Table of Content

05 November 2020, Volume 53 Issue 11

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An Ultra-high-power Electric Arc Furnace Model for Low-Frequency Non-stationary Inter-harmonics Studies

LIN Caihua, ZHANG Yi, SHAO Zhenguo, LIN Fang, HUANG Daoshan, LIN Yan

2020, 53(11):  1-8.  DOI: 10.11930/j.issn.1004-9649.202006170

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Ultra-high-power electric arc furnace is the main equipment of modern smelting, but its low-frequency non-stationary inter-harmonics and high-power impact will seriously affect the power quality of distribution networks. The accurate modeling of ultra-high-power electric arc furnace is of great significance to understand the time-varying and non-linear characteristics of the arc and to conduct the targeted research on power quality issues. Aiming at the problem that the existing models have not considered the low-frequency non-stationary inter-harmonic emission characteristics, a novel ultra-high power electric arc furnace model is established. Firstly, a static arc equation is established according to the law of conservation of energy, and a dynamic mathematical model is constructed for the ultra-high-power electric arc furnace using chaos theory. Then, the genetic ant colony algorithm is used to identify the parameters of the dynamic mathematical model. Finally, a 100 t AC electric arc furnace in Fujian was used for case study, and the feasibility of the proposed model was verified under Matlab/Simulink environment. The simulation results show that the proposed model can better describe the operating characteristics of the ultra-high-power electric arc furnace, and can be used for study of the low-frequency non-stationary inter-harmonic emission characteristics of the ultra-high-power electric arc furnace.

A System Assessment Method for Voltage Sag Severity Based on Average Point-To-Line Distance Index

ZHONG Qing, YAO Weilin, XU Zhong, ZHOU Kai, WANG Gang

2020, 53(11):  9-14.  DOI: 10.11930/j.issn.1004-9649.202006095

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In order to systematically compare the voltage sag severity of different regions, a system method is proposed to assess grid voltage sag severity. The method obtains the time duration intervals through cluster analysis, and gets the medians of residual voltages for different duration intervals, with which the median line is plotted. And then, the average point-to-line distance index is proposed to quantify the voltage sag severity for different regions. The voltage sag levels are graded according to index distribution features of different regions. The correctness and feasibility of the proposed assessment method are verified with the real monitoring data of a city power system. This method can effectively obtain from numerous monitoring data the voltage sag severity information of different regions, and realize the system assessment of the voltage sags from two dimensions: residual voltage and time duration, which is conducive to compare the voltage sag severity among different regions.

Control Strategy for MMC Based on Super-Capacitor Energy Storage

LI Lei, Jun TAO, ZHU Mingxing, FAN Yuan

2020, 53(11):  15-22.  DOI: 10.11930/j.issn.1004-9649.202006159

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In order to resolve the problems of the rapid variation of power system’s active power caused by grid-integrated renewable energy and pulsed loads, which will threaten the stability of adjacent generator units and power systems, a distributed energy storage system is proposed based on the modular multilevel converter (MMC) integrated with super-capacitor energy storage. The bi-directional DC/DC converters are used to control the charge and discharge process of the energy storage system, and the design principles for relevant parameters are provided. The control strategy based on dual-closed-loop PI regulator and the phase shifted PWM technology is used to control the balance of super-capacitor energy and the capacitor voltage stability of the MMC sub-modules. The energy management mechanism is introduced to control the collaborative operation of the MMC and the DC/DC converters, subsequently realizing the real-time compensation for pulsed active power variation in medium- and high-voltage systems. The simulation results with Matlab/Simulink platform have verified the effectiveness of the proposed device and control strategy.

Analysis on Interactive Influences among Multiple APFs Connected to Weak-structured Power System

WANG He, LI Yujia, YIN Zhongdong, WANG Zhangming, ZHAO Haisen

2020, 53(11):  23-30,88.  DOI: 10.11930/j.issn.1004-9649.202007180

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When multiple active power filters (APF) are incorporated into the regional distribution network, there are complex negative interaction effects among the control circuits, resulting in the decline of APF compensation performance and harm to the stability of power system. Firstly, a multi-APF grid-connected mathematical model under the condition of weak power grid is established based on NORTON equivalent circuit. And then, combined with the dynamic relative gain array (DRGA) principle, a quantitative analytical method is proposed for analyzing the interaction degree between multiple APF control circuits, which can analyze the quantitative relationship between the interaction degree, the control circuit parameters and the weak grid equivalent inductance based on different frequency intervals. Finally, the MATLAB/Simulink platform is used to build a grid-connected simulation system model. The time-domain simulation results are consistent with the theoretical analysis, which verifies the effectiveness of the proposed method.

Voltage Stability of Weak Sending-end System with Large-Scale Grid-connected Photovoltaic Power Plants

XIAO Feng, HAN Minxiao, TANG Xiaojun, ZHANG Xin

2020, 53(11):  31-39.  DOI: 10.11930/j.issn.1004-9649.202007045

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With the rapid development of new energy power, the HVDC sending-end system in the northwest of China gradually shows the characteristics of high penetration of new energy, weak AC grid and small rotational inertia, and the voltage stability problems of the weak sending-end system caused by them cannot be ignored. Firstly, based on the static voltage sensitivity of the converter bus with large-scale grid-connected photovoltaic power plants at the sending end, the factors affecting the voltage stability of the converter bus are derived. Furthermore, a calculation process for photovoltaic grid-connected critical capacity is designed to provide a basis for site and capacity selection of PV plants. A sending-end comprehensive short circuit ratio (SCSCR) index is proposed to describe the support strength of AC system at the weak sending-end with large-scale grid-connected photovoltaic power plants. The comparison of SCSCR, SCR and ESCR shows that SCSCR is more relevant to the static voltage stability of the weak sending-end system. The SCSCR index can simplify the analytic process of voltage stability of weak sending-end system, and guide the power source organization and topology design of outbound delivery system with high penetration of new energy.

Optimal Allocation Strategy for Power Quality Control Devices Based on Harmonic and Three-Phase Unbalance Comprehensive Evaluation Indices for Distribution Network

ZHUO Fang, YANG Zebin, YI Hao, YANG Guangyu, WANG Meng, YIN Xiaoqing, ZHU Chengzhi

2020, 53(11):  40-49.  DOI: 10.11930/j.issn.1004-9649.202007181

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Power quality control devices represented by active power filter are mostly used to compensate harmonic and unbalanced current generated by local loads. With the massive access of non-linear loads in the distribution network, the local compensation configuration strategy becomes expensive and inefficient. Therefore, the device configuration strategy for global improvement of network power quality is necessary. Firstly, a global evaluation strategy for power quality indices based on analytic hierarchy process is proposed to evaluate the configuration effects of control devices. Secondly, taking the global configuration effects, the total number and capacity of control devices as the optimization goals, and regarding the harmonic distortions and unbalance degrees of the nodes satisfying the standard as the constraint condition, the optimal configuration node and capacity of each device is determined by multi-objective particle swarm algorithm. Finally, an IEEE-18 node simulation model with non-ideal loads is built to verify the effectiveness and superiority of the proposed global evaluation and power quality control device configuration strategy for the comprehensive optimization of network harmonic and unbalance voltage.

Value-added Service Strategy of Voltage Sag Governance for Mutual Satisfaction of Power Supply Companies and Power Users

ZHANG Bo, TANG Yuzheng, DAI Shuangyin, CHEN Yunzhu, WANG Panpan, XIAO Xianyong

2020, 53(11):  50-59.  DOI: 10.11930/j.issn.1004-9649.202007161

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For realizing marketization of voltage sag governance, two major problems need to be solved: lowering the investment cost and reasonably setting the service fee. A method for whole-process voltage sag risk quantification is thus proposed based on the process immunity time (PIT). Combined with the service cost of sag governance, an optimal configuration model of voltage sag governance equipment considering maximum risk removal is proposed to realize the optimal governance service scheme decision-making to meet the needs of users. On the basis of introduction of the satisfactory price strategy, definition of the satisfaction degree of both power supply companies and users, and clarification of their interest demands, a value-added service pricing model of voltage sag governance is proposed based on mutual satisfaction. Finally, taking a chemical plant in China as an example, the feasibility and rationality of the proposed method are verified through case study. The proposed method is helpful to the implementation of value-added service for voltage sag control, thus solving the voltage sag problems of users and alleviating the contradiction between power supply and consumption, which is of great practical value.

Assessment on Harmonic Emission Level of Residential Loads with Symmetry and Volatility Constraints

ZHANG Huaying, ZHU Mingxing, SHI Hengtong, WANG Qing, LI Hongxin, GAO Min

2020, 53(11):  60-68.  DOI: 10.11930/j.issn.1004-9649.202006003

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The accuracy of harmonic emission level assessment of residential loads is affected by background harmonics or three-phase harmonic unbalance. Based on Norton’s and Davinan’s theorem, a model is constructed for assessment of harmonic emission level. An analysis is carried out on the problems of fluctuation method applied in harmonic impedance estimation of residential load distribution network and the influence mechanism of three-phase unbalance on the system harmonic impedance estimation. A system harmonic impedance estimation method is proposed by combining the harmonic main sequence characteristic extraction and the selection of fluctuation coefficient. The equivalent method of load harmonic impedance and the influence of its estimation error on the calculation of harmonic emission level are discussed. The results of simulation and engineering application show that the 95% confidence interval of the estimated harmonic impedance converges significantly, and the assessment result of harmonic emission level is more accurate.

Optimal Distributed Configuration of Electric Heater and Heat Storage Considering Capacity Distribution

ZHU Ziqi, ZHANG Weitao, HAN Mingliang, ZHANG Wensong

2020, 53(11):  69-77.  DOI: 10.11930/j.issn.1004-9649.201911030

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It is an effective measure to reduce wind power curtailment by making full use of the adjustable capacity of electric heater (EH) and heat storage (HS). However, most of the current research focuses on the concentrated capacity optimization of EH and HS, which is easy to break through the constraints of grid operation. In view of this, this paper proposed an optimal configuration method of EH and HS considering capacity distribution. Taking the symplified 8-node actual power grid as an example, a comprehensive energy system mathematical model is established based on the DC power flow model, which considers the capacity, operation constraints and specific distribution of EH ans HS. On this basis, taking the optimal economic benefits as the goal, the optimal distribution model of EH and HS is given which meets a certain heat load ratio. The simulation results show that the phenomenon of wind power curtailment of the system will be reduced, and the economic benefits of the system will be significantly improved when EH and HS are in the optimal allocation.

Source-Load-Storage Coordinated Rolling Dispatch for Wind Power Integrated Power System Based on Flexibility Margin

HUANG Pengxiang, ZHOU Yunhai, XU Fei, CUI Dai, GE Weichun, CHEN Xiaodong, LI Tie, JIANG Feng

2020, 53(11):  78-88.  DOI: 10.11930/j.issn.1004-9649.201911153

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The high penetration of new energy increases the uncertainty and fluctuation of power system, while the insufficient flexibility of power system significantly affects the accommodation of new energy and the safe operation of power grid. From the perspective of flexibility resource output characteristics and the flexibility supply capacity, a source-load-storage coordinated rolling dispatch model is established based on the power system flexibility margin index, which takes the maximum peak regulation benefit, the maximum wind power consumption, the minimum units operating cost and the minimum interruptible load cost as the objectives, and comprehensively considers the operational constraints of the power system. Based on the philosophy of coordinated dispatch, the output of each flexible resource in each period was optimized, and then the output and status of each flexible resource were revised by using the super short-term wind power forecast data. Finally, a case study is made to verify the feasibility of the proposed dispatch method.

Model of Multi-energy Hub Cooperative Bargaining in Energy Internet

ZHU Xiping, FU Qian, LI Zilin, LUO Jian, WANG Dengsong

2020, 53(11):  89-96.  DOI: 10.11930/j.issn.1004-9649.201912193

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The energy hub (EH) of energy internet is an effective way to realize the coordinated supply of multi energy and the complementary and optimized operation among energy regions. In order to realize the coordinated and economic operation of multiple energy hubs within a region and to ensure the interconnection of information flow and energy flow between different energy hubs, this paper constructs a multi energy hub coordination model based on the traditional energy hub model. At the same time, considering the cost of energy transaction between energy hubs, a cooperative bargaining model among multi energy hub operators is proposed. The simulation analysis shows that it can effectively reduce the operating cost of each operator, achieve the coordinated flow of energy flow between energy hubs, maximize the utilization of resources, and reduce the waste of energy.

Optimal Capacity Allocation of Heat Power Supply and Its Economic Analysis Considering Thermal Characteristics of Heat Energy System

XU Yanping, HUANG Yuehui, SUN Yong, CAO Zheng

2020, 53(11):  97-105.  DOI: 10.11930/j.issn.1004-9649.201912125

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China has actively carried out the pilot work for flexibility transformation of thermal power. Multiple types of heat sources have played an important role in accommodation of wind power, and reasonable heat source capacity planning can effectively improve wind power accommodation. With consideration of the heat storage characteristics that the thermal system naturally has in heat power capacity planning, the joint optimization of power system and thermal system can be realized, and the economic cost of heat source allocation can be further reduced. Firstly, a thermal characteristic model is established for each part of the thermal system, including heat source, heat supply network, and heat load; And then, a time-series production simulation model is constructed for optimal capacity planning of multiple heat sources, with the goal of minimizing the annual economic cost of the heat source; Finally, based on the constructed model, the heat source allocation schemes and their economic comparison are studied under different scenarios, which can provide a reference for heat source planning.

Influence of Converter DC Bus Overvoltage on Wind Turbine High Voltage Ride Through Capabilities

ZHANG Guangru, BAI Runqing, ZHU Hongyi, YANG Yong, XU Honglei, MA Xiping, DONG Kaisong

2020, 53(11):  106-110,125.  DOI: 10.11930/j.issn.1004-9649.201901035

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Off-grid occurred in a wind farm caused by converter fault during field test of the DFIG (doubly feed induction generator) high voltage ride through (HVRT) capabilities. Through an in-depth analysis of the overvoltage of the wind turbine converter, a method was proposed for dynamic adjustment of converter DC bus reference voltage during wind turbine HVRT, which solved the off-grid problem of wind turbines caused by overvoltage protection of converter. Field test verified the effectiveness of the proposed method, which can provide useful experience for improving the wind turbine HVRT capabilities, and a reference for preventing the oscillation of renewable energy DC sending-end power system.

Discussion on Focus of Power Development in Tibet during the 14th Five-Year Plan

ZHANG Lifeng, HE Zhiqiang, ZHANG Jinzhi

2020, 53(11):  111-115,138.  DOI: 10.11930/j.issn.1004-9649.202007192

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Clean energy resources, especially hydropower resources are very rich in Tibet. However, due to the low level of power source development and the mismatch between the power output characteristics and the grid load characteristics, the Tibet Power Grid have long been “surplus in flood season and shortage in dry season” in power supply and demand characteristics since the “12th Five-Year Plan” period. As the development of clean energy resources continues to increase, the overall production scale of Tibet's power supply during the “14th Five-Year Plan” period will far exceed its actual electricity demand, but most of the newly added power sources are far away from the grid load center. It is therefore the primary objective of the Tibet's “14th Five-Year Power Development Plan” to meet the electricity demand, promote the coordinated development of power supply and grid, and lay the foundation for the construction of the national clean energy base. According to Tibet's power grid structure, power sources distribution, and power supply and demand situation, and combined with the characteristics of energy resources, the power development focus of Tibet's “14th Five-Year Plan” are clarified in this paper through study and comparison of the power supply measures in the “14th Five-Year Plan” period and the years afterward.

Evaluation Method for Running State of Electricity Meters Based on Random Matrix Theory and Clustering Algorithm

CHENG Yingying, DU Jie, ZHOU Quan, ZHANG Jiaming, ZHANG Xiaoyong, LI Gang

2020, 53(11):  116-125.  DOI: 10.11930/j.issn.1004-9649.201907096

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With the expansion of intelligent distribution network and the increasing complexity of power grid structure, the amount of data in power system increases rapidly, and new challenges rise from the checking and monitoring evaluation of power equipment. Based on the principle of big data mining analysis, this paper proposes a method based on random matrix theory and clustering algorithm to evaluate the running state of electric energy meter. Firstly, time series data of various indicators are characterized and then integrated by real-time separation window technology. Based on the random matrix theory, the random matrix-based analysis model is constructed to calculate and analyze the characteristics with multi-dimensional statistical timing in real time. Further, an improved DTW (dynamic time warping) clustering algorithm is used to analysis the linear feature statistics of the output of the random matrix. Finally, according to the clustering result, the state of the electric energy meter is obtained and outputted as different classes. The experiments show that compared with the traditional Principal Component Analysis evaluation method, the proposed method has good robustness, reliability and timeliness, which provides a new idea for the application research of power grid detection technology.

Application of Image Recognition in On-Line Monitoring System of UHVDC Valve Element Faults

JIANG Jing, ZHAO Yangyang, FAN Hongwei, ZHOU Zhenyu, DONG Chaoyang, YANG Qingbo, WANG Xiaoli

2020, 53(11):  126-132.  DOI: 10.11930/j.issn.1004-9649.201907086

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UHVDC valve elements are numerous and hard to monitor in real time. An on-line monitoring method is thus proposed for converter valve element conditions based on image recognition technology. The paper introduces the method to extract and pre-process image data, and analyzes the image characteristics of thyristor gate wire shedding and nut torque line displacement. The image recognition technology is adopted to realize intelligent detection of faults such as thyristor gate wire shedding and nut displacement. A prototype is designed for on-line monitoring of converter valve element conditions based on the image recognition and fault feature extraction method, and its software and hardware design methods are introduced in detail. The testing environment is set up on a valve tower of UHVDC to test the image analysis and fault feature extraction method. The testing results show that the proposed method can effectively identify the abnormal working conditions of the valve elements, and realize the early warning of their abnormal operations, and can prevent the further expansion of faults.

Anti-strong Magnetic Interference Technology of Intelligent Miniature Circuit Breaker

XIONG Dezhi, CHEN Xiangqun, LI Wenwen, LIU Xiaoping, YANG Maotao, ZHANG Baoliang

2020, 53(11):  133-138.  DOI: 10.11930/j.issn.1004-9649.201808164

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Aiming at the problem that the intelligent micro-circuit breaker is prone to misoperation and miss trip under strong magnetic interference, the intelligent micro-circuit breaker control system, DC motor, switching power supply and other related modules that may cause magnetic interference are analyzed, and the concrete optimization design scheme is put forward. An optimum design method for anti-magnetic interference of control system based on dual-mode detection of micro-switch and Hall switch is proposed, which is realized in both hardware and software. The optimum design methods for anti-magnetic interference of DC motor and switching power supply output power are also proposed, which improve the anti-magnetic interference ability of DC motor and switching power supply. The experimental results show that the optimized intelligent micro-circuit breaker has excellent anti-magnetic interference performance, high stability and reliability. It thoroughly solves the problem that the intelligent micro-circuit breaker is prone to maloperation and rejection under strong magnetic interference, and has a strong application value.

Subsynchronous Oscillation Time-varying Amplitude Frequency On-Line Monitoring Method Based on Sliding Window FFT

YANG Jing, WANG Tong, TANG Junci

2020, 53(11):  139-146.  DOI: 10.11930/j.issn.1004-9649.201901118

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This paper proposes a time-varying amplitude frequency monitoring method based on sliding window FFT of sub-synchronous oscillation to identify the time-varying modal parameters of the oscillating signals on-line. Firstly, windowed interpolation methods are used to reduce the spectral leakage and fence effect, and decrease the FFT identification errors. Then, through the sliding of the time windows, the signals intercepted by each time window is transformed through FFT to obtain a dynamic sequence of oscillation frequency and oscillation amplitude, that is, varying frequency and amplitude with time. The dynamic sequence of the damping factors is obtained by analyzing and calculating the time-varying oscillation amplitude. Finally, using the ideal non-stationary signals, simulation signals and the measured signals of the power grid as test cases, the comparison results with the Prony and HHT algorithms show that this method not only can eliminate the influence of modal mixing, but also has anti-noise ability. The method can effectively identify the random time-varying oscillation modes and achieve on-line monitoring and analysis of sub-synchronous oscillations.

Insulation Fault Caused by Current Transformer Seal Failure

LI Yuquan, KOU Xiaoshi, ZHANG Shaofeng, SHAO Yingbiao, ZHAO Yongfeng, ZHU Hua, ZHANG Yifan, ZHENG Hanbo

2020, 53(11):  147-153,174.  DOI: 10.11930/j.issn.1004-9649.202004018

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Firstly, the insulation structures of oil-immersed current transformer (TA) and SF₆ TA are introduced. And then, the causes of TA insulation breakdowns are analyzed based on an investigation of the insulation faults of TAs occurring in several cases. It is concluded through further disintegration examination that the seal failure is the root cause for insulation fault. Lastly, operation recommendations and precautions are presented for seal failure caused by the metal expander leakage of oil-immersed TA and the rupture disc damage of SF₆ TA.

Electromagnetic Transient Parallel Simulation of Large-Scale New Energy Grid Connection Based on PSCAD/EMTDC

CAO Bin, WANG Liqiang, ZHAO Yongfei, ZHANG Xiuqi

2020, 53(11):  154-161.  DOI: 10.11930/j.issn.1004-9649.201902012

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As the large-scale new energy is connected into the grid, the electromagnetic transient characteristics of traditional power grid have been greatly changed owing to the access of the massive power electronic components. Therefore, new requirements of the power grid simulation have been put forward from both the algorithm and the calculation scale. The electromagnetic transient model of Inner Mongolia power grid with new energy access is built based on PSCAD software to realize the accurate simulation of the electromagnetic transient of the whole network. Based on cluster parallel simulation technology, the problem that the computing scale is too large to be computed or the simulation efficiency is low is solved. Furthermore, a network equalization integration method based on the size of the node matrix is proposed, which reduces the occupancy rate of computing resources. The simulation results show that the cluster parallel technology can effectively improve the simulation efficiency of the new energy grid-connected electromagnetic transient model. At the same time, under the premise of ensuring the simulation speed, the proposed network equalization integration method can optimize the utilization efficiency of computing resources.

Development of Climbing Inspection Robot for Power Towers

WEI Xiaodong, LI Yi, LI Qi, MEI Jia, LI Qingmei, ZHOU Limin, WANG Zhijun, LIU Da

2020, 53(11):  162-167.  DOI: 10.11930/j.issn.1004-9649.201912197

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Due to the complicated structure of power towers, researches rarely focus on climbing robot for tower inspection. Based on the T-shaped fall-prevention rail of tower, an inspection robot is developed with tower climbing function realized through permanent-magnetic adhesion, which has solved the adhesion and climbing problems for inspection. The robot is operated by inspector through remote-control system. The image and video information is obtained via pan/tilt camera on the robot and can be transmitted to handheld display-terminal through image transmission system. The robot inspection of tower and fall-prevention rail is thus achieved and any defects of towers can be discovered effectively. The developed robot is convenient for inspectors to keep well informed about tower defects and remove them timely.

Short-Circuit Fault Location for Power Cables Based on Improved Two-Terminal Traveling Wave Method

CHAI Peng, ZHOU Hao, ZHANG Yu, ZHAO Wenjie, YANG Xiao, ZHOU Miao, LI Mingzhen

2020, 53(11):  168-174.  DOI: 10.11930/j.issn.1004-9649.201901111

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In order to accurately locate the short-circuit fault on the power cable line, the principle of fault location of the two-terminal traveling wave method has been improved. The traditional long-distance two-terminal synchronization method is replaced by the local clock synchronization method. Thus, the synchronization problem of the two-terminal traveling wave method has been solved from the principle level. The improved fault location method is only related to the time difference between the fault traveling wave arriving at the monitoring points at both ends of the power cable. And the normalized fault point criterion is used instead of the specific numerical criterion to reduce the dependence on the accuracy of the line electrical parameters. Therefore, the improved method has higher fault location accuracy. The double-ended structure of the power cable is simulated by PSCAD. The results show that the improved method has higher location accuracy, and the fault location accuracy is better than the traditional two-terminal traveling wave method under any fault positions.

Method of DFIG Cooperating with SVG to Suppress Low-Frequency Oscillation in Power Systems

YANG Lei, GAN Weigong, LI Shengnan, ZHOU Xin, HE Peng, HE Xin, ZHANG Jie, WANG Delin

2020, 53(11):  175-184,201.  DOI: 10.11930/j.issn.1004-9649.201912045

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With the increasing penetration of wind power, when the power system is insufficiently damped, low-frequency oscillations are likely to occur. Doubly fed induction generation (DFIG) can decouple the control of active and reactive power, which helps enhance system damping. This paper proposes a method for grid connection of DFIG units with static var generator (SVG). It adopts a control strategy of additional damping, accesses the power control node of the control system, changes the active and reactive reference current of the device, and realizes the dynamic output power of the device to offset the power increase to suppress the power oscillations in the system. The control system model of DFIG and SVG connected to 4-machine 2-area system is built. Additional damping control signals are added to control the active power of DFIG and the reactive power of SVG. The characteristic root of the system is changed, and the modalities of the oscillating signals are analyzed based on Prony method. The results show that the method can effectively provide positive damping, suppress low-frequency oscillation of the system, accelerate the stabilization of oscillation waveform and improve the small signal stability of the power system.

Research and Application of Parabolic Sun-tracking System Based on PLC

WANG Ben, NIU Honghai, XU Weifeng, CAI Dan, CHEN Jun

2020, 53(11):  185-194.  DOI: 10.11930/j.issn.1004-9649.201911149

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A novel parabolic sun-tracking system is proposed, in response to inefficiency of solar energy utilization due to the low tracking accuracy of sun-tracking system for parabolic trough solar concentrator. The mathematical model of trough photothermal single-axis tracking is established based on solar position algorithm (SPA) and the influence of different arrangements on the operation characteristics of the system is studied. The prototype test system is built and the tracking control logic is developed based on programmable logic controller (PLC). The application of high-precision algorithms in the PLC controller is realized through custom embedded programming. A strategy of intermittent control based on deviation of tracking angle is proposed. The push-pull drive mechanism with dual hydraulic cylinder and medium and high pressure is utilized to drive the trough concentrator with the purpose of real-time tracking of the solar position. The hardware and software architecture, main function modules and control flow of PLC control system are expounded. Besides, redundancy configuration of the controller and monitoring network are adopted. Operating data shows that the system has features of simple system structure, convenient maintenance and high tracking accuracy.

Optimization of Reactive Power Configuration for Offshore Wind Farms

YANG Yuan, YANG Xi, TAN Jiangping, CHEN Liang, XIN Yanli, CHEN Xia

2020, 53(11):  195-201.  DOI: 10.11930/j.issn.1004-9649.201912153

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According to the reactive power configuration principles of offshore wind farms, this paper analyzes the reactive power losses of offshore wind farms, mainly consisting of wind turbines, submarine cables and main transformers. Under the condition of meeting the requirements of reactive power compensation and power-frequency overvoltage, the capacity of the shunt reactor should be increased as much as possible to obtain the optimal configuration scheme of the shunt reactor and the dynamic reactive power compensation device SVG. Research results show that when the length of the 220 kV submarine cable increases and the wind turbine output decreases, the capacitive reactive power of the offshore wind farm will increase; When the length of the 220 kV submarine cable increases, the no-load overvoltage, the load rejection overvoltage, the single-phase ground fault overvoltage of the offshore wind farm will increase. Finally, based on a real case, the optimal capacity of the shunt reactor and the SVG were obtained. The proposed method is simple, practical and economically feasible, and can be applied in the practical engineering.

Intelligent Full-Digital Technology and Its Applications in Power Plant

ZHANG Wenjian, LIANG Geng, LI Gengda, CUI Qingru

2020, 53(11):  202-211.  DOI: 10.11930/j.issn.1004-9649.201907082

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Although the traditional digital power plant technology can attain the digital mode of enterprise construction and management, it still lacks the conditions and features of high intelligence in the aspects of the breadth of information collection and the depth of information utilization. In this paper, by taking the production characteristics of power plant into consideration, an intelligent all-digital technology system applicable for power plant is proposed on the basis of the technology of digital power plant, and the concept, composition and basic relationship between the elements of intelligent all-digital technology is aslo proposed, expounds the basic structure and principle of intelligent all-digital system, elaborates the intelligent operation mode of intelligent subject, and explores the application scheme of intelligent all-digital technology in power plant. In this system, the intelligent subject and digital technology are organically combined to increase and expand the breadth and depth of data information usage, i.e., expand the paths and means of information acquisition and collection to increase the breadth of information, and conduct in-depth analysis, mining and reutilization of data information by introducing more intelligent calculation algorithms and analysis methods. At the same time, the new data and knowledge generated by the system can be fed back to the existing all-digital system continuously to achieve the self-adjustment and self-organization of the system, in which the traits of intelligence are fully exhibited.

Multi-objective Predictive Control of Gas Turbine System Based on T-S Fuzzy Model

HOU Guolian, DAI Xiaoyan, GONG Linjuan, XU Haixin, ZHANG Jianhua

2020, 53(11):  212-219,226.  DOI: 10.11930/j.issn.1004-9649.201909088

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The traditional control strategy of gas turbine system mainly focuses on the load tracking problem without taking the economic performance into full consideration. In this paper, a T-S fuzzy model-based multi-objective predictive controller is proposed to enhance both the tracking performance and economic performance together. First, regarding the strong non-linearities of gas turbine system, an incremental T-S fuzzy structure is applied and model identification is processed based on some historical data from a combined cycle unit. To avoid possible model mismatch, the parameters of the prediction model is updated in realtime according to the current operation conditions. Next, the multi-objective predictive controller is designed in which the load tracking index and economic index are defined and combined into a comprehensive multi-objective cost function. Then, in order to improve the settling speed of load tracking process, the simultaneous heat transfer search algorithm is employed to optimize the cost function and determine the control variables. Simulated experiment results have shown that this multi-objective predictive control scheme could enhance tracking performance and economic performance effectively.

Numerical Simulations on the Protection of the Molten Salt Thermal Tube under Cloud Occlusion

FENG Lei, XIAO Gang, GUO Lei, YANG Chenggang, LIAO Haiyan

2020, 53(11):  220-226.  DOI: 10.11930/j.issn.1004-9649.201903071

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Based on the single tube model of the receiver, the temperature drop characteristics of the endothermic tube such as the heat flux, molten salt flow rate and surface convection heat transfer coefficient were studied through numerical simulations on the scenarios under cloud occlusion before and after protection schemes implemented. The results exhibit limited impacts of radiation heat flux density on the elapsed time for temperature at the molten salt outlet to decrease to close to the solidification point. Moreover, the greater the convection heat transfer coefficient, the more likely the molten salt solidification may appear; while the lower the molten salt inlet velocity, the longer time it will take for the molten salt outlet temperature to drop close to the solidification point. Without the protective device, the time taken to reach the freezing temperature of the molten salt at the outlet is about 20 seconds. The scheme of adding the protective device and reducing the inlet velocity of molten salt can be applied to extend the time by almost six times, or as much as 130 seconds. Thereby system security can be greatly improved.

Experimental Studies on the Influence of Soot Blowing on the Overheating of Water Wall Pipes under Low Loads in Supercritical Boilers

WANG Yang, ZHU Xiaoxin, CHEN Ming, WANG Huajian, FANG Fan, ZHU Weijian

2020, 53(11):  227-233.  DOI: 10.11930/j.issn.1004-9649.202003111

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In this paper, through the experimental studies on the installation location, operation frequency, time period of load section and corresponding wind distribution mode of the soot blowers under low loads, it is observed that the sudden rise of wall temperature mainly occurs on the two side walls during soot blowing under low loads, and particularly the ash blowers above and under the overfire air port have substantial effects with the maximum temperature surge amplitude of about 100~130 ℃. In the process of ash blowing, with the auxiliary air volume increased by about 20% in the corresponding area, the temperature surge amplitude can be reduced by about 20 ℃. At the same time, choosing the time section of higher load for soot blowing and reducing the operation frequency of the soot-blowers are also helpful for the control of the water wall overheating. The experimental research method and conclusions have appeared to have certain referential significance to the soot-blower operation of units with the same type under low load conditions.

SCR Zone-Based Hybrid Dynamic Leveling Technology and Its Application for W-Flame Boiler

XU Jian, LUO Zhi, ZHOU Xin, LI Wenjie, HUANG Shaobo, CHANG Lei, WANG Xiaobing, NIU Guoping, ZHANG Guangcai

2020, 53(11):  234-242.  DOI: 10.11930/j.issn.1004-9649.202002093

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Burning anthracite in W-flame boiler inevitably results in the emission of high concentrations of NO_x as much as 700~1 200 mg/m³ even with the low nitrogen combustion technology implemented. Therefore, the combination of “SNCR+SCR” is often adopted as the traditional route of ultra-low emission control. However, in addition to the high investment and operating costs, this technical route is extremely likely to cause excessive ammonia slip, which eventually leads to air preheater blockage. In this paper, the SCR transformation project of two 330 MW W-flame boilers are conducted by adopting the “SCR zone-based hybrid dynamic leveling technology”. The test results show that after the completion of the project, the SCR removal efficiencies of both boilers have been raised to more than the super high value of 94% while the ammonia escape rate is controlled under 2 μL/L. In this way, not only the goal of ultra-low emission can be achieved but also 14 million RMB of the SNCR investment and 7.6 million RMB of annual operation costs is saved respectively. Besides, to remove the same amount of NO_x, 19% less ammonia is consumed after the transformation while the differential pressure of air preheater is still maintained stably in the long term.

Comparative Study on the Characteristics of Mercury Emission from Coal-Fired Plants before and after Ultra-Low Emission Retrofitting

WANG Hongliang, XU Yueyang, XUE Jianming, LIU Tao, GUAN Yiming

2020, 53(11):  243-251.  DOI: 10.11930/j.issn.1004-9649.202006328

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In this paper, with the aid of Ontario Hydro Method (OHM), the mercury emissions from the two units with ultra-low emission retrofit are sampled and tested, then the testing data are compared and analyzed with those before the retrofitting. The testing results show that the mercury emission concentrations from the two units are 2.31 and 4.22 μg/m³ respectively, far below the limit value in the current domestic national standard. The mercury emission factors are reduced from 1.76 and 3.13 g/TJ to 0.83 and 2.17 g/TJ correspondingly, 52.84% and 30.67% of decrease. And the total mercury removal efficiencies of the retrofitted air pollution control devices with the combination of SCR+DESP+WFGD+WESP are 87.91% and 82.27%, respectively, 18.34% and 16.66% of increase compared with the efficiency of 69.57% and 65.61% of the combination of SCR+DESP+WFGD before the retrofit. These data indicate that the ultra-low emission retrofit has improved the oxidation rate of SCR to elemental mercury, and enhanced the capability of WFGD to capture mercury oxide and particulate mercury. Besides, the newly added WESP also displays a synergistic removal capability of both mercury oxide and elemental mercury. Furthermore, by improving the uniformity of SCR flow field, strengthening the maintenance of catalyst and inhibiting the reduction of mercury oxide in WFGD, the synergistic mercury removal capacity can be effectively guaranteed for the present air pollution control devices.

Study on Reduction and Recycling of the Regenerated Wastewater from Condensate Polishing

WANG Yuanyuan, GAO Yanfeng, HU Dalong, MENG Long, LI Jianbo, LI Junwan, WANG Jing, LI Zhao

2020, 53(11):  252-256.  DOI: 10.11930/j.issn.1004-9649.201905109

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The regenerated wastewater from condensate polishing treatment system may have significant impacts on the performance of the entire plant in that once the national standard for the discharge of wastewater is not reached, more investment will be needed and operation cost will rise up for zero wastewater discharge system. In order to reduce the side effects of regeneration wastewater, the following two schemes are put forward in the aspects of analysis on the reduction of the regeneration wastewater amount as well as the classification and treatment of the regeneration wastewater. One option is to reduce the volume of recycled waste water through feedwater oxygenated treatment and high-speed mixing bed optimization measures. While the other option is by giving the priority of implementing the H₂SO₄ regeneration technique in the fine treatment of positive resin, the recycled wastewater could be reused in the desulfurization process. For those power plants which are not qualified for H₂SO₄ regeneration process, according to the characteristics of the water quality of each step of regeneration, the recycled wastewater of low saline content can be reused as industrial water, while the high saline content wastewater can be treated to produce sodium hypochlorite or fertilizer to realize resource recovery.