Table of Content

05 October 2021, Volume 54 Issue 10

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A Short-Circuit Current Calculation Method Based on Adjoint Network for DC Grid

WANG Ziwen, ZHANG Yingmin, LIU Tianqi, CHEN Ruochen, LI Feng

2021, 54(10):  2-10,27.  DOI: 10.11930/j.issn.1004-9649.202104070

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When a short-circuit fault occurs in a DC line, the sub-module capacitor is the main contribution unit for the short-circuit current before the blocking of the converter. Its extremely fast discharge speed poses a challenge to the safe and stable operation of DC grid. It is thus urgent to study the calculation method of the short-circuit current in DC grid. Firstly, the trapezoidal integration method is used to discretize the dynamic components in the established DC grid equivalent model. Then, the improved node method is used to construct the network equation, and the short-circuit current of the faulty branch is obtained through iteration. Furthermore, a new method is proposed to calculate the short-circuit current of DC grid based on adjoint network. A four-terminal real bipolar toroidal DC grid model is built in PSCAD/EMTDC software. According to the simulation results of different types of faults in DC lines, the proposed calculation method is compared with the typical state space method in terms of calculation accuracy and calculation efficiency. The error sources of the two methods are analyzed, and the effectiveness of the proposed algorithm is verified.

Analysis of Comprehensive Influence of Main Circuit Parameters on DC Short Circuit Fault Current of MMC-HVDC Grid

YUAN Min, MAO Meiqin, CHENG Dejian, ZHANG Liuchen

2021, 54(10):  11-19.  DOI: 10.11930/j.issn.1004-9649.202105015

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Due to the complexity of the modular multilevel converter based high voltage direct current (MMC-HVDC) grids, it is difficult to build an accurate analytical model for analysis of direct fault currents. In this paper, an analytical method based on central composite design(CCD) is proposed to quantitatively analyze and evaluate the comprehensive influence of main circuit parameters on DC short-circuit fault current (DSCFC). By the proposed method, the multi-factor regression equation is obtained with as few simulations experimental data as possible to realize quantitative analysis and evaluation of the comprehensive influence of multi-factors on DSCFC. Taking the four-terminal DC grid as an example, multi-factor simulation experiments are designed to analyze the influence of the four main circuit parameters including DC inductance, bridge arm inductance, neutral line inductance, and ground resistance on DSCFC. The statistical method is used to analyze and evaluate the main effects and interaction effects of each factor on the peak value of the DSCFC. The analysis results show that among the four main circuit parameters, the DC inductance has the strongest influence on the peak value of fault current within 6 ms after the faults, the neutral inductance has the strongest influence on the peak value of bridge arm fault current within 6 ms after faults, and the interactions between main circuit parameters are weak and can be ignored. The analysis results can provide a theoretical basis for the research of fault current suppression and the comprehensive optimization of multi-component parameters in the case of short-circuit fault in DC grid.

Influence of the VSC-HVDC System Structure on the Overvoltage of Single-Pole Grounding Fault

WU Jialing, LU Tiebing

2021, 54(10):  20-27.  DOI: 10.11930/j.issn.1004-9649.202009036

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Flexible operation mode is one of the VSC-HVDC system’s advantages, and the power system may continue to operate in the future with different grid structures under maintenance and other working conditions. However, for the VSC-HVDC system with overhead transmission lines, the DC lines are prone to single-pole grounding fault. Therefore, it is necessary to study the overvoltage of the DC lines with single-pole grounding fault for different grid structures. In this paper, a VSC-HVDC overvoltage model is built based on the ring-type four-terminal DC grid project, and the transient overvoltage characteristics of the VSC-HVDC system with the DC-side grounding fault are studied. The overvoltage levels of key nodes and equipment inside and outside the converter station are analyzed for four different VSC-HVDC grid structures with single-pole grounding fault occurring in the DC line. The results show that the system has a relatively low overvoltage level for the pseudo-U shaped structure, but has a relatively high overvoltage level for the broken-line shaped and real-U shaped structures. The system structures have the similar impact tendencies on the voltage to ground at the valve-side of converter transformer and on the overvoltage of the converter station bus.

Real-Time Modeling and Simulation of Flexible DC Grid with Various Types of Current Limiting Devices Based on RT-LAB

XIA Shiwei, GAO Chenxiang, SUN Yuhao, FENG Moke, ZHAO Chengyong, XU Jianzhong, CHENG Tingting, GU Huaiguang

2021, 54(10):  28-37.  DOI: 10.11930/j.issn.1004-9649.202105056

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Flexible DC grids have the characteristics of "low inertia and weak damping", and their fault current rises rapidly with high peak value. Current limiting devices usually contain a huge number of power electronic devices, which not only increases the complexity of the system, but also reduces the simulation efficiency of the DC grids. An equivalent modeling method is proposed for different current limiting devices, so as to improve the simulation efficiency of the model and meet the requirements of system design and operation analysis. Firstly, a digital simulation model of a 17-terminal flexible DC grid with various current limiting devices is built in this paper. Then, according to the generalized equivalent modeling method for single-port sub-module, an effective equivalent modeling strategy is proposed for four typical fault current limiting devices respectively. Finally, the digital model of 17-terminal DC grid is processed in real-time based on RT-LAB platform to complete the construction of the real-time model. The simulation results show that the proposed equivalent method and the real-time simulation model can accurately fit various working conditions such as steady-state and fault.

Derivation and Comparison of Modular Multilevel Converter Topologies with DC Fault Ride-through Capability

FAN Shiyuan, YANG Heya, XIANG Xin, YANG Huan, LI Wuhua, HE Xiangning

2021, 54(10):  38-45.  DOI: 10.11930/j.issn.1004-9649.202105101

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The DC fault has become one of the most serious obstacles for the development of DC system because of its wide impact range and large fault current. The modular multilevel converter (MMC) can realize the DC fault ride-through by deploying bipolar submodules, which however sacrifices the characteristics of both low construction cost and high efficiency under normal operation. In order to find out a MMC topology with consideration of hardware cost, operating efficiency and DC fault ride-through capability, four categories including thirteen kinds of modular topologies are derived by abstracting the general structure of DC-AC converter and allocating the basic modules for each arm. A T-type alternate arm multilevel converter is proposed with the arm phase-shift modulation that promises both high efficiency and DC fault ride-through. Various modular topologies with DC fault ride-through capability are compared in terms of construction costs, operating losses, reliability and applicability, which can provide a direction for DC-AC conversion applications.

A High-Voltage Large-Capacity DC/DC Converter Integrated with the Function of DC Circuit Breaker

QUAN Yue, WANG Zhiyuan, LI Binbin, XU Dianguo

2021, 54(10):  46-54.  DOI: 10.11930/j.issn.1004-9649.202105149

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In recent years, high-voltage and large-capacity DC/DC converters have received more and more attention as the key equipment for voltage conversion in the DC grid. This paper presents a high-voltage large-capacity DC/DC converter integrated with the function of DC circuit breaker, which uses the control of the converter itself to realize the function of blocking DC-side short-circuit faults, and has the advantages of simplicity, low cost and high efficiency. At first, this paper conducts a comprehensive analysis of the topology structure and operation principles of the converter, and deduces the parameter design basis for sub-modules, thyristors, diodes and other devices. And then, the mechanism for blocking sub-modules and thyristors is proposed to block the fault current, and the control schemes of the converter are also designed. Besides, simulations and experiments are carried out for normal conditions and fault conditions, and the results have verified the effectiveness of the topology structure and control schemes. Finally, through a comparative analysis with other typical DC/DC topologies integrated with the function of DC circuit breaker, the technical and economic efficiency of the proposed converter is evaluated.

Modeling and Prediction Analysis of Magnetic Field in DC Yard of VSC-HVDC Station under Short Circuit Condition

BAO Meng, SHEN Hong, LI Jialiang, SHI Yan, QI Lei, WANG Qian

2021, 54(10):  55-62.  DOI: 10.11930/j.issn.1004-9649.202012100

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For the voltage source converter based high voltage direct current (VSC-HVDC) station equipped with hybrid DC breakers, the DC circuit breaker has fast speed for breaking fault current and short time for internal commutation. The electromagnetic influence of magnetic field caused by fault current on secondary equipment in DC field is not clear. A multi-path magnetic field calculation model of hybrid DC circuit breaker and a magnetic field calculation model of smoothing reactor and DC bus are established to predict the magnetic field around the secondary equipment in the DC field of Dinghai Converter Station and Zhangbei Converter Station with bipolar short circuit fault at the DC side, and the distribution characteristics of magnetic field in the DC yard is obtained. The calculation results show that the peak value of magnetic field intensity around the existing secondary equipment in the DC field of Dinghai Converter Station and Zhangbei Converter Station can reach 883 A/m and 362 A/m respectively; the peak value of magnetic field intensity occurs at the time when the fault current reaches its peak value or the DC breaker has the second commutation; the magnetic field intensity is higher near the smoothing reactor and at the location where there are many buses in the DC yard. The calculation results can provide a reference for the evaluation of secondary equipment immunity and the layout of electrical equipment.

Calculation and Influencing Factor Analysis of PCOV in ±1 100 kV UHVDC Converter Station

MA Jiangjiang, QI Lei, ZHU Yi

2021, 54(10):  63-71.  DOI: 10.11930/j.issn.1004-9649.202001102

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The peak value of the continuous operating voltage including commutation overshoot is an important parameter that determines the reference voltage of the DC arrester in the converter station. Based on the Changji-Guquan ±1100 kV UHVDC project, the models of main equipment including converter valve, converter transformer, flat wave reactor and PLC filter are established. The PCOV level of each type of DC arrester is simulated according to the equivalent circuit model of the ±1100 kV UHVDC converter station. Compared with the results calculated by the formula method, the simulated PCOV level is consistent on the whole, with the maximum deviation of 13.4%, indicating that the formula method still has a certain safety margin. Based on the models established in this paper, the influencing factors of PCOV are further analyzed, and the results show that the damping resistance, damping capacitance, the iron inductance of the saturated reactor, the leakage inductance of the converter transformer and the inductance of flat-wave reactor have a great influence on PCOV calculation.

A Stabilization Control Method of Supercritical Reduced-Order Medium-Voltage DC Distribution System Based on Taylor Expansion

YUAN Yubo, YI Wenfei, ZHAO Xueshen, ZHU Lin, WANG Yizhen, LIU Haitao

2021, 54(10):  73-80.  DOI: 10.11930/j.issn.1004-9649.202012006

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In this paper, a typical medium voltage DC (MVDC) distribution system is taken as the research object, its supercritical reduced-order model is also established, and the effect of constant power load (CPL) on the system stability is also studied. The negative incremental resistance of CPL will lead to the voltage oscillation and instability of the system, which will adversely affect the stable operation of the system. For this reason, a stabilization control method which can improve the voltage stability of MVDC distribution system is proposed in this paper. The position of the dominant pole in the s-domain plane can be changed, which is due to the decentralized and flexible design of key control parameters in state feedback control, droop control and voltage control, and the stability of MVDC distribution system is improved. The simulation results verify the rationality of the supercritical reduced-order model and the effectiveness of the stabilization control method based on Taylor expansion.

A Control Strategy for High-Power Charging Piles Based on Bounded Uncertainty and Disturbance Estimator

GONG Xiaotong, LV Zhipeng, LIU Lan, SONG Zhenhao, ZHOU Shan, CHEN Quan

2021, 54(10):  81-88.  DOI: 10.11930/j.issn.1004-9649.202009038

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The high-power charging piles integrated into power grid are the key devices for the charging and discharging operation of electric vehicles across multiple voltage levels. However, due to the plug-and-play requirements and impulse charging power characteristics of electric vehicles, the integration of high-power charging piles will bring challenges for the stability of the grid operation. In this paper, a control strategy for high-power charging piles based on uncertainty and disturbance estimator (UDE) was proposed. First, in the dual active bridge converter (DAB) module of the high-power charging pile, the virtual direct current machinery (VDM) control strategy is adopted to realize the basic control of the DC voltage. Then, considering the impact of electric vehicle switching on the stable operation of the power grid, the UDE compensation control link is established based on the LC filter. The dynamic error of the filter unit is fed back with its output taken as the compensation component to achieve the compensation control of the DC bus voltage. In this way the stability of DC voltage can be effectively improved. Finally, by virtue of PSCAD/EMTDC software, a system simulation model with high-power charging pile is built. From the simulation analysis results the feasibility of the proposed control strategy has been verified.

Precise Control of DC Distributed System Based on Finite State Machine Matrix Modeling

LIU Haitao, XIONG Xiong, JI Yu

2021, 54(10):  89-96.  DOI: 10.11930/j.issn.1004-9649.202011130

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Power electronics dominated and multi-voltage are the two basic characteristics of DC power distribution in the future. How to achieve precise control and decentralized control is an urgent problem in the field of DC power distribution control. Aiming at the status of multi-operation state, multi-power electronic equipment and multi-control mode of DC power distribution system, a finite state machine matrix is established to describe the multiple states and mutual switching process of the system. At the same time, the system is based on the coordinated control strategy of each converter to achieve smooth switching and precise control. The proposed method uses the current state and triggering condition as the controller input variable and the new state and coordinated control strategy as output variables to establish a mathematical model. Finally, the correctness and effectiveness of the proposed method are verified by simulation analysis.

Adaptive Timescale Energy Optimal Predictive Control in DC Power Consumption Zone

YI Wenfei, SI Xinyao, CHEN Qing, LIU Haitao, JI Yu, XIONG Xiong, PAN Di, JIN Guobin

2021, 54(10):  97-103.  DOI: 10.11930/j.issn.1004-9649.202011055

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The integration of new energy generation, energy storage and flexible load parallel operation of the power terminal is an important part of DC distribution network optimal operation. In view of the limitations of the fixed energy optimization execution timescale, an adaptive timescale energy optimal predictive control in the DC power consumption zone with flexible dispatching units is proposed. The method is based on the adaptive adjustment of the energy output time scale of the power output ratio of the intermittent power supply in the DC power partition. At the same time, the lower-level comprehensive upper-level DC distribution network optimization instructions which are based on model free adaptive predictive control to achieve adaptive timescale energy optimal predictive control in the DC power consumption zone. The simulation results show that the proposed energy optimal predictive control method can realize the adaptive adjustment of the execution timescale, and under the condition that the flexible dispatching units are abundant, else and it can realize the fast and accurate tracking and execution of the energy optimal dispatching instruction of the superior DC distribution network.

Piezoelectric Materials and Devices for Monitoring and Diagnosis of Power Equipment

CHAI Bin, LIU Fei, JIANG Pingkai, JIANG Xiuchen, HUANG Xingyi

2021, 54(10):  105-116.  DOI: 10.11930/j.issn.1004-9649.202009096

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With the development of China’s smart grid and the expansion of grid scale, piezoelectric material based sensors and energy harvesters are playing an increasingly important role in online monitoring, troubleshooting and wireless sensor network. In this article, from the perspective of grid monitoring and self-powered sensors, an investigation is conducted on the recent research progress of voltage and electric field sensors, acoustic detection and environmental energy harvesters, and an introduction is also made to the traditional and newly-emerged piezoelectric materials such as inorganic piezoelectric ceramics, piezoelectrets, and piezoelectric composites. On the basis of current researches, the challenges encountered in the field of piezoelectric devices for smart power systems are pointed out, including frequency matching, environment stability and integration, etc. The practical application of new piezoelectric devices will need the assistance of emerging nano processing technologies, exploring the coupling effect of piezoelectric effect and other physical effects, designing new piezoelectric materials and energy harvesting circuits.

A Intelligent Method for Insulation Defect Detection of Switchgear Based on Multi-source Sensing

CAO Pei, XU Peng, HE Jianming, GAO Kai, TIAN Haoyang, JI Yiping

2021, 54(10):  117-124,133.  DOI: 10.11930/j.issn.1004-9649.202009053

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Insulation fault accounts for a large proportion in electrical equipment faults, so it is an important strategy to detect and eliminate the faults in their latent phase. Insulation defects are usually accompanied by temperature rise or partial discharge, which can be used as an important basis for judging the insulation status of the equipment. The infrared photoelectric sensor can detect the temperature of the equipment, and the ultraviolet photoelectric sensor can detect the ultraviolet pulse signal generated by partial discharge of the equipment. In this paper, taking the cable terminal defects in the switch cabinet as an example, an infrared and ultraviolet photoelectric sensor synchronous acquisition device is constructed. Based on the adaptive fuzzy neural network, an intelligent detection method is proposed with combining two information sources of temperature rise and partial discharge. The experimental results show that compared to the information detection with single sensor, the diagnosis algorithm based on multi-source sensing significantly improves the accuracy of equipment defect diagnosis. The proposed method can provide a new research idea for identification and diagnosis of insulation defects of switchgear.

Synchronized Switch Harvesting on Capacitor Integrated Circuit for Piezoelectric Energy Harvesting Application

FAN Shiquan, CHEN Yunxiang, XIE Ying, YUAN Chenxi

2021, 54(10):  125-133.  DOI: 10.11930/j.issn.1004-9649.202009095

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An improved efficient energy harvesting integrated circuit is proposed for piezoelectric energy sources. By using the synchronized switch harvesting on capacitor (SSHC) interface circuit, the charge re-sharing is realized when the internal current source of the piezoelectric energy source crosses zero. The charge on the internal capacitance of the piezoelectric energy sources is transferred to the external capacitor, and then the charge on the external capacitor is reversed through switch control, and finally the reversed charge is transferred back to the internal capacitor. This method avoids the energy loss caused by the charge stored on the internal capacitance of the piezoelectric energy source being neutralized after the internal current source crosses zero, which achieves higher energy harvesting efficiency. The designed SSHC circuit is fabricated with standard 0.18 μm CMOS process, with an active area of about 0.06 mm². The post-simulation results show that when the open-circuit voltage (OCV) of the piezoelectric energy sources is 2.8 V, the maximum voltage flip rate of 81.8% can be achieved. Compared with the standard full bridge rectifying structure, the proposed structure achieves the maximum 8.1X improvement in energy harvesting capability.

Design of Matching Network for Magnetic Field Energy Harvesting in Wide Frequency Range

NIE Qingqing, PENG Han, LI Hongbin, KANG Yong

2021, 54(10):  134-143,195.  DOI: 10.11930/j.issn.1004-9649.202009034

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Magnetic field induction energy harvesting is an effective solution for self-powered sensors in electromagnetic environment. Different from electric field and RF energy of single frequency, the electromagnetic field in power conversion systems contain wide frequency ranges. To increase the power density of self-powering module, it is important to harvest as much as power within limited volume. A dual-resonance bandpass filtering network is thus proposed. Furthermore, the output characteristics of the single LC resonant network and dual-resonance bandpass network are compared under different current patterns and load conditions. The suitable application conditions for dual-resonance bandpass network are summarized.

Design and Application of a New-Type Power Cable Multi-parameter Live Detection Device

ZHANG Cheng, WANG Weidong, YANG Yanbin, ZHAO Yang, ZHOU Yi, ZHANG Jingcheng

2021, 54(10):  144-151.  DOI: 10.11930/j.issn.1004-9649.202007084

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Current power cable live detection technology has such problems as complicated operation, high detection difficulty, numerous equipment types and high testing personnel requirement. A new type of multi-parameter live detection device is thus designed for power cables. By configuring different types of sensor devices, it can detect a number of state quantities of power cables such as partial discharge, infrared-measured temperature, ground current and load current. Different sensing devices are modularized to facilitate the field detection. At the same time, the device has the functions of wireless communication transmission, human-computer interaction, data storage, etc. After interconnection with the cloud platform, it can realize the applications of platform detection task release, detection data return, data analysis and abnormal diagnosis, consequently reducing the technical requirements for detection personnel and meeting the requirements of intelligent inspection of power cables under the Internet of things mode. Through practical field application, the device can effectively improve the efficiency and enhance the application level of live detection of power cables, and save about 2/3 of the field workers, which has a wide range of practical and promotional values.

Numerical Analysis of Bolt Preload of Transmission Towers Based on Nonlinear Acoustoelasticity Effect

ZHANG Wuneng, LAN Guangyu, WANG Yi, ZHANG Liubin, WU Kehua, WANG Xiaochen

2021, 54(10):  152-160.  DOI: 10.11930/j.issn.1004-9649.202009013

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Based on the practical needs for developing the structural health monitoring technologies of transmission tower bolted structures, a finite element model of transmission tower bolt is established based on the nonlinear acoustoelastic effect, and a multi-physics coupling numerical iterative algorithm is proposed. Firstly, based on the geometric configuration characteristics of the transmission tower bolts, a two-dimensional axis-symmetric finite element model is established. and then the Murnaghan hyperelastic material that considers the third-order acoustoelastic effect is used to realize the coupling solution analysis of the stress-strain field and the acoustic field of the bolt. Finally, the proposed numerical model and the iteration method is validated through experiment. The results show that the preload and the clamping length have a significant effect on the stress field distribution within the bolt structure, and the wave velocity variation and bolt deformation caused by the bolt preload have a dual impact on the time of flight of the ultrasonic longitudinal wave. The model and numerical method proposed in this paper can provide a numerical simulation platform for bolt preload measurement based on the nonlinear acoustoelastic effect, and can also provide a technical guidance for related experimental research and instrument development.

Numerical Simulation and Optimization of Metal Grid Architecture for Laser Photovoltaic Converters

WEI Jianguo, LIU Weilin, DENG Hui, HUANG Hui, ZHANG Jia

2021, 54(10):  161-168.  DOI: 10.11930/j.issn.1004-9649.202009091

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Laser power-over-fiber (LPoF) technology is one of the crucial means to energize the monitoring sensor nodes for electrical equipment in high-voltage environment. The optimization of metal front contacts grid architecture for laser photovoltaic (PV) converters is crucial to the improvement and application of LPoF technology. Numerical simulation and optimization of metal grid architecture for PV converters is presented with application to devices of relatively small size. The model is constructed based on the open source Solcore library written in Python, and a three-step-process is developed to create a hybrid quasi-3D model for PV converters. In this paper, given different layer architectures of GaAs-based single-junction PV converters, numerical simulation is run to study how the photoelectric conversion efficiency is affected by the base thickness and the metal grid architecture under different illumination profiles. The results show that with the help of the model, the metal grid architecture can be optimized to yield the highest conversion efficiency under different layer architecture and illumination profiles. The model and optimization results can be used to guide the development of laser PV converters.

Mechanical Fault Diagnosis of High Voltage Circuit Breakers Based on Phase Space Reconstruction and Improved GSA-SVM

XIA Xiaofei, LU Yufeng, SU Yi, YANG Jian

2021, 54(10):  169-176.  DOI: 10.11930/j.issn.1004-9649.202004025

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The vibration signals generated by transmission and impact of circuit breaker mechanical components have chaotic characteristics, which are difficult to be analyzed with conventional signal processing methods. Firstly, the vibration signals are reconstructed into a high-dimensional space by mutual information method and Cao algorithm, and the permutation entropy is calculated as the feature vector. And then the support vector machine (SVM) is used to identify the mechanical fault types of circuit breakers. Finally, the PSO improved GSA hybrid algorithm is used to optimize the parameters of SVM, and the measured vibration signals of the circuit breakers are used to verify the results. The results show that the characteristics of circuit breaker vibration signals can be accurately extracted with combination of phase space reconstruction and permutation entropy. The PSO-GSA-SVM can quickly and effectively identify the fault types of circuit breakers, thus providing an effective solution to such problems as path distortion, energy leakage and mode overlap of existing diagnosis methods.

A New Online Monitoring Method for MOA Based on A-VMD and A-SVD

RUAN Ying, YE Xingwen, DENG Mingfeng, WANG Xing, YANG Linyu, SHU Qin

2021, 54(10):  177-185.  DOI: 10.11930/j.issn.1004-9649.202103144

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Resistive current is a key parameter to judge the working state of a metal oxide arrester (MOA). The resistive current of a MOA is very small when the power system is working normally, and the measured resistive current is prone to be affected by the interference of high-frequency noise, white noise and random pulses, leading to the false alarms of alarming devices. At present, none of the existing denoising methods can completely eliminate the influence of the above interference on the resistive current of MOA. Therefore, this paper proposes a new method for eliminating the interference of resistive current of MOA based on adaptive variational mode decomposition (A-VMD) and adaptive singular value decomposition (A-SVD). Firstly, by sequentially changing the secondary penalty factor and the decomposition layer, and with the energy and loss indicators to measure the effect of VMD decomposition, the optimal parameters of decomposition layers and secondary penalty factor are searched out. Secondly, the A-SVD is used to eliminate the residual white noise in the resistive current after preliminary denoising by A-VMD, which provides a reliable basis for judging the insulating state of a MOA. The effectiveness of the proposed method is verified by simulation and measured data, and the processing results meet the requirements of actual projects.

Effect of Temperature on FDS Characteristics of Resin Impregnated Paper Bushing

ZHANG Han, HU Wei, XU Zuoming, WAN Baoquan, YIN Pengbo, MAO Hangyin

2021, 54(10):  186-195.  DOI: 10.11930/j.issn.1004-9649.202105006

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In order to study the influence of temperature on the frequency domain spectroscopy (FDS) characteristics of resin-impregnated paper (RIP) bushings, and more accurately apply the dielectric response technology to the evaluation of bushing damp or aging state, the FDS measurement experiments of RIP bushing insulation at different temperatures are designed in this paper. The dielectric polarization theory is used to analyze the frequency domain dielectric phenomenon, and the extended Debye model is established. The genetic algorithm and interior point method are used to identify the parameters of each branch, and the influence of temperature on FDS characteristics is analyzed through changing the model parameters. The results show that with the increase of temperature, the real part of complex capacitance in low frequency region (10^–3~1 Hz) increases significantly, and the spectrum curves of dielectric loss and imaginary part of complex capacitance move to high frequency; the established 6-branch Debye model is basically consistent with the measured values, and the insulation resistance R₀ decreases with the increase of temperature while the geometric capacitance C₀ basically keeps unchanged; the "frequency temperature shift factor"-based main curve technology can effectively eliminate the influence of temperature and convert the spectrum curve under known temperature to unknown temperature, consequently extending the measurement range of spectrum curve; the "translation factor" of the spectrum curve satisfies the Arrhenius equation, and the activation energy calculated by the "translation factor" is about 31.92 kJ/mol. Therefore, the influence of temperature must be considered when using frequency domain dielectric spectroscopy to evaluate the state of damp or aging, otherwise the evaluation results will be inaccurate.

A Partial Discharge Pulse Extraction and Denoising Technology Based on Random Singular Value Decomposition

WANG Li, ZHANG Wei, LUO Dingnan

2021, 54(10):  196-203.  DOI: 10.11930/j.issn.1004-9649.202103116

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Partial discharge signals are prone to missed detection under low signal-to-noise ratios, and the traditional singular value decomposition algorithm requires massive calculations when extracting partial discharge pulses. In this regard, a technology of partial discharge pulse extraction and denoising based on random singular value decomposition (RSVD) was proposed. It can extract partial discharge pulses and remove white noises. Compared with traditional SVD pulse extraction and calculation, the method requires a shorter time and has higher engineering practical value. Firstly, the sliding short-time data window was used to intercept the original partial discharge signal fragments. The maximum singular value calculated by RSVD was compared with the global optimal singular value threshold to determine the starting and ending points of pulses. Then the singular value decomposition (SVD) was combined with the local optimal singular value threshold to denoise the extracted signal. Experiments on typical simulated partial discharge pulses verified the superiority of the algorithm’s execution efficiency in pulse extraction. Partial discharge tests were performed on the cable defects simulated in the laboratory at power frequency voltage. Moreover, comparative experiments were conducted for the proposed method, discrete wavelet transform (DWT) plus the adaptive double-threshold method. The results show that the proposed method has a low missed detection rate and a remarkable performance in signal denoising.

Cause Analysis and Enlightenment of Global Blackouts in the Past 30 Years

HU Yuan, XUE Song, ZHANG Han, ZHANG Hua, FENG Xinxin, TANG Chenghui, LIN Yi, ZHENG Peng

2021, 54(10):  204-210.  DOI: 10.11930/j.issn.1004-9649.202009134

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Since 2019, Venezuela, Argentina, the United States, Indonesia, the United Kingdom, and other countries have successively experienced blackouts, and power grid security has become the focus of public attention. This paper sorted out 138 major blackouts in the past 30 years and found that after 2008, blackouts entered a period of high incidence, and major economies become the “severely affected areas”. Specifically, natural disasters accounted for 56% of those blackouts, and their scope of impact was relatively clear and controllable. management factors accounted for 31%, and they were easily accompanied by other inducing factors. These global and systematic influences may endanger the energy security of the entire country and region. Emergencies or human error accidents accounted for 10%. In addition, cyber-attacks became an emerging cause, accounting for 3%, and it was concealed, complex, and difficult to defend, easily leading to huge physical damage. After an in-depth analysis of the causes for the four types of blackouts, this paper puts forward measures and suggestions to prevent blackouts in China in light of the current development of China’s power grid.

Mid-and-Long Term Load Forecasting Based on Integrated Power Consumption Data

WANG Xingang, ZHU Binruo, GU Zhen

2021, 54(10):  211-216.  DOI: 10.11930/j.issn.1004-9649.202103108

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Load forecasting is critical for management and security of smart grid system. Traditional methods are usually on the basis of historical power consumption data, and the popularization of multi-meter integration technology makes analysis of integrated energy consumption data more efficient. Towards the issue of load forecasting, with water/power/gas consumption data collected by integrated smart meter as features, two mid-and-long term power consumption forecasting methods are proposed: gaussian process regression (GPR) and relevance vector regression (RVR). Experimental results show the superiority of the proposed method and the significance of integrated energy consumption data for load forecasting problem.

Pole Assignment Based Auxiliary Damping Control for Renewable Generation Integrated into Power System

ZHANG Yi, HU Zhengyang, PENG Peipei, CHEN Ning, TANG Bingjie, GAO Bingtuan

2021, 54(10):  217-222.  DOI: 10.11930/j.issn.1004-9649.202108034

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A large amount of renewable energy has changed the damping characteristics of the power system through the access of power electronic equipment, resulting in the prominent problem of low-frequency oscillation of synchronous generators, posing a threat to the safe and stable operation of the system. Virtual synchronous generator (VSG) is a new type of converter control technology, and it is necessary to analyze the damping characteristics of renewable energy integrated to power system based on VSG. Firstly, the mathematical model of the VSG controller and the model of synchronous generator are established. Secondly, taking the renewable energy into the single-machine infinite bus system as the research object, the coupling mechanism of the operation characteristics of the VSG and the synchronous generator is analyzed based on the small-signal analysis. Furthermore, an auxiliary damping control strategy for renewable energy grid-connection is proposed, and a virtual synchronous generator auxiliary damping controller is designed to suppress the low-frequency oscillation of the system based on the principle of pole assignment of the power system. Finally, the simulation results verify the correctness of the theoretical analysis.

Iterative Optimization and Economic Analysis of Photovoltaic Power Generation Forecasting under Haze Conditions

CHEN Wei, REN Jing, WU Xinfang, YU Wenying, LIU Yongsheng

2021, 54(10):  223-230.  DOI: 10.11930/j.issn.1004-9649.202010098

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Photovoltaic power generation is vulnerable to environmental factors such as temperature and irradiance. In recent years, haze (with high concentration of PM_2.5) has caused serious pollution, greatly reducing the power generation of the photovoltaic system. Therefore, it is of great significance for the photovoltaic market to predict the photovoltaic power generation in the haze weather. In this paper, based on the annual photovoltaic data of a household photovoltaic roof in Shanghai, the relationship between the PM_2.5 concentration and the power generation loss index is fitted and analyzed with controlled variables and the similar days for haze analysis. According to the principle of iteration, the algorithm for photovoltaic power generation prediction is optimized, and the formula for photovoltaic power generation prediction under haze is given to modify the photovoltaic revenue prediction model. The results show that the optimized algorithm can improve the accuracy and stability of the prediction results. Through the revenue analysis of three photovoltaic economic models, the iterative optimization algorithm can improve the accuracy of photovoltaic revenue forecast.