2021, 45(21):2-22. DOI: 10.7500/AEPS20210909007
Abstract:With the rapid development of deep and distant offshore wind power, the transmission and the grid integration of wind power via HVDC systems have become technical hotspots. Focusing on several vital technologies such as AC collection-based DC transmission, low-cost DC transmission, multi-terminal DC transmission and multi-voltage-level DC transmission, the paper comprehensively discusses the current status, existing problems, research hotspots and development trends of the offshore wind power via DC transmission technologies in terms of system topology, equipment, control and protection. It is pointed out that the transmission and grid integration of offshore wind farms via the centralized HVDC system is the mainstream scheme in the near future. For such a system, the broadband oscillation is an urgent problem to be solved, it is the focus of attention to make the whole system perform the characteristics of dominant power sources. In terms of cost reduction, the technical route of the diode rectifier based HVDC transmission system has good features. However, to use pure diode rectifiers as the sending-end converter requires the improvement of wind turbines. For the multi-terminal DC grid with wind power integration, the development depends on the progress of low-cost DC circuit breaker and transient control and protection technology. And the offshore wind power transmission and grid integration via multi-voltage-level DC grid still lacks the support of the DC transformer and other key equipment.
2021, 45(21):23-32. DOI: 10.7500/AEPS20210427001
Abstract:Offshore wind power has been developed in Europe for nearly 30 years. Nowadays, it has been developed into one of the most important sources of renewable energy generation, and occupies strategical importance in future renewable energy planning and adoption. Offshore wind farms start their journey in several European countries including Denmark, Britain, and the Netherlands in the early 1990s from a series of demonstration projects. From the initial size of plants of a few megawatts, the size gradually increases to hundreds of megawatts and to gigawatts nowadays. In this process, European wind power industry has accumulated vast experience in electric design, monitoring, operation and maintenance. In view of the development process of European wind farms, this paper starts from reviewing some of the main technical problems arising from different stages of the development. After that, it focus on several major issues in electrical design, and summarizes some experience in engineering practice from the perspective of electrical design. Finally, the paper reviewes a few technical trends that can impact on the future development and integration of offshore wind power.
2021, 45(21):33-47. DOI: 10.7500/AEPS20210416003
Abstract:With the short supply of onshore and nearshore wind power resources, the site selection of offshore wind farms is from nearshore to the deep distant offshore, the scale reaches gigawatt level, and the capacity of offshore wind turbines is developing to more than 10 MW level. For large-capacity offshore wind turbines, this paper firstly describes the development status of key technologies such as capacity, types and converters of offshore wind turbines in China and abroad. Secondly, this paper analyzes the problems and key technologies of the large-capacity three-phase and multi-phase offshore wind turbines, and gives the development trend of topology design and control strategy optimization method of the future multi-phase offshore wind turbines. Finally, this paper summarizes the large-capacity offshore wind turbines.
2021, 45(21):48-56. DOI: 10.7500/AEPS20210420007
Abstract:Under the coupling effect of offshore wind, wave, mechanical load and other factors, the state data of wind turbines fluctuates rapidly. The sensitivity of wind turbine state characteristics under time-varying conditions leads to the dynamic change of wind turbine maintenance requirements, which increases the difficulty in accurately dispatching maintenance tasks of wind farm. This paper proposes a dynamic dispatching method for maintenance tasks of wind farm considering the risk situation of wind turbine state under time-varying sea conditions. First, the fuzzy C-means clustering algorithm is used to divide the time-varying working conditions of wind turbines. The improved joint-domain adaptive convolutional neural network is used to minimize the difference of feature distribution, so as to realize the adaptive extraction of the wind turbine condition features under time-varying working conditions. Then, the Markov model is used to describe the initial state transition matrix of each component. Considering the influence of incomplete maintenance on the performance of the unit components, the state risk situation prediction model considering the adaptive state assessment of the wind turbine is established by the component performance degradation model. On this basis, with the constraints of maintenance vessels, personnel and working hours, a dynamic dispatching model of maintenance tasks of offshore wind farm is established to minimize the cost of unit power dispatching and maintenance dispatching, which realizes the dynamic dispatching for maintenance tasks of offshore wind farm under time-varying conditions. Finally, the effectiveness and economy of the proposed method is verified by taking an offshore wind farm as an example.
2021, 45(21):57-65. DOI: 10.7500/AEPS20210328001
Abstract:With the large-scale development of offshore wind power, the comprehensive utilization of marine multi-energy has attracted widespread attention. In the background of the dual-carbon strategy, it is of great significance to explore the feasibility of adding wave energy devices in the offshore wind farms. An asynchronous planning model and optimization method for the offshore wind and wave energy combined power generation considering the operation and maintenance requirements are proposed. Considering the electricity consumption demand of the offshore wind farms during shutdown and maintenance, the wave energy power generation devices are added to the existing offshore wind farms. Considering the constraints of the engineering requirements, such as the location of offshore wind turbines, operation and maintenance, and ship berthing and collision avoidance, the layouts of the wave energy power generation devices are optimized. The joint layout scheme of the offshore wind energy and wave energy devices is evaluated, and the optimization method is finally obtained considering the economy and feasibility comprehensively. The results of case study show that the adoption of the planning model and optimization method for the combined power generation can effectively improve the economic benefits of combined power generation and the convenience of operation and maintenance of wind turbine , while ensuring the reliability of the combined power generation.
2021, 45(21):66-75. DOI: 10.7500/AEPS20210326005
Abstract:For the electrical system topology optimization of multi-substation offshore wind farms, the current methods are to divide the overall offshore wind farm into several fixed sub-areas according to the predefined number of substations, then independently optimize the cable connection layout in each sub-area and eventually combine all of them as the overall scheme. However, it is often hard to obtain the global optimal scheme due to the fixed partition strategy. Therefore, this paper designs a cable connection layout model for multi-substation offshore wind farms and proposes a Q-learning particle swarm optimization algorithm based on Voronoi adaptive partition, which aims to minimize the total cost considering substation locations, cable type selection, and power loss. Taking the Q-learning particle swarm optimization algorithm as the key, the proposed method designs an adaptive partition strategy based on the Voronoi diagram, which can realize the cable connection in different partitions with coding and decoding strategies. Finally, the case analysis proves the effectiveness of the proposed model and algorithm.
2021, 45(21):76-84. DOI: 10.7500/AEPS20210412002
Abstract:With the large-scale development of offshore wind power, the harmonic problem caused by the integration of offshore wind power has become more and more prominent, so the modeling and analysis are urgently needed. The modeling of submarine cables in the harmonic frequency range is one of the important work to tackle this problem. The calculation method for electrical parameters of submarine cable in the harmonic frequency range is studied, which provides a basis for the modeling of the submarine cable. First, the definition of electrical parameters of the cable is given, and then the calculation process and formulae for the electrical parameters of the submarine cable are summarized. The applicability of the calculation formulae is also demonstrated. Finally, the electrical parameters of three types of typical submarine cables are calculated. Results show that both the positive sequence parameters and the zero sequence parameters of the cable will change greatly with the frequency, and the characteristics of the electrical parameters of the cable changing with frequency must be considered when performing harmonic analysis.
2021, 45(21):85-91. DOI: 10.7500/AEPS20210412003
Abstract:Because the capacitive effect of power cables at the same voltage level is over 20 times larger than that of overhead lines, the harmonic resonance amplification problem which is not serious in the onshore wind farm integration may become very serious in the offshore wind farm integration. To this end, this paper studies the harmonic resonance amplification problem caused by the offshore wind farm integration and its mitigation principle. Firstly, the harmonic resonance amplification mechanism caused by offshore wind farm integration is analyzed, and the technical path of mitigation is given in principle. Then, taking a real offshore wind power integration project as a study case, the harmonic models of the offshore wind power grid and the onshore power grid are established, respectively, including the harmonic models of cables, transformers and wind turbines, the equivalent harmonic models of the extra-high voltage (EHV) large-scale grid and the low-voltage distribution network. According to the established harmonic models, the causes of harmonic resonance amplification are analyzed, and a mitigation scheme for solving the problem of harmonic resonance amplification is proposed and verified. Finally, the mechanism and mitigation principle of harmonic resonance amplification caused by offshore wind farm integration are summarized.
2021, 45(21):92-102. DOI: 10.7500/AEPS20210530001
Abstract:Modular multilevel converter (MMC) based flexible direct current transmission technology is an important scheme for the power transmission of distant offshore wind farm, but it presents multi-input-multi-output (MIMO) characteristics. The frequency coupling effect of MIMO characteristics poses a new challenge to the stability of the offshore wind farm-MMC interconnection system. Most of the existing MMC models have ignored the frequency coupling effect, and there are few studies on the key mechanism affecting the frequency coupling effect. In view of the above problems, this paper establishes the equivalent sequence impedance model of MMC on the wind field side, including the frequency coupling effect, and analyzes and evaluated the key factors affecting the frequency coupling effect and stability. Meanwhile, an improved MIMO stability criterion based on Gershgorin-circle theorem is proposed to solve the complicated problem of the existing stability criterion, which simplifies the stability judgment process by introducing the distance function. Then, based on the criterion, the influence of the key factors and the frequency coupling effect on the stability of the system is analyzed, and the conservatism of the criterion is analyzed quantitatively. Finally, a simulation example is given to verify the correctness of the theoretical analysis.
2021, 45(21):103-111. DOI: 10.7500/AEPS20210413004
Abstract:In order to reduce the ride-through cost of the flexible high voltage direct current (HVDC) system with wind farm integration when a low-voltage fault occurs in the AC main grid, this paper proposes a grid fault ride-through strategy in which the DC energy dissipation equipment and the DC choppers in wind turbines work together. The wind farm will reduce its DC power output with the coordination of the sending-end converter during the grid fault. The DC energy dissipation equipment is only required to limit the DC voltage increase in the early stage of the grid fault before the output power of the wind farm have been reduced. Therefore, this strategy could greatly reduce the volume of the DC energy dissipation equipment. On this basis, the proposed strategy integrates the energy dissipation resistor of the DC energy dissipation equipment into the receiving-end modular multilevel converter, to further reduce the unloading cost. Finally, a simulation example of the flexible HVDC system with wind farm integration is constructed in the PSCAD/EMTDC simulation software, which verifies the correctness and effectiveness of the proposed fault ride-through method.
2021, 45(21):112-119. DOI: 10.7500/AEPS20200925001
Abstract:Flexible DC integration systems of offshore wind power should have the ability of fault ride-through (FRT) when AC grid faults occur. However, without high-speed communication, the multi-type converter devices in the wind farm and flexible DC system are difficult to be coordinately controlled to achieve DC voltage stability during the low voltage ride-through process of the system. Therefore, a coordinated control method for FRT based on the information transmission by harmonic injection is proposed for flexible DC integration systems of offshore wind power. During the fault process, the converter on the wind farm side detects that the DC voltage exceeds the threshold, and then injects harmonics into the system, which makes the wind power converters cooperatively limit the power injected into the grid according to different harmonic thresholds, so as to realize the coordinated FRT of multiple converter devices in the system without communication. It is also compared with the conventional method of reducing the power only by the converter on the wind farm side. For various fault types of the power grids, the proposed method can limit the voltage of flexible DC within the allowable range more quickly, and the system can realize safe and reliable FRT.
2021, 45(21):120-128. DOI: 10.7500/AEPS20210425008
Abstract:Offshore wind farm has more abundant and stable wind energy resources, and has the advantages suitable for large-scale development. It has great development potential and is the main development trend of wind power in the future. This paper summarizes and analyzes the high voltage direct current (HVDC) transmission topologies for the large-scale offshore wind power involved in the existing engineering and theoretical research, including the flexible DC converter station system using only modular multilevel converter (MMC) and the hybrid HVDC converter station system using diode rectifier unit (DRU-MMC). The operation characteristics of each scheme are summarized. In order to solve the problems of low flexibility and feasibility of the existing schemes, a hybrid cascaded HVDC transmission system with multiple voltage levels is proposed, and the schemes are compared and analyzed from the perspectives of technology and economy. Finally, the coordinated control strategy for the hybrid cascaded HVDC transmission system with multiple voltage levels is designed, and the feasibility of the grid integration scheme is verified by PSCAD/EMTDC simulation analysis.
2021, 45(21):129-138. DOI: 10.7500/AEPS20210426004
Abstract:The offshore wind power grid-connected scheme based on diode rectifier (DR) can greatly reduce the investment cost, but the problems of control and start-up are difficult to be solved. The actively commutated current source converter (CSC) has high power density and great control performance, but its cost is relatively high. This paper proposes a hybrid cascaded DC grid-connected transmission system for offshore wind power based on DR and auxiliary CSC, which can realize the miniaturization of offshore platform and solve the problems of control and start-up. First, the topology and mathematical model are analyzed. Then, the steady-state control strategy and black start control strategy are proposed for the offshore wind grid-connected system, and the simulation verification is performed based on PSCAD/EMTDC. Finally, the economic analysis of the proposed scheme is carried out. The results show that the hybrid cascaded offshore wind grid-connected system can smoothly complete the black start of wind farms, has good steady-state characteristics, and can adapt to the fluctuations of wind power output.
2021, 45(21):139-148. DOI: 10.7500/AEPS20210419001
Abstract:Offshore wind power has promising development potential, and reliable and efficient large-scale distant offshore wind power grid integration system is the key technology to develop the offshore wind power. Aiming at the miniaturization problem of offshore platform for distant offshore wind power HVDC transmission system, an HVDC transmission scheme for the grid-following medium-frequency distant offshore wind farm is proposed. This scheme utilizes the grid-following wind turbines, where the modular multilevel converters (MMCs) are installed on both the rectifier side and the inverter side of HVDC system. Firstly, the grid-following medium-frequency scheme is described from two aspects of topology and control system. Then, the influence of the offshore AC system operation frequency on the offshore wind farm and its HVDC transmission system is analyzed, including influence on the transformer, the AC cable and the MMC. Based on a ±320 kV/1 000 MW offshore wind power HVDC transmission system, the technical and economic efficiency of the proposed 100 Hz grid-following medium-frequency scheme is analyzed, including the main circuit parameters of key electrical equipment, the cost of AC cable, the cost of offshore platform, the cost of wind turbines, the AC cable loss and rectifier station valve loss, and the transmission capability of AC cables. Finally, an electromagnetic transient simulation model of the grid-following medium-frequency scheme is built in PSCAD/EMTDC to verify the effectiveness of the scheme.
2021, 45(21):149-158. DOI: 10.7500/AEPS20210331010
Abstract:To achieve the maximum benefit on the demand side, a hierarchical energy scheduling method based on deep reinforcement learning (DRL) that can cope with complex environments is proposed. Firstly, a two-tier framework for the home energy management system (HEMS) is established. By changing the charging and discharging power of the second-tier energy storage system, the power over-limit of the first-tier is solved caused by the concentration of the load to the low-electricity-price period for meeting the power demand of users and reducing the electricity bill. Then, electrical appliances are classified and modeled according to their load characteristics. Secondly, Markov decision process (MDP) is used to model the energy management problem. The reward function is employed to replace objective functions and constraints. Moreover, Rainbow algorithm is introduced to optimize the strategy with the goal of maximizing the long-term benefits and achieving online scheduling economically and efficiently. Finally, a simulation is performed on a residential house, which includes solar panels, an energy storage system, multiple electrical appliances, and an electric vehicle, to verify the effectiveness and superiority of the proposed method in dealing with the uncertain problems.
2021, 45(21):159-169. DOI: 10.7500/AEPS20210201001
Abstract:Aiming at the uncertainty of renewable energy output in the micro energy grid, based on the “multiple interaction” characteristics of integrated demand response (IDR), a strategy for IDR to smooth the fluctuation of renewable energy is proposed, and a two-stage robust optimization model for IDR day-ahead and intra-day collaborative scheduling in the micro energy grid is built. In the model, the uncertain budget parameters of wind and photovoltaic are introduced for regulating the economy and robustness of the system. In the day-ahead stage, the worst-case scenario is considered to determine the day-ahead scheduling scheme of the system. In the intra-day regulation stage, the regulation scheme in the worst-case scenario is optimized based on the optimization results of the day-ahead stage. The strong duality theory and column and constraint generation algorithm are used to transform and solve the robust optimization problem of min-max-min structure. The results show that the robust optimization method can improve the system ability to resist uncertain risks, and the IDR can improve the economy and self-sufficiency of the micro energy grid.
2021, 45(21):170-180. DOI: 10.7500/AEPS20210603005
Abstract:After a large-scale and long-term blackout accident in the distribution network under extreme disaster events, the active distribution network with distributed energy resource and emergency power supply vehicles (EPS) can form islands to supply power for the load. Before the completion of fault repair, aiming at the problem that the unfairness of the sustainable restoration time of each key load with the same weight leads to the decrease of customer satisfaction. This paper establishes a two-layer model of dynamic islanding balanced recovery, which considering the scheduling of EPS. It sets the optimal balance of critical load restoration and the maximum economic benefit of the distribution network as the upper and lower objective functions, respectively, and realizes the fair and balanced restoration of loads with the same weight in the outage period. At the same time, in order to reduce the cost of EPS scheduling and load outage loss, based on the above two-layer model and Dijkstra algorithm, an optimal scheduling method of EPS supported by the traffic network model is established, and the best candidate access node and optimal scheduling path are obtained. Then, the original mathematical model is transformed into a mixed-integer linear programming problem by using Big-M and other linearization methods. Finally, a modified PG&E 69-bus model and a traffic network are used to verify the effectiveness of the proposed method for improving the reliability, equilibrium and economy of power supply in the fault stage.
2021, 45(21):181-188. DOI: 10.7500/AEPS20210421008
Abstract:A combined forecasting method for the charging and discharging capacity of electric vehicles (EVs) is proposed. Firstly, based on the historical charging data of EVs and the survey data of users' willingness to participate in vehicle-to-grid (V2G), the characteristics of vehicle state of charge (SOC), travel time and users' sensitivity to price are analyzed. Then, a random forest based classification model is established to determine whether the EV participates in V2G scheduling, and the importance of characteristic factors that affect users' decision-making is evaluated. Secondly, the Monte Carlo simulation method is used to simulate the situations of EV travelling and charging/discharging, and the charging and discharging capacities are predicted respectively. Finally, a simulation is carried out by taking an office area as an example to compare and analyze the EV charging and discharging behaviors and load distribution with multiple charging and discharging modes. The constructed random forest classification model has an accuracy of 0.917, which can effectively classify the charging and discharging behavior of EVs during the V2G planning period. Simulation results also verify the effectiveness of the proposed forecasting framework.
2021, 45(21):189-196. DOI: 10.7500/AEPS20210420005
Abstract:In view of the current situation that charging stations for electric logistics vehicles (ELVs) in some cities of China have difficulties in carrying out demand response, a competition map based demand response strategy of charging stations for ELVs is proposed. Firstly, a classification method for ELVs is constructed, and the characteristics of ELV users are analyzed according to the charging temporal-spatial characteristics and charging price curve of ELVs, etc. On this basis, the charging very important person (VIP) system for ELV users is constructed based on the competition map, and the incentive mechanism for ELV users to participate in demand response is formed. Secondly, considering the energy storage, electricity price and other factors of the ELV charging station, the demand response strategy of the ELV charging station is constructed, so as to reduce the impact of disorderly charging of the ELVs on the charging station participating in demand response. Finally, the economy of the proposed strategy is compared with that of the Monte Carlo demand response strategy through an example analysis, and results verify the effectiveness and practicability of the proposed demand response strategy.
2021, 45(21):197-205. DOI: 10.7500/AEPS20200925016
Abstract:With the increase of load and peak-valley difference in power systems, the electric consumption structure is continuously optimized. It is expected that hydropower can use its flexible start-stop feature on the power grid side to fully participate in the peak-shaving of power grid for reducing the peak-valley difference of the system. The power plant side pursues the maximum cascaded electric quantity in the dispatching process to realize the maximum benefit. For the purpose of coordinating the contradictory demands on the cascaded power output process and simultaneously satisfying the complex constraints between the power plant and the power grid, the units of the cascaded hydropower station are taken as the dispatching unit, and the maximum cascaded power generation and the minimum peak-valley difference of the residual load in the power grid are selected as the objective function. The tie-line constraints on the power grid side and the constraints on the power plant side that are related to the water head such as tailwater jacking, output curve limit and irregular vibration areas, are fully considered and linearized. A multi-objective short-term mixed-integer linear programming (MILP) model is constructed under complex constraints to normalize the multi-objective function and transform it into a single objective function by weight method. The model is solved by Gurobi commercial solver.The practical application of the cascaded hydropower plants of Hongshui River in China, shows that the proposed model can get a cascaded generation plan which takes into account the goals and expectations on both sides of the power plant and the power grid.
2021, 45(21):206-214. DOI: 10.7500/AEPS20201230002
Abstract:In the modular multilevel DC/DC converter, the traditional phase-shifted carrier pulse width modulation (PWM) leads to the larger inductor current ripple when the modules have different input and output voltages. As a result, the inductor loss increases and the optimal design of filter devices are difficult as well. Aiming at this problem, through the frequency-domain analysis of current ripple, this paper reveals the problems of traditional phase-shifted modulation technique. In addition, the relationship among the current ripple, input and output voltages of modules and carrier phase-shifting angles are analyzed. The carrier phase-shifting angle of each module is calculated with the goal of minimizing the switching frequency harmonic amplitude in the current ripple. The calculation method of the phase-shifting angle is adjusted according to different conditions, and a modulation strategy for current ripple suppression is realized. Finally, the software simulation and experimental test of the modular multilevel DC/DC converter with three cascaded modules are carried out. Both simulation and experimental results show that the proposed control strategy can effectively reduce the current ripple when the input and output voltages of each module are unequal, which verifies the correctness and practicability of the proposed control strategy.
2021, 45(21):215-223. DOI: 10.7500/AEPS20210407004
Abstract:With the increasing installed capacity of wind power, the impact of the power prediction error of wind farms and wind farm clusters on dispatching and operation is increasing. Current research on the characteristics of wind power prediction error is mainly for a single wind farm, which cannot meet the current demand of power grid dispatching and operation. Therefore, the paper counts and analyzes the distribution characteristics of a large number of wind farms, summarizes the variation law of distribution characteristics of the prediction error for different scales of wind farm clusters, and explores the best fitting model of the probability distribution of cluster prediction error. Then, in view of the guiding role of the prediction error distribution, the evaluation index for the persistence of error distribution is proposed. Finally, a more comprehensive analysis on the role of the prediction error research of wind power is carried out, and it is also pointed out that it has more engineering applicability to carry out the power grid dispatching with wind farm station clusters as a unit.