Power Flow Calculation for VSC-based AC/DC Hybrid Systems Based on Fast and Flexible Holomorphic Embedding

The power flow(PF)calculation for AC/DC hybrid systems based on voltage source converter(VSC)plays a crucial role in the operational analysis of the new energy system.The fast and flexible holomorphic embedding(FFHE)PF method,with its non-iterative format founded on complex analysis theory,exhibits superior numerical performance compared with traditional iterative methods.This paper aims to extend the FFHE method to the PF problem in the VSC-based AC/DC hybrid system.To form the AC/DC FFHE PF method,an AC/DC FFHE model with its solution scheme and a sequential AC/DC PF calculation framework are proposed.The AC/DC FFHE model is established with a more flexible form to incorporate multiple control strategies of VSC while preserving the constructive and deterministic properties of original FFHE to reliably obtain operable AC/DC solutions from various initializations.A solution scheme for the proposed model is provided with specific recursive solution processes and accelerated Padéapproximant.To achieve the overall convergence of AC/DC PF,the AC/DC FFHE model is integrated into the sequential calculation framework with well-designed data exchange and control mode switching mechanisms.The proposed method demonstrates significant efficiency improvements,especially in handling scenarios involving control mode switching and multiple recalculations.In numerical tests,the superiority of the proposed method is confirmed through comparisons of accuracy and efficiency with existing methods,as well as the impact analyses of different initializations.

Preserving scheme for user’s confidential information in smart grid based on digital watermark and asymmetric encryption

As an essential part of the industrial Internet of Things(IoT)in power systems,the development of advanced metering infrastructure(AMI)facilitates services such as energy monitoring,load forecasting,and demand response.However,there is a growing risk of privacy disclosure with the wide installation of smart meters,for they transmit readings and sensitive data simultaneously.To guarantee the confidentiality of the sensitive information and authenticity of smart meter readings,we proposed a privacy-preserving scheme based on digital watermarking and elliptic-curve cryptography(ECC)asymmetric encryption.The sensitive data are encrypted using the public key and are hidden in the collected readings using digital watermark.Only the authorized user can extract watermark and can decrypt the confidential data using its private key.The proposed method realizes secure end-to-end confidentiality of the sensitive information.It has faster computing speed and can verify the data source and ensure the authenticity of readings.The example results show that the proposed method has little influence on the original data and unauthorized access cannot be completed within a reasonable time.On embedded hardware,the processing speed of the proposed method is better than the existing methods.

A novel two-degree-of-freedom spherical ultrasonic motor using three travelling-wave type annular stators

In order to promote the tolerance and controllability of the multi-degree-of-freedom(M-DOF) ultrasonic motor, a novel two-degree-of-freedom(2-DOF) spherical ultrasonic motor using three traveling-wave type annular stators was put forward. Firstly,the structure and working principle of this motor were introduced, especially a spiral spring as the preload applied component was designed for adaptive adjustment. Then, the friction drive model of 2-DOF spherical motor was built up from spatial geometric relation between three annular stators and the spherical rotor which was used to analyze the mechanical characteristics of the motor.The optimal control strategy for minimum norm solution of three stators' angular velocity was proposed, using Moore-Penrose generalized inverse matrix. Finally, a 2-DOF prototype was fabricated and tested, which ran stably and controllably. The maximum no-load velocity and stall torque are 92 r/min and 90 m N·m, respectively. The 2-DOF spherical ultrasonic motor has compact structure, easy assembly, good performance and stable operation.

Autonomous-synergic Voltage Security Regions in Bulk Power Systems

Determining security/stability boundaries is a common and critical means of preventing cascading failures induced by voltage-related issues,which represents one of the major challenges in bulk power systems.However,traditional approaches suffer from conservative issues and heavy computational burdens.To address these challenges,the concept of an autonomous-synergic voltage security region(AS-VSR)and the corresponding dynamic constraint coefficient pruning(DCCP)computation method,which fully consider the volt/var characteristics of bulk power systems,are proposed in this letter.Both linearized and nonlinearized robust optimization problems are introduced to obtain accurate results.The computational accuracy,time cost,and advantages of autonomous-synergic control are observed in the simulation results.

An Energy Efficiency Assessment Method for Distribution Network Containing DG

Energy efficiency assessment of distribution network containing distributed generation is one of the core contents of power grid construction. Aiming at the lack of a quantitative evaluation method for energy efficiency of distribution network containing distributed generation, a novel energy efficiency assessment method based on the super-efficiency model is proposed. Starting from the basic elements and operational requirements of the distribution network containing distributed generation, the energy efficiency assessment metric set is constructed. On this basis, the concept of generalized energy efficiency function is defined, and the super-efficiency model is used to assess the energy efficiency of the distribution network containing distributed generation. Finally, an example is given to evaluate and analyze energy efficiency. The results confirm the validity of the proposed method.

Capacitor Voltage Imbalance Mechanism and Balancing Control of MMC When Riding Through PTG Fault

Modular multilevel converter (MMC) based fault ride through (FRT) control is a promising solution to deal with the pole-to-ground (PTG) fault in high voltage direct current (HVDC) system. However, when MMC switches to the FRT control, capacitor voltage imbalance between upper and lower arms will occur, resulting in the deterioration of FRT performance. This letter provides a comprehensive analysis for the imbalance issue from the perspective of fundamental frequency circulating current (FFCC). It is found the imbalance during FRT stage will not expand continuously, but converge to a certain value gradually. The specific imbalance degree is closely associated with the amplitude of FFCC. In order to solve the imbalance issue, an open-loop balancing control is proposed. By introducing a fundamental frequency feedforward item to the inherent circulating current control, the proposed method can not only balance the capacitor voltages, but also minimize the amplitude of FFCC, and consequently the power loss of MMC during FRT process can be reduced. Finally, simulation results of PSCAD/ EMTDC verify the validity of theoretical analysis.

Defect Detection in c-Si Photovoltaic Modules via Transient Thermography and Deconvolution Optimization

Defects may occur in photovoltaic(PV)modules during production and long-term use,thereby threatening the safe operation of PV power stations.Transient thermography is a promising defect detection technology;however,its detection is limited by transverse thermal diffusion.This phenomenon is particularly noteworthy in the panel glasses of PV modules.A dynamic thermography testing method via transient thermography and Wiener filtering deconvolution optimization is proposed.Based on the time-varying characteristics of the point spread function,the selection rules of the first-order difference image for deconvolution are given.Samples with a broken grid and artificial cracks were tested to validate the performance of the optimization method.Compared with the feature images generated by traditional methods,the proposed method significantly improved the visual quality.Quantitative defect size detection can be realized by combining the deconvolution optimization method with adaptive threshold segmentation.For the same batch of PV products,the detection error could be controlled to within 10%.

Generation dispatch method based on maximum entropy principle for power systems with high penetration of wind power

This paper proposes a generation dispatch model based on the maximum entropy principle. The objective is to find an optimal generation dispatch strategy that minimizes the generation cost and satisfies the security constraints of power systems, while taking into account the uncertainty of wind power. Since in many situations, only partial information of the probabilistic variables can be obtained, the maximum entropy principle is introduced to find the most likely realized probability distributions of the power flow, thus providing an accurate probabilistic circumstance to solve the generation dispatch model. The proposed method is tested on the IEEE 39-bus system, and is compared with the methodologies based on Monte Carlo simulation and Gram-Charlier expansions.

Evolutionary Game-theoretic Modeling of Massive Distributed Renewable Energy Deployment Towards Low-carbon Distribution Networks

This paper proposes an evolutionary game-theoretic model of massive distributed renewable energy deployment in order to shed light on the self-organization sustainable developments of renewable energies in distribution networks towards low-carbon targets. Since neighboring buses can interact in terms of energy exchanges, the return matrices of individual buses in the evolutionary game are established based on profiles of loads and renewable energy generation. More specifically, an evolutionary strategy is proposed based on the return matrices for individual buses to determine whether or not to deploy renewable energies in the next round of the game. Then, a dynamic model is derived for analyzing the renewable energy penetration rate in the distribution network throughout the multi-round evolutionary game. In theory, this model can reveal the self-organization process of renewable energy deployment in the distribution network. With this model, the distribution network operator would be aided in designing the incentives for buses deploying renewable energies toward a pre-defined low-carbon target. Numerical results on an actual 141-bus system and a synthetic 2000-bus system have demonstrated the validity and efficiency of the proposed model.

Event-based Two-stage Non-intrusive Load Monitoring Method Involving Multi-dimensional Features

This paper proposes an event-based two-stage Nonintrusive load monitoring(NILM)method involving multidimensional features,which is an essential technology for energy savings and management.First,capture appliance events using a goodness of fit test and then pair the on-off events.Then the multi-dimensional features are extracted to establish a feature library.In the first stage identification,several groups of events for the appliance have been divided,according to three features,including phase,steady active power and power peak.In the second stage identification,a“one against the rest”support vector machine(SVM)model for each group is established to precisely identify the appliances.The proposed method is verified by using a public available dataset;the results show that the proposed method contains high generalization ability,less computation,and less training samples.

An Electricity Demand-Based Planning of Electric Vehicle Charging Infrastructure

The construction of charging infrastructure is an important prerequisite for the development of electric vehicles （EVs）. In this paper, the classification of charging vehicle models and charging infrastructure was firstly summarized, and the optimal charging mode of each type of EV model and the total electicity demand of charging were then analyzed. Combined with the general principle of the development and application of new energy vehicles in the city H, the model of electric vehicle charging infrastructure planning was designed. The case we proposed fully proved the effectiveness of the model.

Ripple-based matrix modeling and cross-coupling effect analysis of double-input DC-DC Boost converters

This paper deals with the modeling and cross-coupling effect analysis in double-input Boost converters with multiplex current control. A ripple-based multiplex current controlled matrix model is proposed to restore the system's high-frequency domain dynamics information and resolve the coexistence problem of the sample-and-hold effect in multiplex current controllers, which significantly improves the resolution of the conventional average model. Based on the proposed model, both sub-harmonic and low-frequency oscillations are identified in terms of stability analysis, and the inherent mechanism of these complicated nonlinear dynamic behavior is revealed, which not only illustrates the origin of the oscillations but also points out the dominant factors in diverse types of instability situation. Besides, cross-coupling effect analysis is performed to study the interaction between the input ports with the help of the Gershgorin band, and the mechanism of the special unbalanced oscillation phenomenon is revealed. Furthermore, the sensitivity analysis approach is used to identify the key parameters with respect to the cross-coupling effect, which provides more design-oriented knowledge for practical engineering. In addition, the benefits of the proposed model are further illustrated through a comparative analysis. Finally, these theoretical results are verified by experimental ones. These results are beneficial to the improvement of performance as well as the understanding of the cross-coupling effect of multi-input converters.