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Residual current protection can detect and isolate the grounding (leakage) fault of low-voltage distribution networks in time, which is an essential technical measure to reduce electric shocks and fire accidents and improve power supply safety. This paper systematically analyzes the operating characteristics of low-voltage distribution networks and proposes a distributed residual current protection method based on closed sections. It utilizes the capabilities of a distribution IoT platform to provide comprehensive measurement information for the entire substation area. A method was introduced to divide the low-voltage distribution substation into different protection closed surfaces, defining the current phasor at the external contact point of the closed surface and the remaining current of the closed surface. A calculation method for the critical current based on the remaining current of the closed surface for fault detection was proposed. Case studies have shown that this method is less affected by the inherent current and can significantly improve the sensitivity of protection. For TN-C and TN-C-S systems, in the selection of closed surfaces, the repeated grounding point of the neutral line is excluded from the closed surface. This method can also overcome the impact of residual current changes when load switching, demonstrating the flexibility of the new principle. [ABSTRACT FROM AUTHOR]

Eigenvalue Method can analyze the operating features of the system. The trajectory section eigenvalue method combines the model and trajectory, and extends the equilibrium point eigenvalue to the unbalanced point of the cross section at any time on the disturbed trajectory, which well solves the non-linearity and the time-varying issues of the system. Based on the theoretical basis of clarifying the trajectory section eigenvalue (TSE) method, the paper establishes a stability evaluation framework based on the time-varying second-order resonant circuit, which extrates the damping feature and frequency feature information in the eigenvalue sequence of the trajectory section, then compares it with the results of signal processing. The influence mechanism of time-varying factors on system stability is clarified, and the advantages of the trajectory section eigenvalue method in stability evaluation are demonstrated.

Power system analysis (PSA) is the first comprehensive curriculum for the major of electrical engineering at the undergraduate stage. It helps students obtain an overview of the power system, and systematically learn about fundamental theories. However, the traditional teaching of PSA course excessively focuses on the class lecture, and is combined with hardware-based experiments that are restricted due to concerns of security and cost. Enhancing the PSA course has been attempted at Shandong University, China, since 2016, which combines class lecture, laboratory exercises, and practical activities into the daily teaching. In laboratory exercises, project-based learning (PBL) is carried out by using the hardware experimental equipment and self-developed teaching software, VTPCE, hence to enhance students' understandings of knowledge, hands-on abilities, and researching skills. Practical activities are organized by visiting the topic-related institutes to establish the relationships between theories and practices. This paper introduces the architecture, the intended learning objects (ILOs), and the detailed arrangement of the reformed PSA course. The assessment procedures for students are also presented, as well as the satisfaction surveys. [ABSTRACT FROM AUTHOR]

A real-time harmonic estimator is proposed to provide a high-accuracy, computationally-efficient, and low-cache-cost solution that can be applied in embedded system for the online monitoring of numerous harmonics in wide frequency band in electronic power systems. Firstly, the fast-tuning filter bank, named FT-TFT FB, is presented, which can be fast generated by straightforwardly tuning the pseudo inverse of the basic single-frequency Taylor-Fourier basis matrix using the frequency-related diagonal weight matrix. Secondly, the robust FT-TFT estimator, named RFT-TFT, is proposed by estimating harmonics using the FT-TFT FB with a fast bias compensation process. It is verified using the numerical test that RFT-TFT estimator can achieve the same high accuracy as iterative TFT (I-TFT) estimator under steady and dynamic conditions but be much faster than I-TFT. The online application feasibility is further verified by implementing RFT-TFT estimator in C6748 DSP-based embedded prototype. Finally, this prototype is applied for the field signal analysis of $\mathbf {\pm }800$ kV Ultra HVDC transmission system and the real-time harmonic monitoring of one 0.3 MW distributed photovoltaic (PV) system, respectively. [ABSTRACT FROM AUTHOR]

With the accelerating integration of variable renewable energies (VREs), power systems become more vulnerable to active power disturbances, and more drastic frequency dynamics emerge. The battery energy storage system (BESS) is able to handle the uncertainties of VREs, and the decreasing system inertia and frequency regulation capability. This paper proposes a chance-constrained optimal configuration scheme for the BESS to maintain both the uncertain power fluctuations and frequency deviation within predefined limits. First, the required frequency regulation capability of the BESS constrained by the maximum transient frequency deviation (MTFD) and quasi-steady-state frequency deviation (QSSFD) is estimated. Then, the kernel density estimation method is utilized to model the net power fluctuations of VREs and load. A multi-objective chance-constrained programming model accounting for the life cycle cost, energy arbitrage, uncertain power fluctuation, MTFD, and QSSFD is established to optimize the capacity of the BESS. Furthermore, the Bernstein approximation is utilized to process the chance constraint, and transform the optimization model into a deterministic form. Based on the linear weighted method and Benders decomposition, the optimization model is solved through alternating iteration. Case studies were conducted to validate the proposed scheme, showing superior performance in smoothing uncertain power fluctuations, and reducing frequency deviation under contingencies. [ABSTRACT FROM AUTHOR]

The transmission line parameters vary with the change of temperature, which has a significant impact on power system state estimation (SE). Based on the theory of electro-thermal coordination (ETC), this paper proposes two ETC-SE approaches with the consideration of transmission line temperature. The heat balance equation (HBE) is combined with the conventional weighted least square SE for establishing an ETC-SE model. Moreover, an augmented Jacobian ETC-SE approach is developed by integrating the HBE into pseudo measurements and the line temperature into state vectors. The Jacobian matrix is augmented correspondingly and the partial differential coefficients of measurements to line temperature are provided, which enables to calculate line temperature and voltage phasors simultaneously. Furthermore, in order to accelerate the solving process, an improved two-step ETC-SE algorithm is proposed, in which the SE and temperature estimation are decoupled and solved via alternate iteration. The effectiveness of the proposed ETC-SE approaches is verified by the IEEE 14-, 39-, and 118-bus systems. The results show that the proposed ETC-SE approach is effective to reduce the calculation errors and possesses good convergence performance with varying environmental circumstances and ill-conditioned branches. [ABSTRACT FROM AUTHOR]

We present a reflective metalens with a hybrid antenna structure, which can realize the beam splitting function of two wavelengths in free space. A key feature of our design is the presence of phase-independent regulation at two different wavelengths in each nanoresonator, which can generate a specific phase or a continuous sweep in the range of 0° to more than 300°. To demonstrate the wavefront manipulation ability of the array, we simulated the characteristics of wavelength beam splitting, especially the free space wavelength beam splitter with a focal length of 300 μ m and a relative focal position of 100 μ m. [ABSTRACT FROM AUTHOR]

Microgrids incorporate an increasing number of distributed energy resources (DERs), which induce a higher variability and faster dispatch capabilities in power systems. This paper proposes a two-layer real-time scheduling model for microgrids, based on approximate future cost function (AFCF), where the future cost represents the opportunity cost for the microgrid operation in subsequent periods. At the upper layer, the look-ahead rolling scheduling is adopted to optimize microgrid operations, in which the future cost function (FCF) in deterministic and stochastic scenarios is approximated by a piecewise linear function. At the lower layer, a real-time parameter updating strategy based on real-time data is proposed. In this case, the real-time scheduling readjusts the look-ahead schedule using the immediate cost in the current period and the future cost calculated by the updated AFCF. The proposed two-layer real-time scheduling model uses an offline optimization, in which most of the computation tasks are completed at the upper layer, and applies a real-time optimization, in which the time-consuming problem is avoided at the lower layer. The effectiveness of the proposed two-layer real-time scheduling model of microgrids is validated by using a grid-connected microgrid system. For comparison, other existing real-time scheduling methods are also implemented in the same microgrid system. [ABSTRACT FROM AUTHOR]

Practical application of deep learning based non-intrusive load monitoring (NILM) system requires the deep neural network model to generalize on new unseen data. Existing NILM solutions are not suitable for real-world application due to their poor disaggregation accuracy on new unseen data. In order to address this problem, this paper presents a NILM algorithm that uses data augmentation to generate synthetic data for training deep convolutional neural network models for each target appliance. Proposed data augmentation technique works by combining on and off-durations of a target appliance from various datasets, and forms a unified and comprehensive synthetic aggregate and sub-meter profiles. Apart from proposed algorithm, this paper also proposes an evaluation approach that relies on total predicted energy and ground-truth energy of an appliance to provide detailed insights about total overlapping energy, missing energy and extra energy predicted by the algorithm. Comparison results on our proposed evaluation approach showed that proposed disaggregation algorithm was able to predict energy that was 60% overlapping with ground truth energy and 36% energy was extra. Overall results showed that overlapping energy was 2.5 times more, and extra-predicted energy was 60% less than state-of-the-art algorithms in unseen test cases. [ABSTRACT FROM AUTHOR]

Numerical simulation is the key technique for large-scale power system analysis. Redistribution of global renewable power via international interconnections requires new simulation tools to study the interconnected systems with different nominal frequencies as a whole. This paper introduces an open-source simulation toolkit for electrical power systems (STEPS) hosted at Github. Its kernel is coded in C++ with major functions of power flow and electro-mechanical dynamic simulation. Flexible options are provided and configurable to improve power flow solution and dynamic simulation. Common devices and models are supported in STEPS for AC/DC hybrid system studies. The study of interconnected systems with different nominal frequencies is supported in STEPS for research of international interconnection. Application program interfaces are provided and wrapped with Python to enable high-level interfaces for general applications. STEPS is thread-safe, and parallel computation is supported in both kernel-level and task-level parallelization to accelerate simulation. It is portable and works on Windows and GNU/Linux platforms. Cases from small to large-scale systems are thoroughly tested to validate the toolkit with commercial packages as benchmarks. [ABSTRACT FROM AUTHOR]

Detection of the high impedance fault (HIF) in distribution systems is significant for power utilization safety. In addition to the low fault currents, traditional approaches are invalid to detect HIFs also due to the diverse characteristics, including the slight HIF nonlinearity during the weak arcing process, the distortion offset caused by the lag of heat dissipation, and the interference of background noise. This paper proposes a distortion-based algorithm to improve the reliability of HIF detection under various conditions. Firstly, the challenges brought by the diversity of HIF distortions are explained according to the field experiments in a 10 kV real-world distribution system. HIFs are classified into five types according to the distortions of their current waveforms. Secondly, a definition of interval slope is introduced to describe waveform distortions. The interval slope is extracted by combining methods of linear least square filtering (LLSF) and Grubbs-criterion-based robust local regression smoothing (Grubbs-RLRS), so that the distortions under different fault conditions can be uniformly described. Thirdly, an algorithm is proposed to judge the features presented by the interval slope, and distinguish from non-fault conditions. Finally, the reliability and security of the proposed algorithm are thoroughly analyzed with real-world HIFs and the simulated HIFs obtained in IEEE 34-bus and IEEE 123-bus systems. Results show the improvements of the proposed algorithm by the comparisons with other advanced algorithms. [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]

Accurate modeling of arc is significant for the researches on the high impedance arcing fault (HIAF), which performs variations of nonlinear distortions under different fault conditions. The diversity of the waveform distortions during HIAFs is rarely investigated previously. This paper proposes a black-box HIAF model to simulate the nonlinear distortions of currents with improved controllability and higher accuracy. Firstly, with field HIAFs experimented in a 10 kV distribution network, three major characteristics varying in the distortions of currents are summarized and illustrated, including the offset, extent, and duration. Secondly, a distortion-controllable (DIST-C) HIAF model is proposed based on the heat balance equation after mathematical assumptions and transformations. The three characteristics of distortions can be independently controlled by the proposed model, specifically, by the three parameters with practical meanings. After illustrating the implementation of the model in PSCAD, an automatic parameter determination method is presented by taking advantage of the particle swarm optimization in a Python-PSCAD-MATLAB co-simulation platform. Finally, the controllability and accuracy of the proposed model are verified by comparing it with the existing typical black-box models. [ABSTRACT FROM AUTHOR]

Non-synchronous renewable energy sources (RESs) have strong volatility and low inertia, which brings about great challenges on the accommodation of RESs and the security and stability of power systems. This paper proposes a bi-level power system dispatch and control architecture based on the grid-friendly virtual power plant (GVPP), so as to accommodate RESs flexibly and securely. The typical dispatch and control system of the power system in China is presented, and the particular challenges stemming from non-synchronous RESs are analyzed. The functional requirements, concept, and fundamental design of the GVPP are provided, which is distinguished from traditional virtual power plants (VPPs) for its active participation in power system stability control. Based on the cloud platform, a bi-level dispatch and control architecture considering two objectives is established. First, in the inner level, the GVPP operates to promote the accommodation of RESs under normal condition. Then, from the perspective of out-level power systems, GVPPs serve as spinning reserves for power support under contingencies. Besides, the key problems to be solved in the development of the GVPP-based architecture are summarized. Although the architecture is proposed for the power system in China, it can be applied to any power systems with similar challenges. [ABSTRACT FROM AUTHOR]

The increasing permeation of the distributed generators in the power system brings great challenges for fault diagnosis, especially for the distribution networks with ungrounded neutral or grounded by Peterson coil as the fault current is limited and easily affected by the noises and interferences. A single phase-to-ground fault section identification method is proposed based on feature extraction of the synchronous waveforms and the calculation of the eigenvalues for the time-sequenced features. First, several fault features are defined and extracted from the synchronous current waveforms obtained by the fault recorders. Then, the topology related fault feature matrix is constructed using the time-series features obtained from different measurement sites, and the eigenvalues of the matrix are calculated based on the random matrix theory. Lastly, using the distribution characteristics of the eigenvalues, improved K-means clustering algorithm is utilised in classifying the fault cases and identifying the faulty sections. The effectiveness of the proposed scheme is verified by IEEE 34 nodes test system and a multi-feeder distribution network. [ABSTRACT FROM AUTHOR]

Due to the increasing penetration of variable renewable energies, the primary frequency response (PFR) capability of thermal power units is becoming more and more significant for power systems to tackle abrupt active power disturbances. Moreover, the actual thermal states of boilers that are usually assumed to keep rated and constant may degrade PFR capability, and incur unexpected severer frequency deviations and even instability. This study proposes an extended system frequency response (ESFR) model incorporating the influence of thermal states on PFR. The ESFR model is developed by extending the system frequency response (SFR) model with a boiler thermodynamic sub‐model, of which a low‐order and linear time‐domain expression is derived. Then, a quantitative assessment framework is established to assess the influence of thermal states on PFR capability, in which the instantaneous and accumulative quantities are calculated via the expressions of the ESFR and SFR models. Furthermore, based on the developed ESFR model, an improved estimation approach of dynamic frequency response considering thermal states is provided for the preliminary application. Case studies are conducted to validate the proposed model and approach. [ABSTRACT FROM AUTHOR]

High impedance arc faults (HIAFs) happening in the medium-voltage distribution system may result in damages to devices and human security. However, great difficulties exist in identifying these faults due to the much weaker features and the varieties when grounded with different surfaces. This paper presents an integrated algorithm to detect the HIAFs with high-resolution waveform data provided by distribution-level PMUs deployed in the system. An improved arc model is proposed, which can continuously imitate the randomness and intermittence during the “unstable arcing period” of arc faults. The integrated algorithm consists of two branches. First, the variations of HIAFs during unstable arcing period are identified with the unified harmonic energy and global randomness index, which can unify the scales of harmonic content in different fault situations and enlarge the disparities from non-fault conditions. Then, the waveform distortions of HIAFs during the stable arcing period are identified with discrete wavelet transform to extract the detailed distribution characteristics. The reliability and security of the proposed algorithm are verified with numerical simulations and field tests in a 10-kV distribution system. [ABSTRACT FROM AUTHOR]

Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China. [ABSTRACT FROM AUTHOR]

As a result of small fault current, high level of noise and a large penetration of distributed generators (DG), in the neutral non-effectively grounded medium-voltage (MV) distribution networks, it is quite difficult to locate the faulted line section for single phase to ground faults. In this paper, using a technique based on synchronized measurements in distribution networks, a fault location method based on the analysis of the energy of the transient zero-sequence current in the selected frequency band (SFB) is proposed. The equivalent impedance of the distribution network with lateral branches is studied with an equivalent network, and the phase-frequency characteristics of the equivalent impedance are analyzed. The SFB, within which the transient energy of the faulty line section is larger than that of the healthy line sections is determined. A combined fault-section location criterion is proposed and the implementation scheme is illustrated with the distribution level phasor measurement units. Numerical simulations of the IEEE 34 node system and the field experiments of a 10kV distribution network validate the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

With ultra high voltage direct current (UHVDC) and flexible alternating current transmission systems (FACTS) have been widely applied in power system, the AC/DC power grid is constantly extending which makes its composition more complicated. The independent electromechanical transient simulation and electromagnetic transient simulation cannot meet the need of rapidly developing modern power system in simulation technology. Electromechanical–electromagnetic hybrid simulation not only has the advantages of analysing large systems, but also accurate description of power electronic components. Analysing errors in hybrid simulations can help them to quantitatively analyse different hybrid simulation interface technologies to optimise simulation settings. Concerned fault types, fault location, and point fault cleaning time, an electromechanical–electromagnetic hybrid simulation scenarios probabilistic model based on Monte Carlo is put forward. Then adopt Prony method to quantitatively analyse the hybrid simulation errors. Through a typical example of UHVDC power grid in Shandong province, the error evaluation method of the hybrid simulation is demonstrated in detail. The research shows that this kind of assessment method can well analyse the numerical validity of hybrid simulation. [ABSTRACT FROM AUTHOR]

Micro-transmission lines are the physical model elements that own the same physical properties as the real transmission lines in power system, which are the significant components of a micro-dynamic simulation laboratory. This study presents an integrated experiment cabinet for micro-transmission lines (IECMTL). IECMTL integrates 20 micro-emulating transmission lines units, four micro-bus units and two light-synchronised phasor measurement units (PMU Lights). Benefitting from the same size and interface, researchers can change the position and wiring of any two transmission lines units according to the needs of the simulation experiment. PMU Lights are employed to get the operating status of micro-emulating transmission lines, including three-phase frequency, voltage phasor and current phasor. Appropriate software was developed on server to communicate with IECMTL in a two-way manner through the internet. Real-time monitoring and control features of IECMTL provide a flexible physical platform not only for innovative research ideas but also for education engineering of the power system. An example test-bed consisting of two IECMTLs was presented to simulate the power grids containing three generators and nine buses. [ABSTRACT FROM AUTHOR]

Distributed energy resources and new types of electrical loads are morphologically changing the signals of power grids. Grasping the wideband information of field data will play an important role in helping smart grids to operate in a safe manner. A wideband signal model composed of deterministic and stochastic components is proposed to enhance the wideband measurement. Based on the model, an adaptive decomposition scheme is designed for the offline analysis of wideband signals. In the scheme, a modified robust local regression smoothing method is presented to denoise the wideband signal. A totally data-dependent threshold is established to extract adaptively interharmonic components. Further, the Taylor–Fourier transform algorithm is used to estimate the parameters of deterministic components based on the subsignals extracted by the Chebyshev-II infinite impulse response filter bank. Finally, the stochastic components are extracted using the adaptive threshold based on the signal that the deterministic components are removed. The performances of the decomposition scheme under different conditions have been validated. In the end, the proposed decomposition scheme is carried out and verified with the field data of the 24-h current signals at one residence community. [ABSTRACT FROM AUTHOR]

In the active distribution networks (ADN), the interharmonic distortion level is aggravated due to the growing penetration of distributed generations and wide application of power-electronic loads. The interharmonics, even at a low amplitude level, can cause power-quality problems, deteriorate the measurement accuracies of the advanced metering infrastructures, and increase the possibilities of power system oscillations. In this paper, an interharmonic detection scheme is proposed to effectively extract the interharmonic components of the power system even with the extremely noisy signals, and then to estimate their frequencies and magnitudes. In the scheme, a totally data-dependent threshold is proposed to detect interharmonics adaptively. This adaptive threshold is determined iteratively based on the statistical character of the background noise bins. And then the time-varying frequencies and magnitudes of the detected interharmonics are estimated using the time-domain method. Case studies show that the adaptive threshold is robust under the conditions with high noisy signal, and the proposed scheme can estimate precisely the frequencies and magnitudes of the interharmonics detected for both the steady-state and dynamic interharmonic conditions. Furthermore, the proposed scheme is also verified using the field 10-kV current signal of one fuel company and 0.4-kV voltage signal of one electric vehicle's charging station. [ABSTRACT FROM AUTHOR]

The global climate change leads to more extreme meteorological conditions such as icing weather, which have caused great losses to power systems. Comprehensive simulation tools are required to enhance the capability of power system risk assessment under extreme weather conditions. A hybrid numerical simulation scheme integrating icing weather events with power system dynamics is proposed to extend power system numerical simulation. A technique is developed to efficiently simulate the interaction of slow dynamics of weather events and fast dynamics of power systems. An extended package for PSS/E enabling hybrid simulation of icing event and power system disturbance is developed, based on which a hybrid simulation platform is established. Numerical studies show that the functionality of power system simulation is greatly extended by taking into account the icing weather events. [ABSTRACT FROM AUTHOR]

This paper proposes a hierarchical control of thermostatically controlled loads (TCLs) to provide primary frequency control support. The control architecture is composed of three levels. At the high level, an aggregator coordinates multiple distribution substations and dispatches the primary reserve references. At the middle level, distribution substations estimate the available power of TCLs based on the aggregated bin model, and dispatch control signals to individual TCLs. At the local level, a supplementary frequency control loop is implemented at the local controller, which makes TCLs respond to the frequency event autonomously. Case studies show that the proposed controller can efficiently respond to frequency events and fulfill the requirement specified by the system operator. The users’ comforts are not compromised and the short cycling of TCLs is largely reduced. Due to the autonomous control, the communication requirement is minimized. [ABSTRACT FROM AUTHOR]