Table of Content

05 December 2021, Volume 54 Issue 12

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Optimal Allocation of SVG Considering Voltage Mitigation of DG Grid-connected Inverter and APF

XU Jing, TIAN Shuya, ZHAO Tiejun, GAO Xiaogang, YE Ju, BU Lingyan, LIU Wenmiao

2021, 54(12):  2-10.  DOI: 10.11930/j.issn.1004-9649.202104032

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To solve the voltage deviation problem caused by the high penetration of distributed generation (DG) in the distribution networks, a strategy is proposed for optimal allocation of static var generator (SVG) with consideration of voltage mitigation of DG grid-connected inverter (GCIN) and voltage detection based active power filter (VDAPF). Firstly, a partitioning method based on community theory is proposed. And then, the dominant nodes in each region are selected as the candidate nodes of SVG to be installed. A multi-objective SVG optimal allocation model is established with minimum total cost of SVG and optimal mitigation effect of system voltage deviation, and the improved genetic algorithm is used to solve the allocation model. Considering the uncertainties of the remaining capacity of GCIN and VDAPF, the multi-scenario analysis technique is adopted to construct a series of voltage mitigation operation scenarios. A case study of the IEEE 33-bus distribution network is conducted to verify the effectiveness and rationality of the proposed strategy.

SVG Based Supplementary Damping Mitigation Strategy for Sub-Synchronous Oscillation in Grid-connected Photovoltaic Plants

ZHANG Fan, GAO Benfeng, LI Tiecheng

2021, 54(12):  11-19,44.  DOI: 10.11930/j.issn.1004-9649.202010078

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To mitigate the sub synchronous oscillation (SSO) in large-scale photovoltaic (PV) plants integrated to weak AC networks, a static var generator (SVG) based supplementary damping control strategy is proposed. The strategy extracts the voltage at the PV coupling point as input signal, and produces the sub-synchronous current that is in the same phase with the sub-synchronous voltage at the coupling point through supplementary sub-synchronous damping controller (SSDC). Thus, the SVG is equivalent to the positive resistance of the accessing the system in parallel at the sub-synchronous frequency, and dissipates the SSO energy for suppression. In addition, the structure and parameter design method of SSDC are analyzed in detail. Finally, taking a PV plant in northwest China as an example, the mitigation effectiveness of the proposed control strategy is verified under various working conditions through electromagnetic transient simulation, which shows that the proposed control strategy can improve the stability margin of the photovoltaic system.

Maximum Power Transfer Control Strategy for Variable Frequency Transformer

LU Jiahao, HONG Ruiyuan, CHEN Sizhe

2021, 54(12):  20-28.  DOI: 10.11930/j.issn.1004-9649.202103149

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Variable frequency transformer (VFT) is a new type of flexible AC transmission system equipment to achieve the interconnections of asynchronous grids, whose transmittable power capacity is an important performance indicator. In view of the problem that the traditional control strategy does not consider the rotor winding’s overloading caused by the increase of slip rate, a study is conducted on the maximum stator active power under different slip rates and stator reactive power, and the operation characteristics of a VFT system based on series converter are investigated. The detailed working conditions are analyzed for different slip rates. The mathematical relationship between the optimal stator reactive power reference and the slip rate, and the rated apparent power and the actual absorbed reactive power is derived, and the maximum power transfer (MPT) control strategy for VFT is proposed. A hardware-in-loop (HIL) experimental platform is established to verify the effectiveness of the proposed MPT control strategy. The experimental results show that the proposed MPT control strategy can effectively increase the transferable power capacity of VFT and realize the maximum power transfer of VFT.

Control Strategy for Non Grid Side Converter-based Variable Frequency Transformer under Dual-Side Asymmetrical Conditions

LU Jiahao, CHEN Sizhe

2021, 54(12):  29-37.  DOI: 10.11930/j.issn.1004-9649.202102082

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In view of the problem of uncontrollable rotor voltage of variable frequency transformer, a non grid side converter-based variable frequency transformer (NGSC-VFT) configuration is proposed. Based on this configuration, an integrated mathematical model of NGSC-VFT under dual-side symmetrical and asymmetrical grid conditions is established, and the control strategy of NGSC-VFT is further studied. The stator side series compensation converte (SCC) aims to maintain the DC-link voltage, independently control reactive power, and eliminate the stator’s negative sequence voltage, while the objective of the rotor side SCC is to eliminate the rotor’s negative sequence voltage. The simulation results show that, with the proposed NGSC-VFT configuration, the DC-link voltage can be regulated without GSC with faster dynamic tracking effect of the active power and lower DC capacitor voltage, and the reactive power can be independently controlled. In addition, the fluctuations of torque and power under dual-side asymmetrical conditions can be further suppressed. The novel configuration improves the asymmetrical fault ride-through capability of VFT system.

Core Saturation Analysis of Magnetic-Valve Controlled Reactor Based on Jiles-Atherton Inverse Model

HU Caifei, FAN Xueliang, TONG Li, HE Liqun, LI Xiaohui, ZHENG Hong, ZHANG Liusong

2021, 54(12):  38-44.  DOI: 10.11930/j.issn.1004-9649.202010113

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Magnetic-valve controlled reactor (MCR) is a reactive compensation device in power system, and its capacity is regulated through changing the saturation of iron core. In order to accurately analyze the core saturation of MCR in real time for optimizing the output control of MCR and guaranteeing the safe and reliable operation of magnetic valves, a mathematical model of MCR is established based on Jiles-Atherton (J-A) inverse model and the effects of different state variables on the output characteristics of MCR are compared and analyzed with the electromagnetic computing program. The results show that the DC excitation current of the excitation branch presents linear relationship with the saturation degree. Therefore the DC excitation current can be used as an index for core saturation observation and analysis. The reached conclusions are helpful for realizing the on-line monitoring of the MCR core saturation and the operating state of excitation branch, and for optimizing the control of the output reactive power of MCR.

Analysis on Influence of Large Capacity SVC Reactor on Low Voltage DC Power Supply for Station and Preventive Measures

WEN Caiquan, QUAN Jiexiong, ZHOU Kai, PAN Longbin, LI Cheng

2021, 54(12):  45-53.  DOI: 10.11930/j.issn.1004-9649.202102078

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The influence of phase controlled reactors of a large capacity SVC on the low voltage DC power supply of a power substation can be very serious. In order to understand this influence, field measurement of SVC is carried out in a substation. Furthermore, a three-dimension electromagnetic field simulation model is established for SVC and its adjacent area. A full-scale model is established in the laboratory to verify the equivalence of the simulation results through comparison with the field measurement. An analysis is made on the improvement of DC power quality brought about by different shielding materials and related problems. The results show that the influence of phase controlled reactors of a large capacity SVC on the low voltage DC power supply of a power substation has exceeded the allowable value stipulated by relevant code, and the layout optimization of silicon steel sheet, aluminum plate and iron plate can effectively reduce the interference, and the resulting inductance change and heating are acceptable.

Comparison of Assessment Methods for the Connection of Nonlinear Loads to HV-EHV Power Systems

YU Xijuan, LI Hongtao, MA Xihuan, TAO Shun, MA Ning, SONG Xiaoqing, XU Yonghai

2021, 54(12):  54-62.  DOI: 10.11930/j.issn.1004-9649.202010121

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With the "double-high" development of power systems, the capacity of nonlinear loads or equipment is becoming increasingly larger, and the demand for their connection to the HV-EHV power systems is getting increasingly higher. However, the connection of the nonlinear loads will inject harmonic currents into the power systems, causing voltage waveform distortion. In order to assess whether the connection of nonlinear loads to power systems will cause unacceptable harmonic voltage distortion, the IEC/TR 61000-3-6: 2008 technical standard and the UK G5/5 Engineering Guidelines in 2020 have provided corresponding assessment methods. This paper firstly introduces the assessment methods for the connection of nonlinear loads to HV-EHV power systems provided in these two standards, and then makes a comparative analysis, which can provide a different vision and idea for the revision of the national standards on harmonics.

Application Evaluation of Non-linear Load Connected to HV-EHV Systems

MA Xihuan, TAO Shun, XU Yonghai, MA Ning, SONG Xiaoqing

2021, 54(12):  63-72.  DOI: 10.11930/j.issn.1004-9649.202102081

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Currently, China is continuing to rapidly build a high-speed electrified railway network. The demand for high-speed rail loads to connect to the high-voltage-ultra-high voltage power grid is increasing. When they are connected to the power grid, however, harmonic current will be injected, subsequently causing voltage waveform distortion. In order to compare the evaluation methods for connecting the non-linear loads to the HV-EHV systems provided in the IEC/TR 61000-3-6: 2008 technical document and in the British G5/5 Engineering Guidelines, this paper takes the grid connection of a 220 kV high-speed rail traction station as an application case to compare and analyze the application effects of the two methods, which can will provide some references for the revision of China’s national standards for power quality harmonics.

Influence of the Parameter Dispersion of Domestic 1 200 V SiC MOSFET on Parallel Current Sharing

WU Peifei, TANG Guangfu, YANG Fei, DU Zechen

2021, 54(12):  73-80.  DOI: 10.11930/j.issn.1004-9649.202111031

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Based on the 1 200V 20A SiC MOSFET developed by our team, this paper studies the influence of device parameter dispersion on current sharing of parallel components. Firstly, the deviation degree and coefficient of variation of the device are introduced, and the influence of the three wire method and the two wire method test platform on the test results of the device threshold voltage and the on resistance are analyzed. The conclusion shows that the three wire method is more reliable and has more parameters to be measured. Based on the test platform of the three wire method, the basic characteristic parameters of the device are tested, including threshold voltage, on resistance, transconductance, etc, and the dispersion of 30 devices are analyzed. Finally, the threshold voltage and on resistance are taken as the research objects of parallel current sharing. SiC MOSFETs with similar device characteristic parameters and large dispersion are selected to carry out parallel double pulse experiment. On the basis of eliminating the parasitic parameters of the test circuit through the experiment, the influence of the on resistance and threshold voltage on the device parallel current sharing are verified. The results show that the threshold voltage has a great influence on the current sharing of the transient process before and after switching, and the devices with smaller threshold voltage will bear greater overshoot current, which will affect the reliability of the parallel system; Compared with the switching transient process, the on resistance has a greater impact on the current sharing after steady-state. The devices with smaller on resistance will bear greater current and affect the reliability of branch devices.

R&D of 3 300V SiC MOSFET With Embedded SBD

LIU Guoyou, LUO Haihui, LI Chenzhan, Song Guan

2021, 54(12):  81-85,93.  DOI: 10.11930/j.issn.1004-9649.202107055

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In this paper, a 3 300 V silicon carbide(SiC) metal-oxide-semiconductor field effect transistors (MOSFET) with embedded schottky barrier diodes(SBD) is developed, where the traditional MOSFET structure is integrated with a titanium-formed Schottky contact. The optimized surface treatment procedure after nickel annealing is adopted to improve the leakage performance of gate-source in the chip manufacturing process, which contributes to a 58% reduction in gate-source short circuit failure rate. It is found that the switch-on voltage of parasitic diode is about 8V, when the diode current density (J_SD) reaches 100 A/cm², the voltage drop of SBD (V_SD(SBD)) is 2.1V, which indicates the effectiveness of the embedded SBD in suppressing the parasitic diode turn-on and reducing the bipolar degradation risk of MOSFET. In addition, the threshold voltage of the chip is 3.05V, and the specific on-resistance and blocking voltage are 18mΩ·cm² and 3 955V respectively, suggesting a broad application prospect in the high-voltage rail transit market.

Research on dV/dt Reliability Mechanism of 4 H-SiC PiN Power Diode

GUO Dengyao, TANG Xiaoyan, LI Linqing, ZHANG Yuming

2021, 54(12):  86-93.  DOI: 10.11930/j.issn.1004-9649.202107043

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Silicon carbide (SiC) power switches have the advantages of high-speed and high-power, which leads to more severe high dV/dt reliability issues than silicon (Si) devices in pulsed power systems. It has been verified by experiments that high dV/dt stress will cause permanent degradation of the breakdown characteristics of 4H-SiC PiN power diodes. Simulation results show that there is a strong electric field concentration at the junction between the terminal area of the device and the main junction under high dV/dt stress, which causes the avalanche to occur early, and in turn leads to local temperature rise and permanent damage. The electric field concentration is caused by insufficient depletion of the junction terminal extension (JTE) region under high dV/dt stress. This paper proposes high-concentration compensation doping to the JTE region so as to increase the ionization rate of aluminum (Al) atoms, thereby improving the insufficient depletion of JTE under pulse stress. Simulation proves that this method can effectively reduce the electric field concentration at the edge of the main junction under pulse stress. The 6×10²⁰ cm^–3 phosphorus (P) atom compensation doping improves the device's anti dV/dt capability by about 30%, while the static characteristics are not affected. The proposed method could be a viable way to improve the dV/dt reliability of JTE terminal SiC power devices.

Primary Frequency Control Strategy for Wind Farms Based on Variable Parameter De-loading Control

WANG Tianxiang, CHENG Xuekun, LI Weichao, ZHENG Jinxin

2021, 54(12):  94-101.  DOI: 10.11930/j.issn.1004-9649.202007200

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Variable speed constant frequency (VSCF) wind turbines are connected to the grid through power electronic equipment, leading to the decoupling of wind turbine’s rotor speed and system frequency. As a result, the wind turbines cannot actively respond to the changes of system frequency. To cope with the system frequency stability problem resulting from large-scale integration of wind power, a new active power control strategy for wind farms is proposed, which preferentially applies overspeed de-loading control to wind turbines operating in low wind speed zone. Accordingly, the kinetic energy stored in the rotating mass of wind turbines can be maximized. In the meantime, the reserve demand of the system required by the transmission system operator can be met by applying pitch angle control to wind turbines operating in high wind speed zone. Furthermore, a virtual inertial control strategy with variable parameters is proposed, and the droop coefficient is tuned to ensure the full release of the reserve power of the wind turbines during the system frequency regulation. A system simulation model is built in the DIgSILENT, and the simulation results show that the proposed strategy can reasonably allocate the reserve power of wind turbines and effectively use the reserve power to respond to the frequency change of the system, thus improving the frequency control capabilities of the wind turbines.

Parameter Identification of Low Voltage Ride-Through Control Model for Permanent Magnet Direct-Drive Wind Turbine Based on Probabilistic Reliability Assessment

QIAO Teng, ZHANG Yiming, CAO Yijia, WANG Li, YUAN Qing

2021, 54(12):  102-111.  DOI: 10.11930/j.issn.1004-9649.202012112

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The accurate parameter acquisition is helpful to improve the simulation accuracy of the low voltage ride-through control model for permanent magnet direct-drive wind turbines. The parameters of a complex model are numerous and cannot be fully obtained through tests, and the inaccuracy of its non-target parameters will affect the accuracy of target parameter identification. Therefore, the sensitivity analysis is used to evaluate the difficulty of parameter acquisition, providing a basis for the identification order selection. The non-target parameters are randomly set within the empirical range, and the probabilistic reliability assessment method is employed to decide observed variables from them for the accurate identification of target parameters. A stepwise identification strategy that takes into account both the priority order for identification and the selection of corresponding observed variables is adopted for the target parameters, and the identification results with high probabilistic reliability are obtained as the final parameter values. In this way, the influence of inaccurate settings of non-target parameters is eliminated. The proposed method is applied to identify the target parameters of the low voltage ride-through control model for permanent magnet direct-drive wind turbines, and the results have been verified by measured data.

A New Method for Stator Ground Fault Protection with Voltage Injection on Large Hydro Generators Grounded by Arc Suppression Coils

ZHOU Lang, LIU Jianzheng, ZHANG Qixue, ZHONG Bo, GUI Lin

2021, 54(12):  112-120.  DOI: 10.11930/j.issn.1004-9649.202010102

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For stator ground fault protection of large hydro-generators grounded with arc suppression coil at neutral point, the conventional method is to inject 20 Hz signal from the instrument coil attached to the arc suppression coil. In order to improve the effect of the protection and to overcome the difficulties caused by the increase of stator-to-ground capacitance, a new method is proposed to inject the 20 Hz signal from the resistor in series with the arc suppression coil. The proposed method is compared with the conventional one in terms of the 20 Hz equivalent circuit viewed from the injection port, and the corresponding calculation method for transition resistance is also studied. Taking a generator of Tianshengqiao-Ⅱ Hydro Power Station as the research object, an arc suppression coil based on the parameters of the real generators’ arc suppression coil is customized for test, and the stator ground fault experiment is conducted on a dynamic simulation generator to test the effectiveness of the two methods. The results of the experiment show that, compared with the conventional method, the proposed new method has the maximum measured ground fault transition resistance increasing from 5.6 kΩ to 20 kΩ with an error within ±6%.

Internal Short Circuit Protection for Rotor Winding of Variable Speed Pumped-Storage Generator-Motor

CHEN Jun, YE Hong, LI Huazhong, LI Jianchun, CHEN Xiaogang, WANG Kai

2021, 54(12):  121-127,136.  DOI: 10.11930/j.issn.1004-9649.202011097

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To provide a solution to the imperfect protection problem of variable speed pumped-storage units when the rotor winding has internal short circuit faults, a stator-side branch circulation current based protection method is proposed for internal short circuit of AC excitation motor rotor winding. Based on the multi-loop theory, the key technologies for internal short circuit protection are studied in terms of digital filter design, protection criteria, error prevention measures. A protection prototype is developed and a dynamic simulation experiment platform is established for its verification. The experimental results show that the proposed method can reliably reflect the inter-turn and phase-phase short-circuit faults of the rotor winding. The protection devices developed based on the method will be applied in a variable speed pumped storage unit in China.

Research on Production Efficiency of China's Power Systems from the Perspective of Network Structure

XIE Baichen, LU Long, DUAN Na

2021, 54(12):  128-136.  DOI: 10.11930/j.issn.1004-9649.202012113

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Power systems play an important role in China’s energy security strategy. It is of great significance for high-quality social and economic development to comprehensively evaluate the production efficiency of power systems and their divisions in power generation, transmission, and distribution. This article adopts the data envelopment analysis (DEA) method to analyze the production efficiency of China’s power systems from 2014 to 2017. Compared with traditional DEA models, the network DEA model employs the link variables such as the on-grid electricity and distributed electricity to model the internal connections of a power system and explore the internal factors of system inefficiency. The results show that obvious regional differences in the production efficiency exist among China’s power systems, and the new round of power system reform has promoted the power system development. The heterogeneous characteristics of the external environment lead to different development paths for power systems in various regions. The effects of the policies aiming to improve the production performance are heavily dependent on the actual situations and environment where the power system locates.

Environmental and Economic Dispatch Model for Island Microgrid of Combined Cooling, Heating and Power

WU Qiang, HAN Jun, JIN Ting, CAI Chao, MENG Shiyu, YIN Junping

2021, 54(12):  137-142.  DOI: 10.11930/j.issn.1004-9649.202109126

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The wind turbine, photovoltaic, and cooling, heating and power load forecasts have strong random uncertainties, which increase the difficulty of dispatching and operating island microgrids. This paper studies the environmentally and economic dispatch problem of the islanded microgrid with combined cooling, heating and power. The interruptible load is incorporated into the islanded microgrid as a virtual power generation resource to participate in the system operation, while considering the economy and environmental of scheduling. Fuzzy chance constrained programming is used to deal with the uncertainties, and then the model is transformed into deterministic equivalence. A multi-objective processing method based on satisfaction indicators is used to take into account the economic goals and environmental goals. Numerical example verifies the effectiveness of the proposed model.

Investment Allocation for Medium-Voltage Distribution Networks Considering Investment Equilibrium and Benefits

LI Na, LI Chaoyang, ZHOU Jin, GAO Yi, HONG Yunfu, YOU Weiyang

2021, 54(12):  143-149.  DOI: 10.11930/j.issn.1004-9649.202011016

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In view of the lack of reasonable investment allocation methods for distribution networks, this paper proposes an investment allocation model for multi-region and multi-type projects of medium-voltage distribution networks. Firstly, from the perspective of investment demand, regional power demand, problem status and planning targets, investment benefits, project management level, risk-sharing ability and other factors, the indicators and grading methods are investigated and determined for investment allocation of the medium-voltage distribution networks, and the panel regression analysis is introduced to establish the functional relationship between the medium-voltage distribution network investment of each prefecture-level power supply company and the integrated indicator grading. Secondly, based on the multi-objective optimization decision-making techniques, and with the objective of investment balance and overall benefits and the best unit investment benefits, an investment allocation model is proposed for various large-scale projects and various function projects to increase power supply, improve power supply reliability and reduce line losses. The case study shows that the proposed model can provide a support for the investment allocation for various regions and various projects of the medium-voltage distribution networks of prefecture-level power supply companies.

Transformer Fault Diagnosis Based on Hybrid Sampling and Support Vector Machines

LI Liang, FAN Jin, YAN Lin, ZHANG Mi, WANG Pengfei, ZHAO Xiaojun, XIAO Haibin

2021, 54(12):  150-155.  DOI: 10.11930/j.issn.1004-9649.202109153

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Aiming at the impact of transformer imbalanced data set on transformer fault diagnosis model. A transformer fault diagnosis method based on hybrid sampling and support vector machines (SVM) is proposed. It uses synthetic minority oversampling technique (SMOTE) and under sampling method based on nearest neighbor rules to underestimate transformer fault data and normal data, respectively. Sampling and oversampling, and then using the balanced data obtained by hybrid sampling training based on support vector machines transformer fault diagnosis model. The performance of the SVM-based transformer fault diagnosis model is compared through the test set under imbalanced data and balanced data. Finally, the influence of sampling rate on the diagnostic accuracy of transformer fault diagnosis model is analyzed. Experimental results show that this method can effectively reduce the impact of imbalanced data on the diagnostic model and improve the diagnostic accuracy of the transformer fault diagnostic model.

Carbon Emissions Trading Pilot Policy and Power Industry Emissions Reductions

CHEN Jingdong, ZHAO Pei

2021, 54(12):  156-161.  DOI: 10.11930/j.issn.1004-9649.202110046

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In the context of the low carbon strategy, carbon trading system plays an important role in reducing carbon emissions of the power industry. Using a sample of 30 provinces in China and local carbon trading pilot policies as a quasi-natural experiment, this paper examines the real effect of carbon emissions trading policies by difference in differences (DID) estimations. The study finds that, first, carbon emissions of power industry in pilot provinces decline more rapidly than that in non-pilot provinces. Second, the carbon emissions of the power industry in a province are negatively related to the total volume and amount of local carbon trading. These findings imply that China's carbon trading pilot system has a positive effect on carbon emissions reduction of local power industry.

Research on Supplementary Frequency Regulation with Aggregated Electric Vehicles Considering Distribution Network Congestion

CHEN Hao, HU Junjie, YUAN Haifeng, ZHOU Huayanran, LUO Kui

2021, 54(12):  162-169.  DOI: 10.11930/j.issn.1004-9649.202012058

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Electric vehicles (EVs) have attracted wide attention in the frequency regulation of power systems due to their increase in quantity and their fast response capacity. However, current research ignores the fact that during the frequency regulation their load attributes may lead to distribution network congestion when a large number of them are connected to the grid. Therefore, a control strategy for the supplementary frequency regulation with aggregated EVs is proposed considering distribution network congestion. Firstly, the hierarchical scheduling framework of the supplementary frequency regulation with the aggregated EVs is presented. Secondly, a frequency regulation power allocation mode considering the state of charge (SOC) of EVs is proposed. Then, a control strategy based on current congestion indicator factors (CCIF) is established to limit the charging and discharging power of EVs. Finally, the advantage of the control strategy is verified by the MATLAB/Simulink simulation, which shows that this strategy can avoid the distribution network congestion and ensure the frequency fluctuation of the system lower than 0.1 Hz.

Field Observation of Air Density for Transmission Line Corridors in High-altitude Regions

YANG Fengli, ZHANG Hongjie, SHAO Shuai, HUANG Guo

2021, 54(12):  170-176.  DOI: 10.11930/j.issn.1004-9649.202009082

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Field observations of air temperature, relative humidity and atmospheric pressure in high-altitude regions were carried out at the sites near the transmission line corridors in Tibet with an altitude height higher than 4300 m, and the field observation data of three different altitudes with over one year duration were obtained. The impact differences of air temperature, relative humidity and atmospheric pressure on air density in high altitude regions were determined. The mean value, maximum value and minimum value of the observed air density at three typical high-altitude sites were statistically analyzed. Based on three types of statistical parameters, the decrease ratios of the observed air density relative to the standard air density were calculated respectively. The decrease ratios of air density at the three observation sites are basically higher than that proposed in applicable standards, which means that when the altitude heights are in the range of 4300 m to 4900 m, the anti-wind design of transmission lines is on the conservative side if the air density proposed in the applicable standards is adopted. When the observed maximum air density is proposed for design, the basic wind pressure for the regions with the altitude higher than 4300 m will be decreased by at least 31.7% compared to the value determined by standard air density. The results can provide a reference for anti-wind design and maintenance works of transmission lines in high-altitude regions.

An Online Location Method for Local Defects in Power Cables Based on Electromagnetic Coupling Injection

LIU Haoran, ZHOU Kai, WANG Yuhao, LI Rong, FU Yao, RAO Xianjie

2021, 54(12):  177-185,194.  DOI: 10.11930/j.issn.1004-9649.202105175

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In order to solve the shortcoming that the existing frequency domain reflection method can only locate and detect cable defects in offline situations, this paper proposes an online cable defect location method based on frequency domain reflection method and electromagnetic coupling technology. Firstly, based on the cable distribution parameter model, the cables with local defects are simulated and modeled, and the influence of the defect points of different capacitance changes and different test frequencies on the positioning effect of the cable defects is analyzed. Then the influence of the sensor structure on the signal injection efficiency was explored. Finally, it was verified on 105 m and 500 m power cables in the laboratory. In the case of offline testing, the load resistance of the 105 m cable end was changed to simulate different cable loads. The line condition, and analyze its influence on the reflection intensity at the local defect and the end of the cable. In the case of live test, the 500 m cable is energized through the series resonance platform, and the cable is tested using the online positioning method proposed in this article to verify the effectiveness of the method proposed in this article. The simulation and experimental results show that: compared with the cable offline positioning test method, this method can realize the online positioning of the cable defect point when the cable is energized, and has higher positioning accuracy and recognition sensitivity.

Comparative Analysis on Discharge Gas Production Characteristics of Typical Electrodes with Oil Insulation

LIU Qin, XIE Xiongjie, HU Wei, LIU Taiwei, LUO Xiaoqing, YE Qiming

2021, 54(12):  186-194.  DOI: 10.11930/j.issn.1004-9649.202106003

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The oil-less equipment is internally insulated by filling with insulating oil. The discharge or overheating faults may cause the degradation of insulating oil, consequently producing a large amount of dissolved gases such as H₂, hydrocarbon gas, CO and CO₂, and equipment explosion may occur in severe cases due to the small internal space, resulting in huge economical loss. Therefore, a discharge test system is designed for oil-less equipment to obtain the gas diffusion characteristics in the oil after discharge. A comparative study is conducted on the gas production for three typical electrodes, including the plate-plate electrode, ball-ball electrode and needle-plate electrode, with different gap distances and different discharge time. The research results show that under plate-plate electrode discharge, the gas produced by direct pressurization to breakdown is small in volume; the content of C₂H₂ and H₂ produced by the ball-ball electrode and needle-plate electrode discharge tends to be saturated; under same gap distance and discharge time, the needle-plate electrode produces the largest volume of gas, followed by the ball-ball electrode, and the plate-plate electrode produces the least; at 1mm gap, the discharge process of ball pure oil ball is more intense than that of ball paperboard ball with large gas production. The variation characteristics of the discharge gas content in three typical electrode oils are thus obtained. The reached conclusions have great significance for the fault analysis of oil-less equipment under gas production.