Table of Content

28 July 2022, Volume 55 Issue 7

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Power System

Aging Prediction of IGBT Based on Improved Support Vector Regression

CHEN Zhengxiong, PAZILAI Mahemuti, SHEN Wei

2022, 55(7):  1-10.  DOI: 10.11930/j.issn.1004-9649.202201008

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In order to accurately predict the aging state of insulated gate bipolar transistor (IGBT), a novel IGBT aging prediction method is proposed based on improved whale optimization algorithm (IWOA) and optimized support vector regression (SVR). In this method, the time-frequency domain characteristics of IGBT collector-emitter voltage signals are extracted and are integrated into a comprehensive index to characterize the aging state of IGBT through dimension reduction of kernel principal component analysis (KPCA); Against the deficiency of the whale optimization algorithm (WOA), the Sobol sequence population initialization, inertia weight and reverse learning strategy are introduced to enhance the local search ability and convergence speed of WOA; The IWOA is used to optimize the penalty factor and kernel parameters of SVR, and an IGBT prediction model is constructed based on the comprehensive index. The proposed IWOA-SVR method is verified by using the IGBT aging data set of the NASA Ames laboratory. The results show that the constructed IWOA-SVR prediction model can more accurately predict the aging of IGBT.

Key Factors of Medium-High Frequency Oscillation in Chongqing-Hubei HVDC System and Suppression Strategies

LI Qinan, XIA Yongjun, ZHANG Xiaolin, SUN Baokui, SUN Huadong, ZHANG Fan, LI Lanfang, YANG Yuefeng, HAN Qingtao

2022, 55(7):  11-21.  DOI: 10.11930/j.issn.1004-9649.202106054

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For the problem of medium and high frequency oscillations in flexible HVDC projects, the existing solution is to add a filter in the voltage feedforward path to suppress high frequency oscillations, which, however, increases the risk of medium frequency oscillations. For this reason, a method is proposed to avoid medium-high frequency oscillations by adjusting the control link delay based on the south channel of the Chongqing-Hubei flexible HVDC transmission project. Firstly, a impedance model of MMC converter station is established based on the principle of multi-harmonic linearization, and a comprehensive analysis is made on the influence of control link delay and grid voltage feedforward strategy on the impedance characteristics of MMC converter station and system stability. And then, the feasibility of adjusting the delay of the control link to avoid the medium-high frequency oscillation is discussed, and the method for selecting the time delay of the control link is given. Finally, based on PSCAD/EMTDC electromagnetic transient simulation, it is verified that the proposed method can achieve the suppression of the medium-high frequency oscillations in both the Hubei-side and Chongqing-side systems. Meanwhile, the proposed method has minor effect on the dynamic characteristics of the system.

Transformer Hierarchical Fault Diagnosis Model Based on Dissolved Gas Analysis of Insulating Oil and Class Overlap Features

CHEN Tie, LENG Haowei, LI Xianshan, CHEN Yifu

2022, 55(7):  22-32,41.  DOI: 10.11930/j.issn.1004-9649.202202010

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Dissolved gas analysis (DGA) of insulating oil can effectively identify transformer discharge fault and overheating fault. In order to improve the accuracy of transformer fault diagnosis, a transformer hierarchical fault diagnosis method is proposed based on class overlap features. Firstly, the support vector data description (SVDD) is used to divide the overlapping region of transformer fault sample data spaces, and the class overlap rate and class overlap degree are selected as the overlapping features to describe the class overlap degree and the importance of sample points respectively. And then, a hierarchical fault diagnosis model is established based on the class overlap rate. The samples of each diagnosis layer are trained separately by the separate training method, and a two-class fuzzy support vector machine (FSVM) is constructed based on class overlap degrees to diagnose faults. Experimental results show that the proposed method is more accurate than other models.

Energy Storage Control Strategy of Railway Network Side Considering Characteristic Harmonic Control

HE Yanqiang, WANG Ying, CHEN Xiaoqiang, CHEN Jianxiao

2022, 55(7):  33-41.  DOI: 10.11930/j.issn.1004-9649.202012050

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Aiming at the problems of energy saving, efficiency improvement and harmonic control of electric railways, a control strategy of railway traction network-side energy storage system is proposed with consideration of characteristic harmonic control. Firstly, the access scheme of the traction network-side energy storage system is analyzed and its working modes are classified. Secondly, according to the distribution of characteristic harmonics of traction load, a novel method is deduced for peak load shaving and characteristic harmonics control by using the energy storage system at the grid side. And then, a control strategy of supercapacitor energy storage system is proposed, which takes into account the grid-side characteristic harmonic suppression. Finally, the feasibility of the proposed strategy is verified by simulation tests under multiple working conditions. The results show that the proposed strategy can effectively recover the regenerative braking energy, reduce the power fluctuation, and suppress the characteristic harmonics at the grid side.

Comparison of Harmonic Current Limit Determination Methods Between GB/T 14549—93 and IEEE std.519:2014

SONG Yifan, ZHAO He, SHEN Junyan, TAO Shun, MA Xihuan, XU Yonghai

2022, 55(7):  42-48.  DOI: 10.11930/j.issn.1004-9649.202009147

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The allowable harmonic current limits injected to the common connection points are stipulated for users respectively by GB/T 14549—93 and IEEE std.519:2014. However, there are some differences between two standards in determination methods and engineering application. The harmonics national standard GB/T 14549 under revision intends to adopt the emission limits of harmonic current for single user in IEEE std.519:2014. Therefore, it is necessary to fully discuss the rationality of the methods for determining the harmonic current limits. In this paper, the limits of the harmonic current in two standards are introduced, and their determination methods are deduced and discussed respectively. Finally, the similarities and differences between them are analyzed through case study.

Error Analysis and Correction of Voltage Sag Measurement for Capacitor Voltage Transformer

ZHOU Wen, LIANG Jifeng, JIAO Yadong, LI Tiecheng, LU Yanqiao, LIU Yong

2022, 55(7):  49-58.  DOI: 10.11930/j.issn.1004-9649.202011082

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Voltage sag is one of the most prominent power quality problems in modern power grid. When monitoring voltage sag in a large range of high-voltage systems, the adverse effects caused by capacitor voltage transformer (CVT) with energy storage elements must be considered. According to the structure of the CVT, the error of voltage sag measurement is deduced, and the internal and external factors influencing the error are identified. Then, a simulation model is built, and a study is made in detail on the influences and sensitivities of the initial phase angle of sag, residual voltage, CVT parameters and load on voltage sag duration, sag amplitude and phase jump. Finally, a virtual impedance compensation based method is proposed for CVT’s voltage sag measurement error correction, which eliminates the problem of measurement error diversity caused by voltage sag uncertainty. The simulation results show that the method is effective and provides a feasible correction scheme for the accurate measurement of voltage sag in the high-voltage system which widely adopts CVT.

A Voltage Sag Prevention Index Based on Virtual Upper and Lower Limit Tolerance Curve

LV Jinbing, YANG Guochao, FAN Xingguan

2022, 55(7):  59-66,73.  DOI: 10.11930/j.issn.1004-9649.202102074

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Based on the uncertainty of equipment sensitivity to voltage sags, a voltage sag prevention index is proposed for the virtual upper and lower limit curve with single tolerance characteristic. Firstly, the virtual upper and lower limit curves are described according to the uncertainty of the equipment’s tolerance characteristics, and on this basis, a voltage sag severity evaluation model is established based on the virtual upper and lower limit curves to visually present the sag’s impact on the equipment. And then, with consideration of user mitigation expectation threshold, a compensation cost function and an economic benefit based profit function are established, and the voltage sag prevention index is defined. Finally, the effectiveness and rationality of the proposed method is verified through a case study. The proposed method can be used for voltage sag evaluation and governance of actual power systems.

Diagnosis of Deviation Indicators of Distribution Network Engineering Based on Fuzzy Petri Net

YU Haozheng, PAN Zhaolun, LI Ke, ZHOU Peng, GUO Xinzhi, SUN Huijun, YU Jinxiong, LI Cunbin

2022, 55(7):  67-73.  DOI: 10.11930/j.issn.1004-9649.202011033

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With the rapid development of distribution network engineering in recent years, the data of various control indicators for construction of distribution network projects has also become more diverse and complex. At the same time, it is hard to identify the deviation indicators of the distribution network projects due to the limited data processing capabilities, which leads to poor identification and location of some long-standing deviation indicators in the distribution network projects. Based on the theory of fuzzy Petri net, a diagnosis model is designed for the deviation indicators of distribution network projects, and the method for determining the real-time data of each indicator in the process of construction of the distribution network projects is analyzed. A case study is carried out for determining the deviation indicators of the distribution network project, which can provide a good reference for project management personnel to manage the distribution network engineering in a timely and effective manner.

An Analysis of the Driving Factors of Chinese Electricity Consumption from 1997 to 2017

WANG Hailin, WENG Yuyan, ZHANG Chenglong

2022, 55(7):  74-80.  DOI: 10.11930/j.issn.1004-9649.202110070

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The logarithmic mean divisia index (LMDI) factor decomposition method is used to analyze the driving effects of economic factor, technical factor, structural factor, population factor and resident activity factor on the China's electricity consumption growth between 1997 and 2017. The results show that economic factor contribute most to the increase in electricity consumption; the technical factor and structural factor slow down the rapid growth of electricity consumption; and the contribution of population and resident activity factors to the growth of electricity consumption gradually increases. It is suggested that we should continue to maintain high-quality economic growth and formulate more scientific and reasonable electricity development goals and targeted policies and measures. The transformation of household electricity consumption patterns should be guided rationally to meet the residents' high-quality living needs with high-quality electricity development.

Optimization Strategy of Multi-group Inspection Path of Distribution Equipment with Equipment Information Included

CHEN Chaoqiang, GONG Hanyang, ZHANG Di, ZHANG Zhidan, LE Jian

2022, 55(7):  81-86.  DOI: 10.11930/j.issn.1004-9649.202108110

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In view of the low efficiency of the existing inspection strategy, this paper proposes an optimization strategy of multi-group inspection paths of distribution equipment, which includes equipment information, in order to improve the scientific in the development of inspection path and the efficiency of inspection when facing at a large number of inspection equipment. A distribution equipment information model is established, and a multi group distribution equipment routing optimization model is proposed to minimize the total routing cost, which is solved by genetic algorithm. The optimization strategy of a certain distribution network equipment inspection task is given, the total distance of inspection under different strategies is analyzed.

Failure Early Warning and Life Estimation Method of Batch Smart Meter

HUANG Youpeng, LU Tao, CHEN Liang, DANG Sanlei, ZHANG Jie, YU Sida

2022, 55(7):  87-92.  DOI: 10.11930/j.issn.1004-9649.202109151

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With the construction of smart grid, the number of smart meters has increased significantly. Based on the field reliability data, the maximum likelihood method is used to estimate the distribution parameters of batch smart meters, and a batch fault early warning and life prediction method of batch smart meters is proposed. Verification results show that the proposed method can warn the batch of faulty smart meters, predict the reliable life, and provide theoretical and data support for the optimization of smart meter rotation mechanism.

Control Strategy of Power Conversion System under Unbalanced Grid Voltage

OU Mingyong, WANG Yichao, LIU Wenjun, XIA Xiangyang, CHEN Lingbin, LENG Yang, ZHENG Chuyu

2022, 55(7):  93-101.  DOI: 10.11930/j.issn.1004-9649.202110012

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Unbalanced grid voltage will cause voltage fluctuations on the DC side and active power fluctuations on the grid side, which seriously affects the safety and energy conversion efficiency of the energy storage battery system. Aiming at the above problems, this paper proposes a coordinated control strategy of DC side voltage and grid side active power. Firstly, by considering the active power fluctuation absorbed by the filter inductor on the AC side of the power conversion system, the suppression of the voltage fluctuation on the DC side is realized; Then, the weighted idea is used to coordinately control the voltage on the DC side and the active power on the grid side, and the fluctuation amplitude of the two is well reduced; Finally, the effectiveness and superiority of the proposed method are verified in the simulation.

Flatness-Based Control of AC / DC Hybrid Microgrid Interconnected Converter

ZHANG Yu, WANG Hongxi, WANG Pu

2022, 55(7):  102-109,120.  DOI: 10.11930/j.issn.1004-9649.202109160

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An Interlinking Converter (ILC) control strategy based on the differential Flatness-Based Control (FBC) theory is proposed to improve the dynamic and robust performance of the hybrid AC-DC microgrid in the presence of power fluctuations and power loss. First analyzes the distributed power supply in the sub-network under the island mode. It uses the droop control strategy to solve the power distribution problem in the respective network. Secondly, it establishes the ILC mathematical model under the DQ axis and proves that the ILC system satisfies the differential flatness. Then, according to the differential flatness theory, the power controller of ILC is designed, which consists of feedforward control and error compensation. The cascade control structure is adopted in the system. The outer power loop generates the reference trajectory of balanced output, and the current inner loop generates the DQ axis voltage component of the ILC expected output. Finally, the ILC simulation system of FBC and PI control is established in Matlab / Simulink. Under three working conditions, the simulation results verify that the FBC control system has better dynamics and robustness.

Integrated Planning of Optimal Sizing and Siting of Energy Storage Plants Across Wide Area

CAO Xinhui, CHE Yong, SI Zheng, KAI Saijiang, ZHOU Zhuan, YUAN Tiejiang

2022, 55(7):  110-120.  DOI: 10.11930/j.issn.1004-9649.202202024

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Aiming at optimal sizing and siting of energy storage plants in large-scale wind power systems, an integrated planning method was developed to realize the optimal sizing and siting of energy storage plants across wide area. First, based on the four-quadrant operation characteristics of energy storage devices, the effect mechanism of optimal sizing and siting of energy storage plants on static voltage stability and wind energy accommodation, facing simple radiant networks and complex networks, was comprehensively demonstrated by PV curve, and the concept of sizing and siting of energy storage plants was put forward. On this basis, the static voltage stability and wind energy accommodation in the company of network loss and energy storage investment were comprehensively considered. An integrated planning model was developed with annual income of wind power consumption maximal and static voltage stability margin change rate, loss cost and annual investment cost of energy storage plants minimal as the objective. Finally, a Genetic algorithm embedded power flow and particle swarm optimization was designed to solve this model. The validity of the proposed method was verified by the simulation tests carried out on large-scale wind power base in Zhundong, Xinjiang.

Electric Load Forecast

Short-Term Load Forecasting Based on Feature Selection and Combination Model

XU Yusong, ZOU Shanhua, LU Xianling

2022, 55(7):  121-127.  DOI: 10.11930/j.issn.1004-9649.202111045

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A short-term load forecasting method based on feature selection and combination model is proposed. At first, the method divides the feature vectors into two sets according to the individual characteristics. Spearman rank-order correlation coefficient and max-relevance & min-redundancy algorithm are individually employed for selection. Bayesian information criterion is used to get the dimension of the optimal feature vector. And then, three different simple-kernel based support vector regression models are built using three kernel functions respectively and complete prediction. Finally, a neural network is set up for experimental analysis. The simulation results show that the proposed combination model has a great high forecasting accuracy and robustness.

Design of a Short-Term Load Intelligent Forecasting System for Regional Power Grid Based on Accurate Weather Data

LI Dan, ZHANG Yuanhang, LI Huangqiang, TONG Huamin, WANG Lingyun

2022, 55(7):  128-133.  DOI: 10.11930/j.issn.1004-9649.202108026

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Based on the time-zoned refined meteorological data, the regional power grid short-term load intelligent prediction system is developed to realize the accurate forecast of the power curve. The features of this system are to decompose the grid supply load into the superposition of various power components, and to provide a variety of feature selection modes, prediction methods and historical reference dates according to the characteristics and influencing factors of each power component, which improves the accuracy, automation and work efficiency of short-term load forecasting.

New Energy

Fault-Tolerant Control Strategy Based on Multi-Phase Wind Power System

ZHOU Shijia, YANG Guangyuan, PENG Guangqiang, WU Jiyang, XIN Qingming

2022, 55(7):  134-141.  DOI: 10.11930/j.issn.1004-9649.202107095

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Owing to their characteristics of large torque, low torque ripple, and high fault tolerance, multi-phase motors are especially suitable for low-speed high-power application scenarios such as wind power systems. This paper proposes a wind power system based on multi-phase direct-drive permanent magnet synchronous generators. The multi-phase wind turbines are integrated into the high-voltage direct-current (DC) grid via three-phase bridge uncontrolled rectifiers, isolated DC/DC converters, and modular multilevel converters (MMCs). Specifically, an average current reference value of the three-phase bridge uncontrolled rectifiers is obtained by maximum power point tracking (MPPT) control based on an outer speed loop. Then, the current reference values of each set of three-phase bridge uncontrolled rectifiers are reconstructed according to the working state of the converter for the output power balance of the normal phase. The aim is to ensure that overcurrent does not occur in the fault phase, achieve fault-tolerant control of the system, and thereby improve reliability. Finally, the wind turbine is connected to the DC grid via the single bridge arm of the MMC to reduce the complexity of the converter. With 18-phase wind turbines as an example, a Matlab/Simulink simulation model is built to verify the effectiveness of the proposed topology and fault-tolerant control strategy.

Dynamic Coordination Mechanism of DFIGs Based on Inertia Response

LIU Zhaorui, JIA Qi, YAN Gangui, ZHAI Wenchao, SUN Yong, LI Baoju

2022, 55(7):  142-151.  DOI: 10.11930/j.issn.1004-9649.202108078

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Large-scale wind power integration occupies the capacity space of synchronous generators with moments of inertia and weakens the inertia level and frequency regulation ability of the power system. Therefore, the participation of wind power generation in system frequency regulation is urgently needed. On the basis of the frequency response model of the doubly-fed induction generator (DFIG), an inertia expression is obtained analytically, and the influence of phase-locked loop (PLL) control parameters on the inertia characteristics of DFIG is analyzed. It is then concluded that the inertia response of DFIG can be achieved by optimizing PLL control parameters when unbalanced power disturbance occurs in the system. With a parallel system of two DFIGs as an example, the law of unbalanced power distribution between the DFIGs during inertia response is analyzed and extended to multi-machine parallel systems. A frequency response model of a system of integrated DFIGs and synchronous generators is built, and the frequency response characteristics in each stage of power disturbance are analyzed. Finally, the effectiveness of the theoretical analysis is verified by time-domain simulation.

Quantitative Assessment Method for Transient Voltage of Distribution Network with High-Penetration Wind Power

XU Yanchun, JIANG Weijun, SUN Sihan, MI Lu

2022, 55(7):  152-162.  DOI: 10.11930/j.issn.1004-9649.202110085

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With the further penetration of distributed wind power in the distribution network, assessing the factors affecting the transient voltage stability of the power grid becomes increasingly significant. However, the existing indexes for evaluating transient voltage stability are not applicable to scenarios with multiple transient characteristics. In this paper, the assessment idea of a multiple-two-element notation criterion is outlined. For voltage sag, a safety margin index of voltage sag of the bus is proposed. Next, a safety margin index of the overvoltage of the bus is further constructed. A step factor is introduced to combine the two indexes effectively into a safety margin index of the transient voltage of the bus based on an improved multiple-two-element notation. Then, a safety margin index of regional transient voltage is proposed to measure the transient voltage stability of a certain region. Finally, Matlab/Simulink is used to simulate the distribution network with high-penetration wind power according to the proposed indexes. The effectiveness and universality of the proposed indexes are verified while the factors affecting the transient voltage stability of the distribution network are quantified.

Voltage Trajectory Prediction of Photovoltaic Power Station Based on CNN-GRU

FENG Yuqi, LI Hui, LI Lijuan, ZHOU Yanbo, TAN Mao, PENG Hanmei

2022, 55(7):  163-171.  DOI: 10.11930/j.issn.1004-9649.202112009

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The output randomness of photovoltaic power stations can easily cause large voltage fluctuations at grid-connection points. Advance regulation through trend prediction is an effective way to improve voltage stability. To improve the accuracy of voltage trend prediction, this paper proposes a voltage trajectory prediction method based on convolutional neural network (CNN) and gated recurrent unit (GRU). Specifically, a time series is constructed by extracting voltage data from the acquisition unit. Then, the autocorrelation coefficient of the voltage time series and its maximal information coefficient (MIC) relative to external variables are calculated, and the correlations of the voltage time series with external variables in timing are analyzed. Finally, the high-level features of input data are extracted through the CNN network and input into the GRU network to complete voltage trajectory prediction. The measured data of a photovoltaic power station are utilized for verification. The results show that compared with GRU, long short-term memory (LSTM), CNN-LSTM, and support vector regression (SVR) models, the proposed model has higher prediction accuracy.

Optimal Operation of Rural Energy System with Biomass Biogas Power Generation Considering Energy Utilization Efficiency

FENG Kaihui, YAN Hu, DAI Wuzhen, CHEN Jian, LIU Yongliang

2022, 55(7):  172-178.  DOI: 10.11930/j.issn.1004-9649.202109098

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Rural energy is an important part of China's energy system. With the popularization and application of biomass biogas power generation, the application prospect of rural integrated energy system is more extensive. Taking the integrated energy system in rural areas as the research object, an energy system model containing biomass biogas power generation is constructed, and the uncertainty of load and renewable energy output is described by using opportunity constraints, and a comprehensive consideration of system economy and energy utilization is proposed. Efficiency optimization runs the model. Economic optimization operation considers the lowest cost of electricity and gas. Energy efficiency considers the energy efficiency index of standard coal.In order to comprehensively consider the economy and energy utilization efficiency, the target membership function is defined to maximize the satisfaction rate. The index method converts multi-objective problems into single-objective optimization solutions. The simulation results show that the proposed method can take into account the system economy and energy utilization efficiency.

Two-Stage Optimal Scheduling of Regional Integrated Energy System Considering V2 G Response of Electric Buses

YU Zichun, FAN Hong, XIA Shiwei

2022, 55(7):  179-192.  DOI: 10.11930/j.issn.1004-9649.202109022

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Aiming at the problems of wind and photovoltaic power curtailment and total electrical load fluctuation caused by high permeability renewable energy and electric buses access to the system, a two-stage optimal scheduling method of regional integrated energy system considering the vehicle-to-grid (V2G) response of electric buses was proposed. Firstly, under the condition of satisfying the operation plan of electric buses, the V2G response of electric bus cluster is embedded into the optimal scheduling of regional integrated energy system, and the first-stage scheduling strategy is formed with the goal of minimizing the operation cost of the whole system. Then, based on the optimization results of the first stage, the second-stage scheduling model further optimizes the scheduling plan of each equipment and energy interaction strategies with the goal of minimizing the fluctuation rate of total electrical load generated by all electrical equipment in the system. Finally, the results of coordinated scheduling of electric bus cluster and energy equipment in four scenarios are compared and analyzed, the simulation shows that the proposed model can improve the capacity of wind and photovoltaic power accommodation and operation economy of the regional integrated energy system, and reduce the fluctuation of the total electrical load of the system, which verifies the effectiveness of the proposed scheduling method.

Energy Conservation and Environmental Protection

Study of Evaporation Performance Between Centrifugal Spray Evaporation and Bypass Flue Evaporation to Treat Desulfurization Wastewater

ZHAO Ning, FENG Yongxin, LIN Tingkun, YANG Qingshan, XIE Zhiwen

2022, 55(7):  193-200.  DOI: 10.11930/j.issn.1004-9649.202109067

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To investigate the high-temperature bypass flue gas evaporation technologies including centrifugal spray evaporation tower and bypass flue evaporation, spray evaporation experiments were conducted using desulfurization wastewater. Evaporation process of wastewater droplets was revealed by the fluorescent tracer technique and single droplet evaporation characteristics. The influences of inlet temperature and residence time on the evaporation efficiency were also discussed. Differences between the two wastewater treatment technologies were analyzed. The results show that after being exposed to the hot flue gas, desulfurization wastewater droplets would rapidly lose their free water, form shells and turn into semi-dried particles, with temperature in the main evaporation zone of the dryer decreasing significantly. Increasing inlet temperature helps to improve the evaporation rate and reduce the moisture content of the outlet ash particles. By increasing the size of the dryers and controlling the amount of extracted flue gas, the residence time of high temperature flue gas in dryers can be extended, which is about to improve the heat utilization and achieve the drying effect with low gas-liquid ratio. In comparison with the bypass flue evaporation technique, centrifugal spray evaporation tower was better due to better atomization performance and uniform gas-liquid mixing.

A Pilot-scale Research on SO₃ Control in Flue Gas of Coal Fired Power Plant

LIU Yi

2022, 55(7):  201-208.  DOI: 10.11930/j.issn.1004-9649.202109031

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SO₃ emission from coal fired power plant causes serious environment issues. In this paper, the generation and removal of SO₃ in flue gas was investigated on a pilot-scale platform of multi-pollutant removal in a Beijing power plant where the Shenhua coal was combusted. The results showed that increasing the inlet temperature of Selective Catalytic Reduction (SCR) reactor led to an increase in the conversion rate of SO₂/SO₃. The SO₃ removal efficiency of low-low temperature electrostatic precipitator (LLT-ESP) decreases with the increase of inlet flue gas temperature. No obvious effect on improving the removal efficiency of SO₃ with a further increase in the amount of the slurry of the wet flue gas desulfurization (WFGD) system. SO₃ removal efficiency could be maintained in 75 % and 80 % when 3 circulating pumps was in operation and the inlet temperature of SCR and LLT-ESP were 290℃ and 130℃ respectively. Under this circumstance, the emission concentrations of NO_x and SO₂ were far below the ultra-low emission requirements.