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In recent years, there is an increased use of passive filters in both transmission and distribution systems due to the proliferation of power electronic devices. However, the problem of filter design has not been well addressed in the past and one example is the application of high-pass filters in a filter package. Three types of HP filters are widely used to mitigate multiple harmonics, i.e., 2nd HP filter, 3rd HP filter and C-type filter. There is still a lack of research to clearly reveal the characteristics of each filter, which makes it difficult for the designer to select the optimal filter topology under different circumstances. The main goal of this paper is to solve the problem of how to select a proper configuration among three HP filter candidates for a given harmonic problem. Unlike the conventional optimization-based filter studies, this paper investigates three HP filters in an analytical way. As a result, the paper provides a deep insight into the inherent characteristics of the filter and the conclusions drawn in the paper are universal; thus can be used in different applications, ranging from low voltage levels to high voltage levels. The research results lead to a recommended application scope of each HP filter, which is very useful to guide the design of filter packages and help designers to evaluate the design results. [ABSTRACT FROM AUTHOR]

The paper deals with a final experimental validation of three-dimensional Multiconductor Cell Analysis (MCA) developed by the Electric Energy Transmission Laboratory of the University of Padova. This final confirmation comes from the measurement campaign carried out on one of the longest (100 km) HVAC submarine three-core cables: the Sicily-Malta interconnector. The considered frequency measurements range from 0 to 40th harmonic order with both positive and zero sequence supplies. In this overall harmonic interval and with both supplies, the comparison between measurements and MCA results highlights a very good agreement (maximum differences of 10%). These differences are negligible if measurement uncertainties are considered. [ABSTRACT FROM AUTHOR]

Existing non-intrusive load identification methods face with the difficulty of being applied to industrial users whose operation and ON/OFF status are hard to be acquired. Focusing on the above challenge, this paper proposes a non-intrusive identification method of harmonic source loads for industrial users. Utilizing Clarke transform, an equation with respect to voltage and current is formulated to calculate the integrated equivalent impedance, which is then decomposed with the improved complex local mean decomposition (CLMD) and complex fast independent component analysis (C-FastICA) achieving non-intrusive separation of the equivalent impedance signals of individual branches. The separated impedance signals will be used as the basis for load type identification. The proposed method is validated by numerical simulations as well as actual three-phase voltage and current monitoring data. The case study shows that it can effectively identify the types of harmonic source loads while being resistant to noise and impacts of sampling accuracy and diverse load capacities, which obviously demonstrates its practical applicability. [ABSTRACT FROM AUTHOR]

This paper yields a systematic scientific foundation for the evaluation of responsibility of utility and customer to the harmonic distortions at the point of common coupling (PCC). In this regard, this paper proposes a technical process for the evaluation of harmonic distortions. This technical process provides a way to answer some technical questions related to harmonic issues based on the following aim: identifying the dominant side, determining the responsibility of each side to harmonic distortions, determining the influence of the utility on harmonic current distortions at the PCC, and determining the influence of the customer on harmonic voltage distortions at the PCC. This paper deciphers these technical questions in the concrete “relevant factors” for the evaluation of harmonic distortions. These “relevant factors” are systematically classified based on the equivalent circuits of utility and customer side for harmonic studies. In this regard, six well-defined “relevant factors” for the Thevenin equivalent circuit and six analogue “relevant factors” for the Norton equivalent circuit are formulated. Some of these “relevant factors” are proposed, for the first time, in this paper. Furthermore, this paper explains the relations between the “relevant factors” to ascertain their interdependence. In addition, the usefulness of the “relevant factors” to answer the technical questions related to the harmonic issues is illustrated using a calculation example. [ABSTRACT FROM AUTHOR]

High Impedance Faults (HIFs) occur by the contact between an energized conductor and a high impedance surface. Due to the low fault current level, HIFs cannot be detected by conventional protection and there is no fully efficient solution to this problem. HIF detection methods often extract metrics using signal processing techniques, such as Fourier Transform, Wavelet Transform, Stockwell Transform, and Mathematical Morphology. However, these techniques are applied under specific conditions, which hinders comparative and critical analyses among them. Therefore, this paper presents a critical review of HIF detection methods based on the aforementioned techniques, and also shows a detailed investigation of the performance of the metrics commonly used with them. To do this efficiently, the authors proposed a set of assessment indices based on the ratio between the metrics’ characteristics and another one based on the repeating of the metrics features. The proposed indices revealed that some of these metrics fail to distinguish HIF from other typical occurrences in power distribution systems, and their performances are negatively affected by the fault location and by the existence of noise in the measurements. Additionally, the results showed a need to specify system conditions in which any HIF detection technique is valid. [ABSTRACT FROM AUTHOR]

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

Modern electric grid usages extensive non-linear loads leading to the increased harmonic current injection in the system. This paper proposes a data-driven algorithm based on a sparse decomposition approach with the Overcomplete Hybrid Dictionary (OHD) and provides in-depth mathematical analysis to detect and mitigate harmonics in emerging electric power systems. The sparse decomposition method is widely used in image processing applications with significant advantages to big-data applications. However, its application is not properly addressed in electric grids’ applications despite massive data generation in a smart grid environment. Hence, in this paper, the performance evaluation of a sparse decomposition-based algorithm using a greedy approach is proposed and carried out in a real-time simulation environment for Distribution Static Compensator (DSTATCOM) application for the real-time detection, classification, and mitigation of PQ events to show its suitability and effectiveness. Finally, the experimental results on a small-scale laboratory setup are presented to validate the effectiveness of the proposed sparse-based control algorithm for DSTATCOM in real-time applications. The comparative result shows that the proposed sparse-based method is advantageous for off-line PQ signal processing and capable of real-time detection, classification, and control of the power apparatus for harmonics mitigation. [ABSTRACT FROM AUTHOR]

The use of synchronized measurement technology leads to a more reliable and secure operation of the power system. Phasor calculation is needed for all buses where a phasor measurement unit is installed, and fast and precise estimation is necessary for accurate monitoring, protection and control. The Prony algorithm is one promising method due to its capability to estimate phasor adaptively with changing frequency. The algorithm projects the signal on $L$ exponentially damped sine functions, where $L$ is the order of the Prony. However, the order under different conditions should be adapted automatically to reduce computation time. To adaptively specify the order, the empirical mode decomposition (EMD) method is proposed in this paper to be combined with the Prony and named EMD-Prony. The EMD decomposes a signal into finite single oscillatory modes, representing the number of modes in the signal. EMD is also used as a pre-processing step to filter noise from an input signal of Prony. Therefore, EMD is proposed here as an assistant of Prony in a phasor estimation. Finally, the proposed method is tested on the benchmark signals proposed in the IEEE standard, signals obtained from a simulated power system, and measured data from a real-world power system. [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]

Generalized integrator-based orthogonal signal generator phase-locked loops (GI-OSG PLLs) are very popular in single-phase grid synchronization. Generally speaking, the OSG of this PLL is realized from a second-order integrator perspective. As a result, the OSG implementation impacts the PLL dynamics. Improving the PLL performance by employing a reduced-order OSG implementation is the main motivation for this paper. Therefore, a reduced order-OSG PLL is proposed. In the proposed PLL, a single state and two output equations are employed to obtain a pair of orthogonal signals. This results in the loss of orthogonality between the OSG outputs and the single-phase supply voltage. Also, harmonic filtering capability provided by SOGI-OSG becomes non-existent. These issues are resolved by incorporating steady-state phase angle-offset error compensation at the PLL output and a moving average filter (MAF) within the PLL loop. To counteract the influence of MAF on the PLL’s stability and dynamics performance, a phase-lead compensator is designed. Through experimental studies, it is shown that the proposed reduced order-OSG PLL offers excellent dynamic performance. Also, the proposed PLL achieves minimal fluctuation of PV power when employed for grid synchronization of a PV system. [ABSTRACT FROM AUTHOR]

As the weakest components of wind turbine converters, dc-link capacitor withstands high-frequency ripple currents from both rectifier and inverter. Traditional wind turbine control strategy does not consider the influence of the harmonic current generated by both side converters on the dc-link capacitor, and ignores the restriction on the reliability of direct-drive wind turbine converter. For the reason, this paper synthesized the wind turbine model and dc-link capacitor harmonic spectrum analytical approach to derive the connection between mission profile and harmonic spectrum. Regarding with the capacitor lifetime model, the optimal mapping from control strategy to mission profile, maximizing the service life of capacitor, was solved. With the proposed health-oriented control strategy, the core temperature of dc-link capacitor can be effectively reduced, which can increase the service life of capacitor, making the dc-link capacitor no longer the most fragile component in wind turbine converter and ultimately improving the reliability of converters. [ABSTRACT FROM AUTHOR]

The existing methods for multi-point harmonic contribution evaluation usually require some preconditions, such as the node harmonic current is equivalent to the harmonic emission current and the number of measurements is not less than the number of sources. However, these requirements may not be satisfied in practice. In this context, this paper proposes an improved sparse component analysis (SCA) method to solve the underdetermined blind source separation model, which is used to describe the harmonic state under incomplete measurements. The improved SCA adopts a new two-stage technique. First, the mixing matrix containing harmonic transfer impedances is estimated through the synchrosqueezing S-transform. Then, the source signal vector containing harmonic emission currents is reconstructed through the modified subspace projection. By matching the harmonic impedances and currents, the multi-point harmonic contribution evaluation is further achieved. The efficiency of the proposed method is verified by simulations and field case studies. The results demonstrate that the proposed method outperforms the existing methods in terms of accurately estimating the harmonic emission currents and quantifying the harmonic contributions under incomplete measurements. [ABSTRACT FROM AUTHOR]

The periodic characteristic of modular multilevel converter (MMC), which originates from the multi-harmonic couplings among internal dynamics, results in challenges to develop the linear time-invariant (LTI) model of MMC. The linearization of MMC is inherently around time-periodic equilibrium trajectory. In this paper, two different methods to assess small-signal stability of MMC are compared. Both are based on the linear time-periodic (LTP) systems theory, but one is in the time domain (TD) and the other is in the frequency domain (FD). The LTP model of MMC is derived from the linearized arm averaged model. Then, an LTI model for the LTP system is constructed in the FD through the harmonic state-space method. Small-signal stability analysis based on Floquet theory and generalized Nyquist criterion, that accompany TD and FD, are introduced and compared. It is demonstrated that models developed in both domains can effectively capture the small-signal behavior of MMC. Third and higher-order harmonic components of the states have significant effect on the impedance-based stability analysis of MMC around mid-frequency range. Analytical results are validated by electromagnetic transient simulations based on the MMC aggregated model. [ABSTRACT FROM AUTHOR]

Multi-frequency (or harmonic) emissions and interactions are ongoing challenges for converter-dominated power systems. Consequently, multi-frequency models are introduced for power quality and harmonic stability studies. Rotor-Speed-Dependent (RSD) emissions from Type 3 Wind Turbines (WT) can lead to unexpected interactions. However, there are only a few specific studies on Type 3 WTs’ emissions. Besides, the provided analyses have been only based on field measurements. In this paper, the uncertainties of the field measurements are demonstrated experimentally. This paper provides an experimental analysis of emissions in different parts of a Type 3 WT, including stator, rotor, grid-side converter, and DC-link. In this way, theoretical emission patterns are formulated in sequence domain and verified by experimental measurements from a 2.2 MVA Type 3 WT tested by a 15 MVA grid emulator. The tests include emission measurements in different rotor speeds with identical output power set-point. To the best of our knowledge, the provided experiments are unique presentations from different parts of a Type 3 WT using MVA-scale torque and grid emulators. The test results provide a good impression of the existence, patterns, and amplitudes of the RSD emissions. Addressing such emissions in multi-frequency models can be necessary for the improvement of harmonic studies. [ABSTRACT FROM AUTHOR]

This paper investigates the voltage profiles and harmonic impacts of high-speed trains on the traction power-supply system (TPSS) and primary utility system. Power-quality (PQ) assessment has become not only a computing tool for TPSS design and planning, but also an indispensable technique for utilities to estimate the accurate PQ impacts from the railway systems. Therefore, to achieve comprehensive PQ assessment in TPSS, a dynamic fundamental/harmonic power-flow (DF/HPF) method is developed in a companion paper, while further application of the technique is described in this paper. The fundamental and harmonic results calculated in a 24-h period, such as loading levels, voltage profiles, unbalance, power loss, and harmonic distortions have been computed. In addition, the statistical measured background harmonics of the utility system are represented by Weibull function and considered in the harmonic evaluation. The unbalance and harmonic impacts are investigated and checked with national standards in this paper. The proposed method can be effective for excavating and predicting the potential serious PQ problems existing in the TPSS by using a train timetable. [ABSTRACT FROM PUBLISHER]

IEEE Standard 1547 defines grid connection standards for distributed resources (DRs). This standard sets requirements for harmonic performance, which are particularly relevant when converter-interfaced DRs are considered. Specifically, the standard limits the allowable harmonic current injected by a DR, assuming grid voltage at the point of common coupling (PCC) is harmonic free. In this paper, it is demonstrated, both mathematically and experimentally, that the harmonic current injection of a converter-interfaced DR depends highly on the level of PCC background voltage. This paper presents a mathematical formulation that enables the impact of PCC voltage harmonics on the harmonic current injection of a converter-interfaced DR to be determined via experimental identification of the converters frequency coupling matrix. Experimental tests show that an IEEE 1547-compliant solar inverter with measured total demand distortion (TDD) of 0.66% for an undistorted PCC can produce TDD levels in excess of 20% in the presence of 3% voltage distortion. It is concluded that TDD metrics, as employed in IEEE Standard 1547, provide little insight into the field performance of converter-interfaced DRs when background voltage harmonics are present. Improved FCM-based metrics for assessing the harmonic performance of converter-interfaced DRs in the presence of voltage harmonics are provided. [ABSTRACT FROM PUBLISHER]

Powers of three-phase aggregates of single-phase static linear loads supplied with a nonsinusoidal voltage are the subject of this paper. It is demonstrated in this paper that the supply current of such systems with a nonsinusoidal voltage can be decomposed into six mutually orthogonal components associated with distinctive physical phenomena in the load, two of which do not exist at a sinusoidal voltage. Such decomposition enables evaluation of contribution of these phenomena to the supply-current rms value increase and the power factor decline. A power equation and definitions of powers in such systems in the presence of voltage harmonics are proposed. This paper shows that all powers can be expressed in terms of load parameters, which can be identified by a measurement at the load terminals. [ABSTRACT FROM AUTHOR]

With the rapid development of renewable energy resources, harmonics in modern distribution systems have become more serious owing to the wide adoption of power electronic devices. Today, distribution-level phasor measurement units (D-PMUs), which can provide synchronized harmonic data with high-time resolution, are being rapidly developed. To utilize the D-PMUs, a hierarchical harmonic contribution (HC) determination method is proposed in this paper for systems with multiple harmonic sources. The HCs were determined using a widely linear complex partial least squares (WL-CPLS) method, which is both accurate and efficient. In an actual power system, the D-PMUs are not fully installed at all buses and the harmonics monitored by each D-PMU are actually the aggregate effects of multiple harmonic sources in the subarea. Based on the above consideration, a hierarchical HC strategy is proposed to locate the main harmonic contributors, layer by layer. The proposed method can be applied to a radial distribution system with any number of D-PMU placements. The methodologies are explained and verified using the IEEE 37-bus system. The real case study results based on the demonstration project are also analyzed in this paper. [ABSTRACT FROM AUTHOR]

Although the harmonics interaction plays a vital role in causing commutation failure (CF) in an ultra HVDC (UHVDC) system with hierarchical connection, it has not been sufficiently considered in previous work. In this paper, the electric interaction among the inverter valves of the UHVDC system with hierarchical connection during receiving system fault transient process is demonstrated. Moreover, the mechanism of CFs in UHVDC system with hierarchical connection is also investigated via the analysis of harmonics interaction. After the AC fault occurs, the appearance processes of the local commutation failure (LCF) and concurrent commutation failure (CCF) are divided into several stages and interpreted by schematic diagrams. And the characteristics of each stage are also concluded. In addition, a coordinated CF prevention scheme based on harmonics and DC current detection is proposed to mitigate the CFs in UHVDC system with hierarchical connection. Finally, the effectiveness of the proposed scheme is verified via a $\pm 1100$ kV UHVDC transmission system under various short circuit ratio values and AC-side coupling strengths. [ABSTRACT FROM AUTHOR]

In power systems, dense frequency signals (DFSs) comprise several adjacent harmonics or inter-harmonics derived from power electronic devices. Signals that contain two or more significant frequency components within any 15 Hz frequency band are considered DFSs. Existing frequency detection methods cannot detect DFSs with precision in a 10-cycle-period. To solve this issue, an improved dense frequency signal detection (DFSD) method is proposed. Firstly, this paper presents a dense frequency model by which a signal can be decomposed into a series of intrinsic mode functions via complete ensemble empirical mode decomposition (CEEMD). Then, singular value decomposition (SVD) is performed to reduce the dimensions and eliminate noise. Finally, the intensive frequency model is solved via an independent component analysis (ICA), and the parameters of each frequency component are calculated. Simulation and experimental results demonstrate that the proposed method can accurately calculate dense frequency parameters. Considering asynchronous sampling, the proposed method has higher accuracy for harmonic and inter-harmonic detection under the IEC framework. [ABSTRACT FROM AUTHOR]

Voltage source converters (VSCs) have been extensively used in power systems for interfacing renewable energies and other modern loads. Because of their nonlinear nature, a proper harmonic model of VSC devices must be established when conducting harmonic analyses in power systems. This is especially true for low-order harmonics, which are routinely encountered in present power systems. To this end, an analytical harmonic model that represents the low-order harmonic characteristics of VSCs is developed in this paper. The new model takes both the control loop and power loop (i.e., DC-link capacitor) of the VSC into account through rigorous mathematical derivations. The final result has shown that the VSC can be represented by a harmonically coupled impedance matrix at harmonic frequencies, which can be easily used for harmonic analyses of large-scale power systems. It also shows that the DC-link capacitor has a more noticeable impact on the harmonic impedance of VSCs with a smaller DC-link capacitance and low-order harmonics. The appropriateness of the new model was verified using EMT simulations and lab experiments. A complete procedure to construct the proposed VSC model is also established based on the research findings. [ABSTRACT FROM AUTHOR]

The effect of reduction in the terminal voltage of a grid-tied inverter together with minimization in total harmonic distortion (THD) of terminal voltage and current, in the presence of dead-band in the switching remains always challenging for the researchers. With high switching frequency, this issue increases by many folds. The grid-tied inverter used for harmonic compensation acts as a shunt active power filter (SAPF). The implementation of the dead-band elimination technique is based on output current polarity detection, suffers from multiple zero-crossing due to the generation of high frequency switching harmonics during SAPF operation. This paper deals with a dead band elimination methodology together with harmonic compensation using SAPF under non-linear and unbalanced loading conditions. Here, SAPF uses a dual fundamental component extraction (DFCE) scheme with modified complex coefficient filter (MCCF) to extract fundamental components for reference current generation to implement an indirect current control PWM scheme. MCCF extracts fundamental components even in the presence of dc offset and unbalancing at the input. For incorporating dead-band elimination in control, exact polarity detection is done separately through SAPF output current estimation using sensed load current, and modified CCF computed reference source current. Since the estimated SAPF output current contains no high-frequency switching harmonics, the dead-band elimination technique can be correctly implemented. To synchronize the zero-crossing of the estimated and actual SAPF current, an adaptive delay control is used. The proposed MCCF based control logic’s performance together with the dead-band elimination technique is validated for SAPF operating under non-linear, unbalance, and dynamic load conditions. The proposed dead-band elimination scheme based SAPF model is developed and simulated using MATLAB/ SIMULINK. The simulation results show a significant reduction in source current harmonics due to the elimination of odd harmonic injection into the system by incorporating dead-band elimination in SAPF operation. The real-time simulation of the proposed method is also validated using OPAL-RT OP4510 real-time platform. [ABSTRACT FROM AUTHOR]

Nowadays, nonlinear loads such as load commutated inverter (LCI) drives are being used by industrial sector throughout the world. However, despite the LCI drive uses low-cost thyristors and their corresponding controllers, which have high efficiency and low manufacturing cost, there are concerns regarding the use of LCI drives which are two-fold: (i) they generate harmonics and (ii) they are sensitive to voltage sags occurring at their PCC. In the literature, passive harmonic filters have been suggested to address the former concern and therefore various filter designing techniques have been introduced. However, the need to elaborate a cost-effective filter to address both concerns regarding LCI drive operation has been neglected. Therefore, this paper presents an innovative approach for designing passive filters which can decrease the harmonic distortions generated by LCI drives while ensuring more reliable annual operation of these equipment. Besides, other aspects of passive filter designing, such as eliminating detuning effects, resonance elimination and harmonic loading of capacitors, have been taken into account to provide a general and practical filter designing approach for real industrial applications. In order to evaluate the innovative proposed approach, a passive filter is designed for LCI drive used in an Iranian large copper mine company. [ABSTRACT FROM AUTHOR]

The neutral grounding method is one of the crucial technologies used to ensure the safe and stable operation of distribution networks. With the development of power electronics-based devices, the application of highly controllable inverters to grounding systems has become a new research direction. In this review paper, based on the summary and analysis for the research status, the flexible grounding system is systematically classified according to its control strategy and structure. And then, the following core technologies are summarized: faulted line selection, current component detection, high-resistance ground identification, ground parameter measurement, neutral point voltage control, and fault location. Subsequently, deficiencies of flexible grounding systems are summarized from the following four perspectives: reliability, accuracy, speed, and stability. Finally, some discussions and recommendations for future development and research direction are presented. Especially, the reliability-enhanced hybrid grounding system that combines active inverters and low resistance for high-resistance ground faults is recommended. This paper provides a systematic literature review of the crucial technologies for flexible grounding systems and can be used as a technical reference for future research. [ABSTRACT FROM AUTHOR]

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method. [ABSTRACT FROM AUTHOR]

Harmonic loads and unbalanced loading in a distribution transformer lead to higher losses, an increase in temperature that leads to premature failure, and a shorter lifetime. To prevent these difficulties, the rated power of a transformer supplying nonlinear loads or unbalanced load is reduced; commonly known as derating. In this paper, an alternative to traditional loss based derating is proposed. It is shown that derating might actually not be required when contrasted with the traditional approach. A perspective is presented on how to make the kVA rating “adaptive” and utilize the same kVA rating with or without harmonics. Actual field data are presented to support this viewpoint. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

This paper reports a practical challenge in Distributed Energy Resources (DER) operation: open phase detection. The paper presents a wide variety of naturally occurred open phase events recorded by power quality meters in DER commercial operations. The events demonstrate the complexity of open phase detection due to electrical and magnetic interphase couplings through transformer winding and core configurations. The coupling effect could recreate the missing phase in various patterns so that the DER control and protection systems or control room operators may not detect the anomaly for prolonged period of time. This paper shares experiences of timely DER open phase detection by using power quality data and suggests practical detection guidelines based on waveform signature analytics. The original field records provide credible benchmarking references for algorithm development to meet the new DER open phase detection requirement in the IEEE Stdandard 1547–2018 revision. [ABSTRACT FROM AUTHOR]

The harmonic impedance of the grid needs to be calculated for the harmonic assessment of new connections in the high voltage (HV) network. For these studies, one of the elements is the modeling of the downstream distribution networks. In the Netherlands, these medium voltage (MV) networks are cabled with a length between 20 to several hundreds km. These distribution networks could have a significant impact on the network impedance. For the HV network operator, it is almost impossible to model these networks in detail as not all information of the distribution network operators is readily available. The aim of this paper is to present a simplified representation for the distribution networks and develop a method to determine the parameters to be used in the model via a clustering method. To verify the method of clustering and compare the various equivalents, a comparison is made with an exact representation of a distribution grid. A sensitivity analysis has been performed on the most important parameters. It has been found that the Type-2 load model gives the best result compared the conventional ones. If the network information is known by the transmission system operators (TSOs), the Type-2 load model can be constructed directly. Otherwise the approach based on the clustering process as described in this paper can be used as an alternative. [ABSTRACT FROM AUTHOR]

Harmonic resonances in DFIG-based wind parks can be investigated by using extensive electromagnetic transient simulations, which is a time-consuming process. This paper shows that not all wind parks need such a detailed analysis. In response, the paper proposes a practical, easy-to-use method for a first-cut assessment of the risk of harmonic resonance of DFIG-based wind parks. First, simple equations are obtained to estimate resonance frequency and amplification in terms of wind park parameters readily available in practice. These equations reveal combinations of wind park parameters that cause problematic resonance. Such combinations are used to trace regions of concern in a chart. If the wind park is out of all regions of concern of the chart, one can directly declare this park is resonance-free, without running any simulation study. In addition to the analytic (model-based) method, a measurement-based method is provided for obtaining the resonance chart without running any simulation. Examples show how the proposed approach can be applied to facilitate resonance assessment during the wind park planning and operation. [ABSTRACT FROM AUTHOR]

This paper presents a technique to mitigate zero-sequence harmonics in power distribution systems. The method is based on the concept of passive zero-sequence harmonic filters. However, its basic configuration has been expanded to create a double-tuned filtering feature. This feature makes it possible to trap two harmonics with one filter and is especially attractive in solving harmonic-caused telephone interference problems. Furthermore, this paper has shown that common utility service transformers can be used to construct the filter. As a result, a practical and low-cost solution to mitigating zero-sequence harmonics has been found. A method for sizing and loading assessment of filters has also been developed. As an example application, the proposed filter package has been applied to mitigate a telephone interference problem. Issues, such as filter location, the number of filters required, and the effectiveness of filtering harmonics produced by distributed residential loads have been investigated. The results show that the proposed filter is a very promising technique to reduce zero-sequence harmonics in primary power distribution systems. [ABSTRACT FROM PUBLISHER]

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]

Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential protection failure to trip for converter transformer internal faults. The main factor responsible for the protection failure to trip is half-cycle saturation of the converter transformer core, which results from the dc component in the fault current. This suggests that harmonic blocking may not be suitable for differential protection of converter transformers to identify inrush conditions. In this regard, this paper proposes a novel method using mathematical morphology to effectively discriminate between the conditions caused by inrush and internal faults. This method implements the waveform symmetry criterion and improved morphological gradient criterion to distinguish between these two conditions. The effectiveness of the proposed method is evaluated based on extensive simulation, and the presented results show that the proposed method can ensure operation for all types of internal faults. Additionally, this method improves the accuracy of inrush identification, even in extreme cases, such as sympathetic inrush and inrush with current transformer saturation. [ABSTRACT FROM AUTHOR]

This paper presents an extended harmonic domain (EHD) model of a wind turbine generator (WTG) system based on a doubly fed induction generator. The proposed EHD WTG model includes both electrical and mechanical subsystems and permits monitoring step-by-step the harmonics time variation; this allows, among other applications, to detect contingency scenarios, such as resonance problems caused by a specific harmonic. Moreover, control efficiency can be improved by means of the power-quality indices which are instantaneously available by the EHD WTG model. The proposed model is capable of accounting for an arbitrary number of harmonics; however, the more harmonics that are included, the larger the system becomes. This affects the central-processing unit time by the transient simulation. An efficient heuristic remedy is proposed in this paper which consists of accounting for “high-impact” harmonics only, based on the converter's frequency spectrum. The EHD WTG model is validated with the power system computer-aided design/electromagnetic transients including dc (PSCAD/EMTDC) software tool via a case study. [ABSTRACT FROM AUTHOR]

This paper develops a simplified phasor model for predicting harmonics present at the point of common coupling between a voltage-source power converter and the utility network. It employs the frequency coupling matrix (FCM) modeling technique. The FCM is experimentally determined without requiring access to converter internal details, such as control and output filter parameters, which are generally not available for converters used in distributed generation systems. Experimental measurements are conducted on a commercially available two-stage, three phase, grid-tied photovoltaic inverter to assess the accuracy of experimentally determined FCMs to predict the harmonic behavior of grid-connected converters. This paper considers two grid conditions for the experimental analysis: 1) when the grid is stiff and contains only background voltage harmonics and 2) when the grid contains harmonic grid impedances as well as background voltage harmonics. In all cases, a close match between the results predicted by the FCM approach and experimental measurements is achieved, thus demonstrating the practical viability of using experimentally determined FCM models for harmonic predictions. [ABSTRACT FROM PUBLISHER]

This paper presents the design and application of a recently developed damped high-pass (DHP) filter for the line-commuted converter HVDC terminals. The DHP filter can damp resonance at the noncharacteristic harmonic frequencies commonly encountered by multipulse HVDC links. A design method is developed to address two unique issues faced by HVDC filter design: a wide range of system impedances seen by a HVDC terminal and a variable reactive compensation configuration at the terminal. Both are challenging filter design problems even for traditional filter banks. Using realistic industry case and sensitivity studies, this paper has shown that the proposed filtering scheme and design method can result in saving in filter costs and space requirements without sacrificing performance in comparison with the common filtering schemes used at HVDC terminals. In addition, the proposed design concepts are also applicable to the design of other filters facing the same design issues. [ABSTRACT FROM AUTHOR]

Sympathetic interaction between transformers is a quite normal phenomenon in power systems. For the purpose of preventing transformer differential protection relays from malfunction, this paper proposes a morphological method for the identification of sympathetic inrush, which is the first time when mathematical morphology is applied in this field. Since the waveform of differential current is symmetrical in an internal fault case while asymmetrical in a sympathetic inrush case, the proposed method uses a morphological operator to extract the peaks and valleys of the differential current to distinguish sympathetic inrush. Considering the possible current-transformer (CT) saturation conditions, this paper combines a morphological gradient with a weighted mathematical morphological operator to improve the effectiveness of the proposed method. The proposed method is evaluated on data collected from simulation cases established in PSCAD/EMTDC and from laboratory experiments, respectively. Identification results have verified that by comparing with the traditional second harmonic restrain method, the proposed method can distinguish sympathetic inrush from an internal fault current more accurately and more effectively, even when the CT is fully saturated. [ABSTRACT FROM PUBLISHER]

This paper proposes a new fast algorithm to estimate the fundamental phasor of fault current signals, based on discrete Fourier transform (DFT). Since fault currents include decaying dc component (ddc), DFT-based algorithms have inaccuracy in phasor estimation. The proposed algorithm consists of four steps. First, a new auxiliary signal is introduced based on a simple high-frequency modulation of the fault current. Then, DFT of the fault current and sample-summation of both fault current and auxiliary signal are calculated, for each one-cycle-length data stream. Next, DFT estimation error due to the ddc is calculated. Finally, by removing this error, accurate fundamental phasor is obtained. This paper uses an innovative and accurate formulation based on notations of signal processing literature. To validate the proposed algorithm, several computer-simulated signals and an electromagnetic transient program-generated signal are employed. The proposed algorithm is compared with recent phasor estimation methods, using wide variety of test signals. Standard indices of rise time, settling time, and percentage overshoot are used for comparison. These comparisons show that the proposed algorithm has robust performance in off-nominal frequency condition, and presence of harmonics, noise and multiple ddc components. Moreover, it provides faster convergence speed and lower computation burden. [ABSTRACT FROM AUTHOR]

Fifth harmonic voltages and currents have been measured in some extra high voltage (EHV) power transmission systems. This paper contains a discussion and explanation of this phenomenon. The emphasis is on the impact of overexcitation of power transformers in EHV systems. Examples of measurements taken in 500 kV substations are illustrated and representative data collected are shown. [ABSTRACT FROM AUTHOR]

Inverter transformers are one of the key components in a PV plant and their operating patterns are considerably different from that of transformers in a substation. Moreover, these transformers experience higher level of harmonics generated by the inverters. Therefore, investigating the ageing profile of the inverter transformer is a timely requirement. In this paper, the ageing of inverter transformers in a medium-scale solar PV plant is studied by utilizing various real-time measurements including load current, top-oil temperature (TOT), ambient temperature, moisture in top oil, solar irradiance and individual current harmonics. An updated moisture-dependent thermal model (MDTMH), with consideration of current harmonics is developed to estimate the hot-spot temperature (HST). The influence of weather conditions, which directly determine the load current of inverter transformers on the loss of life (LOL) of an inverter transformer is also investigated. Furthermore, the effect of current harmonics levels on the inverter transformer's HST and LOL is explored. Finally, comparisons of HST and LOL of inverter transformers connected to PV arrays with three different tracking technologies, i.e., fixed-tilt, single-axis and Dual-axis PV arrays are performed. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC) based devices have become widely accepted by the power industry in recent years as a result of renewable energy initiatives and other developments. Due to their nonlinear v∼i characteristics, it is essential to understand and model the harmonic impact of VSC devices in a power system. In response to such a need, this paper proposes a rigorous harmonic model of VSC devices suitable for harmonic power flow studies of large-scale power systems. Through detailed mathematical derivations, this research has shown that the model takes the form of a coupled frequency-dependent impedance matrix. Formulas to compute the impedance elements have been established. The results show that the impedance is independent of the harmonic voltages experienced by the VSC device, and thus it can be easily integrated into existing harmonic power flow tools. The accuracy of the model, as well as its usefulness for harmonic power flow studies, have been verified using EMT simulations. [ABSTRACT FROM AUTHOR]

This paper deals with the useless active powers and the reactive powers included in the unbalance power (SU1), and in the nonfundamental effective apparent power (SeN) defined in IEEE Standard 1459. Only the fundamental positive-sequence active power (P1^{+}) must be delivered to loads under ideal supply operating conditions. Fundamental negative- and zero-sequence active powers (P1^{-} and P1^{0}, respectively), and the harmonic active power (PH) are considered useless active powers because they increase the power losses in the load and in the distribution lines, reducing the global efficiency of the electrical power systems. SU1 and SeN are calculated in this paper for two cases, resolving the effective apparent power (Se) by means of the product between the terms of the effective voltage (Ve) and the effective current (Ie). It yields to expressions of some useless active powers and reactive powers that differ from the definitions included in IEEE Standard 1459. New definitions of Ve, Ie, and Se are proposed to avoid the disagreements detailed in this paper. The new definitions are compared with the IEEE Standard 1459 definitions by means of one numerical example. Magnitudes calculated by means of the IEEE Standard 1459 definitions present a general overvaluation that arrives at a maximum of 83% for SU1. [ABSTRACT FROM AUTHOR]

This paper presents measurements of harmonic and interharmonic current (with 5-Hz frequency resolution) in the complex plane. The complex currents are spread around a center point. For interharmonics, this center corresponds to the origin of the complex plane. For integer harmonics, this center is offset from the origin. A goodness-of-fit test reveals that phase angles of most interharmonics are uniformly distributed. A Monte Carlo simulation based on the measurements has been performed to study the aggregation of the emission from individual turbines to the public grid. Low-order integer harmonics show less cancellation compared to high-order harmonics. Interharmonics aggregate close to the square-root rule for uniform phase angles. [ABSTRACT FROM AUTHOR]

For an LCC–MMC hybrid HVDC transmission system with reverse current blocking diodes, this paper presents a method for dc-side harmonic current calculation and dc-loop impedance calculation. First, in the calculation of dc-side harmonic currents, the line-commutated converter at the rectifier side is replaced by the three-pulse harmonic voltage sources; the modular multilevel converter is represented by an equivalent passive circuit based on the linearization theory; an improved calculating method for the coupled line model is introduced into calculating the admittance matrix of the dc transmission lines to improve the computational efficiency; and then this paper describes the complete procedures of the proposed method. Second, based on the dc-side equivalent models mentioned before and the nodal voltage analysis method, this paper presents an analytical method for the calculation of dc-loop impedance according to the definition of dc-loop impedance Finally, the PSCAD/EMTDC simulation verifications have been carried out based on a 1500 MW/+ 500 kV MMC-HVDC system, a 3000-MW/\pm500-kV LCC–MMC hybrid HVDC system, and its dc network. The simulation results and the analytical results coincide with each other, and the effectiveness of the proposed methods is proved. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of a recently proposed online transformer internal fault detection technique and examines the impact of harmonics through detailed nonlinear simulation of a transformer using 3-D finite-element modelling. The proposed online technique is based on considering the correlation between the instantaneous input and output voltage difference (\Delta \bf V) and the input current of a particular phase as a fingerprint of the transformer that could be measured every cycle to identify any incipient mechanical deformation within power transformers. To precisely emulate real transformer operation under various winding mechanical deformations, a detailed 3-D finite-element model is developed. Detailed simulations with (non)sinusoidal excitation are performed and analyzed to demonstrate the unique impact of each fault on the \Delta \bf V\-\bf I locus. Impacts on harmonic order, magnitude, and phase angle are also investigated. Furthermore, practical measurements have been performed to validate the effect of a winding short-circuit fault on the proposed \Delta \bf V\-\bf I locus without and with the impact of system harmonics. [ABSTRACT FROM PUBLISHER]

Decaying DC (DDC) current due to impact loads causes detrimental consequences in weak grids. In this paper, the active power filter (APF) is enhanced to simultaneously compensate for the undesired harmonics, negative sequence components and the DDC component in the event of a transient. To this end, first, a novel DDC component detection method is mathematically deduced for single-phase transient current, followed by its extension to the three-phase system. With respect to existing techniques, the proposed method has a significantly reduced detection time from 1 grid cycle to approximately 0.5 grid cycle. By further exploiting the transient signal properties in dq-frame, the three-phase fundamental-frequency current in positive sequence can be separated. Finally, the enhanced APF control scheme is developed based on the proposed detection algorithm. Experiments verified the effectiveness and advancement of the proposed technique. [ABSTRACT FROM AUTHOR]

One of the impacts of harmonic resonances in wind parks is to overload wind park components. The process of identifying a problematic component may be time-consuming due to the number of scenarios and different components (capacitors, transformers, cables and generators) that must be analyzed. In this context, this paper develops a first-screening approach to narrow down the number of scenarios that must be studied in detail. It first identifies that the capacitor bank is the most critical component. Then, it uses the operating limits of the capacitor bank and the harmonic distortions measured at the wind park entrance to obtain a chart that maps the problematic operating conditions of the park. Detailed investigations are needed only if the wind park condition is in specific regions of the chart. The proposed chart is attractive for practical applications as it is obtained with a simple equation, without the need to run any computer simulation, and can be consulted with few information from the wind park that is readily available to engineers. [ABSTRACT FROM AUTHOR]

Harmonic pollutions in residential distribution system has become an emerging concern for utility companies. It is however difficult for the utility to have a systematic understanding on the severity of the distortion due to the lack of monitoring infrastructure. To address this issue, this paper proposes a practical solution to estimate harmonic distortions in residential distribution systems. The measurement is first taken at several representative service transformers and a unified harmonic model is developed for the secondary residential system. Then a probabilistic harmonic load flow algorithm is proposed with the network model provided by the utility. Importantly, the available harmonic measurements are considered, which significantly reduce the uncertainty of the estimation. Finally, the proposed method is verified using a real three-phase four-wire residential system in North America. Practical issues are also discussed to show the robustness and limitations of the proposed method. [ABSTRACT FROM AUTHOR]

The stator ground fault location of large generators can guide rapid maintenance. In this paper, a location criterion without fault transition resistance is constructed by using the fundamental potential and the third harmonic potential of the faulty winding simultaneously. The above potentials of the faulty winding are corresponding to the fault position. To precisely locate the grounding fault, it is necessary to clarify the analytical relationship between the faulty winding potential and the fault position. However, the existing methods often take the coil potential as the unit to analyze the winding potential, which has theoretical errors for the generators with short-pitch winding. Aiming at this problem, a more precise analysis method for winding potential is established by taking the slot conductor potential as the unit. To solve the location criterion, the optimization model for each slot conductor of the faulty phase is constructed. The slot conductor with the smallest optimal objective function value is judged as the faulty slot conductor, and the corresponding decision variable is judged as the fault position. Simulation and experiments verify the effectiveness of the proposed method, which can still meet the needs of engineering applications under multiple adverse conditions. [ABSTRACT FROM AUTHOR]