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Available online:July 23, 2021 DOI: 10.7500/AEPS20210210001
Abstract:Aiming at the problem of hysteretic nature existed in the primary frequency regulation of the generator unit based on the feedback control law after large power deficits, an event-driven fast frequency response (EFFR) control method is proposed. First, the connotation of EFFR is elaborated, of which the system architecture and action scheme are designed. The specific disturbance event signals are used to trigger the emergency control mode and adjust the output of generator units promptly. Then, an extended system frequency response model is established to extract the coupling relationship among the disturbance power, EFFR power and the under-frequency load shedding action by the periodic pre-decision method, on which the required EFFR power of the system can be estimated in the expected accident scenario. Additionally, with the objective of minimizing the control cost, the required EFFR power is distributed optimally, and the control commands for response generator units are calculated. Finally, a case study is carried out to validate the effectiveness of the proposed method.
Available online:July 23, 2021 DOI: 10.7500/AEPS20210329005
Abstract:Aiming at the phenomenon of high-frequency oscillation in DC distribution system, the frequency domain models of voltage source converter, DC line and constant power load are established, and the frequency domain reduced model of DC distribution system is further established. The analytical expression of the influence of system parameters on high-frequency oscillation frequency in frequency domain is derived. The influences of droop control coefficient, DC line and load parameters on system oscillation frequency are analyzed. It is found that the negative damping characteristics of droop controller and constant power load are easy to cause high-frequency oscillation. Based on this, the droop control method of feedforward compensation based on line parameters and the controller design method of improved virtual resistance are proposed, and the stability of the system before and after compensation is compared. The correctness of the theoretical analysis and the effectiveness of the proposed method are verified by simulation in frequency domain and time domain.
Available online:July 23, 2021 DOI: 10.7500/AEPS20210122004
Abstract:DC line protection is the key to the development of multi-terminal flexible DC transmission technology. The boundary conditions provided by DC current limiting reactor facilitate the design of fault protection algorithms. Combining the voltage drop characteristics of current limiting reactor with the shunt characteristics of adjacent lines, a protection scheme for multi-terminal flexible DC lines based on boundary transient energy is proposed. The protection scheme uses the difference of transient energy ratio between the two ends of the line to identify internal and external faults, and uses the transient energy ratio of positive and negative poles to select the fault poles. The implementation of the protection scheme is simple, the data of two terminal does not need to be synchronized, and the operation time is short. The simulation results show that the scheme can reliably identify faults and select fault poles in different fault scenarios, and has strong resistance to transition resistance and strong anti-noise ability. This work is supported by National Natural Science Foundation of China （No. 51877135），National Key R&D Program of China （No. 2016YFB0900601）， and Shanghai Rising-Star Program （No. 18QA1402100）.
Available online:July 21, 2021 DOI: 10.7500/AEPS20201206001
Abstract:The power generation of upstream power stations directly affects the power generation of downstream power stations through the upstream and downstream hydraulic connection. This hydraulic connection increases the difficulty of the establishment and solution of the spot market clearing model. To this end,a clearing model for the cross-regional transaction spot market coupled with complex hydraulic connection is proposed. Firstly, the nonlinear constraints are linearized in this model. Secondly, the cascaded hydropower family is introduced, and the complex hydraulic connection is transformed into the maximum and minimum power generation of cascaded hydropower stations. Furthermore, the hydropower units participate in the spot market clearing by means of the equivalent thermal power units which meet the maximum and minimum power generation constraints of the hydropower station groups, so the complex hydraulic connection is eliminated. Taking the hydropower transmission from Yunnan Province to Guangdong Province as an example for verification, the results demonstrate that the clearing results can meet the various operation constraints of the units, and can reflect the market price signals truthfully; the congestion components of the power generation of cascaded hydropower stations can reasonably represent the congestion cost caused by the basin inflow.
Available online:July 21, 2021 DOI: 10.7500/AEPS20210105005
Abstract:With the liberalization of the power sales side, distributed power generation and electricity sale companies compete together to form a trading market with multiple electricity sale entities. However, existing trading model has many drawbacks. For example, the uniform pricing of network fees is difficult to fairly deal with the impact of different transactions on grid operating costs, and the centralized transaction form is difficult to efficiently and economically process a large number of distributed transactions. Therefore, it is especially important to establish a reasonable trading model. For this reason, this paper discusses the peer-to-peer transaction model of multiple electricity sellers with the dynamic network fees mechanism. Considering the small-volume and high-frequency characteristics of peer-to-peer transactions, the min-max problem of dynamic network fees based on sensitivity coefficients is established and solved. Using the matching mechanism of continuous double auction and considering the general-customized service provided by electricity sellers, the multi-round transactions mechanism of two continuous double auctions with dynamic network fees is proposed. In multi-round transactions, a real-time rolling network verification method is proposed for each round of transactions. Considering the psychological behavior of both parties to the transaction, an improved disappointment-regret avoidance theory is proposed to update the quotation. Simulation results show that the proposed dynamic network fees mechanism can allocate the operation cost of the power grid more reasonably according to the characteristics of each transaction, and the designed peer-to-peer transaction mechanism can safely, economically and efficiently realize the multilateral bidding transactions considering the dynamic network fees.
Available online:July 21, 2021 DOI: 10.7500/AEPS20210220003
Abstract:Current research on the security region of distribution networks generally focuses on the AC distribution networks. This paper proposes the convex hull based security region (CHSR) for AC/DC hybrid distribution networks. Firstly, a nonlinear security region model for AC/DC distribution networks is constructed by taking into account the voltage constraints, feeder capacity constraints and critical equipment output constraints. Secondly, the security boundary points of the AC/DC hybrid distribution network are solved based on the nonlinear programming model. Finally, the convex hull fitting approach is adopted to generate the observable security region space. The results of the example verify the rationality and superiority of CHSR. Meanwhile, the impact of the control modes of the converters, the reactive compensation volumes of the converters and the lower limit of the system voltage on the constructed security region are investigated.
Available online:July 21, 2021 DOI: 10.7500/AEPS20210331004
Abstract:A universal mathematical description and a unified energy flow calculation method suitable for different energy forms are the theoretical basis for the planning and operation of multi-energy networks. On the basis of constructing the three-dimensional topological diagram for the multi-energy network, this paper proposes a unified description for energy transfer, and constructs the energy transfer equations of branches in the multi-energy network by borrowing the concepts of intensity quantity and extension quantity in thermodynamics. Taking the most common cylindrical transmission pipeline as an example, this paper derives the energy transfer processes of branches of the AC and DC power transmission, incompressible viscous fluid, compressible gas, heat conduction and fluid heat transfer, and then establishes a network equation describing extension quantity conservation of nodes. The steady-state energy flows of the independent energy sub-networks and multi-energy network are solved by the Newton-Raphson method, and the calculation examples verify the feasibility of the proposed model.
2021,45(14):1-12, DOI: 10.7500/AEPS20200628003
Abstract:A scientific and reasonable electricity regulatory system can effectively prevent potential market risks and promote healthy development of electricity markets. With the increasing development of China’s electricity spot markets, the regulatory mechanism of electricity markets has once again attracted the attention of electricity market designers. As a mature electricity spot market, the Australian National Electricity Market has accumulated rich practical experience in dealing with the transition of energy structure, opening the electricity market competition, and enhancing the electricity regulatory system and so on. Firstly, the characteristics of the Australian National Electricity Market and the electricity regulatory system are introduced, and the history of its electricity regulatory reform and the functions of the electricity regulatory agencies are elaborated. Secondly, the electricity market evaluation index system and evaluation methods adopted by the Australian electricity regulatory agencies are analyzed in depth. Then, the assessment mechanisms of the regulatory agencies for the electricity markets in Australia, Europe and the United States are compared and analyzed, and the advantages and current problems of Australian electricity regulatory mechanism are summarized. Finally, combining with the development of domestic electricity markets in China, relevant suggestions are made on the energy and power legislation and regulation system in the electricity market, the improvement of the functions of the regulatory agencies, the deepening of the degree of the regulatory informatization, and the sound evaluation mechanism.
Short-term Photovoltaic Output Forecasting Based on Diffusion Kernel Density Estimation and Improved Max-relevance and Min-redundancy Feature Selection
2021,45(14):13-21, DOI: 10.7500/AEPS20201126004
Abstract:With the growth of the installed capacity of the photovoltaic (PV) power generation, its proportion in energy consumption keeps increasing. Accurate forecasting of PV power generation is of great significance to the development plan and dispatching operation of power systems. At present, there are still few researches on feature selection in PV forecasting. Unreasonable feature selection often leads to loss of information, and it is difficult to effectively mine the mapping relationship between meteorological parameters and the power output, which results in low forecasting accuracy. Therefore, this paper proposes a feature selection method for PV forecasting based on improved mutual information calculation and improved max-relevance and min-redundancy (mRMR). Aiming at the problem that it is difficult to directly calculate the correlation and mutual information of continuous random variables, based on the theory of diffusion kernel density estimation (DKDE), an interval division method based on the probability density is proposed and applied to the discretization of variables, which improves the ability of mutual information to represent actual limited data sets. Then, the incremental search process of the traditional mRMR is improved,and an improved mRMR algorithm is proposed, which can select multiple feature subsets in parallel. The XGBoost (eXtreme gradient boosting) algorithm is applied to each feature subset to construct the weather information and PV power forecasting model. Finally, the effectiveness and accuracy of the proposed method are verified by the measured data of an actual photovoltaic power station.
2021,45(14):22-29, DOI: 10.7500/AEPS20201127004
Abstract:Mining the causal correlation between wind power and wind energy, as well as the spatio-temporal correlation among multiple wind farms from multi-source, multi-dimensional, and multi-modal wind power data, is an effective way to improve the accuracy of wind power prediction. A data-driven hybrid deep-learning model is proposed. First, in the data preprocessing stage, the scene of the wind farm operating at full power output and wind speed overflowing is regarded as the abnormal and fault state of wind power prediction, and the corresponding data-cleaning method is proposed to enhance the correlation between the wind speed and wind power. Then, aiming at the causal correlation between meteorological data and wind power, multi-channel convolution is designed to mine the coupling relationship; aiming at the spatio-temporal correlation between adjacent wind farms,the multi-scale convolution is designed to mine wind power characteristics with different spatio-temporal scales, and these spatio-temporal characteristics are weighted, fused, and reduced through the
convolution layer. Finally, the fusion characteristics are input into the gated recurrent network to mine the time correlation and output the prediction results of wind power for multiple wind farms. The effectiveness of the proposed model is verified by real datasets.
2021,45(14):30-37, DOI: 10.7500/AEPS20201005003
Abstract:The reasonable cognition of the influence of typhoons on regional wind speed forecasting is crucial to the maximum utilization of wind power in the future. Based on the multi-model ensembled numerical weather forecasting information of typhoon, a multi-step wind speed forecasting model considering the influence of typhoons is proposed. In view of the noise of wind speed data during typhoons, empirical wavelet transform (EWT) is used to deconstruct the historical data of wind speed and the noise disturbance is eliminated based on the adaptive threshold method. The sequence signal of wind speed is reconstructed. Then the multi-step prediction of the reconstructed wind speed series is carried out by using gated recurrent unit (GRU) network to obtain the prediction information of wind speed without consideration of the influence of typhoons. For the lack of data during typhoons, deep belief network (DBN) is introduced to realize correction under the condition of typhoon and improve the accuracy of wind speed forecasting considering the influence of typhoons. Finally, a case study is carried out based on the actual data of a weather station in southern China and compared with the fundamental case without consideration of numerical forecasting information of typhoons. The result shows that compared with the baseline model without considering the influence of typhoons, the proposed model can effectively reduce the forecasting errors of wind speed.
Analysis on Oscillation of Multi-infeed System with Renewable Energy and Static Var Generator and Calculation of Its Generalized Short-circuit Ratio
2021,45(14):38-46, DOI: 10.7500/AEPS20200826008
Abstract:The interaction between static var generator (SVG) and renewable energy equipment becomes obvious in weak grids, and it is a difficult problem to analyze the oscillation characteristics of multi-infeed system with renewable energy and SVGs. To deal with this issue, a system dynamic model with unified operation of SVG and renewable energy which can preserve the network structure is established. Also, a generalized short-circuit ratio calculation method for multi-infeed system with renewable energy considering SVG effect is proposed to judge whether the system is stable and to quantify the stability margin of the system. Firstly, the system transfer function matrix and characteristic equation of the network structure preserving model are derived. Then, based on the modal perturbation theory, an equivalent homogeneous multi-infeed system which approximates the dominant characteristics of the system is constructed. And it is strictly proved that the generalized short-circuit ratio can be used to analyze the oscillation stability margin of multi-infeed system with renewable energy and SVGs. Finally, the calculation method of generalized short-circuit ratio and its critical value in multi-infeed system with renewable energy considering SVG effect is given. Besides, the quantitative method of the power grid strength and system oscillation stability margin is presented. The simulation results verify the validity of the proposed analysis and calculation method.
Short-circuit Fault Current Analysis of AC Transmission Line of MMC-MTDC System for Wind Power Intergration
2021,45(14):47-55, DOI: 10.7500/AEPS20201020002
Abstract:With the development of offshore wind power and the increase of installed capacity of regional wind power, large-scale renewable energy sources are intergrated to the grid with modular multilevel converter based multi-terminal direct current(MMC-MTDC), which has become a trend of wind power grid intergration. However, the fault ride-through coordination of the multi-converter system with MMC-MTDC and permanent magnet generator and its fault characteristic analysis have not been effectively resolved, thus needing urgent research. First, the traditional fault ride-through strategy of MMC-MTDC system intergrated wind power is optimized in this paper, and a feasible ride-through strategy is put forward. Furthermore, combined with the respective fault analysis of multiple converters, the principle of power transmission balance between converters and permanent magnet generator converters is used to propose an analytical expression of steady-state short-circuit current, which can fully reflect the AC fault characteristics of the MMC-MTDC system intergrated wind power. The problem that the fault characteristics of the cascaded multi-converter are difficult to analyze is solved. Finally, based on the actual engineering parameters, modeling is carried out on the PSCAD/EMTDC simulation platform, and the simulation results verify the effectiveness of the proposed fault ride-through control and the correctness of the steady-state short-circuit current calculation formula.
Pre-synchronization Scheme with Voltage Fluctuation Suppression and Frequency Out-of-limit Avoidance for Grid Connection of Islanded Microgrid
2021,45(14):56-64, DOI: 10.7500/AEPS20201104009
Abstract:During the traditional pre-synchronization process for grid-connection of islanded microgrids, there exist the voltage fluctuation caused by the output variation of the wind and photovoltaic distributed generators, and the frequency out-of-limit risk caused by the improper proportional-integral (PI) parameters. To solve the above problems, firstly, the feedforward disturbance compensation of the load voltage and current is introduced into the V/f control strategy of the energy storage system to suppress the voltage fluctuation. Secondly, based on the Simplex algorithm, the PI parameters are optimized to take into account the grid-connected speed and frequency fluctuation in the pre-synchronization process, and the optimized PI parameter selection method combining off-line optimization and on-line lookup table is designed. Thirdly, a pre-synchronization optimized control scheme for the islanded microgrid considering both the voltage fluctuation and frequency out-of-limit risk is proposed. Finally, the simulation experiment proves that the pre-synchronization scheme for the grid connection of islanded microgrids can realize the fast phase synchronization and effectively suppress voltage fluctuation and avoid frequency out-of-limit risk.
Transient Voltage Stability Assessment and Risk Quantification Based on Convolutional Neural Network
2021,45(14):65-71, DOI: 10.7500/AEPS20200615002
Abstract:A method is proposed for assessing transient voltage stability and quantifying its risk. Firstly, the correlation and matching between the convolutional neural network (CNN) and transient voltage stability assessment are discussed and the CNN based assessment model for the transient voltage stability is built. Secondly, the quaternary assessment structure is introduced in the credibility framework, which can effectively solve the dependence of CNN on the time-domain simulation in the stable boundary recognition. Then, the transient voltage stability margin is obtained through the assessment results. Combined with the credibility, a risk function is constructed to realize the quantitative classification of the risk of transient voltage stability. Validity of the method is verified by the analysis results of an actual power grid example.
2021,45(14):72-81, DOI: 10.7500/AEPS20210107005
Abstract:According to the proposed operation model of hydrogen-power hybrid energy storage systems and secondary adjustment process of voltage-power droop characteristics in multi-terminal flexible DC distribution networks, a multi-time-scale optimal scheduling strategy for source-storage-network coordination is proposed. The objective of day-ahead scheduling is to minimize the operation cost and voltage deviation. It also optimizes the operation mode of hybrid energy storage, the on-off of micro-turbines and the secondary adjustment parameters of droop characteristics to provide the reference for intra-day optimal scheduling. Intra-day rolling optimal scheduling takes the economy as its objective to fine-tune the output power of equipment and compensate the power fluctuation caused by forecast errors. A case study is undertaken in an annular multi-terminal DC distribution network to verify the feasibility of the proposed scheduling strategy, the advantages of hybrid energy storage, and the improvement of operation by the secondary adjustment of droop characteristics.
Optimal Configuration of Distributed Energy Storage for Distribution Network in Peer-to-peer Transaction Scenarios
2021,45(14):82-89, DOI: 10.7500/AEPS20200930002
Abstract:The integration of high-penetration renewable energy into the distribution network not only promotes distributed generation technology to participate in market competition in the peer-to-peer (P2P) form, but also poses a challenge to the safe and stable operation of the power grid. In order to ensure the safe and stable P2P transactions in the distribution network and to improve the benefits of investing in energy storage for both users and storage suppliers, a two-layer optimal energy storage allocation method is proposed in two transaction scenarios. The upper-layer model aims at minimizing the total investment cost of energy storage, collects one year of operational data of the distribution network for optimization, and calculates the configuration specifications of energy storage to transfer to the lower layer model. The lower-layer model considers two P2P transaction modes, i.e. centralized-bidding and user-driven modes, introduces the concept of regional marginal price to maximize the benefits for both buyers and sellers, and optimally dispatches the energy storage charging and discharging strategies to minimize the energy storage investment costs. Finally, the effectiveness of the proposed method is verified by taking the IEEE 15-bus distribution network as an example.
2021,45(14):90-96, DOI: 10.7500/AEPS20200802003
Abstract:With the continuous development of provincial-regional power grid integration and transmission-distribution network integration in China, the dimension of power system calculation is getting higher and higher. As a basic component of power system situation awareness, state estimation needs to ensure its real-time performance. Weighted least squares (WLS) method is the most widely used state estimation method in power systems. Therefore, according to the time-consuming characteristic when solving matrix multiplication and linear equations in the Newton iteration by WLS, this paper designs a state estimation algorithm of power system based on preconditioned conjugate gradient iteration with the idea of conjugate gradient method in Krylov subspace method. This method uses incomplete LU decomposition to preprocess the original linear equations, and adopts graphics processing unit (GPU) parallel acceleration technology to accelerate matrix multiplication, linear equation preprocessing, and conjugate gradient method iteration. The case analysis shows that the method in this paper has obvious acceleration effect, low memory and video memory requirement, and less iterations of the linear system of equations preprocessed by the incomplete LU decomposition method, which can meet the real-time requirements of large-scale power system state estimation.
2021,45(14):97-103, DOI: 10.7500/AEPS20200208001
Abstract:With the advancement of the electricity market reform in China, electricity side market is gradually opening up. The electricity retailers can aggregate a large number of distributed loads to participate in the demand response in the electricity market environment. In this paper, the modeling and solving methods of incentive demand response based on deep reinforcement learning are proposed to maximize the comprehensive profits of both retailers and customers. Firstly, the demand response models for retailers and customers are established. By introducing the time-price elasticity, the recent customer response model is improved, which takes the customer response to the subsidy price difference between adjacent periods into account. Then, based on the Markov decision process framework, an optimization model of subsidy price decision is constructed, and a solution algorithm based on deep Q-learning network is designed. Finally, simulation calculation is performed using one retailer and three different types of customers as examples. By analyzing the convergence of the algorithm and comparing the optimization results of different models and parameters, the rationality of the improved model and the effectiveness of the generated strategy are verified, and the impact of incentive demand response on the retailer and customers is analyzed.
Operation Mechanism of Inter- and Intra-provincial Electricity Spot Markets to Promote Renewable Energy Accommodation
2021,45(14):104-113, DOI: 10.7500/AEPS20201111002
Abstract:With the rapid growth of renewable energy installed capacity in China, the accommodation task becomes more difficult. The construction of electricity markets also needs to combine the characteristics of the power grid development and find a market mode in the background of high proportional of renewable energy. Based on the analysis of the problems existing in the current operation mechanism of the two-level peak regulation auxiliary service market in Northwest China, this paper proposes the key problems and their solutions in the coordination operation of inter- and intra-provincial electricity markets in the future regional power grid. And an inter-market optimization coordination mechanism aiming at maximizing the accommodation of renewable energy is established. The designed mechanism can promote the orderly cooperation of multiple inter- and intra-provincial markets, and also promote the optimal allocation of electric energy and the effective use of peak regulation resources. Based on the actual operation data of a provincial electricity market, the simulation calculation and the comparative analysis of the proposed two-level market coordination mechanism are conducted, and the results verify the effectiveness of the designed mechanism.
Inter-basin Scheduling Method of Cascaded Hydropower Plants Considering Multi-grid Peak Shaving and Hydropower Accommodation
2021,45(14):114-122, DOI: 10.7500/AEPS20200729002
Abstract:The power structure of Yunnan Province in China is dominated by hydropower, and the problem of hydropower accomodation is extremely prominent. In addition, the peak shaving in receiving-end of Yunnan hydropower transmission is under great pressure. In order to fully tap the potential of power transmission lines from the west to the east in Yunnan power grid, maximize the accomodation of Yunnan hydropower and respond to the peak shaving demand of multiple power grids, this paper constructs a mixed-integer linear programming model for the inter-basin scheduling of cascaded hydropower plants, which takes into account multi-grid peak shaving and hydropower accomodation. The model uses the target weight method to transform the multi-grid peak shaving problem into a single target problem. By adjusting the weight corresponding to the average value of the absolute value of the residual load anomaly for each power grid, the target meets the demands of peak shaving among multiple power grids and hydropower accomodation. Meanwhile, by introducing the special order set constraint, an simple linearization method for the minimum duration time constraint is introduced. The method can be applied to other constraints with the same type. Finally, taking 18 hydropower plants on the main stream of the Lancang River and the Jinsha River as examples, the feasibility and effectiveness of the model are verified. The results show that the proposed method can meet the needs of multi-grid peak shaving and hydropower accomodation.
Analysis and Suppression Measures for Resonance on Interconnection Line Between Mircogrids Based on Harmonic Impedance Matching
2021,45(14):123-131, DOI: 10.7500/AEPS20200623004
Abstract:Neighboring microgrids are usually interconnected by long transmission lines. For the system where both sending and receiving ends of transmission lines have independent harmonic sources, the harmonic interaction characteristic and the harmonic amplification mechanism are more complicated, and the conventional resonance suppression method of resistive active power filter is not suitable. Based on the transmission line theory, this paper analyzes the harmonic propagation mechanism of interconnected lines, and proposes a resonance suppression method based on harmonic impedance matching. To overcome the uncertainty of line parameters and phase angle information of harmonic sources, this paper proposes a coordinated control method for harmonic impedances of active power filters based on the line middle-point harmonic voltage detection. According to the harmonic voltage content, the phase angles of harmonic impedance at both ends of the line are adjusted online to realize resonance suppression. Meanwhile, an improved control strategy of active power filters is proposed to ensure that the harmonic impedance phase angle can be adjusted arbitrarily. Simulation results verify the effectiveness of the proposed scheme.
Characteristics Analysis and Suppression Strategy of Voltage Unbalance in Areas with Integration of Wind Power
2021,45(14):132-139, DOI: 10.7500/AEPS20200702001
Abstract:The wind power in China mostly adopts the mode of concentrated development in large-scale and long-distance transmission and there are often no local loads in the wind power integrated areas. Therefore, three-phase voltage unbalance frequently occurs in the areas with large-scale integration of wind power, which can cause wind turbines to disconnect from the grid in severe cases, threatening the safe and stable operation of the power system. In response to this problem, the voltage unbalance mechanism and suppression strategy in the areas with integration of wind power are proposed. Firstly, key features of voltage unbalance phenomenon in the areas with integration of wind power are extracted based on the actual data. Secondly, the equivalent circuit model is constructed in the areas with integration of wind power. Considering the static voltage stability limit and unbalanced three-phase impedance of untransposed transmission line, a method for estimating the voltage unbalance factor (VUF) is proposed. Finally, with the objective of minimizing the negative sequence voltage of the integrated substation, a three-phase reactive power optimization model for the areas with integration of wind power is proposed, and the negative-sequence voltage suppression in the areas with integration of wind power is achieved through adjustment of the dynamic reactive power compensation device to each phase. Taking the actual wind power integration system as an example, simulation analysis verifies the effectiveness of the mechanism analysis and suppression strategies.
2021,45(14):140-148, DOI: 10.7500/AEPS20201030011
Abstract:Since the flexible DC power grid with teed lines is only installed with DC circuit breakers (DCCBs) at the sending and receiving ends, the DCCBs at each end need to be disconnected when an internal fault occurs. Compared with the double-terminal DC power grid, its fault current characteristics are more complicated, and conventional DC power grid protection methods cannot be directly applied. In order to meet the high protection requirements of the flexible DC power grid with teed lines, a fault identification method based on modulus phase plane for flexible DC lines is proposed in the paper. Firstly, the network topology is studied, and based on the equivalent circuit of internal and external faults before the converter is blocked, the characteristics of fault current modulus are derived. Then, combined with fault locations and types, the protection operation principle and threshold setting are analyzed. At the same time, the adaptive setting of protection is realized by constructing a threshold scaling factor. Finally, the simulation model of the flexible DC power grid with teed lines is established in PSCAD/EMTDC platform to verify the rapidity and reliability of the proposed method. The proposed method is simple in algorithm with low computational complexity, only uses single-end information to quickly identify faults, and has a strong ability to withstand system disturbances.
2021,45(14):149-157, DOI: 10.7500/AEPS20210115001
Abstract:Aiming at the difficulty in detection of tree-contact single-phase-to-ground faults (TSFs), the modeling and detection method for TSFs are studied. By analyzing temperature changes of trees during faults, a time-varying model for transition resistance of TSFs is established based on actual physical parameters of trees, i.e., resistivity, height, radius, and resistivity-temperature characteristics. Using this model, the TSF characteristics of zero-sequence voltage and current variation with time for different 10 kV systems are analyzed and concluded. Based on the characteristic that TSF zero-sequence current rises slowly and monotonously, a TSF detection method is proposed which monitors the time duration of the zero-sequence current exceeding the threshold, and the measurement reliability is improved by using multi-cycle measuring data. The model and the method are verified with digital simulations and 10 kV TSF experiments. The verification results indicate that the model can accurately simulate the slowly time-varying curves of transition resistance for TSFs caused by size difference of trees , and the detection sensitivity of the proposed method can reach tens of kiloohms, which can greatly improve the detection ratio of TSFs.
2021,45(14):158-164, DOI: 10.7500/AEPS20200925009
Abstract:The existence of interference signals, such as decaying direct current (DDC) components and high-order harmonics, makes certain errors in detecting the amplitudes and phases of fundamental frequency component in the distortion signals of the power grid. The time constant of the DDC component usually exceeds 45 ms, and its duration is long. To solve this problem, this paper proposes a half-cycle four-point sampling detection algorithm for DDC components in distorted signals. The proposed algorithm can greatly shorten the detection response time for the DDC component. For the distorted signals that contain both DDC components and high-order harmonics, the above DDC component detection algorithm is combined with the high-order harmonic detection algorithm in the dq coordinate system after the half-cycle integration of the distorted signals. The influence of interference signal can be filtered out at the same time in a half-cycle of the power frequency, and the fundamental-frequency component in the distorted signal can be detected accurately. Finally, a semi-physical real-time simulation model in MATLAB/Simulink is built to verify the effectiveness of the proposed algorithm in terms of detection accuracy and response time.
Single-stage Inductive Power Transferred Battery Charger Based on Minimal Three-capacitor Compensation
2021,45(14):165-172, DOI: 10.7500/AEPS20201015003
Abstract:To adapt to the charging profile of lithium-ion battery consisting of an initial constant current (CC) and a subsequent constant voltage (CV), an inductive power transferred battery charger should provide the required output as well as resistive input impedance to reduce device stresses and reactive power during the whole charging process. In view of the fact that the four basic compensation topologies cannot meet the above requirements at the same time, and the high-order compensation network must rely on more devices to satisfy the above requirements, the design freedom of single-stage inductive power transferred battery chargers based on the minimal three-capacitor compensation is systematically derived, which has the characteristics of resistive input impedance and constant output, and the constraints of its compensation parameters and operation frequency are analyzed. Meanwhile, the output characteristics of the S/SP and S/PS structures of three-capacitor compensation are analyzed and compared, and the influence of the compensation component parameters on the input impedance angle and output gain is analyzed to guide the soft switch design. Finally, a single-stage inductive power transferred battery charger with an output of 48 V/2 A is built to verify the feasibility of the above design.
Overview on Principle and Application of Shifted Frequency Based Electromagnetic Transient Simulation for Power System
2021,45(14):173-183, DOI: 10.7500/AEPS20200727009
Abstract:Efficient and accurate electromagnetic transient (EMT) simulation is the key technology for supporting the operation and control of the power systems. To cope with the contradiction between the speed and accuracy of the traditional EMT simulation, a shifted frequency (SF)-based EMT simulation method was proposed. This paper reviews the key technology of principles for the SF-based EMT simulation and analyzes some existing problems of it. First, the principle of the SF-based EMT simulation is introduced. Then, three key points of the SF-based simulation are analyzed, including the formulation method of a complex signal, the concrete forms of frequency shifting, and the SF-based EMT simulation models as well as their characteristics. Next, the research status of applications for SF-based simulation technology is introduced. Finally, some critical problems in the SF-based EMT simulation are discussed.
Volume 45,2021 Issue 14
- Hot Topics
ZHANG Boming, LUO An,
WEN Fushuan, WANG Qing