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      • ZHOU Yongliang, YU Guangzheng, LIU Jiangfeng, SONG Ziheng, KONG Pei

        Available online:July 08, 2020  DOI: 10.7500/AEPS20191212003

        Abstract:Accurate wind power prediction is of great significance to the safe grid connection of offshore wind power. Different from the land, the sea has the characteristics of complicated meteorological factors and significant fluctuation of wind turbine output, which makes the prediction accuracy of offshore wind power difficult to meet the actual engineering requirements. Aiming at the above problems, this paper proposes a prediction model based on the improved long-term recurrent convolutional network (LRCN) for ultra-short-term offshore wind power prediction. Firstly, the improved LRCN is used for preliminary power prediction, that is, the multi-convolution channel is constructed to extract the time series characteristics of variables at different layers, and the network convergence effect is improved by the forward-looking improved Adam optimizer. Secondly, the swing window algorithm and the clustering of fluctuation characteristics are used to classify the types of output fluctuation in the predicted period. Thirdly, error correction models are established for different fluctuation types, and the strongly correlated feature factors screened by Xgboost algorithm are input to achieve error correction. Finally, experiments with data in actual offshore wind farm is put forward, the results show that the proposed method can effectively predict the ultra-short-term offshore wind power, and the prediction accuracy is higher than that of many traditional prediction models.

      • ZHANG Zhijian, JING Long, ZHAO Yuming, WU Xuezhi, WANG Jingjing, YANG Jinsheng

        Available online:July 08, 2020  DOI: 10.7500/AEPS20191119001

        Abstract:In order to prevent the resonance problem caused by the grid-side LCL filter of high-power grid-connected inverter, the passive damping method is usually used to suppress the resonance. However, the conventional passive damping method ignores the effect of digital delay on the system stability. Through the modeling in the discrete domain and the continuous domain combined with Nyquist stability criterion analysis, it is pointed out that, when quasi-proportional-resonant (QPR) control is adopted, the digital delay angle at the low switching frequency will lead to negative crossing near the resonant frequency. This characteristic will lead to the unstable operation of current loop. And it is further pointed out that the stability of the passive damping system under the control of QPR will not be affected by the change of grid impedance. In order to ensure the stability of current loop at the low switching frequency, this paper proposes a method to compensate the phase lag of resonant frequency of QPR controller by using lead links with low-pass characteristics, and gives the design steps of lead compensation. Finally, the correctness and effectiveness of the proposed method are verified by simulation and experiment.

      • YANG Long, WU Hongbin, DING Ming, BI Rui

        Available online:July 08, 2020  DOI: 10.7500/AEPS20200202002

        Abstract:Loads in the renewable energy grid is more sensitive to various feature factors, it is a new challenge for short-term load forecasting when facing the massive feature data. Aiming at the in the renewable energy grid with high-dimensional feature data, a short-term load forecasting method based on bi-directional long-short-term memory(Bi-LSTM) network considering feature selection is proposed. The sample data are firstly clustered and mapped into the weight-induced space according to density, by defining a structure of interval measurement data, the maximum interval sum is used as the objective function. In order to achieve the sparsity of the solution space, a regular term is added into the objective function, and the feature weights are solved by the gradient descent algorithm. With the pre-tests for determining the hyper-parameters such as the feature selection threshold, the required feature factors are selected. Finally, the Bi-LSTM network is used to carry out load forecasting based on the selected data. Taking the renewable energy grid in a certain area of China as an example, the effectiveness of the proposed method is verified, compared with the traditional methods, the proposed method is more accurate and applicative.

      • SONG Yunong, LIN Shunjiang, TANG Zhiqiang, HE Sen, LU Yi, MAO Tian

        Available online:July 02, 2020  DOI: 10.7500/AEPS20200302005

        Abstract:A probabilistic modeling method for the spatial-temporal distribution of electric vehicle (EV) charging load based on dynamic traffic flow is proposed. The model is composed of the traffic road network model, the spatial-temporal transfer model of vehicles and the probabilistic model of residents’ traveling. Considering the influence of traffic lights, the road is divided into three sections to calculate the traffic density in the traffic road network model. The spatial-temporal transfer model of vehicles reflects the influence of traffic condition on the speed and power consumption of EVs. The driving vehicles can dynamically adjust the speed and traveling path according to the traffic condition. The probabilistic model of residents’ traveling establishes the joint probability distribution of travel destination and departure time based on the NHTS2017 data set, and the trip chains are obtained by sampling simulation from the model. Simulations are performed based on the road network model of a typical urban area, and the Monte Carlo method is adopted to predict the spatial-temporal distribution characteristics of EV charging load. Compared with the results of the existing methods and the actual EV charging load curve, the correctness and effectiveness of the proposed probabilistic model are demonstrated.

      • GAO Huisheng, ZI Peng, HUANG Linbin, WANG Guanzhong, XIE Huan, ZHAO Tianqi, XIN Huanhai

        Available online:July 02, 2020  DOI: 10.7500/AEPS20191022007

        Abstract:With the connection of large-scale renewable energy sources, the rotational inertia of modern power system is reduced, and the dynamic characteristics of frequency are deteriorating. In order to support the system frequency, the power generation devices of renewable energy sources are usually required to have additional control functions, such as inertial simulation or primary frequency regulation. However, there is little research on the optimal frequency support of renewable energy sources. Based on the optimal control theory, this paper preliminarily discusses the optimal power support trajectory and approximate optimal control structure of power electronic devices as grid-connected interfaces, when there are energy constraints on the renewable energy sources participating in frequency regulation. Firstly, the common mode component of frequency of multi-machine system is deduced, and it is pointed out that this component dominates the frequency drop characteristics of power grid. Secondly, based on the Gauss pseudospectral method, this paper studies the optimal support trajectory of power electronic devices with energy constraints when the system frequency drops. Thirdly, the optimal feedback control structure for grid-connected power electronic devices supporting power grid frequency is studied. It is pointed out that the control structure combined with virtual inertia control and droop control is close to the optimal control structure, and the virtual inertia control is better than the droop control when the available energy is small. Finally, the simulations verify the rationality of the theoretical analysis and the discussed optimal control structure.

      • GUI Lin, CHEN Jun, WANG Kai, WANG Bo, WANG Guang, WANG Xiangheng

        Available online:July 02, 2020  DOI: 10.7500/AEPS20200211004

        Abstract:Generator main protection renovation projects of large and medium hydropower stations in China are gradually increasing. Limited by the existing branch wiring mode and installation condition for electromagnetic current transformer (CT), there is still a protection dead zone in the final selection of the main protection configuration scheme, despite the quantitative design for the generator main protection. Taking the main protection renovation project of Tankeng Hydropower Station in China as an example, a suboptimal main protection configuration scheme has to be selected due to the installation limitation of electromagnetic CT. As a result, safety risks still exist in generator operation after the renovation. Because of the high speed and small foundation space of large generator-motor, the problem above is more prominent. In the main protection renovation project of Shenzhen pumped storage generator-motor, based on the advantages of the flexibility of flexible optical CT winding, the wiring mode of branches is virtually changed through the flexible installation of optical CT without changing the existing branch grouping or the copper ring wiring mode of the generator. And the branch currents and branch group currents connected to the generator protection devices are recombined. Then the main protection configuration scheme of Shenzhen pumped storage generator motor is optimized based on quantitative analysis, thereby the dead zone of main protection is eliminated.

      • LI Ji, ZHANG Huiyuan, CHENG Jiehui, WANG Ziqi, GAO Ning, LI Wei

        Available online:July 01, 2020  DOI: 10.7500/AEPS20200319002

        Abstract:With the development of inter-regional interconnection structure and large-scale distributed generators connecting to power grid, the coordinated and optimal scheduling of inter-regional interconnection system based on source and load status is studied, in order to fully utilize the complementary benefits of the existing inter-regional resources. Firstly, taking the total operation cost and network loss of power grid at the sending end and receiving end as the objective functions, the charging and discharging state of energy storage device at the sending end and the inter-regional power flow state are judged according to the source and load status of the sending and receiving end. Secondly, considering the operation constraints of the actual system, the improved multi-objective search algorithm based on particle swarm optimization algorithm is used to optimize the charging and discharging power of energy storage at the sending end, the inter-regional interconnection plan and the output of the peak-shaving thermal power unit. Finally, the actual power grid in Xinjiang of China is used as an example for simulation calculations. The simulation results show that the proposed inter-regional optimal scheduling strategy can significantly improve the operation efficiency of the sending-end and receiving-end power grids, and give full play to the collaboration of interconnected regions.

      • WANG Yingxin, XU Yonghai, TAO Shun, SONG Xiaoqing, AN Zhe, WANG Tongxun

        Available online:July 01, 2020  DOI: 10.7500/AEPS20200217010

        Abstract:With the deepening of power electronization in power system, the supraharmonic components of voltage and current signals in the power system continue to increase. The supraharmonic has wide measurement frequency domain, and its amplitude is small but has a large fluctuation. The conventional harmonic real-time monitoring equipment is difficult to meet the demand. This paper theoretically deduces the existing typical sampling methods for supraharmonic detection (i.e. full sampling, equidistant sampling, first and last cycle sampling). The errors of the above three sampling methods are compared in the case of amplitude fluctuation. On this basis, a combined equidistant sampling method is proposed, which combines the characteristics of waveforms to reconstruct the sampled data. The proposed sampling method can effectively reduce the data storage while ensures the error accuracy. Finally, the above sampling methods are verified by cases.

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      Volume 44,2020 Issue 13

        >Energy Transformation and Power Support
      • XUE Yusheng, HUANG Jie, WANG Fang, ZHANG Cuiyun, QIAN Feng

        2020,44(13):1-8, DOI: 10.7500/AEPS20191008004

        Abstract:To achieve zero net emission as soon as possible and actively mitigate climate change, it is necessary to make more revolutionary breakthroughs in both carbon emission reduction and carbon sink expansion. Climate change mitigation requires a variety of emission reduction measures, but different measures may have large differences in both cost and effectiveness. Existing carbon markets have provided different commodities and incentives for different emission reduction behaviors. However, there is a lack of unified analysis frameworks and decision support methods to answer questions like ‘how to achieve differentiated incentives’, ‘how to coordinate the trading between different commodities’. This paper re-examines carbon market regulatory mechanism design from the perspective of cyber physical social system in energy. Based on a detailed analysis of the multiple attributes and interaction mechanisms of carbon emission reduction behaviors, the interaction logics among different commodities in carbon market are re-designed. Each commodity market is regulated according to its respective market rules and different commodity markets are coordinated by multiple equivalents. The proposed mechanism focuses on the multiple benefits of carbon emission reduction and carbon offset behaviors to avoid market shocks and disturbances caused by rigid market links, discrete regulation and multi-end regulation. Sandbox derivation based on hybrid simulation can be utilized to provide decision support for market framework design and regulation.

      • >Basic Research
      • XIAO Jucheng, HE Guangyu, ZHANG Siyuan, WANG Zhihua, CHANG Naichao, LU Qiang

        2020,44(13):9-16, DOI: 10.7500/AEPS20191027004

        Abstract:Due to the increasing proportion of the installed renewable energy capacity, conventional generation units have to keep massive contingency reserves, which leads to poor economy of unit operation. To solve this issue, this paper proposes the concept of decentralized transfer of contingency reserve (DTCR), which refers to transferring part of the centralized contingency reserve from the supply side to the demand side. Then, the benefit assessment method and reference indicators of DTCR are proposed. The observed diminishing marginal benefit (DMB) of DTCR serves as a reference to select a reasonable capacity of decentralized reserve. The economic load rate threshold (ELRT) is proposed to quantitatively assess the economic load rate range of producing DTCR benefits. Finally, the benefit, its mechanism, impact factors and application scenarios of DTCR are analyzed using 5 IEEE standard systems with 14 buses to 300 buses and considering wind power generators and demand response. Results indicate that the proposed DTCR system can continuously generate economic benefits, optimize power flow distribution, and reduce network loss in daily dispatching.

      • LI Xiangyu, ZHAO Dongmei

        2020,44(13):17-24, DOI: 10.7500/AEPS20190909011

        Abstract:The optimal configuration of various resources in virtual power plant (VPP) is an important prerequisite for its participation in the optimal dispatching of power grid. Aiming at the difficulty of dynamic aggregation and quantitative evaluation of resources with different characteristics, this paper proposes a multi-level optimal configuration system including flexible adjustable resources, aggregation adjustment characteristics and optimization target modeling. At the level of flexible adjustable resources, the flexible regulation characteristics of different distributed generators, controllable loads and energy storage are studied. At the level of aggregation adjustment characteristics, on one hand, the flexibility of various resources is integrated to obtain the external adjustable flexibility of VPP. On the other hand, the randomness model of VPP is obtained by convolving the probability density function of the randomness of various resources. In the optimization target modeling, based on the portfolio theory, this paper establishes a net present value model including construction, operation and maintenance costs of VPP, and the profits of VPP participating in the electric energy and auxiliary service markets. Based on the static optimization configuration, the dynamic effects of key elements are analyzed. Taking an actual power grid in a certain region of China as an example, the correctness and effectiveness of the proposed method are verified.

      • CHENG Xin, XU Liang, ZHOU Shucan, LIU Zhengchao, LIN Yong, GONG Xianfu

        2020,44(13):25-31, DOI: 10.7500/AEPS20190717008

        Abstract:The integration of large-scale renewable energy to the power grid makes the uncertainty caused by power supply higher than the uncertainty of load demand. The probabilistic power flow of power system makes the management of operation department even more difficult. To solve this problem, this paper proposes an energy storage configuration method based on trajectory sensitivity analysis of guaranteed rate for renewable energy output. Firstly, an over-limit equation of probabilistic power flow for assessed components is set up according to the uncertainty of new energy output. Secondly, an analysis model of trajectory sensitivity for probabilistic power flow is built by setting the guaranteed rate of new energy output as a parameter. Thirdly, a configuration strategy of energy storage is proposed to make the probabilistic power flow not exceed limit. Meanwhile, power and energy requirements of energy storage are calculated for component-related nodes which have low trajectory sensitivities. Finally, the proposed method is simulated and verified on a practical power system in an area of South China.

      • HE Chenke, WEI Gang, YUAN Ding, ZHU Lan, LI Yang

        2020,44(13):32-38, DOI: 10.7500/AEPS20190920001

        Abstract:An interval load model of electric vehicles and an emergency support model of interval load for a charging-swapping-discharging-storage integrated power station (CSDSIS) are established. An emergency support strategy of interval load and an island partition strategy for power supply range of interval load for a CSDSIS are proposed. Based on the analysis of interval theory, a multi-layer optimization interval model of fault recovery for active distribution network (ADN) is established by taking the island load of system, times of breaker switching and power loss as the optimization objectives, respectively. Chaotic simulated annealing imperialist competitive algorithm is proposed to optimize the fault recovery interval model, and an interval Dist-Flow method based on affine number is proposed for power flow calculation. Results of the example show that the proposed method is fast and effective.

      • CHEN Lu, YANG Yongbiao, XU Qingshan

        2020,44(13):39-47, DOI: 10.7500/AEPS20190829004

        Abstract:Coordinated control of load for aggregated air-conditioners (ACs) has been included in the normal operation of power system due to better characteristics of spatial thermal storage. However, it leads to many problems such as aggregated power oscillation, unstable response potential and excessive rebound load, because the diversity of load groups is destroyed by the control strategy. To solve these problems, the influence of temperature control on the power and response potential of single and aggregated ACs is analyzed, then the time-variant power characteristic model and polymerization method of aggregated ACs are established based on the temperature control. To improve the aggregated quality of air-conditioner groups, the two-stage coordinated control model of aggregated ACs with time-variant complementary characteristics is built based on the framework of time-phase coordinated control and the corresponding response and recovery strategies are proposed. Finally, based on simulations with 15 000 ACs, the influence of external temperature, temperature strategy and other factors on the aggregated power is analyzed. Meanwhile, the coordinated control strategy of time-phase air-conditioner groups is applied in demand response events. The results verify the effectiveness of the proposed method.

      • XU Guangda, ZHANG Li, LIANG Jun, SUN Donglei, ZHAO Long, HAN Xueshan

        2020,44(13):48-55, DOI: 10.7500/AEPS20191118003

        Abstract:Energy Internet construction can realize the state awareness of residential appliances, and provide new data support for the estimation of price elasticity for demand. On this premise, an estimation method of price elasticity for residential electricity demand is proposed based on the electricity consumption features of appliances. Firstly, the correlation between the features and self-elasticity as well as cross-elasticity of residential appliances is analyzed according to the definition of price elasticity for demand. The objective constraint on behaviors of residential price response subjected to adjustability of appliances is revealed. Secondly, considering that the adjustment of appliances is uncertain due to the large number of residents and subjective differentiation of their response willingness, an analysis model based on fuzzy reasoning of electricity consumption adjustment of appliances is constructed to simulate the price response behavior of residents, which takes advantages of the classification of the adjustable features for appliances. Finally, the response results are used to calculate the price elasticity matrix of demand, and the residential price elasticity of electricity demand is estimated. Simulation results show the rationality and effectiveness of the proposed method.

      • HUANG Liping, WANG Yifei, GUO Juntao, XU Genli, DI Fangtao

        2020,44(13):56-63, DOI: 10.7500/AEPS20190923003

        Abstract:The increasing frequency of extreme meteorological disasters is posing a great threat to the safe and stable operation of power grid. The traditional economic dispatch method of power system aims at pursuing the economics of the generation costs, while ignoring the load distribution condition of power grid. The outage of heavy or full load lines will lead to the cascading failure and the blackout of power system. To solve this problem, a blackout-prevention economic dispatch method considering load rate homogeneity of power grid and N-1 security constraints is proposed for extreme whether conditions. Based on the traditional economic dispatch model, and comprehensively considering the load rate distribution of the whole power grid and the transmission lines affected by extreme weather, the load rate distribution balance is improved by reducing the average absolute deviation of the load rate of the entire power grid and the load rate of transmission lines in extreme weather areas, the operation security of power system is consequently enhanced. To ensure the robustness and high solving efficiency of the model in extreme weather, a novel method for dynamically checking and adding N-1 security constraints of transmission lines is proposed based on a check-feed-back-check idea and the line outage distribution factor (LODF) of transmission lines. Finally, a reliability evaluation method for dispatch strategies of power system in typhoon disaster is proposed based on the existing cascading failure simulation model considering hidden failures. Simulation results of IEEE RTS-79 test system show that, compared with the traditional economic dispatch method, the proposed dispatch method can effectively increase the load rate homogeneity of power grid, and improve the blackout distribution of the power system under extreme whether condition, thus improving the system operation safety and reliability.

      • SUN Lixia, BAI Jingtao, ZHOU Zhaoyu, ZHAO Chenyun

        2020,44(13):64-72, DOI: 10.7500/AEPS20191225003

        Abstract:In order to further improve the accuracy of transient stability assessment (TSA), a TSA model based on the bi-directional long-short-term memory (Bi-LSTM) network is established according to the sequential characteristics of data in the power system transient process. This method uses the Bi-LSTM network to establish a non-linear mapping relationship between the basic measurement data and the transient stability category of power system. The performance of Bi-LSTM network model is evaluated by accuracy, F1-measure (F1) index and false positive rate (FPR). On this basis, the t-distribution stochastic neighbor embedding (t-SNE) dimension-reduction method and the k-nearest neighbor (KNN) classifier are used to further improve the accuracy of TSA. The example based on the New England 10-generator 39-bus system show that the proposed method has better performance than conventional machine learning models and some deep learning models. The assessment model is analyzed by visualization methods and network prediction scores. The results show that the Bi-LSTM network has a strong ability to extract the characteristics of power system transient process, which is suitable for the TSA of power system. Further, the influence of the normalization mode and method of basic input data on the TSA model is studied. The results show that the z-score normalization method is better than min-max normalization method, and the assessment performance of the model using the total dimension normalization mode is better.

      • LENG Yajun, WU Zongyu, ZHAO Wenhui, LI Zhongxue

        2020,44(13):73-80, DOI: 10.7500/AEPS20191005003

        Abstract:Best selection and assessment of black-start schemes play an important role in fast restoration after a large-area blackout in power system. The entropy method is one of the most popular methods for determining the weights in black-start scheme assessment. The entropy method suffers from a range of problems such as large deviation of weight distribution, failing to reflect small changes of the decision matrix, etc. A novel weight determination method based on affinity propagation (AP) clustering is proposed, which is applied to evaluate black-start schemes. Firstly, the AP clustering algorithm is used to cluster the black-start schemes with all indices, and the clustering results based on all indices are obtained. Then, each of the indices is deleted one by one, the AP clustering algorithm is used to cluster the black-start schemes, and the clustering results based on the remaining indices are obtained. Moreover, according to the structure similarity, the similarities between the clustering results based on all indices and incomplete indices are calculated, and the weights of indices are determined. The linear weighting method is used to sort all black-start schemes. Finally,based on the data of Guangdong power system of China, experiments are carried out to validate the effectiveness of the proposed method. The results show that the proposed method has high accuracy.

      • LIU Qi, WANG Shouxiang, JI Xingquan

        2020,44(13):81-88, DOI: 10.7500/AEPS20190509002

        Abstract:The large-scale integration of distributed generations, energy storage systems and flexible loads in the distribution network causes frequent changes in power flows of the distribution network. It is very important for the planning and operation of active distribution networks to build an accurate and concise function model of node injection power and operation parameters. Combined with the radial topology of the distribution network, a power flow sensitivity analysis model with wide adaptability to the change of node injection power is proposed. The proposed model includes a quadratic sensitivity model of nodal power injections to line losses, a linear sensitivity model of nodal power injections to nodal voltages, a quadratic sensitivity model of nodal power injections to branch power. The effectiveness of the sensitivity model applied to the distribution system with weak ring network is analyzed. The IEEE 33-bus distribution system and the IEEE 123-bus distribution system with ring network are used for simulation analysis. The simulation results prove the effectiveness of the proposed sensitivity model.

      • SHI Yong, XU Dong, YU Hongru, SU Jianhui

        2020,44(13):89-97, DOI: 10.7500/AEPS20190712002

        Abstract:Parameter optimization of secondary voltage and frequency regulation controllers plays a key role in improving voltage and frequency quality in microgrid, especially for a microgrid system which consists of inverters controlled by droop or virtual synchronous generator (VSG) control. In order to complete the process of parameters optimization, the first step is to construct the voltage and frequency response model of microgrid. However, due to the complex structure of the microgrid system, multiple kinds of micro-sources, loads and widely-used commercial power supply, it is difficult to obtain the source model, which brings about lots of difficulties in microgrid mechanism modeling. To solve the above problems, a model structure of voltage and frequency at point of common coupling (PCC) for microgrid system is established. The transfer function in the model is identified by system identification algorithm using operation data sampled in the microgrid, and the parameters of secondary voltage and frequency controller in microgrid system are optimized using identified model. The proposed method can model the microgrid without knowing the internal structure of microgrid or source model, and the order of built model is low which is convenient for parameter planning of regulator. Finally, effectiveness of the proposed method is verified on the test platform for microgrid.

      • >Application Research
      • CHEN Xia, XIN Yanli, TANG Wenhu, ZHENG Ming

        2020,44(13):98-105, DOI: 10.7500/AEPS20191024002

        Abstract:The construction of offshore wind farms in China has been accelerated with the promotion of wind power parity policy. In the future, large-scale offshore wind farms will become an important power supply for local power grids in coastal areas. When power outages occur in coastal local power grids, if offshore wind farms can act as black-start power sources, the restoration process of power grids will be significantly accelerated. In order to start coastal thermal power generation units without the black-start capability, a black-start scheme is proposed, which uses small-capacity auxiliary diesel generators equipped in offshore wind farms as supporting power to start offshore wind turbines, and then supplies power to onshore thermal power plants via main transformers and high-voltage submarine cables. In order to improve the frequency regulation ability of the black-start system and maintain the system voltage stability when the high-voltage submarine cable is put into operation, a coordinated frequency modulation control method of wind turbine and diesel generator and a dynamic reactive power compensation control method considering the power limit of diesel generator are proposed, respectively. The latter uses wind turbine grid-side converters as the sources of static reactive power. Finally, a black-start model of offshore wind farm is built on PSCAD/EMTDC to verify the feasibility and effectiveness of the proposed operation scheme and control strategy.

      • ZHANG Bingxu, XU Gang

        2020,44(13):106-114, DOI: 10.7500/AEPS20191014006

        Abstract:An optimal control strategy for grid-connected electric vehicles (EVs) considering demand difference is proposed, which considers the different demands of EV users. Firstly, the grid-connected EVs are divided into three subsets, including rigid set, reducible set and flexible set according to the demand differences, and the charging-discharging control model is established, respectively. Then, the behavior model of EV users is constructed to describe the distribution characteristics of the user behavior, and then the planned power demand is formulated. Finally, the rolling horizon optimization strategy for grid-connected EVs is proposed with the objective of minimizing deviation between actual power and planned power and power fluctuation. The power increments of EVs in reducible set and flexible set are taken as the optimal control variables and solved under the constraints of the charging-discharging control model in every optimization round. Experimental analysis shows that the proposed optimization strategy ensures that the actual power matches the planned power while taking into account the different demands of users. Moreover, it is robust to random dynamic factors during the course of grid integration.

      • DU Wenlong, YANG Honggeng, MA Xiaoyang

        2020,44(13):115-122, DOI: 10.7500/AEPS20190812007

        Abstract:Under the condition of asynchronous sampling, if the harmonics and interharmonics in the sampling signals of power grid are adjacent, serious spectrum interference will occur. The actual frequency components of the signals cannot be identified. Therefore, a spectrum separation algorithm based on fast independent component analysis (FastICA) is proposed to measure harmonic and interharmonic parameters. Firstly, the model of multi-frequency components is constructed. Spectral lines in the spectrum are represented as the superposition of multiple frequency components. Secondly, frequency component parameters are obtained by using FastICA and least squares method. Finally, the measurement of adjacent multi-frequency components is realized. The simulation results show that the algorithm can accurately identify the frequency components with a small number of required spectral lines, and has good measurement accuracy and certain anti-noise ability.

      • YANG Jinggang, LIU Yang, LIU Ruihuang, YUAN Yubo, MA Dajun, CHEN Wu

        2020,44(13):123-134, DOI: 10.7500/AEPS20191108001

        Abstract:The medium voltage direct current (MVDC) port is essential to power electronic transformer (PET) applied to DC distribution network. The available PET topologies based on modular multilevel converter (MMC) can simultaneously provide four kinds of ports: medium voltage alternating current (MVAC), MVDC, low voltage direct current (LVDC) and low voltage alternating current (LVAC). However, the PET topologies need four power conversion stages, which lead to the complex circuit structure, large volume and heavier weight of PETs. A multi-port resonant PET topology based on MMC applied to DC distribution network is proposed. With a mixed-frequency modulation strategy, a set of LC resonant branches, medium-frequency transformers and full-bridge circuits are connected in parallel in the MMC bridge arm branch to realize the power decoupling outputs of the MVDC ports and LVDC ports. The proposed PET topology can provide the essential multi-port characteristics with only three power conversion stages, and has the advantages of the simplified circuit structure, smaller size and lighter weight. Moreover, the equivalent circuit analysis, control strategy and parameter design process of the proposed PET are presented. Finally, the simulation and experimental results show that the proposed PET topology is feasible.

      • YUAN Jiaxin, ZHANG Zhewei, ZHOU Hang, GAN Pengcheng

        2020,44(13):135-142, DOI: 10.7500/AEPS20190920011

        Abstract:With the increase of the short-circuit current level in high voltage direct current (HVDC) system, the fault current may make circuit breakers have either insufficient capacity, or larger size and higher cost. Using fault current limiter (FCL) can slow down the rising rate of fault current and reduce the performance requirements on DC breakers. FCL with permanent-magnet-biased saturation has advantages of automatic response, low power loss, and does not need external control and exciting circuit. The parameter design methods in previous works mainly focus on inductance in steady state, while the parameter design and optimization method of characteristic in transient process still needs to be studied. An optimization design method of FCL with permanent-magnet-biased saturation for DC system is proposed. Firstly, the working principle of the proposed FCL is introduced. Then, constraints and optimization object functions of optimal design for FCL are given, and an optimal design of parameters for FCL is obtained with the lowest cost. Finally, the simulation model of finite element and circuit is established and experiment with an FCL prototype is conducted. The simulation and experiment results show that the proposed method can be used to design the FCL, which can meet the requirements, reduce the cost of FCL and the electrical stress of the DC breaker during fault breaking.

      • ZHANG Jianmin, XIA Yan, NI Linna, HU Yingjun, YANG Sijie, QIU Chengfeng

        2020,44(13):143-149, DOI: 10.7500/AEPS20190627003

        Abstract:Situation visualization is a core technology for monitoring and management of low-voltage distribution network (LVDN). Therefore, a new concept of transformer centralized low-voltage distribution network (TCDN) is suggested as an object and target to guide corresponding research and engineering application. Auto-generation of a clear and beautiful phase-separated connection diagram becomes a pre-condition for situation visualization. For wiring asymmetry and load asymmetry being core characteristics of LVDN, two types of situation-oriented connection diagrams are designed. An auto-generation principle is proposed for phase-separated connection diagram of TCDN, so that not only the connection symmetry of three-phase four-wire main branches and the connected three-phase customers is guaranteed, but also the connection asymmetry caused by single-phase two-wire customers can be clearly perceived. For the proposed whole phase virtual connection diagram, based on the provided parameters of fan-shape layout and the arrangement order of each wiring harness, the initial layout and routing methods for each wiring harness by in-out zooming and rotating algorithm are proposed to yield an initial diagram, followed by further force-direction based beautifying algorithm to generate a nice connection diagram. The generated whole phase virtual connection diagram will be finally disassembled into the phase-separated connection diagrams according to the proposed auto-generation principle. The case study verifies the efficiency and effectivity of the proposed algorithm. Moreover, the time sequence pictures of voltage profile based on the generated phase-separated connection diagram show a very good perceptible understanding of dynamic operation characteristics of TCDN caused by variations of power outputs from single-phase household photovoltaics and power assumptions from single-phase and three-phase load.

      • LENG Hua, QIAN Yulin, ZHU Jiran, TANG Haiguo, ZHANG Weiwei, ZHANG Haiwen

        2020,44(13):150-156, DOI: 10.7500/AEPS20190529007

        Abstract:The construction of distribution automation is still in a large-scale pilot stage. In many areas, the means of fault information perception and location are limited. The information of different power outage events come from multiple business systems. It is particularly urgent to achieve accurate analysis of power outage events under limited conditions. Combining with the actual business process of power outage management of the electric utilities, this paper designs the function of power outage information resource pool, comprehensively analyzes the data of multi-layer main network and distribution network of substations, feeders, distribution transformer areas and users, and establishes the signal credibility model on the basis of hierarchical study and judgment. Combining fusion analysis of DS evidence theory algorithm, multi-dimensional flexible mining and analysis of power outage events, the fault tolerance of fault location is improved, and a reliable structured power outage information resource pool is built to achieve data sharing and publishing in different scenarios.

      • >Egineering Application
      • TU Jingzhe, YI Jun, WANG Chao, PAN Yan, LU Xiaodong, ZHANG Jian

        2020,44(13):157-165, DOI: 10.7500/AEPS20190809002

        Abstract:Under the background of centralized access of large-capacity DC systems and high-penetration renewable energy, this paper studies the problem of power angle and voltage interaction instability resulted by the large power disturbance of DC system considering the impact of dynamic characteristics of photovoltaic generator. Firstly, the motion equations of generator rotor and motor load of the two-machine equivalent system are derived in theory. Then, the physical mechanisms of the system instability caused by power angle and voltage interaction after large power disturbance of DC system are analyzed. Finally, the impacts of different active/reactive power characteristics of photovoltaic generator during low voltage ride-through (LVRT) process on the system stability are studied. The results are verified through the simulation of Changji-Guquan high voltage direct current (HVDC) system in China. The study results show that the system instability is eventually resulted by the vicious circle between the relative power angle increase of generators and the load-bus voltage drop (i.e. the decrease of motor resistance). And the impact of different active/reactive power characteristics of photovoltaic generator during LVRT process on the system stability should be comprehensively analyzed by theory combined with simulation.

      • LI Lili, WANG Zhicheng, WANG Gang, ZHOU Wenjun, YANG Leyong

        2020,44(13):166-171, DOI: 10.7500/AEPS20190313005

        Abstract:A two-stage optimization method for medium- and long-term electricity security check based on security-constrained unit commitment (SCUC) is proposed to quantify and evaluate the security risks that the medium- and long-term electricity plan and trading contract may bring to the operation of power grid, and to solve the problems of locating and adjustment of the restrictive factors in the implementation of the electric quantity contract. In the first stage, the electricity constraints of the power plant are strictly analyzed. In order to minimize the over-limit of section power flow, the optimization model of electricity security check is established to obtain the optimal plan of unit commitment scheme. In the second stage, the power grid operation constraints are considered and the optimization model of electricity correction is established to eliminate the over-limit of the section by adjusting the plan of unit commitment. By coordinating the two-stage optimal dispatch of electricity balance and power grid security, the conclusion of feasibility for electricity contract can be acquired, or the orientation of causes and adjustment schemes can be obtained when it is not feasible. The effectiveness and practicability of the method are verified by the analysis of the practical systems.

      • >Survey
      • LI Jianlin, LI Yaxin, LYU Chao, ZHAO Wei, ZHOU Jinghua

        2020,44(13):172-183, DOI: 10.7500/AEPS20191125006

        Abstract:With the advent of the first retirement peak period of electric vehicle power battery, the subsequent treatment of used power battery is posing a serious challenge to the environment and social resources. The energy storage power station is an effective way for cascaded utilization of decommissioned power battery. It is of great practical significance to study the key technology and development status of decommissioned power battery cascaded utilization. In view of the main problems and technical difficulties faced by the battery cascaded utilization, this paper firstly introduces some echelon energy storage demonstration projects at home and abroad, and summarizes and combs the relevant policies and standards in the current industry. On this basis, the key technologies of battery classification, screening, reorganization, characteristics of the thermal runaway, equalization control and electrode material recovery in the process of cascaded utilization are studied, and the advantages and disadvantages of different methods and control strategies are emphatically analyzed. Finally, combining with the development trend of electric vehicles in China, the commercial application mode of battery cascaded utilization is discussed and prospected.