2020, 35(4):3-12. DOI: 10.19781/j.issn.16739140.2020.04.001
Abstract:The disordered planning and unreasonable allocation of electric vehicle charging stations have a serious impact on the safty，stability and economy of the distribution network operation．To solve this problem, this paper constructs an optimal planning model of electric vehicle charging stations based on the voltage stability index of distribution network．The model not only could balance the construction investment of charging stations，the economic operation of distribution system and the quality of power supply，but also take full account of the demand and convenience of the electric vehicle charging．The improved adaptive particle swarm optimization (APSO) algorithm with fast speed and high accuracy is adopted to solve the nolinear optimization problem with multiple constraints. Finally, the simulation results based on IEEE33bus system show the effectiveness of the proposed method.
2020, 35(4):13-19. DOI: 10.19781/j.issn.16739140.2020.04.002
Abstract:With the construction of smart grid, power companies gradually use unmanned aerial vehicles (UAV) to replace manual inspection of transmission lines. This paper proposes a method for processing aerial images of transmission line insulators captured by UAV. Firstly, the threshold and range of RGB components in the color model are used to segment the target and background areas. Secondly, mathematical morphology and nonoverlapping window texture features are applied to roughly mark the target area. Finally, a minimum circumscribed horizontal rectangular frame is generated. Then the texture features of all the patterns within the minimum circumscribed horizontal rectangular frame are identified to locate the minimum horizontal rectangular area of the aerial image of the insulator string. In the end, we use two images to verify the algorithm and compare with the algorithms in the literature. The results show that the algorithm proposed in this paper can better identify the position of insulator strings.
2020, 35(4):20-26. DOI: 10.19781/j.issn.16739140.2020.04.003
Abstract:The accuracy of the impedance model of the converter station directly affects the small interference stability analysis of the flexible DC transmission system. However, the more accurate the model, the higher the order, which increases the complexity of the analysis. To analyze the influence of the phaselocked loop on the impedance characteristics of the converter station, we firstly establish the admittance matrix of the AC side of the converter station under the two conditions of dq coordinates with and without consideration of the phaselocked loop, and then establish the same order and mirror admittance of the converter station at the static coordinates. By comparing and not considering the input admittance of the phaselocked loop, it is concluded that the influence of the phaselocked loop on the input admittance value is directly related to the cutoff frequency of the phaselocked loop transfer function. Finally, the simulation model is established in PSCAD software. We use the signal injection method to obtain the homologous and mirror admittance of the converter station to verify the correctness of the theoretical analysis.
2020, 35(4):27-32. DOI: 10.19781/j.issn.16739140.2020.04.004
Abstract:In order to reduce the cost of traveling wave detection without affecting the accuracy, a traveling wave detection method based on electric field probe is proposed. According to the conditions of the space electric field around the transmission line, the electric field distribution near the transmission line is analyzed by using the mirror theory. Then the appropriate detection position is selected to install the probe to detect the space potential. The probe is not in direct contact with the transmission line, and can be installed and repaired without stopping the operation. The induced current generated by the potential is obtained through the ground wire. Using the inherent time scale decomposition method, we analyze the induced current signal to extract the fault traveling wave signal. The simulation results show that the method of using the probe induced current to characterize the voltage signal on the transmission line is feasible. Moreover, the method has small positioning error and high precision, and can achieve fault detection for longdistance transmission lines.
2020, 35(4):33-41. DOI: 10.19781/j.issn.16739140.2020.04.005
Abstract:In order to study the relationship between the aging and the polarization/depolarization current (PDC) of transformer oilpaper, a prediction method of transformer oilpaper aging is presented based on the BP neural network with the chicken swarm optimization algorithm. Firstly, the relationship between extended Debye parameters and the polymerization degree (DP) of oilpaper is examined. With the variation of atmosphere temperature, PDC changes and it leads to a failure of extended Debye model to response the aging status of oilpaper. In order to eliminate the error caused by temperature, a BP neural network is trained through fitting PDC and DP of oilpaper. Then, in view of the slow convergence and low efficiency of BP neural network, the chickens swarm algorithm is utilized to optimize weights and threshold of the BP neural network. After the optimization, the network convergence is speeded up and the possibility of trapping into local optimal is also reduced. Finally, the simulation results show that the environment influences to polarization/depolarization current are reduced and the oilpaper polymerization degree is predicted accurately.
2020, 35(4):42-48. DOI: 10.19781/j.issn.16739140.2020.04.006
Abstract:The effect of the oscillating wave on the interfacial and internal defects of cable from the perspective of the electric field is studied in order to understand the test results of the oscillating wave voltage method for different defects in cables. Firstly, an oscillating wave test platform is set up to partial discharge tests the artificial defective models including longitudinal knife defects and external semiconductor lap defects. Then, the excitation characteristics of oscillation wave are analyzed primarily by PDIV and PRPD. Furthermore, the electric field distribution of cables with defects are simulated by utilizing a finite element simulation software in threedimension. Artificial defects with different sizes are fabricated to represent the diversity of the defect site and the characteristics of defective discharges are explained by the electric field distortion at the defects. Experimental and simulation results show that: it is hard to detect the internal defects due to the filling of hard grease by using the oscillation wave voltage method, whereas interfacial defects of poor connection of outer semiconductor has better characterization results.
2020, 35(4):49-57. DOI: 10.19781/j.issn.16739140.2020.04.007
Abstract:Singlephasetoground fault section location in small current grounded system is generally realized by a master station comprehensively calculating several different electrical quantities. This method is unsuitable for distribution networks with complex structures since its large workload and complicated calculations easily brings large errors. Under the background, a fault location method of distribution network based on threephase current amplitude analysis is proposed. In views of the analysis on the threephase current of singlephasetoground fault in small current grounded system, the current changes of the two nonfault phases on the fault path are approximately equal and less than the current change of fault phase. The phase current change of the two nonfault phases on the nonfault path are approximately equal and also equal to the current change of fault phase. By calculating the amplitude change of threephase current before and after fault and setting the criterion, the fault phase can be selected and the fault section can be located. ATP simulation results verify the applicability in neutral ungrounded system and neutral point grounding system via arc suppression coil. The local location of fault is realized successfully. The proposed method is simple in location criterion and reduces the computational complexity of the master station.
2020, 35(4):58-65. DOI: 10.19781/j.issn.16739140.2020.04.008
Abstract:In order to improve the reliability of lineselection and faultlocation for singlephase grounding failures in distribution network, the current waveform at different stages were studied and stimulated. Firstly, in terms of the summarized common features of current waveforms during their development, the incremental current of the subtransient sawtooth wave in the grounding phase and the incremental current in the sound phase are selected as the criterion signal sources for the fault location of the insulation loss in distribution networks. The general characteristics of all grounding currents reflected by the saw tooth currents are analyzed. Then, the difference of waveform characteristics is analyzed between singlephase grounding current and operationinduced disturbance currents. It provides a criterion for identifying the authenticity of singlephase grounding failure. The line selection and location method are proposed for the singlephase grounding including the insulation loss to avoid misidentification. Finally, the proposed method is applied to the grounding locator in a low voltage distribution network model for verification.
2020, 35(4):66-75. DOI: 10.19781/j.issn.16739140.2020.04.009
Abstract:In the traditional electromagnetic transient simulation, the computation speed of SVC (Static Var Compensator) and TCSC (Thyristor Controlled Series Compensation) is relatively slower. In order to overcome this deficiency, a new method of fast simulation for the electromagnetic transients of SVC and TCSC is proposed. For the fact that the state equations of SVC and TCSC are unchanged when the state of TCR (Thyristor Controlled Reactor) branches stay the same, the piecewise timeinvariant state equations of SVC and TCSC are developed firstly. Then, auxiliary variables are introduced and the model is transformed from a set of nonhomogeneous linear equations into homogeneous ones to obtain the unified expression suitable for varied working conditions. Finally, the scaling and squaring method is utilized to compute the matrix exponent and the response of the model is obtained. The feasibility and high efficiency of the proposed method is verified by comparing with the results from classical electromagnetic transient simulations using the PSCAD/EMTDC software package.
2020, 35(4):76-83. DOI: 10.19781/j.issn.16739140.2020.04.010
Abstract:As the country proposes the "rural revitalization" strategy in 2017, the rural power grid inevitably faces a new round of upgrading and transformation.How to balance the benefits and risks of rural network investment becomes an important research topic in the future.Based on the analysis of traditional distribution network investment benefits, this paper integrates environmental impact, ancillary services, and rural power grid investment benefits, and proposes a new type of rural power grid investment evaluation system that includes project benefits, management benefits, energy conservation, emission reduction, and valueadded services. At the same time, taking into account factors such as large investment volume and long period of capital recovery, the investment risk evaluation system of rural power grid is introduced. Based on the TOPSISgray correlation degree model, the advantages and disadvantages of the data samples are ranked to reflect the completeness of the evaluation system. In order to reflect the completeness of the evaluation system and the adaptability of the evaluation method, different power supply companies are selected for example verification.In summary, this paper provides power grid investment decision makers with a comparison of different investment schemes based on multiparty risks to better maximize revenue.
2020, 35(4):84-90. DOI: 10.19781/j.issn.16739140.2020.04.011
Abstract:At present, whether a distribution network needs to be improve is solely determined by its power supply reliability, which will make it difficult to enhance power consumption experience effectively. This paper proposes a reliability promotion investment method for distribution network based on object selection for the first time. Firstly, an index system to evaluate demands of distribution network reliability promotion is established including power supply reliability index, region characteristic index, customers’ expect index and economic effect index. Based on the index system, the reliability promotion necessity of distribution network is evaluated and ranked by improved order relation method. According to evaluation results, final reliability promotion planning is determined by total investment and technical transformation cost of each distribution network. Finally, an application example proves that the planning method proposed in this paper shows effectiveness and practical application value.
2020, 35(4):91-98. DOI: 10.19781/j.issn.16739140.2020.04.012
Abstract:Windthermal bundling external power transmission use thermal power to suppress the fluctuation of wind power and improve the utilization rate of the line. It is an important way to absorb wind power on a large scale. Therefore, the choice of windthermal bundling external power transmission lines and supporting thermal power is crucial. And the transmission capacity of the external power transmission line is related to its cross section and environmental temperature. Considering the line heat capacity or namely the influence of environmental temperature, this paper proposes a method to optimize transmission line crosssection and thermal power capacity of windthermal bundling external power. This method comprehensively considers the impact of wind power and thermal power income, total cost of thermal power, line investment cost, and wind curtailment cost on social benefits. The calculation example shows that this method can give full play to the transmission capacity of the line and increase social benefits compared with the method without considering the thermal load capacity.
2020, 35(4):99-106. DOI: 10.19781/j.issn.16739140.2020.04.013
Abstract:The threephase load imbalance seriously influences the safety, economic and reliable operation of distribution network. On this background, a control strategy of the SCR is proposed in this paper. Firstly, on the basis of the seamless commutation device, a mathematic model of threephase load imbalance treatment system in distribution network is established. Then, the vectorgenetic algorithm is utiliazed to realize the threephase load imbalance treatment and the optimal phase allocation scheme of the distribution area is obtained. Finally, the feasibility of the algorithm is verified by the simulation. The seamless commutation devices are installed in the low voltage distribution area and the experimental results validate the correctness and feasibility of the proposed control scheme.
2020, 35(4):107-113. DOI: 10.19781/j.issn.16739140.2020.04.014
Abstract:In the microgrid with the energy storage system as the main power source, the output characteristics of the energy storage system directly affect the power quality of the microgrid. When the output of the energy storage system can not be adjusted quickly, the problem occurs that the power quality of the microgrid does not meet the requirements. The traditional V/f control strategy adopts the reactivevoltage droop control strategy. When the island is running, there is a voltage offset, which will affect the overall power quality of the microgrid. And in the droop control, the droop coefficient parameter selection inevitably leads to misadjustment. In order to solve these problems, this paper discusses the relationship between the DC and AC power of the inverter. Based on the implementation of the control strategy of the energy storage inverter, DC current control is added to solve the problem of voltage offset and misadjustment to achieve nonamplitude difference control of microgrid. Finally, the PSCAD software is used to build the wind and solar storage microgrid model for simulation and verification, proving the effectiveness of the control strategy.
2020, 35(4):114-121. DOI: 10.19781/j.issn.16739140.2020.04.015
Abstract:With the continuous development of the power system, the largescale interconnected power grid has greatly increased the allocation of energy resources, but also brought new problems to the synchronous operation of the power system on the mean time. Partial failures of large power grids may spread to the entire network, leading to cascading failures in the power system, which are generally accompanied by largescale transfer of line active power. In view of this, aiming at the power flow transfer situation after the node is removed, this paper gives the concept of node power flow transfer degree. Considering that the node removal may cause the system to be disconnected, the concept of node position importance is also proposed. Meanwhile, based on these two indicators, and borrowing the concept of electrical coupling connectivity, combining with multiattribute decisionmaking method leads to the ranking method of key node comprehensive indicators. Finally, this paper prevents the occurrence of cascading failures or inhibit the largescale propagation of cascading failures by formulating an active defense strategy for the weak node split operation for a specific network structure. For a provincial system in Central China Power Grid, the identification method and active defense scheme mentioned in this paper are applied. The results show the effectiveness of the method and scheme.
2020, 35(4):122-127. DOI: 10.19781/j.issn.16739140.2020.04.016
Abstract:Power consumption is an important indicator of the economic and energy efficiency of thermal power units, in this paper, an improved weighted fuzzy Cmeans clustering algorithm is proposed. First of all, according to the historical operation data of auxiliary equipment of thermal power units, the factor analysis method is used to obtain the strong and weak correlation between the operation indicators and the influence on the target value of plant power, and extract the important indicators that affect the energy consumption of the unit, the operating indexes are assigned to the corresponding weights, and the weighted fuzzy Cmeans clustering is performed. Secondly, to overcome the shortcomings of the single weighting method, the combination of the analytic hierarchy process and the entropy weight method is further used to modify the indicators in the unit energy consumption assessment weight value. Finally, a sixpoint load interval of 10 months of historical data of a 600 MW unit in a thermal power plant is verified to that the model and algorithm are correct and effective.
2020, 35(4):128-132. DOI: 10.19781/j.issn.16739140.2020.04.017
Abstract:The LLC resonant converter can achieve softswitching while it operates in highfrequency and then improves the power density and efficiency of power supply. The fundamental approximation and mode analysis are two classic analysis methods of LLC resonant converter, but the comparative studies in the voltage gain accuracy, efficiency and application is not intensively discussed. On this background, the principle of the above methods is illustrated firstly. Then, the operation mode of LLC resonant converter is analyzed in PSIM simulation environment. Based on the operation modes, the voltage gain accuracy of the two methods is compared in simulation. Finally, the resonant parameters are selected based on the peak voltage gain, and a 1.5 kW/28 V prototype is set up. The experimental results show that actual gain is higher than the gain curve based on FHA 10% to 15% below the resonant zone, and have high accordance with the voltage gain based on the mode analysis in whole frequency range. And the iterative parameters based on mode analysis have higher efficiency with the experimental prototype.
2020, 35(4):133-140. DOI: 10.19781/j.issn.16739140.2020.04.018
Abstract:Nowadays, the condition assessment of transformer doesn't consider the dynamic characteristics and variational tendency of information in different stages. On this background, a dynamic gray target assessment model based on approaching degrees is proposed in this paper. First of all, the index data of benefittype and costtype are standardized. Secondly, the variance and the mean deviation of the interval grey number are introduced to evaluate the data volatility of the interval index. Then, the best weight can be determined. After that, a dynamic grey target evaluation model of interval grey number is proposed considering the accumulation of transformer phase information and the dynamic variation of index. The approaching degree is regarded as the basis of condition assessment. At last, the multistage operation data of multiple transformers in a substation is analyzed to verify the validity of the proposed method.
2020, 35(4):141-146. DOI: 10.19781/j.issn.16739140.2020.04.019
Abstract:Island detection is an important function of photovoltaic gridconnected power generation system. In order to reduce the dead zone detection effect of the island detection device and its impact on power quality, this paper proposes an improved SMS algorithm based on the research of traditional sliding mode frequency offset (SMS). Its working principle of the algorithm and detection dead zone are analyzed. Then it is applied in the digital signal processing (DSP) environment. The simulation study is performed in the case of a single inverter and multiple inverters in parallel operation. The simulation results show that the method can not only ensure a short detection time, but also reduce the impact on the power quality of the power grid, thus verifying the effectiveness and practicability of the method.
2020, 35(4):147-153. DOI: 10.19781/j.issn.16739140.2020.04.020
Abstract:When itoccurs load mutation in cophase power supply system, conventional detection method in detecting the fundamental active current and reactive current will exist a time buffer, which in turn directly affects the detection effect of fundamental active current and reactive current at power grid side. It may lead to not timely compensate the reactiveand harmonic current in load side. Based on the above problems, this paper proposed a Scott balance transformer combined balance transform device of cophase traction power supply system mode. The twophase voltage and current analysis Through the special power supply mode of the twophase balance transformer. According to the instantaneous reactive power theory of twophase circuit, the whole singlephase fundamental wave active and reactive current signals aredetected.Theactive current in the whole singlephase circuit is decomposed. Thus,the amount of negative sequence can be obtainedin the case of mutation, and then the positive sequence under the condition of fundamental wave stability is recombined with the negative sequence. Therefore, the dynamic performancegets greatly improved when it occurs load mutation. Simulation and theoretical comprehensive analysis verify the rationality of this method.
2020, 35(4):154-160. DOI: 10.19781/j.issn.16739140.2020.04.021
Abstract:The main steam temperature in the thermal power plant boiler has the characteristics such as large delay, strong inertial, time varying parameters and so on. When the condition changes greatly, it is difficult to maintain satisfactory control results using traditional PID control based on a single model. To solve this problem, this paper presents a predictive control method based on multimodel switching. Firstly, the object models are established under a number of typical conditions. Secondly, the best dynamic matrix controller under different models are designed. Thirdly, the switching strategy is designed to select the most appropriate controller to mitigate the effects of varied operating conditions (especially load changes) on the main steam temperature system. Finally, the validity of the proposed method is verified through Matlab simulation. The simulation results show that the proposed method is superior to the traditional PID control, and the multimodel switching improves the robustness of the fixed parameter model predictive control.
2020, 35(4):161-168. DOI: 10.19781/j.issn.16739140.2020.04.022
Abstract:Due to the lack of secondary cable circuit model, the information model of intelligent substation is incomplete. It is not only harm to intelligent substation comprehensive online monitoring and operational diagnosis, but also infect the development of smart substation Informationization. In this paper, a modeling method for secondary cable loops in the intelligent substation is proposed firstly. Then, the secondary cable loop configuration process is designed and the secondary cable loop model structure is introduced. The multidimensional visual display of secondary subcircuit for the intelligent substation is realized by the secondary cable loop model file visualization tool. Finally, the secondary cable loop information model is associated with the physical model by mapping the information model in the SCD file to the secondary cable loop model. The online monitoring and fault location of the secondary cable loop of the intelligent substation is realized successfully.
2020, 35(4):169-175. DOI: 10.19781/j.issn.16739140.2020.04.023
Abstract:It is difficult to analyze and verify the semantics of existing modeling methods for the objectoriented protection relay service such as UML. In this paper, according to the characteristics of Petri net, which can quantitatively analyze the security, accessibility and boundedness of system, a protection relay service model based on the Petri net is proposed. Through the UML activity diagram, the protection relay service model is initially constructed, and the activity diagram is mapped into a Petri net model. And then the Petri net is further transformed into an occurrence sequence and the structure and semantics of the occurrence sequence are verified. The branch or process in Petri net that is inconsistent with the actual procedure is searched and corrected until the occurrence sequence corresponding to the Petri net is verified. Finally, the Petri net is transformed into a protection relay service model. The case application of Petri net proves that the method can significantly improves the effectiveness of the model of protection relay services.
2020, 35(4):176-181. DOI: 10.19781/j.issn.16739140.2020.04.024
Abstract:Aiming at the nondirectly grounded distribution network of neutral point, this paper proposes to connect the small current fault line selection device to the distribution automation system. When a singlephase ground fault occurs, through the onoff control of the intelligent switch on the line and sequential logic of singlephase grounding alarm signal to determine the fault interval.Then the method isolates the fault area through the distribution automation system, restores the power supply in the nonfault area, and realizes the singlephase ground fault selfhealing of the distribution grid. In the end, this paper verifies the effectiveness and necessity of this strategy by experiments.
2020, 35(4):181-182. DOI: 10.19781/j.issn.16739140.2020.04.025
Abstract:Based on the analytical expression of the static voltage stability limit derived from apparent power, this paper gives an index to identify the weak point of static voltage stability, and proposes an improved engineering calculation method for the static voltage stability limit. The power system simulation software is mainly used in the project to calculate the static voltage stability. This method simulates the load growth in the stability calculation program, and solves the problem of limit misjudgment caused by the difficulty of the convergence of the operating point near the static voltage stability limit in the ordinary power flow algorithm. In the power flow calculation program, the PV node of the generator in the region is modified to a PQ node, which solves the problem of excessive output of reactive power caused by the dynamic calculation characteristic of the generator in the stable calculation program. Finally, a certain district power grid in Beijing is used as an example to verify the adaptability of the calculation method.