Abstract:The fault characteristics of direct current (DC) distribution network with small current grounding mode are inconspicuous when single-pole grounding fault occurs. In order to accurately and reliably select the fault line,a grounding fault selection method based on the injected signal waveform correlation is proposed. Firstly,a predesigned additional control strategy is applied to the modular multilevel converter (MMC) of the DC distribution network to achieve the detection signal injection. Then,in combination with the zero-mode network,the fault characteristics are as follows. The specific frequency zero-mode current flow at the beginning of each sound feeder are same,which are opposite to zero-mode current flow at the beginning of the faulty feeder. Finally,based on this fault characteristic,the fault selection criterion is designed,and the Pearson correlation coefficient is introduced to compare the correlation between the normalized zero-mode current of each feeder and the total normalized zero-mode current. The fault selection is completed based on the characteristics that the correlation coefficient of the sound feeder is close to 1 and the correlation coefficient of the faulty feeder is close to -1. The simulation results show that the proposed method can reliably identify single-pole grounding fault occurring at any location on the bus or each feeder under high-resistance grounding fault.

Abstract:The stability of opening process of fast mechanical switch is the key factor affecting the breaking performance of DC circuit breaker. Based on the finite element method,the multi-field coupling calculation of electromagnetic,thermal and momentum is carried out for the opening process of high current fast switch with permanent magnet operating mechanism and electromagnetic repulsion mechanism. The influence of the driving coil input for the permanent magnet operating mechanism and different driving coil currents on the opening process of double-action mechanism are analyzed. The calculation results show that the permanent magnet suction force can be cancelled in advance when the driving coil of the permanent magnet operating mechanism is put in,thereby avoiding the opening spring back phenomenon caused by the electromagnetic repulsion force due to the short working time. Because the electromagnetic repulsion force is very large at the initial stage of opening,the driving coil input of the permanent magnet operating mechanism has little influence on the initial speed of opening. After the electromagnetic repulsion disappears,the greater the driving coil current of the permanent magnet operating mechanism is,the greater the speed to reach the rated opening distance after the electromagnetic repulsion force disappearing. In order to avoid opening rebound phenomenon,the current value of the driving coil should be selected reasonably. The comparison between the simulation results and the opening process characteristic curves of the actual prototype shows that two curves are in good agreement, which verifies the correctness of the simulation method.

Abstract:The characteristics of overvoltage are closely related to the system grounding mode,which is of great significance to the selection and design of key equipment. However,the research on grounding mode selection combined with overvoltage analysis in DC distribution network is still imperfect. Firstly,the mechanism of overvoltage generation is analyzed for ±10 kV dual-terminal DC distribution network based on modular multi-level converter (MMC),and the overvoltage peak values at key positions of the system under three typical grounding modes are compared by fault simulation in PSCAD/EMTDC. Then,the fault recovery ability,economy,voltage stability and overvoltage peak value are proposed as the indexes for the comprehensive selection of grounding mode. Grounding by resistance through neutral point of the transformer in AC side and ungrounding in DC side is evaluated as the preferred grounding mode of the studied system. Finally,the arrester parameters and configuration scheme are determined based on the chargeability calculation design method,and the validity is verified by simulation. References for the selection of grounding mode and insulation configuration of DC distribution network are supplied by the research.

Abstract:An effective way is provided by the electric-hydrogen hybrid energy storage to improve the reliability of islanded DC microgrids. However,the reliability level of the system will also be affected by different electric-hydrogen hybrid energy storage strategies. According to different operation sequences of electric-hydrogen hybrid energy storage,the impact of three energy storage strategies on the reliability of islanded DC microgrid is compared and analyzed. Firstly,the hydrogen storage model,battery storage model and microgrid reliability evaluation index system are established. Then,considering the fault of system components,based on the sequential Monte Carlo method,a reliability evaluation method of islanded DC microgrid considering electric-hydrogen hybrid energy storage is proposed. Finally,the improved RBTS BUS6 F4 feeder system is applied to evaluate the reliability of the islanded DC microgrid,and the influence of different energy storage strategies and different capacities of electric-hydrogen hybrid energy storage on the system reliability are compared and analyzed. The results show that system reliability can be effectively improved with hydrogen energy storage. Different electric-hydrogen energy storage strategies and capacities have different effects on system reliability.

Abstract:With the development of new energy,energy storage,and flexible DC technology,AC/DC hybrid distribution networks have gradually become an important development direction for future distribution networks. The access of a large number of power electronic devices has brought dynamic problems such as low frequency oscillations to the distribution network. In order to deeply study the low frequency oscillation problem on the DC side of AC/DC distribution networks,detailed modeling of key links is conducted such as H-bridge series and isolated bidirectional active bridge structures,three-phase inverters,and three-phase Buck converters in the main valve tower of power electronic transformer. Through time domain simulation,the 4 Hz low frequency oscillation phenomenon on the DC side of the AC/DC distribution network in the actual project is reproduced,and the correctness of the established mathematical model is verified. Based on the established mathematical model,qualitative analysis is conducted on key influencing factors such as power and controller parameters,and the final conclusions are as follows:power level and Buck link voltage loop control parameters affect system stability,the integral coefficient of the current loop in the Buck link affects the oscillation frequency.

Abstract:To realize power dispatch and equilibrium in DC distribution networks,a distributed adaptive coordinated control strategy is proposed. Based on the consistency theory,photovoltaics,energy storage and other units in the network can be considered as the same nodes involved in control,where power deviation and equalization coefficient are chosen as state variables in each node. By the interaction and iteration among adjacent nodes,each node can adaptively adjust the operation state according to the dispatching command. On the premise of effectively accepting the real-time dispatching command and optimal control of energy storage,the regulation ability of distributed power supply is fully utilized to suppress the power imbalance of internal load in the system. Meanwhile,the power provided by each distributed source is proportional to its rated capacity under different operating scenarios by adjusting the power reference,and the the prediction the output of each distributed sources is not required. When each individual distributed power supply loses its adjustment ability,the balance of utilization rate of other units can still be ensured. Simulation results under various scenarios verify the effectiveness of the proposed strategy.

Abstract:The arc is generated upon contact breaking-off resulting in surface erosion in the terminals and affecting the performance of electrical contact. As there is no natural zero crossing point in the DC power supply system,the erosion caused by arcs is often more severe in DC contactors than that in AC contactors. The arc erosion effect on the contact terminals is analyzed in this paper. Based on the theory of magnetohydrodynamics,a dynamic coupling model of arc and contact terminals considering energy coupling is established. Then,the impacts of the electric current level and breaking speed on the arc erosion in contact terminals are studied by using the model. Simulation results show that the arc temperature near the anode is higher than that near the cathode. It can be noted that the arc temperature and arcing time rise significantly by increasing the current from 20 A to 30 A,which improves the arcing energy by 75.93% and thus worsening the contact erosion. Simulation results also indicate that increasing the breaking-off speed from 0.1 m/s to 0.2 m/s,results in an increase of voltage and current variation rates, an reduction of the arcing time and molten pool volume,a decrease of the arcing energy by 47.83%,and an reduction of the impact of arc erosion on the contact terminal. The experimental results are consistent with the simulations confirming the accuracy of the simulation model.

Abstract:With the continuous application and popularization of distributed energy,DC power distribution will become the mainstream form of future power distribution system. As a new type of power distribution system,the overvoltage and insulation coordination of flexible DC power distribution urgently needs further research and improvement. The addition of medium-voltage DC circuit breakers has also led to fundamental changes in the operating overvoltage transient characteristics of the system. It is necessary to analyze the operating overvoltage distribution characteristics and insulation coordination of the flexible DC distribution network with DC circuit breakers. Firstly,the electromagnetic transient simulation model of ±10 kV ring-network flexible DC power distribution network is constructed,and the protection action scheme based on DC circuit breaker is designed. Then,the ±10 kV ring network flexible DC power distribution system is simulated and analyzed for operating overvoltage,horizontal spatial distribution characteristics of its amplitude and the decisive working condition of the maximum overvoltage at key positions are obtained,insulation coordination scheme of the ±10 kV ring network flexible DC power distribution system is proposed. Finally,the simulation analysis of the transient induced overvoltage of the DC cable sheath under the influence of the action of the DC circuit breaker is carried out. The results show that the addition of DC circuit breakers improves the reliability and flexibility of DC distribution network,and the layout of arrester and cable metal shield protector is further optimized,which is of great significance to the design of insulation matching scheme of flexible direct distribution network of ring network.

Abstract:In high voltage direct current cable system,the cable joint and termination are the weakness to stand high voltage,thus making their breakdown strength the critical property to the reliability of power transmission system. In this work,the breakdown mechanism of cross linked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) interface in the cable accessory during the load cycle test of ±320 kV HVDC cable is analyzed. Firstly,the voltage withstand test of the cable accessory suggests that the weakness locates at the XLPE/EPDM interfaces under applied electric field. Secondly,the temperature and electric field distribution under no load and full load are calculated through finite element simulation. This result indicates that the highest electric field is about 29.5 kV/mm located at the cable insulation material near the stress cone,which is far below the breakdown strength of each insulation material. Finally,the electric field distortion in the interface of XLPE and EPDM induced by space charge is measured by a pulsed electro-acoustic system. It is found that the electric field distortion rate reaches 100%~200% in the interface,suggesting that the space charge in the interface is responsible for the breakdown behavior of accessories. This work might provide a breakdown mechanism for further development of high voltage materials and structures for HVDC cables accessories.

Abstract:The flexible DC interconnection technology can effectively improve the power quality of the power grid,and enhance the reliability of power supply,thus improving the level of new energy consumption. In a flexible DC interconnection system with master-slave control,sudden load increase and short-term fluctuation in new energy output are likely to cause the actual power of the master station to exceed its capacity. It is difficult to timely suppress power fluctuation through traditional power limiting or scheduling methods,which leads to abnormal DC side voltage and threatens the stable operation of the power system. Therefore,a master-slave control strategy that takes into account the capacity margin of the master station is proposed. The strategy optimizes the operation mechanism of the master station and incorporates the capacity margin factor into the operation mode of the master station to adjust the voltage of the master station. The loop structure and variable definition of the loop are added. A closed suppression loop based on overcapacity power is added,which ensures the continuous regulation ability of the main station to the DC bus voltage. Since the master station needs the coordinated operation among the slave stations when balancing the system power,a power support mechanism among the slave stations is introduced based on the traditional operation mode and original scheduling of the slave stations,and a system under the constraints of system power balance and other constraints is established. The multi-objective optimization model with the smallest system economic loss and the smallest source-storage equipment loss is established,and the non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ) is used to solve the optimization model quickly. Finally,it is verified by simulation that the proposed strategy can effectively prevent the overcapacity of the master station and ensure the stable and efficient operation of the flexible DC interconnection system.

Abstract:Flexible multi-state switch (FMSS) has the characteristics of nonlinearity and strong coupling. It is difficult to achieve complete decoupling of output power through traditional proportional integral (PI)control. There are some problems in PI control such as difficulty in adjusting PI parameters,slow system dynamic response speed,and poor robustness. Firstly,feedback accurate linearization is uesd to transform the FMSS from a high-order nonlinear system into two completely independent first-order linear systems,realizing the complete decoupling of active and reactive currents. Then,aiming at the problems of slow approach speed,poor system convergence performance and large motion chattering of traditional sliding mode control,an improved sliding mode control based on adaptive approach rate is designed to optimize the voltage outer loop and the current inner loop,and the stability of the proposed algorithm is verified by Lyapunov function. Finally,the simulation analysis of the three-port FMSS are carried out in the Simulink software under the conditions of output power disturbance,AC voltage sag and system parameter perturbation. The results show that the proposed control enhances the robustness and anti-interference ability of the system,and it can effectively increase the dynamic response speed of the system.

Abstract:In order to realize the intelligent prediction of valve inlet water temperature of a voltage sourced converter-high voltage direct current (VSC-HVDC) valve cooling system,a prediction model of inlet water temperature of VSC-HVDC based on a hybrid model of the random forest (RF) and bi-directional long short-term memory (BiLSTM) is proposed,and the cooling capacity of the cooling system is evaluated on the basis of the proposed prediction model. Firstly,a RF algorithm is used to analyze the importance of high-dimensional feature sets,which consist of all the monitoring variables of the valve cooling system. Then the important characteristic parameters affecting the inlet water temperature are filtered out to form an input feature vector with the historical inlet water temperature. Secondly,the feature vector is input to the developed BiLSTM prediction model to train the model for accurately predicting inlet valve water temperature and quantitatively evaluating the cooling capacity. Finally,a VSC-HVDC converter station in Guangdong power grid is taken as an example to verify the effectiveness and superiority of the proposed method. Simulation results indicate that the accuracy of the proposed hybrid model based on RF-BiLSTM is higher than that based on BiLSTM model,RF model,support vector machine (SVM) model and auto-regressive and moving average (ARMA) model. Moreover,the cooling capacity is evaluated quantitatively and accurately. Analysis results show that the cooling margin of this converter station is up to 98%,which indicates that there is a problem of overcooling and energy waste. The evaluation result of the cooling capacity is well consistent with the field operation result of the converter station,which confirms the effectiveness and the accuracy of the proposed method.

Abstract:Direct-drive wind farms (DDWF) merged into weak AC systems have the risk of sub-synchronous oscillations (SSO). Because the DDWF model has a higher order,it needs to be reduced for actual analysis. When analyzing the SSO problems,the existing balanced reduction method cannot accurately keep SSO modes of the system during the reduction process. To solve this problem,a balanced reduction method for SSO analysis is proposed. This method is based on the participation factor analysis and retains the state variables which strongly related to the SSO model. Combining with the Hankel singular value to determine the reduction order,a reduced-order system model consistent with the SSO characteristics and dynamic characteristics of the full-order system model is established. Firstly,a balanced system model by balancing the small signal model of the full-order system is established. Then,the participation factor analysis is performed on the balanced system model,combined with the Hankel singular value to ensure the set of retained state variables,and the residual order reduction method is used to establish the reduced-order system model. Finally,by comparing the full-order system model and the reduced-order system model,the results show that the proposed reduced-order method is suitable for the study of DDWF integration into weak AC system SSO.

Abstract:The volatility impact and insufficient level of new energy accommodation brought by the high proportion of new energy connected to the grid have become an urgent problem to be solved under the background of new type power system. Therefore, based on the characteristics that the increase of prediction accuracy of wind,photovoltaic power and load improves step by step with reduced time scales and the flexible regulation ability of pumped storage plant,a multi-time scale coordinated dispatching model of wind-photovoltaic-pumped storage zero-carbon power system is established. In order to minimize the cost of power generation,the power generation plans of day-ahead 24-hour,the intraday 1-hour and the real-time 15-minute are established. Through the coordination of multiple time scales,the wind power,photovoltaic power and pumped storage output track the load well,thus revising the power generation plan step by step. Taking the wind-photovoltaic-pumped storage zero-carbon power system with 6 pumped storage units as an example,the simulation analysis shows that the proposed multi-time scale coordinated dispatching model is conductive to reducing curtailment of wind and photovoltaic power,and the ability of power system to accommodate wind and photovoltaic power is related to the installed capacity of pumped storage plant.

Abstract:To solve the problem that the traditional single-ended phase selection elements are no longer suitable for AC long lines at the receiving end of the AC/DC hybrid system,a time-domain double-ended phase selection method based on model recognition is proposed. Firstly,the time domain model of thenon-fault phase and fault phase of long AC lines with shunt reactors at the receiving end during fault is studied by using the idea of model identification. It can be seen from the analysis that the differential current and differential voltage of the non-fault phase satisfy the capacitance model but the fault phase does not. Secondly,the capacitance model conformity of each phase is calculated by Spearman rank correlation coefficient,and the fault phase is selected by comparing it with the threshold value. Finally,an AC/DC hybrid system model is built in PSCAD/EMTDC,and the proposed fault phase selection method is simulated and verified from the aspects of fault type,fault location,transition resistance,sampling frequency and noise interference. The results show that the fault phase selection method can quickly and accurately select the fault phase during the fault at each position of the receiving end AC line,without relying on the characteristics of the DC-side power supply,without the influence of the shunt reactors,and with good anti-transition resistance and anti-interference performance.

Abstract:Power to gas (P2G) technology realizes the mutual coupling of electric energy and natural gas,and plays an important role in improving the economy of multi-energy system and reducing the carbon emissions of the system. In order to solve the problem that the oxygen produced by electrolyzed water in the process of P2G conversion can not be fully utilized,A multi-energy system optimal scheduling model is proposed based on the joint operation of P2G and oxygen enriched combustion power plants. Firstly,the P2G process is divided into power to hydrogen process and methanation process. The oxygen generated in the power to hydrogen process is delivered to oxygen enriched combustion power plant for use. Then,the CO₂ captured by the oxygen enriched combustion power plant and the hydrogen generated during the power to hydrogen conversion process are used as the raw materials for the methanation reaction,and the natural gas generated is supplied to the gas turbine unit for use, so as to realize the full utilization of resources. Secondly,the joint operation model of P2G and oxygen enriched combustion power plants is introduced into the multi-energy system,and a low carbon multi-energy system architecture based on the joint P2G and oxygen enriched combustion power plants is built. Finally,a low-carbon economic scheduling model with the goal of minimizing the operating cost of the multi-energy system is established and verified by setting up a scenario comparison. The simulation results show that the proposed model effectively reduces the system cost and carbon emissions.

Abstract:The VSR-VSI dual three-phase pulse width modulation (PWM) converters based on the cascade connection of a front-end three-phase voltage source rectifier (VSR) and a back-end three-phase voltage source inverter (VSI) have been widely used in elevator energy feedback systems,but the front-end three-phase VSR using the traditional proportional integral (PI) double closed-loop control structure usually has the problem that the mid-end DC voltage cannot take into account the disturbance immunity and the follow-ability. Given this,an improved DC voltage regulation strategy of the three-phase PWM rectifier based on two-degree-of-freedom proportional integral differential (PID) is proposed. Firstly,the structure and working principle of the three-phase VSR-VSI dual three-phase PWM converters are expounded. Then,the PI double closed-loop control scheme of the front-end three-phase VSR and its parameter design process are given to analyze the reasons why this traditional scheme cannot have good disturbance immunity and follow-ability of the DC voltage regulation system. On this basis,an optimized DC voltage regulation improvement strategy for the front-end three-phase VSR based on two-degree-of-freedom PID and its corresponding parameter design method are given. Finally,the correctness and the superiority of the proposed three-phase PWM-VSR DC voltage regulation strategy are verified by using the simulation and experimental comparison results.

Abstract:The optimization of capacity allocation of traditional microgrid has limited effect on improving economic efficiency. To solve this problem,a microgrid model based on the coupling of landfill gas power generation (LGPG) and power to gas (P2G) is propoed to improve economic efficiency. Considering the influence of carbon trading and the time value of capital on the optimal allocation of capacity, the full life cycle theory and net present value analysis method in economics are introduced. Then the mutation particle swarm optimization algorithm is used to obtain the optimal allocation of capacity in four scenarios for maximizing the benefit of the whole life cycle. The optimal operation scheme is obtained by Cplex under the optimal allocation of capacity of the traditional microgrid model and the proposed microgrid model. The simulation results show that the economic efficiency of proposed microgrid model based on LGPG-P2G is higher than that of tranditionl microgrid model under the background of carbon trading.

Abstract:The synchronous phasor measurement technology offers data for dynamic security monitoring in power systems. The non-contact voltage sensing technology has the advantages of safety,convenience and low cost,which is helpful for massive distribution of measurement devices. The defect of the existing non-contact voltage measurement technology is that the voltage probe may cause voltage distortion,and it is difficult to figure out the primary voltagephasor. In order to solve the problem of voltage distortion,the distortion law of voltage is revealed by analyzing the non-contact probe's equivalent circuit and transmission characteristic,and a phasor measurement algorithm for each in-band signal is proposed. Signal pre-processing is used to filter the out-of-band signals and noise first; The matrix pencil method is used to calculate the in-band signal's frequencies,which is used to build signal model,then the in-band signal phasors are figured out by fitting in time-domain; Lastly,the secondary phasors are restored to obtain the primary in-band signal phasors and synthetical phasor. The simulation results show that the proposed method can figure out the primary voltage phasor by using the sample of the probe output voltage. Experiment data show that the amplitude measurement error is less than 4.5%,the phase error is less than 1°,the frequency error is less than 0.04 Hz, and the frequency change rate error is less than 4 Hz/s.

Abstract:In order to improve the conversion efficiency of photovoltaic cells and reduce the energy loss,the maximum power point tracking (MPPT) method needs to be studied. Aiming at the problem that the tracking speed and steady-state accuracy of the traditional perturbation observation method (P&O) cannot be balanced,and misjudgment occurs when the environment changes greatly,a variable-step P&O control strategy that can adapt to the environmental changes is proposed. Firstly,the short-circuit current under current illumination is obtained by using the characteristics of the photovoltaic cell similar to the constant current source when it first starts,and the reference voltage of the maximum power point (MPP) is derived by the fixed current method. Secondly,when the illuminance changes abruptly,the power correction method is proposed,and the variable step size adjustment strategy is given. Finally,a fractional order proportion integration differentiation (FOPID) controller based on linear extended state observer (LESO) is designed,which can further track and compensate the reference voltage output by the algorithm. Simulation results show that the proposed control strategy can improve the steady-state accuracy and tracking speed,and effectively improve the output power of photovoltaic cells.

Abstract:Partial discharge is closely related to the insulation state of power equipment. Accurate identification of partial discharge types is of great significance to ensure the operation of the power grid. A partial discharge pattern recognition method based on deep learning and multi-model fusion is proposed in this paper. Firstly,four kinds of typical partial discharge defect models in the switchgear are designed and built,and the phase resolved partial discharge (PRPD) spectrum is collected to estalish sample set. Then,a deep residual network based on transfer learning is built to identify partial discharge defects. Finally,the deep residual network (DRN) model is fused with the traditional recognition model by Sugeno fuzzy integral. The experimental results show that the transfer learning model has better update ability and generalization performance than the non-transfer learning model does,and the fusion model has higher recognition accuracy than the single model does. The proposed method can accurately identify the types of partial discharge defects,and has a certain reference value for the operation and maintenance of power equipment.

Abstract:In order to solve the accurate calculation problem of spoke-type permanent magnet machines with multi-layer winding structure in the magnetic field,based on the two-dimensional subdomain analytical modeling idea,a magnetic density analytical calculation method considering the saturation effect at the rotor magnetic bridge and stator multi-layer winding structure is proposed. In this method,the magnetic vector potential calculation of multi-layer armature winding can be calculated by subdomain division in the stator slot. Then,the saturation effect at the spoke-type rotor core magnetic bridge is equivalent by using an equivalent air gap. To ensure the accuracy of calculation,the error check and cyclic iteration are applied in this method. The key contribution of this method is effectively solving the interior permanent magnet machines' magnetic field calculation problem in the traditional subdomain modeling method due to the inability to consider the finite magnetic permeability of the core. Taking 8-pole 9-slot and 10-pole 12-slot permanent magnet machines as examples,finite element models with different winding structures are built to validate the validity of the proposed method. The results show that the analytical method achieves a reduction in the computation time while maintaining the calculation accuracy.

Abstract:The high-order harmonics caused by traditional fixed switching frequency space vector pulse width modulation (SVPWM) technology cause serious electromagnetic interference problems in electric drive systems. Random switching frequency (RSF) SVPWM can be used to solve the problem of severe electromagnetic interference. The principles of SVPWM technology and RSF-SVPWM technology are introduced,and the influence of the increase of the spread spectrum range in RSF-SVPWM strategy on the system is discussed. To achieve good dispersion of high order harmonics in a small spread spectrum range,a novel RSF-SVPWM strategy based on dual frequency division is proposed. Taking the original central switching frequency as the midpoint, the initial complete variation range is divided into two local frequency bands,and their local central frequency points and local spread spectrum range are set. Taking the positive and negative of the ideal β axis voltage component in the two-phase static α-β coordinate system as the standard,after judging the specific frequency division band of each switching cycle,the switching frequency is randomized around the local central frequency point in the frequency division band. The simulation and experimental results show that compared to the traditional RSF-SVPWM strategy with a wider spread spectrum range,the new RSF-SVPWM strategy has equivalent high order harmonic dispersion effect. At the same time,the new RSF-SVPWM reduces the pulsation of the system current loop and outputs a relatively stable electromagnetic torque. The new RSF-SVPWM promotes the application of electric drive systems in high-end industry fields.

Abstract:Energy storage technology is the key technology to building a new power system with new energy as the main body and an important guarantee to maintain the reliable and stable operation of microgrids. Existing theoretical research and projects at home and abroad are mainly concentrated on single-level cascaded energy storage systems. The power module and battery system are connected with passive devices. The structure is simple without enough control degrees of freedom. In the meantime,aiming at an unbalanced state of charge (SOC) of the battery in cascaded energy storage,the existing intra-phase SOC balance control strategy has disadvantages such as poor adaptability to different load ratesand possible over-modulation in case of extreme imbalance. Based on the double-stage cascaded energy storage system,its overall control strategy and the inter-phase and intra-phase SOC balance strategies are illustrated in this paper. An adaptive intra-phase state of charge balance strategy is proposed and its design principle of the balanced control parameters is explained in detail. The adaptability and balance effect of the cascaded energy storage system under different working conditions such as light loads and heavy loads are improved effectively. Finally,the feasibility and effectiveness of the proposed control strategy are verified by simulation,thus providing theoretical reserves and technical support for project implementation.

Abstract:In the wireless charging system,the power transmission follows the magnetic coupling relationship between the transmitting coil and receiving coil. When there are metal foreign objects between the transmitting coil and receiving coil, the eddy current effect or magnetic focusing effect of the foreign objects will affect the transmission performance and operational stability of the wireless charging system,and in severe cases,it can also endanger charging safety. In order to clarify the influence mechanism of metal foreign objects with different materials on the performance of wireless charging system and effectively analyze the influence law of foreign objects, an accurate modeling method of metal foreignobjects is urgently needed to improve the detection accuracy of metal foreign objects. Therefore,an equivalent modeling method of metal foreign objects field is proposed,taking into account the key factors of metal material,location,equivalent area and so on. By constructing the coupling equation of wireless charging system under the influence of metal foreign objects,the expression of phase difference between the transmitting coil current and receiving coil current before and after the introduction of foreign objects is obtained. On this basis,the equivalent coil model of ferromagnetic metal and non-ferromagnetic metal and its parameter configuration method are further proposed,which can accurately obtain the current phase difference under the condition of metal foreign objects intervention. Finally,the accuracy and effectiveness of the modeling method are verified by simulation and experiment. This method provides a new idea for the characteristic analysis and online detection of metal foreign objects introduced in wireless charging system.

Abstract:The gas pressure of gas insulated switchgear (GIS) is the key factor affecting the sensitivity of partial discharge (PD) detection. In this paper,four defect models,tip,suspension,void,surface,are constructed on the PD experimental platform. PD experiments are carried out at different pressures,0.2 MPa-0.5 MPa. Ultra-high frequency (UHF),high-frequency current transform (HFCT) and acoustic emission (AE) methods are used,and the sensitivity of HFCT and AE methods for PD signal detection is compared. The results show that SF₆ pressure is a key parameter affecting tip discharge,suspension discharge and surface defect discharge. Under these defects,the partial discharge initial voltage (PDIV) is proportional to SF₆ pressure,and the discharge amplitude and pulse number are inversely proportional to SF₆ pressure. For the void defect,the change of external SF₆ pressure does not affect the void gas pressure,and thus does not significantly affect the cavity discharge intensity. The HFCT method can effectively detect suspension defects, however,it cannot detect the PD signals of void and tip defects. As the SF₆ pressure decreases,the sensitivity of HFCT method to detect surface defects increases,although the sensitivity is not comparable to that of the UHF method. The AE method can detect suspension and tip defects effectively, but cannot detect PD signals of void and surface defects. The detection sensitivity of the AE method for tip defects gradually increases with the decrease of SF₆ pressure. When the pressure is 0.2 MPa,the detection sensitivity is equivalent to that of the UHF method.

Abstract:Ablation failures of buffer layer in high voltage cross-linked polyethylene (XLPE) cables occur frequently,which has attracted widespread attention in the industry. The optimization of cable structure is conducive to solving the problem of discharge ablation. From the perspective of the discharge burn mechanism of buffer layer,the finite element model of cable is established for electric field simulation to analyze and discuss the variation of air gap electric field distribution under different structural parameters,thus designing and studying the structure optimization scheme. The results show that,on the premise of satisfying the design requirements of cable,it is beneficial to reduce the air gap electric field distortion because of poor contact,by reducing the thickness of the buffer layer and corrugated depth,as well as increasing extrusion depth between the metal sheath and the buffer layer. The smooth aluminum sleeve structure has low contact resistance with the buffer layer,so it has advantages in restraining fault of buffer layer discharge ablation.

Abstract:To further improve the performance of distance protection in renewable energy systems,a distance protection method is proposed,which considers the effects of control system and the distributed capacitance effect of the line. When an asymmetric fault occurs in the system,combined with the control characteristics of the converter,the sequence impedance characteristics of the grid and the pure resistive feature of the fault resistance,three unknowns in the fault loop equation can be eliminated,and the solution of the fault distance can be solved in the R-L model. Then,the setting point is set based on the calculation results in the R-L model,and the real electrical quantity at the setting point can be deduced by the calculation formula in Bergeron model. Finally,the equation from the setting point to the fault point is simultaneously solved to reduce the distributed capacitance effect on the distance protection. The simulation results testify that the method is less influenced by the fault features of the converter and fault resistance. It still has good performance in long line.