Abstract:我国光伏资源丰富、分布广泛。高渗透率分布式光伏接入模式具有较高的灵活性，能够实现就近式负荷供电，在配电网中的应用潜力巨大。大规模分布式光伏的接入有利于整合资源实现集约开发、削减电力尖峰负荷、节约优化配电网投资、引导居民绿色能源消费，但也给配电网造成了巨大冲击。为了保障分布式光伏安全接入与高效消纳，配电网稳定、低碳运行，用户可靠、经济用电，亟须开展新型配电网网架规划设计及安全性评估、光伏并网稳定控制与系统优化、源网荷储协同自治运行、含高渗透率分布式光伏的配电网数字化转型升级及电力市场机制创新等研究，推进高渗透率分布式光伏试点应用落地，加快实现能源结构清洁绿色转型。

Abstract:With the expansion of grid-connected photovoltaic power generation, the problem of small-disturbance stability caused by the dynamic characteristics of its converter control system becomes increasingly prominent. Due to the small capacity of a single photovoltaic power generation unit, the grid-connected photovoltaic power generation system often contains a large number of photovoltaic power generation units. As a result, the model of the system is high-order. And the large numbel of photovolatic power generation units greatly increases the complexity of stability analysis. Therefore, in order to propose a relatively simple and effective stability discrimination method, a reduced-order model of small-disturbance stability analysis of grid-connected photovoltaic power generation system under DC voltage control timescale through the timescale decomposition method is established in this paper. Then the small-disturbance stability criterion of the system is deduced by applying the Routh-Hurwitz criterion on the basis of this model. From the criterion, the key factors affecting the small-disturbance stability of the system under DC voltage control timescale are analyzed. The main unfavorable affecting factors are heavy loading condition, condition of weak grid connection, improper setting of parameters of converter control systems. The correctness and effectiveness of the proposed stability criterion are verified by simulation examples. The results show that the stability criterion can effectively evaluate the small-disturbance stability of the grid-connected photovoltaic power generation system under DC voltage timescale. The evaluation method does not need to establish a complex high-order model of the system, which greatly simplifies the calculation and analysis process.

Abstract:In view of the inconsistency between the maximization of revenue pursued by photovoltaic (PV) investors and the minimization of network losses pursued by distribution network operators, a PV hosting capacity (HC) evaluation method with PV curtailment constraints is proposed to determine the PV access capacity. Firstly, PV curtailment constraints are integrated into the HC evaluating model based on DistFlow to minimize the network losses. The hosting problem is transformed to a second-order cone programming (SOCP) problem under different PV curtailment rate limits. Then, under the framework of this model, the total access capacity and PV curtailment rate at the time of maximum revenue are estimated by interpolation of the difference relationship between revenue and total access capacity. Finally, the estimated total access capacity and PV curtailment rate are brought into the model as new constraints by the back substitution strategy, and the access capacity of each candidate node can be calculated. The verification results of the improved IEEE 33-bus system show that the access capacity determined by the proposed method balances the revenue and the network losses.

Abstract:The joint planning of wind photovoltaic storage can fully consider the characteristics of renewable energy resources, and the planning results are more scientific in a global perspective. In this paper, the medium-term and long-term time-series power balance is considered for joint planning of wind photovoltaic storage, and the planning results are more reasonable and reliable. The multi-region wind photovoltaic storage joint planning problem is essentially a high-dimensional complex stochastic planning problem with wide area multiple power sources, multiple variables and multiple time sections, which is very time-consuming to solve and even impossible to solve due to dimensional disasters. Based on this, a multi-region wind photovoltaic storage joint planning model is established, and a hierarchical optimization algorithm is proposed based on regional decomposition coordination in this paper, which decomposes the multi-region wind photovoltaic storage joint planning model into a two-layer problem. The configuration capacity of power sources and energy storage in each sub-region is determined in the lower-layer problem. The power sources in each sub-region are determined in the upper-layer problem according to the capacity planning scheme given by the lower-level problem operation. The two layers iterate with each other to coordinate and obtain the optimal capacity of wind photovoltaic storage in each region. Finally, the rationality and effectiveness of the proposed model and method are verified by the arithmetic case analysis.

Abstract:With the continuous development of renewable energy, photovoltaic power stations show a trend of large-scale grid connection, but the disorderly development of photovoltaic grid connection induces the static voltage instability in photovoltaic grid-connected system. Firstly, an equivalent model of photovoltaic large-scale grid-connected system using the equivalent admittance of photovoltaics is constructed in this paper, and the impact of photovoltaic grid-connected system on static voltage stability is quantified as the impact of photovoltaic equivalent admittance on the eigenvalue of the admittance matrix. It is concluded that the decrease of the minimum eigenvalue of the admittance matrix reduces the static voltage stability of photovoltaic grid-connected system. Further an evaluation index for static voltage stability of photovoltaic grid-connected system based on eigenvalue-active power sensitivity is proposed. Considering the strong correlation between the admittance matrix and the network topology of photovoltaic grid-connected system, the influence of the network topology on static voltage stability is analyzed. Photovoltaic grid-connected schemes which can improve the static voltage stability are proposed from AC and DC perspectives, and it is concluded that changing the topology of the transmission network reasonably can improve the static voltage stability of photovoltaic grid-connected system. Finally, based on a IEEE 14-node system example, the eigenvalue index and the photovoltaic grid-connected scheme proposed in this paper is validated to ensure the static voltage stability of large-scale photovoltaic grid-connected systems and promote orderly grid connection of photovoltaic system.

Abstract:The energy system of solar town is a comprehensive energy system with the photovoltaic as the main body, combined with other local energy sources. To fully utilize the solar energy and geothermal energy in such system, a two-stage robust optimization method considering uncertainties of both energy supply and demand is proposed. Uncertainty adjustment parameters are introduced to avoid sacrificing economic efficiency in order to ensure power supply reliability. A max-min-max two-stage robust optimization configuration model is established with the objective of minimizing the overall system cost. The optimization includes the allocation of photovoltaics, combined heat and power, ground source heat pumps and storage in the integrated energy system of the solar town. The uncertainty of photovoltaic and load data is described by a box-type uncertainty set independent of probability distribution. The upper and lower boundary interval as robust constraints represents the fluctuation range of photovoltaic and load. The column and constraint generation algorithm and strong duality transformation are employed to reduce the complexity level of solving the problem. A case study is conducted on a solar town located in northern China. By adjusting the uncertainty adjustment parameters, the conservatism of the configuration plan can be effectively controlled. The scheme has strong applicability, mainly embodied in not only ensuring the reliability of power supply, reducing the load power loss rate, but also reducing the configuration cost and improving the light rejection rate.

Abstract:Aiming at the voltage violation problem caused by distributed photovoltaic connected to the distribution network, a distributed voltage control strategy based on consistency algorithm is proposed considering the economy of voltage regulation of battery energy storage and photovoltaic inverters. Firstly, the voltage regulation mechanism of photovoltaic inverter and energy storage is analyzed, and the voltage sensitivity parameters of distribution network which can be used for distributed iterative calculation are deduced. Secondly, life loss models for energy storage batteries and photovoltaic inverters are established separately. These models are employed to derive unit power cost models. Unit voltage regulation cost models are then constructed by combining the voltage sensitivity parameter. Finally, with the unit voltage regulation cost as the consistency variable, a distributed voltage control strategy based on the consistency algorithm is designed, resulting in power regulation schemes for each node's energy storage and photovoltaic inverter. The simulation results demonstrate that the proposed control strategy can consider the lifetime loss of each node equipment and the overall voltage regulation economy, and fully reduce the voltage control cost of distribution network on the premise of effectively inhibiting voltage violation.

Abstract:With the proposal of the new infrastructure concept, new loads represented by 5G base stations and electric vehicles are connected to the urban distribution network on a large scale, which has a significant impact on the safety and economic operation of the distribution network. In order to evaluate the capacity of urban distribution network to accommodate the new infrastructure load, a method for evaluating the available capacity of distribution network based on load growth demand and distribution network carrying capacity is proposed. The total amount of increased load of each load node is caculated as the available capacity under the condition of new infrastructure load and distributed energy access. Considering the constraints of voltage and power flow, the second-order cone relaxation method is used to solve the model. In addition, considering the influence of the location and sequence of distributed photovoltaic access on the available capacity, the grid-connected position of distributed photovoltaic is optimized according to the dynamic programming method with the objective of maximising the available capacity. The simulation analysis is carried out in a regional distribution network grid, and the results show that the proposed method can effectively evaluate the available capacity level of the urban distribution network grid, and realize the dynamic optimization of available capacity through the orderly access of distributed photovoltaic.

Abstract:When the distributed photovoltaics enters the stage of extremely high proportion penetration, the problem of over-limit voltage in the distribution network becomes more prominent. A variety of adjustable resources in the distribution network are used in the common method to improve voltage distribution, but there is little consideration given to the information interaction between feeder layer and substation in practical engineering. Therefore, a voltage optimization method for distribution network considering the coordination between feeder layer and substation double layer is proposed in this paper. An optimal power flow model with the goal of minimizing the line loss is established in the feeder layer. The voltage at the head of the feeder layer and the injected power of the feeder layer are obtained by second-order cone relaxation, and fed back to the upper substation. A regulating voltage model is established in the substation with the goal of minimizing the voltage adjustment amount of PCC, and the adjusted voltage is returned to the feeder layer to update the power flow distribution. Then, the scheduling plan of adjustable resources are obtained. Finally, the extended IEEE 33-node power distribution system is taken as an example. The Cplex optimization solver is used to verify that the method can control the voltage and reduce the number of switching equipment. The economy of system operation on the basis of solving the distribution network voltage over-limit problem is improved.

Abstract:Due to the large-scale development of rooftop photovoltaics and the continuous improvement of the national carbon trading market, the carbon credits generated by rooftop photovoltaics power generation can obtain additional income through carbon allocation offset or participation in carbon trading market. Firstly, the path and methods of participating in the carbon trading market for rooftop photovoltaic projects are studied, focusing on how large-scale rooftop photovoltaic projects participate in the carbon trading market and the degree of impact on the project's economy. Secondly, the life-cycle economic model of rooftop photovoltaic with carbon trading income is established based on the power generation model. Thirdly, in order to quantitatively analyze the income of carbon trading, an improved grey prediction GM(1, 1) model based on the differential threshold filtering method is proposed to predict the future carbon price. Finally, an example of large-scale rooftop photovoltaic is analyzed. The results show that large-scale rooftop photovoltaic considering participation in the carbon trading market has a relatively significant economic improvement.

Abstract:With the integration of large-scale photovoltaic power stations into power grids, the traditional strategy of the backup automatic switch is affected. When a grid fault occurs, due to the existence of photovoltaic power sources, the busbar voltage at the fault point cannot meet the ‘no voltage’ criterion, and the traditional strategy cannot operate correctly. In order to improve new energy utilization and ensure normal input of the backup power supply without disconnecting photovoltaic power sources, a new backup automatic switch scheme based on distributed buffer resistors is proposed. Firstly, the influence of the mismatch between photovoltaic output and load power on the grid-connected point voltage is analyzed. Secondly, distributed buffer resistor branches are connected in parallel at both ends of the DC capacitor in each photovoltaic power generation unit, using buffer resistors to suppress the power imbalance formed in the islanded state after the main power supply is disconnected (before the backup automatic switch operates), realizing safe and fast operation of the backup automatic switch. Finally, MATLAB/Simulink is used to simulate and verify the scenario of extreme mismatch between photovoltaic and load power, and the inrush current can be limited to 1.5 times rated current, which meets the relevant specifications and verifies the feasibility and effectiveness of the proposed new backup automatic switch scheme.

Abstract:The high permeability and uncertainty of distributed photovoltaics have a great impact on the node voltage and power flow distribution and pose a new challenge to the safe operation of distribution network. How to effectively and accurately optimize and control distribution network containing distributed photovoltaics is an urgent problem to be solved. A model predictive control optimization method for distribution network containing distributed photovoltaics is proposed. In order to reduce the voltage deviation of nodes and power loss, the prediction information is continuously updated and the optimization model is constructed based on the model predictive control, and the rolling optimization control of distributed photovoltaics and energy storage devices is carried out. A modified IEEE 33-node system is used as an example to verify that the voltage deviation of nodes and power loss of distribution network are effectively reduced. The proposed method has the better optimization effect than the day-ahead optimization method does for fitting the actual resuts. Based on the model predictive control, the proposed method is helpful to the safe operation of distribution network under the distributed photovoltaics access, thus reducing the deviation between the control dispatch and the actual situation.

Abstract:To calculate the harmonic impedance on the system side is crucial for the safe and stable operation of the new power system. Existing algorithms for harmonic impedance estimation usually assume that the harmonic impedance on the system side is constant during the measurement period. However, the voltage on the system side always changes according to the working mode of the power grid or access of large power load demand, resulting in fluctuations of the harmonic impedance on the system side. Luckly, the harmonic impedance on the system side does not change much in the adjacent sampling time interval. On this basis, the paper proposes a new non-intervention solution for the variable harmonic impedance on the system side. In this method, the second-order change of the harmonic impedance and the second-order change of voltage in the adjacent sampling time interval are taken as the objective function. The harmonic impedance on the system side is solved by minimizing the target function. The proposed method is proved to be accurate and reliable by simulation and practical analysis, especially in the condition that the strong background harmonic or user-side harmonic impedance is not much stronger than the harmonic impedance on the system side.

Abstract:The maintenance cost of offshore wind farms accounts for a high proportion of the total cost of power generation. In order to reduce the operational and maintenance costs of offshore wind farms, based on a reliability threshold, a maintenance strategy is proposed in the paper that considers the fault-relatedness among subsystems and the subsystem fault risk levels of wind turbine units. This strategy includes fault risk levels, dual reliability thresholds, and a maintenance matrix. Firstly, a fault chain model is used to describe the fault relationships among subsystems of wind turbine units, and reliability models for each subsystem are established. Secondly, the concept of multi-level maintenance is introduced, and the optimization model of wind farm opportunity maintenance strategy considering the fault risk level is proposed. The risk factor of subsystem opportunity maintenance is optimized using particle swarm algorithm, and then the maintenance strategy of a single unit is determined. Thirdly, an offshore wind farm maintenance strategy optimization model based on the maintenance matrix is proposed by considering factors such as offshore accessibility and spare parts inventory. The model takes the minimum total maintenance cost per unit time as the objective function and dynamically adjusts the maintenance strategy according to the subsystem's spare parts inventory and failure risk level. Finally, a single wind turbine in an offshore wind farm is used as an example to analyse the impact of factors such as accessibility and multilevel maintenance on the wind farm opportunity maintenance strategy. The results show that the opportunity maintenance strategy considering the failure risk level can reduce the cost by 27.34% compared with the traditional opportunity maintenance strategy, which verifies the effectiveness of the proposed opportunity maintenance strategy in reducing maintenance costs.

Abstract:Meteorological factors have an impact on the operating temperature of line, and the operating temperature of line is closely related to line impedance and fault outage rate. The influence of meteorological factors on the multiple parameters of line is not fully considered when evaluating the reliability of power systems. In this context, based on weather-dependent heat balance equation of transmission line, the calculation method of line temperature considering meteorological factors and the real-time capacity model of transmission line dependent on meteorological conditions are firstly studied. And then, the impedance parameter model and the real-time outage rate model of transmission line are established, which are dependent on the line temperature. Next, a power system operation reliability evaluation method considering meteorological factors is proposed based on Monte Carlo simulation. Finally, taking the IEEE 14-bus and IEEE RTS96-bus modified systems as examples, the influence of meteorological factors on the real time parameters of transmission lines including capacity, temperature, impedance, failure rate, and on the operation reliability indices of power systems are analyzed. Case results verify the correctness of the proposed operation reliability model of transmission line and the operation reliability evaluation method of power systems considering meteorological factors.

Abstract:DC line protection is the key to the development of flexible DC grids, and fault identification within 3 ms is required. Existing scholars have proposed a digital twinning based flexible DC line protection method with high speed and sensitivity advantages. However, its reliability is easily affected by transformer measurements, which may lead to protection misoperation. The requirements of DC control protection equipment can be hardly meted by existing bad data detection methods with insufficient accuracy and rapidity. Therefore, in order to improve the reliability of this protection method, a self-correcting method of bad data based on the moving average method is proposed in this paper. The predicted value of the measured data is obtained by using the moving average method according to the time sequence characteristics of the steady change of the measured data, and the bad data is detected and corrected by comparing the predicted error and the actual error, without iterative calculation and pre-trained model. The results of a simulation test using the four-terminal flexible DC grid show that the proposed method has higher accuracy and rapidity with good error correction performance than any existing method does, and it can be adapted to the protection method and improve the anti-interference capability as well as the reliability of protection in an effective manner.

Abstract:The controlled-type soft-switching technology based on critical conduction mode (CRM) is an effective way to achieve zero voltage switching (ZVS) of switches. However, the high reverse inductor current of the conventional constant boundary conduction mode (CBCM) method leads to high conduction losses. In this paper, the single-phase three-level neutral point clamped (3L-NPC) inverter is taken as an example for analysis, and a minimum reverse inductor current control is proposed. Firstly, the requirement for achieving ZVS of switches is quantitatively analyzed, and the model of the equivalent resonance circuit is built. Accordingly, the theoretical minimum reverse inductor current for achieving ZVS in the fundamental period is derived, which lowers the conduction losses of switches. Then, by establishing the power losses analysis model of the 3L-NPC inverter, the power losses of the proposed scheme and the conventional CBCM method are calculated and compared in detail. Finally, a 1 kW single-phase 3L-NPC inverter prototype is built, and experimental results verify that the proposed method features lower conduction losses than that of the CBCM method. The maximum efficiency of the 3L-NPC inverter is increased by 0.5 percentage point.

Abstract:The frequency stability of the wind power grid-connected system may be improved by the participation of wind turbines in frequency regulation. However, it is difficult for the existing droop control to coordinate the frequency response characteristics and the operating state of the wind turbine. An adaptive droop control strategy is proposed to make full use of the rotor kinetic energy to participate in frequency regulation and ensure the stable operation of the wind turbine considering the rate of change of frequency (ROCOF) and the rotor kinetic energy. Firstly, a coupling function between the droop coefficient and ROCOF is established by the piecewise function with the intervals of ROCOF according to the system frequency, which can be released to ensure more energy from wind turbines at the initial stage of the disturbance. In this condition, the frequency drop is slowed down due to the support of the wind turbine for frequency regulation. Besides, to avoid the over-deceleration of the wind turbine and secondary frequency drop, an influence factor on the rotor speed is introduced to adjust the droop coefficient according to the operating state of the wind turbine. Finally, a wind-thermal combined system simulation model is built on the MATLAB/Simulink platform to verify the effectiveness of the proposed control strategy. The simulation results show that the proposed strategy can effectively apply the rotor kinetic energy of the wind turbine to improve the frequency response characteristics of the system while ensuring the stability of the wind turbine speed.

Abstract:Due to its multi-energy coordinated operation and absorption of clean energy for power generation, the micro-energy network has the broad application value in the context of ‘double carbon’ goal and new power system. However, with the increasing scale of wind power and photovoltaic power access, the uncertainty of wind power and photovoltaic power output would inevitably affect the stability of the operation of the micro-energy network. Therefore, the uncertainty and correlation of wind power and photovoltaic power generation output are fully considered in this paper, and a two-layer optimal allocation method of micro-energy network considering market planning is proposed. Firstly, based on Wasserstein distance index, the probability distribution is optimally discretized, and a joint distribution function is established using Frank-Copula function to generate a typical daily output curve. Then, considering the planned economy of electricity market and natural gas market, a two-layer optimal configuration model of micro-energy network is established to realize the close coupling between equipment configuration and system operation. Finally, based on the measured data of a micro-energy network in a certain area, a typical wind and solar output curve is generated for optimization calculation. The results show that the two-layer optimal configuration method can reduce the annual cost of the micro-energy network and increase the inclusiveness of the wind and solar limit information, thus improving the economy and stability of the system operation.

Abstract:To explore the bidding strategy of a virtual power plant (VPP) that combines economy and low-carbon, a two-level Stackelberg game bidding model is proposed from the point of view of VPP as the price maker, which takes into account carbon trading and risk to participate in the energy market and spinning reserve market. Taking VPP including wind power and photovoltaic as the research object, the base-line method is firstly used to allocate a free carbon emission quota to VPP, and the carbon trading model of VPP is established. Secondly, a two-level bidding model based on the Stackelberg game theory is established. The upper-level is the VPP operator participating in the carbon, energy power, and spinning reserve markets. The lower-level follower is the electricity market operator. At the same time, conditional value at risk (CVaR) is used to transform the upper-level problem into a multi-objective optimization problem taking risk into account. Finally, the genetic algorithm and solver are combined to solve the problem. The example shows that the model can provide economical and low-carbon bidding strategies in multi-market environments, and it can also provide output plans for different markets. The influence of different market types, carbon trading and different risk aversion coefficients on VPP bidding results are analyzed, which provides a new way to improve the revenue of VPP operators.

Abstract:In order to solve the problems such as significant frequency fluctuation and slow dynamic response under load disturbance in islanded microgrid which has a diesel generator (DG) as the primary source, an auxiliary frequency regulation method is proposed to improve the dynamic response process of the system. Firstly, the model of diesel generator system is established to analyze the impact on transient state due to main parameters, including inertia and response delay. Secondly, the energy storage system (ESS) with auxiliary frequency regulation based on droop control is proposed, and the engineering parameter and ESS's power margin design method are given. In addition, a small signal model of the DG-ESS system is established to prove the stability of the system. Finally, the experimental platform of DG-ESS microgrid is built, and the results verify that the proposed control strategy effectively reduces the transient frequency fluctuation under the change of static or motor load.

Abstract:Overheating fault caused by poor contacts of plum blossom contact is one of the typical defects of switchgear and gas insulation combined appliances. The study of current-carrying friction characteristics and remaining life prediction of plum blossom contacts in different gas medium is of great theoretical and applied significance for the accurate evaluation of the health status of this type of equipment. The test samples are processed according to the real plum blossom contact materials and process. The current-carrying tests in air, N₂ and SF₆ gas medium are carried out by using a self-built current-carrying table, and the contact resistance degradation curves are obtained. The analysis of the surface morphology of the contact shows that the sample surface shows obvious adhesive wear under the effect of large current and mechanical displacement, the gas medium affects the degradation process of the contact by changing the morphology and element composition of the wear region. The contact resistance failure threshold is determined according to the V-T relationship of the contact. The Savitzky-Golay filter is used to smooth the original curves of the contact resistance. The life of plum blossom contacts in three gas medium is predicted by using Wiener degradation model updated by Bayesian. The results show that the model is more accurate than the conventional Wiener model. The remaining life of the plum blossom contact in the air is 9 500 cycles. The life of the plum blossom contact in N₂ is 61 300 cycles. The life of the plum blossom contact in SF₆ is 32 200 cycles.

Abstract:In order to effectively reduce the damage of short-circuit electromagnetic force to cables and cleats, and scientifically and reasonably determine the loop spacing of cables, this paper adopts electromagnetic coupling finite element method to calculate and analyze the short-circuit electromagnetic force of high-voltage large-section cables. The finite element model of three-phase short-circuit electromagnetic coupling for single-loop and double-loop cables in flat formation and trefoil formation is established, and the changes of short-circuit electromagnetic force in time and space are calculated and analyzed. The equation that the maximum value of three-phase short-circuit electromagnetic force varies with the distance between two circuits is obtained. The results show that the short-circuit electromagnetic force of double-loop cables satisfies a specific functional relationship with the loop spacing, and there is an inflection point of the change slope of electromagnetic force, which can be used to determine the loop spacing. This study can provide theoretical and technical support for the theoretical research of cable short-circuit electromagnetic force and engineering design or construction.

Abstract:An energy harvesting power supply based on AC electric field induction is introduced, which is used for power supply of online monitoring devices for transmission lines. However, the output power of this type of power supply is relatively low, which cannot meet the electricity demand of online monitoring devices. Therefore, based on theoretical analysis, a coaxial dual column inductive electrode structure is designed in the article to improve the output power of the power supply. A simulation model is established using COMSOL Multiphysics software and electromagnetic field simulation analysis is conducted. The simulation results show that the designed inductive electrode can generate an induced current of over 3.35 mA at a voltage level of 10 kV. To verify the accuracy of the simulation results, an experimental verification is conducted using a self built testing platform in the laboratory, and the experimental results are compared with the simulation results. The experimental results show that when the value of load resistance is higher than 5 kΩ, an energy harvesting power supply can output continuous power exceeding 630 mW, which is basically consistent with the simulation results. Through the research in the article, the problem of low output power of electric field induction energy harvesting power supply has been successfully solved, and a new design concept and technical solution of energy harvesting circuit has been proposed.

Abstract:The adjacent interharmonic pairs or single adjacent interhamonic symmetrical to the fundamental/harmonic component can be caused by the large-scale grid connection of renewable energy. It is usually necessary to confirm the type of adjacent interharmonics to build an accurate calculation model when estimating the frequency components in electric signal. However, the spectrum leakage interferences caused by the grid frequency deviation makes that requirement hard to achieve. Towards the problem, an effective criterion to confirm the type of the adjacent interharmonics and an accurate estimation method of the harmonics and adjacent interharmonics based on the criterion even there is frequency deviation are proposed. Firstly, the spectrum leakage interferences are suppressed to obtain the adjacent interharmonic spectral lines by analysing the relationship of the frequency deviation and spectral lines. Then, the type criterion of the adjacent interharmonics is proposed by comparing the spectrum difference between each type of adjacent interharmonics. Finally, the spectral line equation for each type of adjacent interharmonics are constructed according to the confirmed type to accurately estimate the harmonics and adjacent interharmonics. Even there is frequency deviation, the type of the adjacent interharmnics can be confirmed accurately and a more precise estimation of the harmonics and adjacent interharmonics can be achieved with the proposed method. The accuracy of the proposed method is verified by the simulation.

Abstract:Under the ‘dual carbon’ goal, the transformation of the power system is accelerating, and wind power prediction technology is of great significance to the construction of a new power system with a high proportion of new energy. In order to improve the accuracy and robustness of wind power prediction, an numerical weather predicition (NWP) implicit correction algorithm based on multi-objective collaborative training is proposed. Firstly, the necessity of NWP correction and the problems of the two-step prediction method based on NWP explicit correction are analyzed. Then, aiming at the problems existing in the two-step prediction method, the optimization method based on multi-objective collaborative training uses the neural network to perform NWP implicit correction, train the model in an end-to-end manner, and realize the functions of NWP implicit correction and wind power prediction at the same time. Combined with the measured data of a wind farm, the specific calculation case analysis proves that the proposed algorithm has an improving effect on short-term, medium- and long-term wind power prediction. In addition, the algorithm only requires one network and avoids secondary calculation, saving computing and storage costs.

Abstract:With more and more renewable generation integrated into power system, the number of thermal units decreased and their reserves are not enough, so it is necessary to utilize the reserve capacity from third-party entities. Two kinds of day-ahead power market clearing mechanism incorporating reserve ancillary service of third-party entities are proposed and compared in this paper. In the first clearing mechanism, energy and reserves provided by the thermal units are jointly clearing at first and then the reserves provided by third-party entities are cleared. In second clearing mechanism, joint market clearing mode for electric energy and reserve on both sides. At first, based on the aspects of market development stage and shortage of reserve, the advantages and disadvantages of 2 clearing modes are analyzed. Then the clearing models are presented and the impacts of third-party entity on bidding results in 2 modes is analyzed. Furthermore, the costs and benefits of different market members are analyzed. Finally, the numerical results of the modified IEEE 30-bus system are given to show that the reserves of third-party entities can be purchased fully to decrease total costs of generation and reserves under the second clearing mechanism.

Abstract:As an important part of the national security system, energy security is of great importance to the construction of a modern and powerful socialist country in China. Based on the definition of energy security by the International Energy Agency, an evaluation index system of energy security is constructed from three dimensions:production guarantee capacity, import diversification degree and economic affordability. On this basis, analytical hierarchy process (AHP)-entropy weight method-improved technique for order preference by similarity to an ideal solution (TOPSIS) evaluation method are combined to obtain relatively fixed energy security index which is not affected by evaluation range and evaluation period. Through the comprehensive evaluation of energy security levels of China and the United States from 2015 to 2021, the change trends of energy security indices of China and the United States are obtained. The results show that China has vast improvement in energy supply guarantee capacity and energy consumption cost, and still needs to further improve China's energy production capacity, accelerate the construction of a new energy system, enhance energy self-sufficiency, expand the scale of international energy cooperation, reduce energy consumption costs, and ensure energy security.