fed system which is usually adopted by high-speed railways, needs to install an auto-transformer on the output side. To eliminate the auto-transformer and to obtain the more suitable and superior co-phase power supply schemes to AT-fed system, two improved schemes are proposed in this paper based on the novel MMC-based scheme (MMC-6 L) we proposed before. While establishing the mathematical model, creating the control system, and carrying out simulation verification of the proposed schemes, the proposed schemes are compared with MMC-6 L and MMC-5 L in terms of topological characteristics, control system complexity, power loss, and functionality. The results show that while the proposed schemes can fulfill the functions of the co-phase power supply scheme well like the MMC-5 L and MMC-6 L, they are more suitable and superior for the AT-fed system in terms of control system complexity and power loss. Among them, MMC-5L1CB performs best in power loss, and MMC-4L2CB performs best in control system complexity. [ABSTRACT FROM AUTHOR]

Autotransformer (AT) traction network (ATN) expands widely with the rapid development of high-speed railway (HSR). The fault occurrence in ATN will affect the safety and reliability of power supply for electric multiple units (EMUs). However, due to the nonlinear relationship between measured impedance and distance, the accuracy and efficiency of the fault location for ATN is facing serious challenges, especially with the diverse fault types and operation modes. In this paper, the general circuit topology of subsections is given and the characteristics of voltage distribution are analyzed according to the impedance matrix of ATN. Through the decoupling method, a comprehensive equivalent model adopting composite impedance is established, which can illustrate a series of characteristics for all fault types. Furthermore, the fault-location method is proposed to express the mapping between the fault position and the relevant electrical quantities for the subsection, and then a reasonable compensation for the effect of mutual impedance between faulty and non-faulty subsections is provided to improve the accuracy. Consequently, the multi-terminals synchronous information is employed for unified fault-location for ATN. Case studies including simulation and field testing have verified that the method is accurate and quite robust to various fault scenarios. [ABSTRACT FROM AUTHOR]

In this part of the paper (Part II), available technical solutions to minimize or prevent wildfires caused by power networks are reviewed in more detail. These methods include fundamental changes to power system equipment, periodic checkups and maintenance, identification of faults via voltage/current waveforms, changes to protection and relaying settings and coordination, and installation of new power system apparatus. The strengths and drawbacks of each method are discussed. The discussion section brainstorms some future possible solutions that smart-grid technologies can enable. The paper concludes that there has neither been enough research in universities nor enough investment by industry to develop viable preventive solutions for wildfires that initiate from the daily operation of electrical power grids. This, in the authors’ opinion, is the crucial time for investment in research and development to tackle the lethal wildfire problem while electric power networks offer “the lowest hanging fruits to pick.” [ABSTRACT FROM AUTHOR]

Focuses on the reliability and power quality at Champion International Corp. paper mill in Deferiet, New York. Numerous process interruptions experienced by Champion; Voltage sags emanating from the Niagara Mohawk Power Corp. transmission system; Causes and location of faults; Mitigation techniques.

Modern power systems face several problems at frequencies above the range of traditional stability studies due to the increasing penetration of power electronic devices. This phenomenon has been recently referred to as electromagnetic transient stability and is a growing issue in systems with power electronic converters at any power and voltage level, such as FACTS, high-voltage DC, interfaces for grid connection of renewable energies, custom power devices for power quality enhancement, among others. The study of power systems stability is usually carried out by simulation tools that represent the power system through phasor models. However, due to the faster dynamics of modern systems, the harmonic interaction among the controllers, converters, and electric power components, recent research results suggest that some stability analyses should be conducted with models suitable for wider frequency ranges, e.g., electromagnetic transient models. This paper reviews the use of frequency domain methods to analyze the influence of the modeling approach of passive power components on the results of electromagnetic transient stability studies. [ABSTRACT FROM AUTHOR]

HVDC technologies are widely acknowledged as one of solutions for the interconnection of renewable energy resources with the main electric power grid. The application of the latest modular multi-level converter (MMC) makes power conversion much more efficient. Due to the relatively low impedance in a DC system, DC fault currents may rise to an extremely high level in a short period of time, which can be very dangerous for HVDC converters. To improve the sustainability and security of electricity transmission, protection solutions for HVDC systems are being developed. Nevertheless, they have various drawbacks on fault signal detection and timely clearance. This paper proposes a protection method that provides a fast and reliable solution addressing those drawbacks. A protection algorithm based on travelling wave simulation and analysis is proposed to detect abrupt transient signals. The algorithm shows high efficiency, reliability, selectivity and has low sampling frequency requirements. The proposed protection method has been validated through a cyber-physical simulation platform, developed using a real-time digital simulator (RTDS) and IEC 61850 communication links. The obtained results show that the proposed method has good potential for practical applications. [ABSTRACT FROM AUTHOR]

Due to limited amplitude and controlled phase of current supplied by inverter-interfaced renewable power plants (IIRPPs), the IIRPP-side distance protection of lines connected to IIRPPs fails to detect the fault location accurately, so it may malfunction. The composite sequence network of a line connected to an IIRPP during asymmetrical faults is analyzed, and an adaptive distance protection based on the analytical model of additional impedance is proposed in this study. Based on open circuit property of negative-sequence network at the IIRPP-side, the equivalent impedance of power grid and current flowing through fault point are calculated in real-time using local measurements, which are substituted into the analytical model of additional impedance to calculate fault location. In the case of negative-sequence reactive current injection from IIRPPs during asymmetrical faults, the error of calculating fault point current from local measurements is analyzed and corrected to ensure reliability of the proposed protection. The proposed protection alleviates the effect of fault resistance in a system with weak sources. In addition, the proposed protection can adapt to different grid codes (GCs), the operation mode change of the power grid, and the capacity change of the IIRPP. PSCAD/EMTDC test results verify the effectiveness of the proposed protection. [ABSTRACT FROM AUTHOR]

The dc current injected into the earth from the grounding electrodes of an HVdc system could flow into the ac electric power grids and gas pipelines through their grounding points. The dc current distribution either in ac power grids or in pipelines would be affected by each other. In this paper, a numerical approach to simulate the dc current distribution in ac power grids and gas pipelines considering the interaction is put forward. The approach is validated by field test. The dc current distribution in ac power grids and gas pipelines is discussed based on the calculation result in a real case. It is found that the effect of ac power grids on the current distribution in gas pipelines is greater than the effect of gas pipelines on the current distribution in ac power grids. [ABSTRACT FROM AUTHOR]

The flow of electric power between the two power system networks needs an interconnection. This interconnection should be simple, cost-effective, reliable, and secure for electric power supply. In this paper, a flexible asynchronous ac link (FASAL) is presented for interconnecting two asynchronous independent power system networks. The FASAL system consists of a doubly fed induction machine (DFIM) operating as an ac link wherein two separate power system networks are connected to its stator and rotor. The electrical power is transmitted between the two networks by magnetic coupling through the air gap of the DFIM. It controls the power transmission between these power system networks, which are asynchronous under different operating conditions. The direction and the magnitude of power flow are controlled by controlling voltage and frequency. The performance and effectiveness of the FASAL system during steady-state and the fault conditions are evaluated and validated using PSCAD/EMTDC software. It is also compared with conventional back-to-back line-commutated converter-based high-voltage direct current (BTB LCC-HVDC) system, and it exhibits better performance under blackout and severe fault conditions. The FASAL system is also verified experimentally for two asynchronous power sources. [ABSTRACT FROM AUTHOR]

Consumers’ transformers play an important role in power systems, and they are essential for operation reliability and commercial benefits. In the past, maintenance personnel had to spend plenty of time on examining consumers’ transformers one by one. Nowadays, with the wide deployment of power user electric energy data acquire system (PUEEDAS), informative metering data are becoming available, which can be utilized for further condition monitoring. Thus, this paper proposes a data-driven abnormal condition monitoring algorithm of data acquisition for consumers’ transformers, which could timely send abnormal condition alerts to operators and maintenance personnel. In the proposed algorithm, Spearman's rank correlation coefficient is utilized to show the degree of correlation among phase currents, and its t-Statistics is used to determine whether abnormal condition of data acquisition exists based on the hypothesis testing. Finally, actual acquisition data from Zhejiang power system in China are employed to validate the effectiveness of the proposed algorithm, and to analyze the characteristics of normal and abnormal conditions, respectively. Sensitive analyses on different significant levels and sampling rates are performed for considering its impact on monitoring results; the application in real power systems is also given to demonstrate the practicality of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In this paper, a mathematically rigorous method is presented to calculate the impedance of FCLs based on a desirable change of short circuit currents. The method is found on the principle of superposition and can be easily incorporated into standard short circuit current calculations of power systems. The changes of the short circuit currents for each FCL installed are easily calculated without having to build a new impedance matrix for each new case. In addition, the method gives a clear graphical representation of the impact of the FCL on the power system taking advantage of the FCL impedance locus in the complex plane. The resultant plots provide valuable insight and information to utilities, system planners, and FCL manufacturers. FCL manufacturers can use these plots to identify the FCL impedance value and angle subject to the manufacturing and system constraints. The plots also allow utilities and system planners to compute the changes of fault current distribution caused by the FCL quickly and accurately. Results show that FCLs can increase short circuit currents in other branches. High-limiting FCLs may cause larger increase in short circuit currents of other branches and therefore a compromise may have to be made between fault current limitation and the maximum additional currents that are acceptable in other branches. Examples on a three-bus system and an eleven-bus system are presented for illustration and validation. [ABSTRACT FROM AUTHOR]

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]

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer specifications can result in increased equipment dimensions, thus complicating their installation, especially during refurbishment projects. This paper presents a study to assess the potential of various low-power instrument transformers (LPITs) for generator protection applications. Two types of LPITs were evaluated: the optical current transformer (OCT) and the stand alone merging unit (SAMU). To test the OCT, a 65-kA-peak test source was developed using a real-time simulator. This source was used to inject symmetrical and asymmetrical currents into LPITs for the assessment of protection accuracy class. Low-current tests were also performed to assess their metering classes. This paper presents the design of a high-current test bench, measurement uncertainty analysis, and analysis algorithms used for device evaluation. Analysis results are then presented and discussed in terms of their applicability to generator differential protection using SAMUs and OCTs. [ABSTRACT FROM AUTHOR]

The need for economic, reliable, and effective delivery of electric power has lead to the search for fast, efficient, and effective methods for diagnosing the insulation of high-voltage (HV) equipment in the power industries. The recent dielectric techniques that have been carefully considered by major industries for transformer insulation condition assessment are the recovery voltage method (RVM) and the polarization and depolarization current (PDC) measurement. However, due to the complexity of the transformer insulation structure and various degradation mechanisms under multiple stresses, insulation condition assessment is usually performed by experts with special knowledge and experience. In this paper, an expert system (ES) is developed, which imitates the performance of a human expert, to make the complicated insulation condition assessment procedure accessible to plant maintenance engineers. The structure of the ES is described in detail including knowledge base, inference engine, and human-computer interface. Examples of the application of the ES are also presented to confirm that the system can provide accurate insulation diagnosis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Water is damaging for power transformers since it accelerates aging processes, reduces dielectric margin, decreases partial-discharge inception voltage, and, therefore, increases the probability of unexpected failures. Taking into account that power transformers are the most expensive equipment in an electrical plant and that the electrical supply guarantee is closely related to their reliability, it seems clear that transformer water content is a variable that must be kept under control. Since water is a byproduct of aging reactions in cellulosic insulation, the amount of water contained in a transformer usually increases from levels below 0.5% in weight, after its manufacturing, to levels of 6% in weight at the end of its life. However, it is not unusual to find high humidity levels in not so-old transformers, for example, in those that have been subjected to repairs in field. These days, some techniques are available to dry transformers in the field. Most of these methods achieve proper drying of transformer oil, but a great amount of water usually remains in cellulosic insulation and water in oil rises slowly some months after finishing the process. In these cases, drying treatments, which are very expensive; would have limited effectiveness. In this paper, a finite-element study is presented that analyzes different field-drying methods that are currently widely used. The efficiency of these methods will be studied and compared, and recommendations about drying parameters will be given. [ABSTRACT FROM AUTHOR]

Preventive diagnosis and maintenance of transformers have become more and more popular in recent times in order to improve the reliability of electric power systems. Dielectric testing techniques such as return voltage measurement (RVM) and polarization-depolarization current (PDC) measurement are being investigated as potential tools for condition assessment of transformer insulation. A better understanding and analysis of the dielectric test results are only possible with a clear understanding of the physical behavior of the insulation system in response to moisture and aging. A circuit model, which describes the dielectric behavior of the transformer's main insulation system, has been parameterized in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been developed between the physical condition of the insulation and the equivalent model parameters that enable a clear and transparent interpretation of the dielectric test results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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”. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

As an important power system protection device, the condition of metal oxide surge arresters (MOSAs) in service could be monitored, and online monitoring might be considered in a few critical installations. In this paper, a new resistive components measurement method termed as the remote nonsimultaneous resistive current measurement method (RNS) for MOSAs online condition monitoring is proposed. The RNS remotely and nonsimultaneously measures all types of resistive current parameters of MOSAs by employing the remote nonsimultaneous phase difference measurement method and noninteger harmonic analysis algorithm. The simulation and application results indicate that the proposed method accurately measures various parameters of MOSAs under complicated field circumstances. [ABSTRACT FROM AUTHOR]

The world's growing population needs good-quality electric power to be efficiently transported to populated areas. High-voltage power capacitors are an important equipment in substations for efficient transport of energy or its quality improvements. Depending on the load conditions, capacitors may generate substantial amounts of sound, which can be disturbing for the people in the neighborhood. This paper presents a prediction model capable of estimating the audible performance of certain capacitor designs under certain load conditions. The predictions have the same accuracy as can be expected at certified acoustic laboratories (±1.5 dB). [ABSTRACT FROM AUTHOR]

Inductive coordination between power lines and pipelines has focused on the 60/50 Hz component in the past. Through field measurements and circuit-model based analysis, this research shows that harmonic currents in power lines can cause more significant voltage induction on pipelines. It is due to the relatively higher frequency and zero-sequence nature of some of the harmonic currents. In view of the findings, this paper presents two methods for assessing the severity of harmonic voltage induction. One is a circuit-model-based method and the other is a field measurement-based method. [ABSTRACT FROM AUTHOR]

Substation asset health has always been one of the utmost concerns for utilities. The substation secondary assets such as current transformers, potential transformers, and coupling capacitor voltage transformers play vital roles for system situational awareness and grid operations. This paper proposes a novel on-line substation instrument transformer health monitoring system (SITHMS), which merely uses time-synchronized and high-resolution phasor measurement unit (PMU) data. The SITHMS can provide early warning and alert utility personnel to diagnose the equipment malfunction and take proactive actions for asset management to prevent equipment failure. The core component of the SITHMS is a model-based three-phase substation-level linear state estimator, which features a built-in largest normalized residual (LNR) module that can detect anomalies in PMU data by using substation model and configuration, so as to probe if there is malfunction or potential failure of these secondary equipment. In addition, we present the system's implementation architecture and testing environment setup. Case studies with the PSCAD simulated data and field PMU data from actual substations at American Electric Power (AEP) demonstrate the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

The technology of a presaturated core fault current limiter (PCFCL) has particular importance due to its instantaneous limitation of the fault current in electric power grids and keeping its value below the ratings of the switchgear or circuit breakers with a reasonable safety margin. In this paper, a design methodology of single-phase, simple nonsuperconducting magnetic coils, PCFCL is proposed based on extensive electromagnetic time-domain finite-element simulations taking into account the relative performances of the constructive design parameters. These controlled finite-element simulations allow fine realistic details, such as the nonlinear magnetic saturation behavior of the iron core to be included, and studying the effect on general performance during dynamic reactions of the system. Moreover, the dynamic behavior of PCFCL is characterized in terms of steady-state voltage drop across its terminals, counter electromotive force induced on the dc coil terminals, and limitation factor of the fault current. Dual-core and single-core designs are investigated with a comparison of their ability of limiting the fault current. Results reveal that the dual-core design has superior performance than that of the single-core one. Finally, a design methodology flowchart has been proposed which depends on the extensive simulation results of different PCFCL topologies and published experimental results, where changing either the governing constructive parameters or the magnetic iron core design are considered. [ABSTRACT FROM AUTHOR]

Centralized control, which can overcome the ill interactional effects among decentralized controllers, impels a real need for fast and reliable computational algorithms to improve the accuracy of phasor and power measurements. This paper deals with discrete Fourier transform (DFT)-based measurement techniques. Based on the practical frequency tracking method proposed by the same authors in another paper, this paper develops practical compensation method to improve the phasor and power measurement accuracy of the DFT algorithm. The method proposed by this paper takes full advantage of the DFT algorithm. It simply modifies the DFT results. With notable accuracy improvement, the calculation burden of the proposed method is little more than the DFT algorithm. The method can be used for on-line purposes. Though the analog signal studied in this paper is sinusoidal signal, the method proposed by this paper has good performance in the presence of harmonics. Some examples are given to demonstrate the feasibility, simpleness and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper describes the investigation of the insulation condition of a number of power and distribution type transformers of different manufacturing dates and with different operating histories. Return voltage measurements were conducted for these transformers. Effects of moisture and aging on the oilpaper insulation of these transformers were investigated and results are presented in this paper. Moisture in oil samples at known temperature was measured for these transformers. Previously accelerated aging experiments were performed on paper wrapped insulated conductors in different environments over a temperature range of 115 to 145°C. Results from the measurements on transformers are compared with those of the accelerated aged samples and are described in this paper. [ABSTRACT FROM AUTHOR]

In order to make the best use of the existing assets, monitoring of power transformers provides the ability to continuously assess the overload capability of transformers. This can be more economically advantageous than the extension of the network by installing a new transformer. During the overloading periods, the cooling system of a power transformer plays a decisive role to transfer the heat generated in the windings to the surrounding ambient. The faulty operation of the cooling system should be detected by an online monitoring system before the transformer is overloaded. In this paper, an online algorithm is presented for malfunction detection of the cooling system based on the calculation of the standardized error in the calculated top-oil temperatures. Moreover, for calculation of the top-oil temperature, a new thermal model is proposed which uses a few design-dependent variables and considers different heat-transfer modes inside a transformer. For validation of the algorithm, the measured data during the normal operation of three transformers are used—two of them experienced failures in a part of their cooling system. The proposed model and algorithm can be easily implemented and integrated into monitoring systems due to their simplicity and good accuracy. [ABSTRACT FROM PUBLISHER]

This paper presents a methodology for the central-processing unit/field-programmable gate-array (FPGA)-based real-time simulation of a two-terminal MMC-HVDC of up to 501 levels using a detailed equivalent model. The proposed modeling approach is validated using case studies of 101-level and 501-level multilevel modular converter setups. FPGA-based models are designed using low-latency custom-made floating-point operators. The FPGA model is entirely reconfigurable in terms of the number of submodules per arm, as well as circuit parameters on an individual submodule basis. Real-time simulation performance is demonstrated in single-rate and multirate simulation environments and under various operating conditions. Limitations to the detailed equivalent model are identified and discussed. [ABSTRACT FROM PUBLISHER]

The total harmonic current emission in a public low-voltage (LV) network is mainly determined by the large number of electronic devices, which use different circuit topologies with different harmonic emission characteristics. Consequently, the total harmonic current measured at the LV busbar can provide valuable qualitative information, for example, about dominating circuit topologies. This paper describes and applies a methodology to characterize the harmonic current emission of an LV network by a prevailing phasor. Using measurements from 130 German public LV networks, the “prevailing” location of 3rd and 5th harmonic current is analyzed with respect to different types of consumers. The results are compared with initial measurements received from other countries. Based on the prevailing phasor, a simplified method for an approximate estimation of the impact (cancellation potential) of a particular LV network or a new equipment technology (e.g., electric-vehicle charging) on the harmonic levels in a public distribution network is proposed. [ABSTRACT FROM PUBLISHER]

Development and type testing of gas-insulated switchgear (GIS) disconnectors are supported with the simulation-based analyses of very fast transients (VFTs) that are associated with the disconnector switching operations. In order to analyze local field values in the entire GIS geometry, the full-Maxwell approach needs to be involved for simulating the VFT generation process. According to power substation layout studies, 90 ${}^{\circ }$-angled GIS disconnectors are often used in GIS projects, since they offer most layout options and, at the same time, require the lowest number of GIS components. This implies that the test setups are asymmetric, thus the direct use of the full-Maxwell approach requires 3-D models, the application of which is considered to be highly demanding from a numerical point of view. The paper shows a comparison of the results obtained from full-Maxwell numerical simulations for a development test setup of 1100 kV GIS. The analyses are conducted for full 3-D geometry and for the corresponding 2-D-axisymmetric geometry. Example simulations of the VFT overvoltage (VFTO) waveforms are presented for both geometries, together with a comparison of the numerical effort needed for solving the associated field equations. The presented approach based on the 2-D-axisymmetric GIS model allows one to significantly reduce the numerical effort involved to support design work and development tests. [ABSTRACT FROM AUTHOR]

As an industry consumer of electric power, an electric arc furnace (EAF) is a major voltage flicker source that causes significant power-quality (PQ) problems. The fast response of the static synchronous compensator (STATCOM) makes it an efficient solution for improving PQ in distribution systems facing this kind of very nonlinear load. In this study, for voltage flicker mitigation of a 100-MVA EAF installed in Europe, a voltage-source-converter (VSC)-based STATCOM with high bandwidth is designed. The STATCOM's current reference includes both active and reactive current components. The performance of the STATCOM in voltage flicker mitigation is highly improved by applying this control strategy which uses the dc voltage margin of the VSC to provide for an active compensating current without oversizing the STATCOM converters capacitor. In this paper, first, the proposed STATCOM is studied through a generalized steady-state analysis. Second, the STATCOM's control strategies for voltage flicker mitigation are introduced and simulation results are provided and compared. Finally the complete model is validated by field tests over a 60-MVAr industrial STATCOM. Static and dynamic performance of the STATCOM is evaluated and voltage fluctuation mitigation studies are performed and discussed. The voltage flicker mitigation is evaluated through onsite measurements respecting international standards. [ABSTRACT FROM PUBLISHER]

Multicore cables are widely used in medium- and low-voltage electric power systems. Current measurement of individual conductors in such cables is especially useful for various advanced applications but technically difficult to implement. Since existing current sensing devices are designed for a single conductor, one has to break the cable enclosure or connector to access individual conductors for current measurement, which is practically infeasible. A noninvasive current sensor for multicore cables is proposed, designed, implemented, and verified in this paper. It is able to measure the magnetic field outside the cable using a sensor array and reversely calculate the currents on individual conductors. It is easy to install and adapt multiple cable specifications. Such a device is able to determine current phasors, perform harmonic analysis, and detect current disturbance with sufficient accuracy. The effectiveness of the proposed approach is validated by both laboratory experiment and field test. [ABSTRACT FROM AUTHOR]

Reliable protection of modern distribution systems is challenging due to various technical difficulties, mainly related to the need for more efficient use of existing networks (e.g., shorter interruption durations and fewer affected customers, integration of distributed generation, meshed network operation, etc.). Such challenges require, among others, the reconsideration of conventional protection techniques. A potential solution under examination concerns the adoption of distance protection for distribution feeders, due to its advantages compared to overcurrent protection. However, as distance relays are mainly designed for transmission networks, there are several issues to deal with in distribution applications, such as the proper setting of the zero-sequence compensation factor (K 0). The latter issue is critical in order to guarantee correct relay operation during single-line ground faults. This paper examines the effect of K0 on the operation accuracy of distance relays protecting inhomogeneous distribution feeders. Theoretical analysis, as well as investigation of various influencing factors, result in the determination of a pattern, which relates relay accuracy to K 0. Based on this analysis, a simple methodology for setting K0 properly is proposed, which is implementable with commercially available relays. The methodology is applied on a test distribution feeder and meaningful conclusions are derived. [ABSTRACT FROM AUTHOR]

In order to solve the frequently occurring lightning flashover problems on ±500 kV dc transmission lines in China, the authors have conducted the research on the dc line arrester since 2011. In this paper, the structure and installation, technical parameters, and protection performances, as well as their deterministic processes of the arrester are presented and analyzed in detail. In 2013, the authors successfully developed the arrester products, and the type tests show that they have excellent performance parameters and can act effectively against the lightning impulse and cutoff the dc follow current. By now more than 500 dc line arresters have been installed and used on seven ±500 kV dc transmission lines in eight provinces of China. About four years of operating experience shows that the arresters work safely and reliably over a long period of time, and have been effectively reducing the lightning flashover rate of the ±500 kV dc lines. [ABSTRACT FROM PUBLISHER]

For installation in the public electric power grid of the city of Augsburg, Germany, a resistive superconducting fault current limiter (SFCL) was designed, constructed, tested and operated in the framework of a partially funded project ‘ASSiST’. Right from the beginning, it was agreed to aim for a prolonged operation even after closure of the project. This endeavor has been started due to a fundamental need to limit the prospective fault currents at a feeder point of the grid and in order to create long-term operational experience on a device seamlessly integrated into commercial grid components and switchgear, thus demonstrating a high level of readiness for widespread application. It revealed that the SFCL was of robust design – withstanding all electrical tests including a special two-phase fault and sustaining stable cryogenic conditions even during partial outages of the cooling system – and that the technology is reducing loss by shunt reactors. Furthermore, due to the control equipment on medium voltage level, valuable insights on grid behavior could be gained. The extended operation for 4 years has clearly shown the performance and robustness of SFCL technology in feeder points on MV level and provides a reference point for further commercialization. [ABSTRACT FROM AUTHOR]

This paper presents an inverter-interfaced distributed generation (DG) device, dubbed the premium power supply (PPS), which is designed to provide reliable electric power, and to improve power quality. This paper proposes an effective configuration for the PPS, using inverters, an inter-tie breaker, and a small generating unit. When the DGs are operated in parallel with the utility systems, some problems occur due to interconnections on such categories as reverse power flow and island protection. This paper advances both an operating strategy and a control scheme for the PPS, with respect to the interconnection standard, IEEE Standard 1547. Simulation results demonstrate the efficacy of the PPS. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy for harmonic power sharing in an islanded microgrid, without affecting the voltage quality at the load bus. The harmonic power distribution in an islanded microgrid depends on the line impedance. The proposed control strategy employs negative virtual harmonic impedance to compensate for the effect of line impedance on harmonic power delivery. The influence of mismatched line impedances on nonlinear load distribution is minimized by virtually varying the effective line impedance at harmonic frequencies. The fundamental active and reactive power sharing is implemented using the droop technique and the proposed method operates independently, without affecting the linear load sharing. Furthermore, a new droop method, called Zh - H droop, is also proposed to estimate the virtual harmonic impedance value adaptively. The proposed concept of negative virtual harmonic impedance and Zh - H droop is validated using simulation and digital signal processor-based experimental studies. [ABSTRACT FROM PUBLISHER]

This paper investigates the economic value of voltage sag mitigation devices to end users. The focus is on sensitive industrial plants. Different types of power system faults are simulated on a typical British distribution network model to obtain voltage sag profiles at various locations in the network. The cost-effectiveness of various mitigation devices is investigated for industrial-size end users with various process characteristics and sensitivities, to obtain the optimal investment profile for each end-user plant. The sensitivity of the assessment to various levels of initial financial loss resulting from process interruption and network fault rates is also investigated. The findings would serve as valuable references for industrial end users who are considering investment in mitigation devices to reduce the financial losses caused by voltage sags. [ABSTRACT FROM PUBLISHER]

For fault ride-through requirement, solar photovoltaic (PV) plant remains connected to the grid during a fault. In such a situation, a PV plant operates in different modes and supplies current to comply with the requirements of the grid. The relative magnitude of phase current contributed by a PV plant during a fault is at par with the nominal current. Such a fault behavior restricts the application of available phase current based relaying approaches while protecting the distribution system with a PV plant. A protection method using local positive sequence voltage and current is proposed to protect a distribution system with PV plant. The method selects pickup current using proposed current-voltage curve for fault detection. A three-dimensional time-current-voltage characteristic is proposed for trip time calculation. Performance of the proposed method is tested for relays on both PV plant side and grid end using DigSilent PowerFactory simulation data for a CIGRE 14-bus distribution system with PV plant having different modes of operations. The method is found to be accurate and comparative assessment reveals its superiority. [ABSTRACT FROM AUTHOR]

Sharing the same corridor of the HVAC and HVDC transmission lines can efficiently increase the power transfer capacity of the limited corridor and will be erected in East China. An accurate and efficient numerical analysis method for analyzing the ion-flow field of the ac–dc hybrid lines is helpful for engineering design. In this paper, an upwind finite volume-based algorithm for the solution of the ac–dc hybrid ion-flow field is proposed, which can take the influence of the space charges generated by corona into account. The electric field is solved by the charge simulation method and the finite-element method, while the advection and recombination effects of the space charges are solved by a time-dependent finite volume method. The reduced-scale ac–dc hybrid models are analyzed and the calculating results by the proposed approach in this paper have good agreements with the experimental ones in the literature and in this paper. Finally, the ground-level electric field and ion current density of the hybrid UHVAC and UHVDC transmission lines in parallel are analyzed, providing a reference for the engineering design. [ABSTRACT FROM AUTHOR]

This paper discusses the need to simultaneously monitor voltage unbalance and harmonic distortions in addition to root-mean-square voltage values. An alternative way to obtain the parameters related to voltage unbalance at fundamental frequency as well as voltage harmonic distortions is here proposed, which is based on the representation of instantaneous values at the dq axes and at the instantaneous Euclidean norm. A new power-quality (PQ) index is then proposed to combine the effects of voltage unbalance and harmonic distortions. This new index is easily implemented into existing electronic power meters. This PQ index is determined from the analysis of temperature rise in induction motor windings, which were tested for long periods of time. This paper also shows that these voltage disturbances, which are harmful to the lifetime expectancy of motors, can be measured by alternative ways in relation to conventional methods. Although this paper deals with induction motors only, the results show the relevance for further studies on other pieces of equipment. [ABSTRACT FROM AUTHOR]

This paper describes a new algorithm to calculate the per phase impedance between the measurement and fault points on a transmission line, featuring robustness to the uncertainties in load current and fault path resistance. The algorithm, which is ideal for distance relaying, is derived based on a justifiable assumption of the fault path being purely resistive, implying that the unmeasured fault point voltage and fault path current are in phase with each other. This paper illustrates the characteristics of the proposed algorithm and evaluates its performance by using PSCAD/EMTDC simulation. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a framework for assessing information security in power communication systems. The framework consists of dividing the communication system to be analyzed into its subcomponents and linking these to relevant evaluation criteria. In this study, the information security standard ISO 17799 has been used as a point of reference to define such evaluation criteria. The framework involves collecting data to evaluate each individual criterion and aggregating these evaluations using a robust algorithm. To cater for the many uncertainties in evaluating information security, the evaluation of the individual subcomponents is aggregated using a Dempster-Shafer based algorithm for evidential reasoning. This algorithm incorporates the many insecure facts and incomplete data that are inherent in large scale systems. The overall result is a set of indicators which highlight the level of information security within a studied communication system. The paper is concluded with a description of a case study in which the framework was applied to a communication system used for automatic meter reading (AMR). Experiences from this application are described in the paper. [ABSTRACT FROM AUTHOR]

Power quality indices have been developed to assess the performance of a single site or an entire system. They can guide utilities to improve their systems and can be used by manufacturers to decide about equipment immunity. This paper is the third part of a joint paper aimed at predicting the voltage sag performance of distribution networks by estimating voltage sag indices. Two types of indices are analyzed in the present paper, they give, respectively, the average number of events and the average interruption duration of each kilovolt ampere served at the low-voltage level over the assessment period. Their values are obtained from a stochastic prediction using a time-domain simulation tool. The methodology used to obtain these indices and the analysis of the influence of the protective devices are the main contributions of this paper. The limitations of the present work are also discussed. [ABSTRACT FROM AUTHOR]

The second part of this paper presents a practical implementation and application of the methodology for the stochastic assessment of the annual financial losses due to interruptions and voltage sags discussed in the first part of this paper. The costs of interruptions and voltage sags are determined separately and then combined in order to estimate the total financial losses in the network. The methodology is illustrated on a generic realistic distribution network with all relevant network components modeled appropriately. Finally, different network topologies are compared taking into account total financial losses in the network. It is shown that the inclusion of the financial losses due to voltage sags that may account for up to about 20% of the interruption costs may alter the network topology ranking. [ABSTRACT FROM AUTHOR]

In contrast to most other power-quality phenomena, it requires several years of measurements to obtain a good insight of the voltage sag problems at a given grid connection point. In some countries, sag measurements at high- or medium-voltage level of several years are available. This paper analyzes how to calculate the sag characteristics at lower voltage levels if the sags measured at higher voltage levels are only characterized as the largest value of the voltage drop in each phase. Whether these values are based on phase-to-phase or phase-to-neutral voltages turns out to be an important parameter and is therefore analyzed in detail. Based on this analysis, this paper gives guidelines on how new sag meters should preferably be connected (measuring phase-to-phase or phase-to-neutral voltages). [ABSTRACT FROM PUBLISHER]