Abstract:The development of contemporary electrical power system causes the frequency characteristics more intricate and makes frequency stability more significant. Therefore, power system frequency problems is more and more significant, research on system frequency characteristics emerge and are applied to make system work more securely and steadily. Firstly, it reviews the process and causes of domestic and overseas frequency events in recent years from perspective of frequency dynamic characteristics. Then, combined with the characteristics of modern power system, the impacts on the frequency characteristics of large-scale power electronic interface are analyzed from multiple perspectives. After that, utilizing the single machine model to analyze the influence of generator parameters on multiple interactions of power system frequency dynamics, and summarize the related research and application of system inertia estimation, frequency regulation performance and frequency dynamic characteristics monitoring. Finally, some problems and directions worthy of exploration and research in the field of system frequency characteristics are proposed.

Abstract:After the large-scale renewable energy is connected to the grid, the traditional reserve system has the problem of poor economy and reliability when dealing with the bilateral randomness of the power supply side and the load side of the power system. To ensure the safe and stable operation of the system, it is necessary to study the configuration and optimization methods of the system reserve. This paper expounds the impact of large-scale renewable energy grid-connected on power system reserve, fully explores various types of reserve resources of the system and expounds the three types of reserve resources of source, load and storage and their research status as cross-regional reserve of interconnected power grids, in order to deal with various long-term, large disturbance, large-capacity power shortage risk events faced by the system under large-scale renewable energy grid-connected environment. This paper analyzes in detail the various methods for configuring the reserve capacity of power systems, reserve dispatching optimization, and points out the research direction of the next stage, which provides reference for the research on the reserve configuration and optimization method of the power system in the context of large-scale renewable energy grid connection.

Abstract:With the urgent demand for dynamic wireless charging capabilities with mobile loads, dynamic wireless power transfer(DWPT) has gradually entered people′s field of vision and become a new research hotspot. The article firstly compares the main structure of the existing dynamic wireless power transfer system and its advantages and disadvantages. Secondly, the key technologies of DWPT are discussed from the aspects of high frequency inverter circuit, compensation network, electromagnetic coupling mechanism, coil array switching method and electromagnetic compatibility. Finally, combining the research results of scholars at domestic and abroad in recent years, it summarizes the progress of the engineering application of dynamic wireless power transfer, and the future development trend of dynamic wireless power transfer is predicted, which shows that dynamic wireless power transfer has broad application prospects in the fields of AGV, substation intelligent inspection robot, mobile electromechanical equipment, rail transit and electric vehicle.

Abstract:With the continuous improvement of the voltage level and transmission capacity in voltage source converter-high voltage DC(VSC-HVDC), the VSC-HVDC transmission technology using voltage source converters has been widely used in China, and its impact on the power grid is further enhanced. Firstly, based on a newly developed VSC-HVDC control system which takes into account fault crossing strategy in the electromechanical transient simulation program PSD-BPA, the differences of disturbance recovery characteristics of weak receiving-end power grid under different HVDC interconnection schemes are compared. On this basis, the influence of VSC-HVDC control mode, transmission power and fault traversal control strategy on dynamic active and reactive power characteristics of converter station is studied. Finally, an optimization scheme is proposed to solve the voltage stability problem of Tibet weak receiving-end power grid. The simulation results show that optimizing the key parameters of VSC-HVDC fault traversal control can improve the recovery performance of weak receiving-end grid after fault.

Abstract:As a means to solve the problems of low reliability and high maintenance cost of flexible AC transmission system (FACTS) devices, distributed flexible AC transmission system(D-FACTS) has become a research hotspot in FACTS area with its flexible and rapid features. Based on the structures and principles of distributed series reactor (DSR) and distributed static series compensator (DSSC), a distributed power flow controller(DPFC) scheme is proposed in this paper. The DPFC sub-control unit makes the impedance adjustment of transmission lines both continuous and hierarchical. At the system level, the overall switching and continuous control of the sub-units are carried out by centralized control. Based on the scheme, the structure of a "DSSC+" type DPFC system is proposed, which has the feature of centralized control and distributed installation. The system control strategy based on online negative feedback is studied in detail. The simulation results verify the effectiveness of the strategy which can provide theoretical support for engineering applications.

Abstract:When there is unbalanced AC voltage in Wind farm with voltage source converter-high voltage direct current (VSC-HVDC) integration system, the output power to the grid will fluctuate with twice of the grid frequency, and the grid voltage and current waveforms, even more, will be distorted, hence the power quality of the wind farm provided will deteriorate badly. To improve the performances of wind farm VSC-HVDC system under unbalanced AC voltage, a doubly-fed wind farm VSC-HVDC system with unbalanced AC condition is discussed in this paper. Based on the positive and negative sequence double dq decoupling shafting, and taking into account the influence of equivalent impedance of the relevant reactor and transformer, the unbalanced mathematical model of the wind farm VSC-HVDC system is established. Moreover, an improved control strategy is realized with the construction of the positive and negative sequence current setting. Finally, a DFIG wind farm VSC-HVDC system is implemented on the Matlab/Simulink, comparisons of the simulation results with and without the proposed control strategy show that the proposed improved control strategy can effectively suppress the fluctuation of output power and improve the waveform of grid voltage and current.

Abstract:For inland wind power, there are many problems with AC interconnection, and the double terminal voltage source converter based high voltage direct current(VSC-HVDC)system can not satisfy the requirements. Multi-terminal VSC-HVDC(VSC-MTDC)is the tendency of wind power integration. The coordination of adaptive DC voltage droop control and HVDC additional frequency control was proposed. The droop coefficient varies adaptively according to the value and direction of DC voltage change and the power margin of the converter station, optimizing power distribution between converter stations. And the additional frequency control is introduced into VSC-MTDC. Takeing full advantage of frequency adjustment capacity. Controller parameters are easy to set and simple. The simulation results show the effectively of coordinated control strategy.

Abstract:Considering the isolated power grid state changing between active power grid and passive power grid, an integrated control strategy is proposed for VSC station to supply power to isolated power grid. Based on the principle of VSC control, the difference of control modes of VSC-HVDC inverter in active and passive power grids is analysed. When isolated power grid changes from active power grid to passive power grid, the control mode of VSC station changes. Considering the stabilization of AC voltage and fast restoration of active power balance, two control strategies, angle following and active power priority, are proposed in the paper. A comprehensive control system is designed to ensure the minimum fluctuation of each electrical quantity of islanded power grid and DC system at the time of switching. The PSCAD/EMTDC simulation model of VSC-HVDC transmission system is established. The simulation results verify the validity of the integrated controller and control strategy proposed in this paper. The continuous and reliable power supply can be realized when the isolated grid state changes.

Abstract:LCC and VSC are essentially different in the steady-state model and working principle. When the two converters are mixed and applied to the same multi-terminal HVDC transmission system, the steady-state mathematical model and the power flow calculation method have also changed accordingly. Firstly, the steady-state model of the hybrid system is described. Then, based on the comprehensive consideration of the working principle and control mode of the two converters, the mathematical model of the power flow calculation of the hybrid system is derived. Furthermore, an alternating solution algorithm suitable for the hybrid system is proposed. Finally, the effectiveness and accuracy of the proposed algorithm are verified by a hybrid DC transmission system example and a hybrid three-terminal system.

Abstract:The online security and stability analysis data is composed of local state estimation data and data of other dispatching agencies issued by the superior. The power deviation of the regional tie line is caused by the synchronization of the time between the two. In this paper, a method of regional tie-line power flow adjustment is proposed for data splicing of internal and external networks. On the basis of aggregation of interconnection lines into contact channels and several nodes with greater sensitivity to the aggregate nodes, the adjustment quantity is obtained by solving the linear equations of aggregate node adjustment quantity and connection channel power flow deviation quantity. When the system is ill-conditioned, the problem of no solution of linear system is avoided by solving minimum norm least squares solution. This method can meet the requirements of speed and accuracy of online safety and stability analysis. The validity of the method is verified by the case of actual power system.

Abstract:Unified power flow controller (UPFC) provides a new control method for reactive power optimization in power system. Based on this, the UPFC steady-state model suitable for the new UPFC topology is first established and introduced into the reactive power optimization problem. Then, the objective function and constraints of the multi-objective reactive power optimization problem are defined by considering the constraints of the number of reactive power equipment actions, and a multi-objective reactive power optimization model considering UPFC is established. Then, a multi-stage method is proposed to solve the problem. In the first stage, the original problem is relaxed and normalized to unify the dimensions of multiple objectives; in the second stage, the Pareto optimal candidate solution set of the relaxation problem is obtained based on the normalized plane constraint method, and the selection method of the compromise solution is given. In the third stage, the integer variables in the compromise solution are reorganized based on the triangular penalty function method to obtain the optimal compromise integer solution of the original problem. Finally, the optimal compromise integer solution of the original problem is obtained. The actual equivalent system of Nanjing western power grid is tested by an example, which verifies the effectiveness of the algorithm and the application prospect of UPFC in reactive power optimization.

Abstract:To explore the applicability of traveling wave fault location in half-wavelength transmission line (UTL), based on the amplitude attenuation feature and the reflection and refraction characteristics of the travelling waves, the critical fault distance within which the single-end traveling wave method can be applied is estimated. Then, the impacts of traveling wave distortion on traditional traveling wave fault location in the UTL are investigated, and a novel fault location scheme applicable to UTL is proposed. In the scheme, an additional measuring point is mounted in the middle of the line, and the polarity of the current traveling wave at the point is used to detect the fault section. The single-end or double-end traveling wave method is selected to pinpoint the fault in the determined fault section. An UTL simulation model is constructed by PSCAD/EMTDC and various fault simulations are conducted. The results indicate that the critical fault distance for single-end fault location is about 100~240 km. The proposed scheme eliminates the dead zone of fault location and shows high accuracy. In addition, the fault condition, tuning network, and load condition barely have any impact on the scheme, which demonstrates its high reliability.

Abstract:This paper reviews the research status of existing parallel multi-terminal HVDC line protection, based on the topology of parallel multi-terminal HVDC system with HSS configuration, the line protection configuration scheme is proposed. In order to solve the selectivity problem of parallel multi-terminal HVDC line protection, the reason for the selective blind area of the existing multi-terminal main protection is analyzed in depth. It puts forward a new line protection method, line protection is divided into two segments, through adjusting protection setting value and adding auxiliary strategy, the selectivity problem under normal and faulty COM munication is solved. Finally, RTDS simulation tests prove that the proposed line protection method can effectively distinguish the faults within or outside the area and solve the selectivity problem, when a permanent fault occurs on one line in parallel multi-terminal HVDC system with HSS configuration, all HVDC terminals are not necessary to be stopped, only the fault line needs to be isolated and the remaining lines can continue to run. This proposed line protection method is suitable for parallel multi-terminal HVDC project with HSS.

Abstract:Aiming at the current situation of rapid application and development of the power system cloud platform, the power grid data analysis and processing resources are unevenly used and the power system data center structure is complex. The current cloud computing has unreasonable resource scheduling problems. So a method for cloud computing virtual resource allocation based on power analysis computing task division is proposed in this paper. This method introduces the concept of membrane computing, and divides the complex analysis tasks under the cloud platform into multiple categories according to the different time of resource allocation. The improved ant colony algorithm is used to achieve the optimal allocation of virtual resources according to the utilization rate of resources and computer energy consumption. The simulation results show that the resource optimization scheduling method based on task classification and ant colony algorithm is efficient and balanced.

Abstract:Aiming at the problems of complex fault location method, poor reliability and high equipment requirements of existing distribution network, a fault segment identification of distribution network based on zero-sequence current distribution characteristics is proposed by means of low-sampling and low-cost fault indicators method. Firstly, the composition and distribution characteristics of zero-sequence current in different locations and different neutral grounding modes of the line are analyzed qualitatively. Then the difference between the zero-sequence current amplitude of the healthy section and the fault section in each case is derived quantitatively, thereby constructing the fault segment identification criterion. Finally, combined with the zero-sequence current distribution feature and the clustering process, the fault line is determined by the ratio of the distance between the last two classes, and the fault segment is determined according to the last two types of boundary numbers of the fault line. The method is simple and effective. The 10 kV low-current grounding system is built in PSCAD/EMTDC for simulation verification. The results show that the proposed method has nothing to do with the initial phase angle of the fault and has strong anti-interference ability, and is not affected by the fault resistance and fault location.

Abstract:Adjustable regenerative micro-turbine has many complex characteristics, such as multivariable, strong coupling, non-linearity, time-varying parameters and so on, which conventional control methods are difficult to meet the control requirements. For the design of micro-turbine control system with two inputs and two outputs, how to achieve stable output under disturbance accurately and quickly is the key to design control system. Firstly, a predictive controller is designed based on dynamic matrix prediction algorithm, and the key control parameters of error correction matrix, error weight matrix, control matrix and softening coefficient matrix are analyzed. Then, at the stable operating point, the predictive control strategy is designed under the given speed disturbance and the internal fuel disturbance. Finally, the simulation control analysis based on transfer function model shows that the control method can meet the control requirements of rapidity, robustness and anti-disturbance, and the overshoot is smaller than the conventional PIDcontrol method.

Abstract:Aiming at the problem that description of physical link information in SCD file is incomplete, description of physical link information in SPCD file is non-intuitive, correlation between SCD and SPCD is little, and due to the lack of mapping information between virtual and real loops in substation operation and maintenance, link fault detection is difficult, this paper proposes a virtual-real link mapping scheme based on SPCD, includes using SPCD to code physical link, extending physical link model to SCD, supplying the electrical (optical) link model to SCD, and constructing the mapping between virtual link and real link. It provides data support for advanced applications such as full-site physical loop visualization, virtual-real link mapping visualization and fast fault diagnosis, provides a feasible scheme for switch configuration-free, at the same time, ensures the consistency of link information between SCD and SPCD.

Abstract:In order to study the typical fault overvoltage characteristics of ±1 100 kV DC filter and its protection measures, this paper uses PSCAD simulation software to calculate the DC filter short circuit, C₁ and C₂ breakdown in the bipolar full voltage operation mode, unipolar-ground line operation mode and unipolar-metal line operation mode in the background of ±1 100 kV Changji—Guquan UHVDC Transmission project. Besides, the transient overvoltage of the arrester is calculated under the most representative fault condition. Based on this, the impact of protection control strategies on overvoltage and energy is further studied. The results show that the overvoltage is more serious when the DC filter short-circuit fault occurs on the rectifier side; when the DC filter short-circuit fault occurs under the unipolar-metal operation mode, the overvoltage is more serious and the arrester absorbs more energy. After the arresters and protection control, the overvoltage values of the critical nodes are lower than the insulation level at the location, and the energy absorbed by the arresters does not exceed its capacity.

Abstract:In order to accurately evaluate the performance of lightning protection equipment, the measurement level and response characteristics of transient current measurement sensor are very important. A design scheme of multi-layer PCB Rogowski coil with good high frequency performance, strong anti-interference ability and suitable sensitivity is proposed in this paper. The frequency range of lightning current waveform and the transmission characteristics of Rogowski coil are analyzed. According to the relationship curve between the size of Rogowski coil, turns number, and the frequency characteristics of coil, the structure size of coil is determined. A new shielding and signal output mode is designed to enhance the anti-interference ability. The sensitivity of output voltage is increased by increasing coil thickness and connecting two coils in series. According to the standard requirements, the dynamic characteristics of the measuring device, the measurement error of standard lightning current and the linearity test are carried out. The step response test of the measuring device shows that the response time is less than 90 ns and the relative overshoot is less than 10%. Compared with the standard Pearson coil, the peak error is less than 0.2%, the time parameter measurement error is less than 0.3%. The linearity of the measuring device is about 0.3% with current range from 5 kA to 40 kA. The test results show that the impulse current measuring system can measure high frequency current signal.

Abstract:In the gas insulated switchgear (GIS), the existence of insulation defects will cause partial discharge, which results in degradation of the equipment and endangers the stability of the power system. The defect of surface discharge along the insulator is relatively severe among all kinds of insulation defects. In order to study the discharge development process and regularity of this kind of defect under constant power frequency voltage, this paper sets up a "triple-combination" model to simulate the surface discharge in SF₆. With the power frequency voltage kept unchanged at 95% of the flashover voltage, a set of partial discharge signal is recorded every 5 minutes until the flashover occurs frequently, and the statistical parameters of each stage is calculated. The results show that before flashover, the discharge development process of the surface model follows the sequence of the increase of discharge repetition rate, the increase of the discharge quantity, the intermittence of discharge and the rising of discharge again, and at last the flashover happens. The occurrence of discharge intermittence may have relationship with the strong electronegativity of SF₆, the uniformity of the electric field and so on. Therefore, long-term discharge detection is required to accurately determine the stage of equipment degradation, and measures should be taken accordingly.

Abstract:In order to determine insulation thickness of DC cable in Zhangbei HVDC flexible project, a design method of insulation thickness of the DC cable is presented. The influences of life index, breakdown field intensity conversion coefficient and Bahder coefficient on insulation thickness are analyzed. The insulation thickness of ±535 kV DC cable is calculated by establishing equations. The electric field distribution and electric field distortion rate under different conductivity temperature coefficient electric conductivity electric field coefficient and insulation temperature difference are calculated by finite element simulation. The results show that the insulation electric field distortion rate of ± 535 kV DC cable meets the requirements of national standards under different conductivity temperature coefficient, conductivity electric field coefficient and insulation temperature difference, and the rationality of the design method is verified. The research results provide theoretical guidance for the insulation thickness design and validation of DC cable and provide technical support for the construction of Zhangbei project.

Abstract:Mechanical vibration signal can reflect the running state of on-load tap-changer. In order to realize effective mechanical fault diagnosis for on-load tap-changer, a fault diagnosis method based on the parameter-adapted variational mode decomposition (VMD) and extreme learning machine optimized by simulated anneal (SA-ELM) is proposed. Firstly, the signal is decomposed by VMD method, and the number of modals is selected based on energy criterion. A group of modal components with narrow band and great discrimination is obtained. Then the energy features of each modal component are calculated, which form the feature vector group, and the modal features of different fault states are clearly distinguished. Finally, the feature vector group is input to the extreme learning machine (ELM) optimized by simulated annealing algorithm to realize the recognition and fault diagnosis of the vibration signals. An experiment is carried out on the simulation experiment platform and the collected signals are processed. Compared with the method based on VMD and ELM, the fault diagnosis method proposed can effectively improve the diagnostic accuracy of mechanical fault of on-load tap-changer.

Abstract:Accurate estimation of lithium ion battery state of charge(SOC)is the key to ensure safe and stable operation of battery management system. The commonly used ampere-hour integral method has cumulative errors. And the Kalman filter algorithm needs to establish an accurate battery model. The neural network method does not rely on a specific lithium battery model and can reflect the nonlinear relationship of lithium batteries, and thus has received extensive attention. However, traditional neural network has long training time and low precision when estimating SOC. For the low accuracy of SOC estimation in the past, the particle swarm optimization(PSO) of extreme learning machine(ELM) neural network method is proposed. In the PSO-ELM model, voltage, current and temperature are used as input vector and the value of SOC is used as output vector. The experimental data is imported into the model for training and testing, and the input weight matrix and hidden layer threshold of ELM are optimized by PSO. In addition, the simulation results show that compared with the prediction results of BP neural network, the method of estimating SOC in this paper has higher precision.

Abstract:The interaction between the direct drive permanent magnet synchronous generator (D-PMSG) and the grid is paid great attention in weak grid connection condition. At present, impedance-based analysis method is the main research means for the SSO issues of the system. The influence of DC voltage perturbation and DC voltage loop are ignored in the traditional impedance modeling process based on dq frame, and the influence of the control parameters of the wind turbine on the stability of the system are not analyzed. Therefore, a direct drive permanent magnet synchronous generator-based wind farm connected to an AC grid is selected as an example, the impedance of D-PMSG based on dq frame is established, and the dynamic characteristics of the PLL, DC voltage perturbation and DC voltage loop are taken into account, and the correctness of the impedance model is verified by the simulation "scanning". Then, the influence factors of the system stability are analyzed based on the determinant of the total impedance matrix of the system. The foundation for the optimization of the control parameters of the wind turbine are proposed. Finally, the correctness and effectiveness of the theoretical analysis are verified by the simulation results in time domain simulation.

Abstract:With the refinement and intelligentization of power grid optimization and the extensive adoption of advanced applications of power grid security and economy, and the large-scale access of distributed energy, the accuracy requirements of bus load forecasting are constantly increasing while uncertainty and nonlinear of the load are further enhanced. Aiming at the above problems, an ultra-short-term bus net load forecasting model based on phase space reconstruction and deep belief network is proposed in this paper.firstly, the phaes space reconstruction of the original time series is carried out by C-C method, and then the reconstructed data is fitted by the deep belief network to obtain the predicted value of the load. In this paper, the effectiveness of the proposed ultra-short-term bus load forecasting model is tested by using the measured load data of a substation in a city. It is proved that the proposed model still has high prediction accuracy under the condition of high distributed power penetration rate and large fluctuation of bus load.

Abstract:The differential current waveform of the transformer has a large offset due to the influence of non-periodic components and higher harmonics in the first period after the internal fault, which causes the protection to be misjudged. In order to solve this problem, a transformer differential protection simulation model is established in PSCAD. The absolute value of the three-phase differential current is summed as the characteristic current. It is found that there is a significant difference in the waveform discontinuity angle and longitudinal offset of the characteristic current between the magnetizing inrush current and the internal fault current, which can be measured by setting a reasonable threshold above the time axis, and the width of the waveform whose characteristic current waveform is below the threshold is taken as the identification of inrush current and internal fault current. Therefore, a new method based on characteristic current waveform width criterion of Transformer Differential Protection is proposed. The simulation shows that the scheme can effectively improve the speed of protection action and maintain it at around 20 ms, which makes up for the lack of reliability and quickness of differential protection based on the principle of waveform symmetry.

Abstract:In order to prevent the impact of electric vehicles′charging uncertainty on the distribution network, a joint modeling method for charging demand considering large-scale and multi-type electric vehicle access is proposed based on dynamic traffic information. This paper starts from the characteristics of electric vehicles as forms of transportation, combining the driving characteristics of different types of electric vehicles with traffic information, and establishes the corresponding operation spatio-temporal model. Considering the two modes of fast charging and slow charging, this paper studies the charging behavior of various electric vehicles, in order to construct corresponding charging demand models. Combined with the user′s daily mileage, the basic operation cycle is proposed, and the Monte Carlo simulation method is used to calculate the distribution of the comprehensive charging demand of multiple types of electric vehicles under different working days. The simulation results verify the feasibility of the modeling method and show that the access of multiple types of electric vehicles will bring multiple peaks of power demand to the distribution network as a whole.