Search

Modular multilevel converter (MMC) tends to cause resonance instability in interconnected systems. Current studies on the stability of MMC mainly focus on high-voltage and large-capacity power systems, while the impedance modeling process of MMC ignores the harmonics of the submodule (SM) capacitor voltages and the influence of voltage feedforward control. The applicability of the MMC impedance model with fewer submodules is doubtful. According to the circuit structure and control mode, the AC-side sequence impedance model of the MMC is established, which takes into account the capacitor voltage balance control of submodules and voltage feedforward control. Based on the RT-Lab control hardware-in-loop (CHIL) test platform, the MMC impedance frequency scanning was carried out to verify the accuracy of the impedance modeling method. The influence of control parameters in different frequency bands on the impedance characteristics of MMC was studied. The AC terminal voltage feedforward causes phase lag in the mid-/high-frequency range and increases the risk of resonance. In the low-frequency range, the dynamics and control of capacitor voltages reduce the impedance magnitude of MMC. Using the resonance phenomenon of an MMC connected to a weak grid as an example, the high-frequency resonance mechanism caused by control parameters is analyzed from the perspective of the negative damping effect. The simulation results show that the detailed wideband impedance model can improve the accuracy of the stability analysis results.

Based on the working principle of high-speed electromagnetic repulsion mechanism, the main factors influencing the motion characteristics of the repulsion mechanism and their variation rules are analyzed and verified through calculation and simulation. Because of its fast moving speed and large kinetic energy of moving parts, it will produce large operating impact and bounce. Buffer performance is of great significance to the safe and reliable operation of high-voltage circuit breakers. Moreover, aiming at the above problems, this article studies the repulsive mechanism and its buffer problem. First, the mathematical model of the electromagnetic repulsive opening mechanism is established, and the electromagnetic repulsive force of the electromagnetic repulsive opening mechanism is deduced. The simplified model of the electromagnetic repulsive mechanism and the finite element simulation mesh are divided, and the discharge circuit model of the energy storage capacitor is designed. Then, the simulation results show that as the repulsion disk spacing, radius, storage, capacitor voltage, turn ratio, and other parameters adjust, the peak value of electromagnetic repulsion force increases, which can reach more than 60 kN, and the displacement of the mechanism can reach 84 mm within 5.5 ms. Finally, the drop weight contrast test is carried out. When the landing height is 300 mm, 500 mm, and 700 mm, respectively, the energy absorption rate of the buffer in the three groups of tests is higher than 85%. Through the speed curve simulation after the installation of buffer, it is verified that the elastic clay buffer is suitable for the buffer mode of fast switch, and a high-speed buffer device based on the elastic clay material is proposed.

In this paper, based on the pseudo-totem pole (PTP) circuit, a family of five-level PTP dual boost converters (PDBC) is proposed. Dual boost converter has some unique advantages such as no bridge arm shoot-through risk and no switch body diode reverse recovery problem, so that it has a good application prospect. Compared with conventional three-level PTP converters, PDBCs has higher power density, higher efficiency, and low voltage and current stress. The topological derivation process, working principle, modulation and strategy of the topology are analyzed firstly. Further, the number of power devices, the switches voltage and current stress of the proposed topology is analyzed. Finally, a representative five-level PDBC experimental prototype is designed with AC input 220V/50Hz, DC output 400V/1kW, and peak efficiency of 98.27%. The experimental results show that the five-level PDBC proposed in this paper has higher efficiency and verify the correctness of the topology.

The study of ground motion effects at river valley sites is an important guide to site selection and seismic design. In this paper, FLAC3D numerical simulation is used to explore the influencing factors of a river valley site. By selecting four influencing factors with different input ground motion spectrum characteristics, ground motion intensities, site slope angles and depth-to-width ratios, orthogonal test forms are designed to determine the calculation conditions, and numerical simulation analysis is conducted. The study obtains the ranking of influencing factors for the top section of the slope, slope section and valley bottom section at different locations of the river valley site. The main influencing factor is the spectral characteristics of the input ground motion; the main influencing factor of the slope section and the valley bottom section is also the spectral characteristics of the input ground motion; and the overall analysis of the amplification effect pattern of the trapezoidal valley site shows that the amplification effect of the valley bottom site is the smallest and that the amplification of the slope top site is the largest.

Among the distribution network faults, single-phase grounding faults have the greatest probability. The faults are often accompanied by arcs in the grounding point soil. This type of fault current has a small amplitude and seldom can obtain field record data. A soil arc grounding fault is tested on a realistic-distribution-network-experimental-platform (RDNEP), and it is concluded that the soil-arc-grounding-fault (SAGF) has three main characteristics: hysteresis, nonlinearity, and asymmetry. By comparing with the characteristics of common arc models, it is pointed out that common arc models cannot accurately fit the characteristics of SAGF. This paper proposes and establishes a double exponential function arc model. Through the comparison of simulation waveforms with experimental data, it is verified that the numerical simulation method proposed in this paper can simulate the development process of SAGF more accurately. Furthermore, the equivalence of RDNEP is verified on the real distribution network system (RDNS). On this basis, analyzed the arc characteristic changes of different SAGF development cycles. Finally, by studying the applicability of the proposed model in simulating ground faults in grass and gravel roads, it is verified that the model proposed in this paper has a strong generalization capability. The research has laid a theoretical foundation for a detection algorithm that is based on the characteristics of SAGF.

The increasing global energy and environmental problems are encouraging to the development and utilization of renewable and clean energy in various countries. Wind power is one of the major source in large-scale renewable energy applications. However, the frequency regulation becomes a critical issue while the technology is spreading. Research on the frequency modulation (FM) technology of wind turbines and its control strategy for future power grids become significant. The paper proposes a novel coordinated frequency control strategy with the synchronous generator to solve the unmatched state between the output power of the doubly-fed wind turbines (doubly-fed induction generators) and the grid frequency, combined with the frequency response characteristics of the synchronous generator. The FM coordination strategy is formulated by the modulation coefficient from current wind speed and operation mode of each wind turbine. By coordinating the FM output of the doubly-fed wind turbine and the synchronous generator within the allowable range of frequency deviation, it will achieve the dual goal of reducing the frequency regulation pressure of the synchronous generator and indirectly reducing the abandoned wind volume of the wind turbine. The simulation is carried out on the MATLAB/SIMULINK platform. The results show that the presenting variable coefficient frequency modulation strategy could significant smooth the wind power fluctuation, and allow the reserve power of the doubly-fed wind turbine can fully engaged in frequency modulation which will reduces the frequency modulation pressure of the synchronous generator in the system.

The essence of the mathematical model of disconnectors' multiple reignition arc is the repeated breakdowns of SF6. In this study, the kinetic properties of microparticles are studies based on the streamer discharge theory, and a fluid arc mathematical model applied in centimetre clearance is established. Ultra-High Voltage (UHV) disconnector manufactured by Pingao Electric Co. Ltd of Henan Province is selected as the object and the resistance change process of contact clearance multiple reignition arc is simulated by finite difference method-finite element method-flux corrected transport self-programming in MATLAB based on the chamber structure and operating characteristics. UHV gas-insulated switchgear very fast transient overvoltage (VFTO) test platform is built to gain the characteristics of VFTO waveforms generated by the disconnector's operation. Compared with simulation results, the test data shows that the fluid arc mathematical model can scientifically explain the dynamic development process of microparticles in the contact clearance and the calculation results are consistent with the test data. In addition, the arc fluid mathematical model combines the structure and operating characteristics of the switch with the development process of microparticles, which can better guide the design of the switch, and provide a theoretical basis for the suppression of VFTO with the disconnector itself.

In this article, we take a 126 kV single-break vacuum circuit breaker as the research object and study the application of high-energy-density PM motor in the high-voltage circuit breaker for the first time. The PM motor maintains maximum power density and torque density during the start-up phase. Note that most of the faults of high-voltage circuit breakers are mechanical faults. We designed a set of mechanical fault prediction systems for high-voltage circuit breakers. We present the prediction method of the opening and closing action curve of the high-voltage circuit breaker. It is inspired by Chaos Ant Colony Algorithm (CAS) and an optimized Long- and Short-Term Memory (LSTM) cycle neural network. We constructed the main structure of the neural network expert system and established the fault prediction model of the high-voltage circuit breaker, based on the LSTM cycle neural network, optimized by CAS. We used the improved least-square method to achieve the operation accuracy of the phase control switch. Finally, we completed the development and experiment of the prototype.

Sub-synchronous oscillation may occur when the wind-thermal-bundled system sent out via AC/DC transmission (WTBH) is integrated in power grid. Causing large-scale wind-turbine tripping, even lead the system to collapse. First, in this study the mechanism of sub-synchronous oscillation of this system is analysed. Second, the influencing factors such as the ratio of the wind-thermal-bundled installed capacity, the series compensation parameters of AC transmission line, the different operating conditions of wind power etc., are analysed, which can cause different degree impact on the sub-synchronous oscillation. Finally, the system is simulated under the Power Systems Computer Aided Design (PSCAD) environment. The results show that under the same conditions, the higher series compensation, the greater transmission capacity of the AC line, the greater risk of the sub-synchronous oscillation. Under the condition of full load, the greater proportion of installed capacity of Doubly fed Induction Generator (DFIG) is, the less risk of sub-synchronous oscillation; Under the lower wind speed condition, reducing the number of wind turbine can eliminate the risk of sub-synchronous oscillation, and the oscillation frequency vary with the number of wind turbine.

In order to ensure the high-efficient and stable operation of the MMC-HVDC transmission system, it is necessary to analyse the failure mode and failure mechanism of MMC-HVDC in detail. First, this article briefly introduced the MMC-HVDC system working principle. Then, we deeply analyse characteristic investigation under internal AC bus fault including single-phase ground fault and two-phase short circuit fault. It is proved theoretically that when asymmetrical fault occurs in AC bus, negative sequence components will appear in the system and cause double frequency fluctuations in the active power. Finally, a simulation model of modular multilevel converter DC transmission is built in the PSCAD/EMTDC electromagnetic transient simulation platform, which validates the correctness of the theoretical derivation, and provides a basis for the research of system protection strategy.

Aiming at meeting the requirement of balancing the fluctuating wind power, this study proposed an optimal control strategy for wind power hybrid energy storage system considering output power amendment. Firstly, fluctuant wind power is decomposed into low-frequency, sub-high frequency and high-frequency components. Low-frequency part is used as the expected value of wind power integration. Sub-high frequency and high-frequency components are used as the target value of output power of storage batteries and ultra-capacitor energy storage system. Secondly, a double power amendment is proposed. In the first-time power amendment, the amendment coefficient is calculated by fuzzy controller whose inputs are the target value of power and state of charge (SOC) of the energy storage system. On the other hand, complement of batteries and super-capacitors are utilised in the second amendment. Through simulation analysis, it can be indicated that the configuration of the hybrid energy storage system and the control system design are feasible. This system can be widely used in wind farm, undertaking the task of balancing the fluctuating wind power.

Dividing wall column (DWC) was used to separate the system (1,2-propylene glycol + 1,3-butanediol + 1,4-butanediol). No research in a DWC has focused on such systems which were difficult to be separated for their great similarity in both molecular structures and properties. The rigorous steady-state and dynamic simulations, as well as the optimization of DWC alternative were performed with Aspen Plus and Aspen dynamics. Sequential quadratic programming (SQP) optimization method and the efficient sensitivity analysis tool were used to minimize the total reboiler and condenser duties. Moreover, a flow rate-composition cascade control structure was first developed to maintain targeted product purity under various external disturbances as feed flow rate and side product composition changed. DWC was compared with both a side rectifier configuration and conventional two-column distillation sequence in energy requirements and total annual costs (TACs) with the same number of stages. It showed that the energy requirements and TAC by DWC were reduced 47.60 % and 35.40 % respectively, compared with conventional distillation sequence, energy savings of 48.00 % and 34.11 % lower TAC compared with side rectifier configuration. This study provided a potential solution for the distillation of the systems with similar molecular structures and properties in a DWC.

This paper proposed demand-driven personalized product configuration design method. A variety of customer orders were clustered and fuzzy transformed; using ontology's feature to establish customer demand ontology model; in order to enable the product family to meet the dynamic demand of customers, established mapping relationship of customer demands and product family; using ontology to express product family model, achieved mapping of customer needs ontology and product family ontology, and improved efficiency of product configuration. Finally, we take planetary reducer as an example to demonstrate the feasibility of the method.

As the standard equipment of the commercial aircrafts, air-ground data link system is widely used in modern aviation transport. However, its validity was unsatisfactory in our testing, especially in error of transmission. In this paper, some influencing factors during air-ground data transmitting were analyzed from the signal transmission. Considering ACARS was based on ARINC-618 protocol, a fundamental study in this article on transmission signal and demodulating signal of the ground air data link system was done on the basis of ACARS signal generated by the transient aircrafts. Keywords-ACARS, Very High Frequency (VHF), Signal Analysis

As the math model and the noise statistical information are difficult to be addressed precisely and the Scene Matching Navigation System (SMNS) output is stochastic, limited and probably mismatching, only a few algorithms suitable for Inertial Navigation System /Scene Matching Navigation System (INS/SMNS) integrated navigation system for information fusion were developed. In order to find new algorithms suitable for this system, this paper studies the issue as follows. Firstly, this paper presents the improved Kalman filter by applying the methods of extrapolation, discretizing system model in unequal interval and eliminating the measurement output delay to the common Kalman filter. Secondly, this paper studies the variable step-size LMS algorithm and normalized LMS algorithm basing on  H optimal estimation. Lastly this paper applies them to the INS/SMNS integrated navigation. Simulation results demonstrate that they are suitable for INS/SMNS integrated navigation. Keywords-Information fusion, Improved Kalman filter, Variable step-size LMS, Normalized LMS

To improve the operating characteristics of disconnector, a limited rotating angle permanent magnet motor actuator is presented in this article. The mathematical relationship between the moving contact stroke and rotation angle of motor is obtained by analyzing the dynamical coordination characteristics of motor actuator used in 550 kV gas-insulated switchgear disconnector. The electromagnetic field equation of motor, motive equation of rotor, and circuit equation of winding are solved simultaneously for acquiring dynamical characteristics of motor. The electromagnetic-mechanical coupled calculating model of motor is established by finite element method, and magnetic saturation of stator tooth is calculated and analyzed. According to the operating principle of motor, the double-closed control system which collects the motor speed and contact stroke signal during the operation of disconnector is developed. The experimental results show that the opening and closing speeds of disconnector are 1.1 and 1.2 m/s, respectively, which meet design requirements. The speed-regulating operation of disconnector is also carried out so that the switching speed of disconnector can adjust by motor actuator and control system.

The morphology, composition and mechanism of the non-NBT phase in the Sr-doped NBT [SNBT, (Na0.5Bi0.5)1-xSrxTiO3] thin films by chemical method were investigated using XRD, SEM, crystallization analysis and thermodynamics analysis. The results showed that the non-NBT phase is the porphyroblastic grains in the matrix and looks like a plate-shaped polyhedron. The mechanism of the non-NBT phase formation is that the Sr2+ and Na+ combined with ${\rm TiO}_{3}^{2-}$ to form initial simple titanate, such as SrTiO3, Na2TiO3, or NBT phase decomposed because of the volatilization of Bi under a higher annealing temperature or longer annealing duration, and then transform into the secondary simple titanates (Sr2+,2Na+)TiO3 or (Sr,Na)xTiyOz. Moreover, the NBT phase disappeared for the SNBT films annealing at temperatures higher than 600°C and for 15 min.