Abstract:The interconnection between asynchronous power systems can promote the consumption of clean energy andthe optimization of resources by high voltage direct current ( HVDC) transmission systems. However, the frequencystability problem becomes a potential threat to power systems, which can be considered as a major challenge of thesending power grid. A novel total transfer capability assessment model (FTTC) of H VDCs is proposed by taking intoaccount the frequency stability of sending- end grid. Considering the inertia reserve requirements and frequency stabil-ity constraints of the sending-end grid, the model can produce maximum acceptable TTC of HVDC tie-lines. Moreo-ver, a optimization model of the HVDC tie-line operation is developed by considering the coordination of the HVDCcorrective control and the generation trigger after contingency. Finally, a case study of the modified RTS-79 system isused to verify the effectiveness of the proposed method.

Abstract:The line loss of distribution network is an important assessment evaluation index of electric power company. For complicated circuits and many branches of distribution network, it is difficult to calculate the cost and benefit brought by refining line losses. Under this background, aiming for the comprehensive energy conservation and loss reduction of distribution network, the life cycle cost and energy conservation and loss reduction benefit indexes are put forward from the aspects of time dimension, component dimension and expense dimension on the basis of the spatial-temporal elements of energy-saving features. The comprehensive energy conservation and loss reduction results are evaluated to derive the optimal design scheme of distribution network comprehensive energy conservation and loss reduction by the improved VIKOR method based on AHP and CEDEA analysis. Then the feasibility and validity of the proposed evaluation method are verified and validated by the actual distribution network in China.

Abstract:For the purpose of popularization in application scenarios, this paper presents an economic dispatch method for the integrated energy systems considering the uncertainty of photovoltaic output and power load. Firstly, a mixed-integer nonlinear probability model for integrated energy systems is established. Secondly, the nonlinear model is segmented and linearized by the special sequence set (SOS-2) method. Then, the symmetric sampling strategy-based traceless transformation method is used to perform deterministic transformation, and to reveal the underlying probabilistic information. Finally, the calculation results of MILP and particle swarm optimization are compared and analyzed. The PuLp open-source modeling framework is utilized to model and solve the potential problems in a demonstration integrated energy project. The results show that, on one hand, the traceless transformation method which is based on the symmetric sampling strategy is efficient, and the generated probability density distribution can reveal the underlying probabilistic density distribution so that the operation information can be available for the system operator. On the other hand, compared with the particle swarm algorithm, the efficiency of solving MILP problems via the PuLp open-source framework can increase several times, which is suitable for real applications.

Abstract:As the voltage level of the power system increases, the DC component influence on the breaking capacity of the circuit breaker has been closely concerned. However, there are few studies on the calculation method of DC component of short circuit current in the circuit breaker branch. In this paper, a calculation method is proposed for the DC component of branch short-circuit current and its decay time constant based on transfer impedance. Firstly, in the case of knowing the impedance matrix of the original network and considering the characteristics of the network when the system is short-circuited, the network is transformed based on the principle of forming the node impedance matrix using the branch-adding method.Then, the transformed network can easily calculate the DC component of the short-circuit current and the decay time constant of each branch. In addition, a unified algorithm for forming the node impedance matrix is concluded when considering the changes of the network topology. It has the advantages of fast calculation speed, clear physical concept, and it is easy to program so as to achieve scanning calculations. The accuracy of the proposed method is verified by EMTP simulation. Compared with results from PSS/E, the superiority of the proposed method is also verified.

Abstract:The construction and component of integrated energy system has changed with fast development of new energy emergence. Under the background, a multi-objective scheduling model is proposed which takes into account source and load uncertainty and multiple types of energy storage demand response. Firstly, an uncertain model of distributed wind power and electric heating gas load is established. The multiple types of energy storage incentives and price demand response models of electricity, gas and heat are then determined. Secondly, the purchase cost of electricity and gas energy, the cost of abandoning wind and the cost of environmental pollution are chosen as the objective of multi-objective optimization. Under the uncertainty of source & load side and the response of multiple types of energy storage demand, the constraints of multiple types of energy power balance, conversion and coupling between cooling and heating systems are considered to establish a source, grid and storage multi-objective optimal scheduling model for integrated energy system. Then the multi-objective Particle Swarm Optimization algorithm is utilized to optimize the constructed model. Finally, a park energy system is simulated. The results show that the proposed optimization is effective. The model can provide a new strategy for the mutual energy service of the park's comprehensive energy source storage and demand.

Abstract:The continuous improvement of wind power penetration has greatly reduced the consumption of fossil fuels and greenhouse gas emissions. However, the uncertainty and intermittent nature of wind power make the solution to the traditional unit commitment infeasible. In order to describe the uncertainty of wind power generation, this paper introduces an ellipsoid ambiguty set based on moment information, and applies the chance constraint to the unit combination model to change the power balance constraint into a soft constraint. Then, the distributionally robust optimization method is utilized, and the unit commitment model is reformulated into a mixed integer linear programming problem by linearization method. In addition, two improved methods, the limiting the distribution of ambiguty set with unimodality and adjusting confidence level according to time, are proposed to improve the economics of the model. Finally, case analysis and numerical results verify the practicality and feasibility of the proposed model and method.

Abstract:Aiming at the problem that the current market power evaluation of generators ignores the uncertainty and ambiguity of the index data, a comprehensive evaluation method of generators' market power based on the combined weighting and trapezoidal cloud model is proposed. Firstly, the method establishes the market power evaluation index system of power producers, and uses the combination weighting method to obtain the weights of each index. Then, the trapezoidal cloud model is utilized to calculate the membership degree of each index to each level, and the evaluation degree vector is obtained. Combining the combination weighting with the trapezoidal cloud model, the comprehensive evaluation of the power generation market power is realized according to the principle of maximum membership degree. The feasibility and accuracy of this method are verified by the case study.

Abstract:Heavy wind is usually accompanied by rainfall, the strong wind and rain load will have negative impacts on the reliability of transmission lines, which could result in windage yaw discharge accidents and leads to significant losses of power enterprises and the consumers. Therefore, a probability distribution model is proposed in this paper based on a three-dimensional joint distribution model, including the wind speed, wind direction and rainfall intensity. Firstly, the distribution characteristics of wind speed, rainfall intensity and wind direction are described by the extreme wind speed, extreme rainfall intensity and wind direction angle. Then, considering the probability of wind speed, rainfall intensity, and direction, Archimedean Copulas function is utilized to build the multidimensional joint probability distribution, which can further be used to build the real-time probability prediction model of windage yaw discharge of transmission line. Finally, the prediction model is realized by Matlab programming, and the effectiveness of the proposed probability prediction model for the windage yaw of transmission lines is verified through the realistic power systems data.

Abstract:Aiming at the differences in maximum wind speed and wind pressure distribution between typhoon wind field and conventional wind, this paper conducts wind tunnel test and wind-induced response analysis of 1 000 kV transmission tower in southeastern coastal areas. Based on the real data of typhoon, the wind profile index is fitted and the turbulence intensity is modified in landform category A for more fulfilling the real wind field. Through the high-frequency force balance test on 1 000 kV transmission tower, the average wind coefficients vary with different wind direction and the shape coefficients of whole tower, tower head and tower body is acquired. By using the harmonic wave superposition method to simulate turbulent wind velocity of all nodes in tower, equivalent static wind loads and wind vibration coefficients in varies operation conditions are obtained. Then, the sensitivity analysis of the structure damping, structure frequency, tower acceleration and maximum base load of the 1 000 kV tower in typhoon wind field is performed. Results indicate that acceleration response is sensitive to damping. Displacement response and acceleration response are sensitive to natural frequency.

Abstract:The stator winding-to-ground capacitor is an important parameter of generator, and its accurate measurement can provide important basis for the stator grounding protection scheme. On the background, the fault branch of generator stator winding is divided, and the internal structure of voltage transformer is studied. Then, a real-time measurement method for the stator winding to ground of generator based on the injected current signal is proposed. Firstly, the current signal is injected from the low-voltage triangle side of voltage transformer for the generator and flows through the high-voltage side of the voltage transformer and the capacitance of the winding to ground. Secondly, the low-voltage triangle-side voltage and the low-voltage star-side voltage of the voltage transformer is measured respectively. The simplified circuit and the injected current signal and the measured voltage signal calculate the stator winding capacitance to ground. This method avoids the influence of the short-circuit impedance of voltage transformer on the measurement results. The PSCAD/EMTDC simulation and experimental results show that the measurement method is safe in process, simple in wiring, and accurate in results.

Abstract:Distributed energy and controllable load can be aggregated in virtual power plants to participate in the electricity market, ancillary service market and carbon trading market. In this paper, an operation model including wind energy, micro gas turbine, photovoltaic, electric vehicle and battery energy storage system are constructed in virtual power plant. Through further analysis the carbon-electricity integrated market characteristics, a carbon-electricity integrated trading strategy model of the virtual power plant is constructed, and the objective function and operational constraints of each unit are listed. Simulation results show that the introduction of carbon trading mechanism can significantly reduce the bidding output level of high-carbon units, and then affect the energy output structure and bidding strategy in virtual power plants. The simulation results validate the effectiveness of bidding strategy for virtual power plants with integrated carbon-electricity transaction.

Abstract:In order to diagnose the typical mechanical looseness and insulation fault of gas insulated switchgear (GIS) in operation, this paper sets up experimental platform to simulate insulator loosenesses, tip discharges and suspended particle discharge faults. Acceleration signals of GIS vibration in different fault patterns are collected. Then the Hilbert marginal spectrum can be obtained by analyzing vibration signal using virtue of Hilbert-Huang transformation (HHT) after the modified ensemble empirical mode decomposition (MEEMD). The mechanical looseness or partial discharge of GIS can be judged through the energy of signals in different frequency bands. The wavelet transform is applied to calculate discharge factors, which reflects the characteristics of different types of partial discharge fault. Results obtained by experimental data analysis indicate that, the above methods can distinguish the faults of GIS and reflect the characteristics of partial discharge faults effectively. Thus providing a useful method for diagnosis of mechanical looseness and insulation fault of GIS.

Abstract:This paper proposes a fault diagnosis method for distribution automation switch cabinet based on the Fuzzy C-means (FCM) clustering and gray correlation, aiming at exploring the effect of the distribution automation switch cabinet fault diagnoses on the safety and reliability of distribution networks. Firstly, through various telemetry information, the membership function is calculated, the weight is determined by the AHP-entropy method, and thus the evaluation of hierarchical fuzzy operations of the distribution automation switch cabinet state can be completed. Moreover, according to the typical information of the distribution automation switch cabinet faults, the gray correlation analysis combined with the FCM method are applied to diagnose six common types of fault, which can greatly reduce the calculation complexity and ensure the diagnosis accuracy simultaneously. A case study based on the distribution automation switch cabinet faults in a certain area of Beijing are presented, where the proposed method is compared with the traditional gray correlation analysis. The status evaluation and fault diagnosis results show that the accuracy of the proposed method can reach 90%, which can support the fast, accurate and objective real-time monitoring of distribution automation switch cabinet status and the further development of smart grid automation.

Abstract:For three-phase voltage-based PWM converters, finite set control model predictive current control (FCS-MPC) is a kind of novel and high-performance control strategy. However, there exist the disadvantages of large amount of calculations and unfixed switching frequency for NPC three-level converters. Thus, in order to overcome the disadvantages, a simplified fixed switching frequency PWM strategy based on MPC is proposed in this paper. The algorithm does not need to perform rolling calculations on 27 three-level vectors, and directly obtains the optimal target voltage vector by calculating the cost function containing the current deviation and the midpoint potential deviation, which greatly reduces the amount of calculation. Experimental results show that the proposed simplified modulation strategy can achieve fixed-frequency control and mid-point potential adjustment of the NPC three-level converters, and has good dynamic and static performance.

Abstract:n view of the operating characteristics of PET when the AC grid voltage sags, this paper proposes a LVRT strategy for bidirectional PET. During the voltage drop, the injected reactive power is automatically adjusted according to the positive sequence voltage drop to assist the grid voltage recovery. According to the active power of PET before the voltage sag, the level and direction of the active power are maintained as far as possible without exceeding the current limit to reduce the disturbance to the AC/DC grids. In the case of asymmetric voltage drop, the negative sequence current suppression is regarded as the supplementary method to ensure the symmetry of the injected grid current. Finally, the effectiveness of the proposed LVRT strategy is verified by the case study.

Abstract:Considering the differences between the source-grid-load coordination in different operation modes of power grids, a source-grid-load coordination identification and control method are proposed in this paper based on trend theory. Firstly, the directional derivative is used to investigate the relationship between the overall power angle safety index and the load balance degree of the grid. According to the difference analysis between the equilibrium of power grid operation and the load balance degree of path, the imbalance between source-grid-load can be identified. Then, based on mapping elasticity potential energy model of power grids, the physical analysis of local grid can be performed. It is found that the minimum singular values of the Jacobian matrix and the output or load of power generation have the same trend. Finally, a source-grid-load coordination identification and control method are proposed. The IEEE 9-bus system is selected to verify the theorical result that the overall power angle security index and the path load balance have the same trend and has no relationship with the balance of power grid operation. The IEEE 39-bus system is used to verify the effectiveness of the source-grid-load coordination control method.

Abstract:In the open power market, there is a surplus of producers (generators) or consumers (large consumers), which will directly affect the pricing strategy of large consumers direct-power-purchasing. Under the background, with the Rubinstein model, this paper constructs the bargain models of pricing of large consumers direct-power-purchasing in single generator-various consumers and various generators-single consumer transaction under the circumstance of incomplete information and asymmetric pressure, and then gives the equilibrium profit of generators and large consumers. The result indicates that the profit of both parties is related to the discount rate and the estimation of the reserve price. Generators and large consumers can calculate and evaluate the profit of the agreement with different objects with the conclusion of the paper, and choose the cooperative object reasonably. The model and conclusion are also suitable to the bargaining game under the circumstance of the producer (generator) surplus or the consumer (large consumer) surplus in the market.

Abstract:The scientific location of the VPSS is of great significance to improve the efficiency of emergency repair and reduce the loss of users. To reflect the impact of the load-loss level on the selected location of the power supply, this paper analyzes the service characteristics of the VPSS and compares whether the corresponding repair distance and cost change when the two conditions of the load-loss level are considered to establish the power substation emergency repair multi-objective model. After that, the pareto-dominated multi-objective particle swarm optimization algorithm is adopted to solve the model and the crowd distance sorting is utilized to update the non-inferior solution set and determine the final optimization scheme. Finally, the calculation example shows that the weighted distance and the comprehensive cost are better than the case of not considering the load level when the load level is taken into account. The results show that the multi-objective site selection model for emergency repairs constructed and the proposed solution method have certain guiding significance for the emergency repair strategies of the VPSS.

Abstract:The Z-source inverter has been widely used because it overcomes the shortcomings of the conventional voltage-type inverter that can only be stepped down and cannot be boosted, and must be added to the dead time. This paper proposes a single-cycle and LCL active damping control scheme to control the grid connection of three-phase Z-source inverter. Compared with the traditional PWM control, the singlecycle control has the advantages of simple structure, low-harmonic content and high output power quality while the LCL filter performs better than the LC and L filters, but the resonance peak reduces system stability. Therefore, the active damping method is adopted and the filter capacitor current loop and the grid-connected current loop are introduced to form a double loop control structure to suppress the adverse effect of the resonance peak on the system stability. Finally, simulations under Simulink verify the feasibility and effectiveness of the proposed scheme.

Abstract:Influenced by the power grid structure, generator parameters, and load characteristics, the post-fault frequency, and voltage at nodes reflect spatial distribution characteristics, resulting in different effects of the load shedding and load recovery strategy performed at different nodes on the system stability. Besides, the influence of the interaction between coupled electrical quantities of frequency, voltage, active power, and reactive power should be considered to improve the efficiency of the load regulation. In this paper, a novel load-control-sensitivity based low-frequency and low-voltage load shedding and load recovery strategy is proposed, which considers the spatial distribution characteristics of frequency and voltage, and the interaction of coupled electrical quantities. A time-domain simulation based on a provincial power grid shows the proposed load regulation strategy improves the power grid stability during the periods of both load shedding and load recovery.

Abstract:In the process of porcelain insulator infrared detection, the missing judgment may happen due to the temperature difference of a zero value insulator failing to reach the threshold specified by the relevant standards. In order to improve the accuracy of the infrared detection, this article proposes a new method combining the iron cap temperature difference threshold method and the temperature gradient correlation coefficient method. Infrared detection field experiment of disc suspension insulators in substation is conducted.The test results show that under the suitable environment conditions, the zero value insulator can be confirmed by studying the dispersion characteristics of temperature gradient distribution correlation coefficients, while the result of difference threshold method for iron cap temperature is unstable. Thus, the missing judgment probability can be decreased. By contrast, under the unsuitable environment conditions, the dispersion of the temperature gradient correlation coefficient increases with the increase of the ambiguity for the temperature difference threshold method. Reduction occurs on the discrimination degree of the temperature gradient correlation coefficient between the normal insulator string and the zero value insulator string.

Abstract:The fault-generated traveling waves will bounce back and forth between the beginning point and the discontinuity points on the distribution line. The energy of frequency components in the corresponding characteristic which are related to such reflected paths will be significantly higher than that of other frequency components. The traveling waves are decomposed into different frequency bands according to the characteristic frequencies in discontinuity points of the distribution line which includes the branch points, the load terminal points, and the connection points of overhead lines and underground cables. The fault scenarios can be divided into three categories: the fault section around the beginning point, fault section between one and another branch point of the main power wire, and the fault section between one branch point and the terminal load. The energy spectrums within the divided frequency bands vary significantly in different scenarios, and thus these energy spectrum differences can be used to identify the location of fault sections. Once the fault occurred, the similarity between the energy spectrum of the recorded fault-generated waveforms and that corresponds to the typical positions was analyzed. The fault section can then be identified based on the constructed fault location criteria. Since the proposed method is not affected by of the following parameters of the distribution line, including the actual length error, incomplete transposition, and heterogeneous surface, the proposed method provides high reliability in the fault location.The simulation results have proved the accuracy and feasibility of the proposed fault section location method.

Abstract:The short-circuit capacities of buses are key parameters required for the operation and control of power systems. On-line measurement of short-circuit capacity is an effective method to achieve the observability and controllability of power grids. This paper proposes an on-line measurement method to estimate the short-circuit capacity of the grid bus according to the bus voltage fluctuation triggered by switching the parallel capacitors. Firstly, based on the circuit model of substation and the substitution and superposition theorem of the linear circuit, an accurate calculation method for measuring the bus short-circuit capacity is derived. It reveals that the phase-angle differences of the voltage vector caused by the capacitor switching is a crucial factor affecting the measurement accuracy, which is an important improvement compared with the approximate formula of previous studies. The proposed measurement method has high precision. The proposed iterative calculation method for the voltage vector phase difference is more convenient and reliable than the direct measurement. The effectiveness of the proposed on-line measurement method is tested on a detailed substation bus model on Matlab/Simulink. According to the tests in various situations, the simulation results show that measurement error of the proposed method is less than 1%.

Abstract:With the emergence of global environmental and climate issues, the advantages of clean energy, such as hydropower, wind and solar energy are highlighted. In a power system with high penetration of hydropower, the cascaded hydropower station needs to serve the peak load and the base load at the same time. How to coordinate the peak-shaving optimization operation of the cascaded hydropower station and the uncertain wind-photovoltaic power stations is of great significance. This paper considers the uncertainty of the wind and photovoltaic power stations, and proposes a coordinated peak-shaving optimization operation strategy of cascaded hydropower stations and wind photovoltaic power stations. On one hand, the adjustment capability of the cascaded hydropower stations is utilized to increase the accommodating generation of the wind-photovoltaic power stations, and the load fluctuation is minimized as much as possible to reduce the peaking pressure of the thermal power units. On the other hand, the nonlinear water-to-power conversion function is linearized to facilitate the model solution. The simulation analysis results of a system with high penetration of hydropower verifies the correctness and effectiveness of the proposed optimization strategy.

Abstract:Aiming at the problem that the efficiency of magnetic coupling resonance-wireless power transmission (MCR-WPT) is reduced due to load changes during the transmission process, an efficient inverter is designed.The improved structure can significantly widen the load adaptation range of MCR-WPT system under high efficiency power transmission. After analyzing the relationship between the load and input impedance of MCR-WTP system and using the E-type inverter as the power supply, the inverter of the MCR-WPT system is fully soft switched by changing the structure of inverter.The operation efficiency of MCR-WTP system keep on the high level. Simulation results verify the effectiveness of the proposed structure.