Abstract:Low power density and high cost are the key factors that prevent the solid-state transformer (SST) from further application in the distribution network. Therefore, a novel power integrated modular multilevel solid-state transformer(PI-MMSST) is proposed, aiming at reducing the device volume and increasing the overall power density through power integrating the input stage and intermediate stage of the conventional three-stage SST. Furthermore, as compared to the recently proposed mixed-modulation SST topologies, the PI-MMSST reduces the use of filters and saves the cost of whole system with the help of the new power distribution concept. Based on the common carrier modulation, the working principle of the topology is analyzed, and a dual-layer control strategy is proposed to achieve the voltage balance of the submodule capacitors. Simulation results from Matlab/Simulink validate the feasibility and effectiveness of the PI-MMSST topology and dual-layer control strategy.

Abstract:The sub-modules of the multi-level DC-link power electronic transformer(PET) are usually composed of half bridge and double active bridge circuits. Different structures and control strategies will cause fluctuation and instability for DC-link voltage of the power electronic transformers. Traditional control strategy require a large number of current sensors, and the low-pass filter on the control link affects the dynamic response of the system. A control strategy for the multi-level DC-link power electronic transformer is proposed. The voltage sharing of DC link between power modules is realized by half-bridge circuit, and the current sharing of output current between modules is realized by DAB circuit. At the same time, the proposed control strategy only needs a current sensor at the inductance of the medium voltage DC port. All power modules do not use current sensors, which saves the cost of the system. The simulation results verify the correctness and effectiveness of the proposed control strategy.

Abstract:When the DC transformer based on the traditional input series output parallel(ISOP) topology has a bipolar short-circuit fault, the module capacitors discharge quickly if no fault current limiting measures are taken. After the fault is cleared, it must be recovered by complicated timing such as slow start charging failed to fault ride-through. A method based on improved ISOP topology, using fault current blocking and current limiting control to realize system bipolar short-circuit fault ride-through is proposed. Firstly, the three stages and the mathematical models of the DC-voltage electronic transformer based on the improved ISOP topology are analyzed. Then, the low-voltage side fault ride-through method based on fault current limiting principle and the design principle of fault ride-through time are analyzed. Finally, the RTDS is built on the hardware-in-the-loop simulation platform. The results demonstrate the feasibility and effectiveness of the proposed fault traversal method.

Abstract:In order to ensure fast response of power electronic transformer port fault removal, DC solid-state switches are usually equipped on the DC port side. When the DC port voltage of power electronic transformer has been established, the capacitance of DC port of power electronic transformer will charge the capacitance of solid-state switch at the moment of solid-state switch closing. Considering the line impedance characteristics, the current sensor at the traditional position of DC port will oscillate due to the line impedance and capacitance in solid-state switch, which will cause serious over-current protection rectification. The machine is locked. The DC port of power electronic transformer is modeled and analyzed, which is simplified to the series response characteristics of RLC circuit. The current oscillation waveform on the DC side of power electronic transformer is analyzed, and the principle of optimal selection of measuring points of current transformer is proposed. The correctness of the theory is verified by simulation and experiment.

Abstract:When a bus short circuit fault occurs in DC system, the fault current will rise rapidly. At present, the bus protection of the DC distribution network basically refers to the AC protection scheme, and the operation time is long, which is not conducive to the rapid isolation of the DC fault. In order to achieve reliable and fast protection of bus faults in DC distribution systems while ensuring economy, firstly the fault characteristics of the bus distribution of the DC distribution network are analyzed, and then the bus protection wiring scheme, protection algorithm and resource allocation of the DC distribution network are studied. Methods or recommendations for differential currents are directly collected by DC transformers, using current rate-of-change algorithms, hierarchically processing data, different exit relays and so on, which the effectiveness is verified by the real time digital simulator(RTDS). The proposed method can effectively improve the reliability and rapidity of bus protection of DC power distribution system as well as balancing economics.

Abstract:On 9 August 2019, a major power blackout occurred in the great Britain power grid, causing power outage in some cities including London, and affected about 1 million people. It is necessary to find the reason to provide reference for the gird operation of China. The accident is summarized and the oscillation in the wind farm is discussed in detail. The reflection and analysis about the oscillation is presented considering the serial oscillation event, including typical characteristic and remaining problems. The new characteristic of oscillation about power grid which connected a large scale renewable plants is described, the shortcoming of wide area measurement system(WAMS) is demonstrated, and a new method of real-time monitoring of oscillation based on wide-frequency measurement is presented. The scheme about implement provides new method for operation and supervisory of power electronics dominated power system and improves the security and stability of future grid.

Abstract:With promotion and application of ultra high voltage direct current(UHVDC) and continuous increase of renewable energy power generation proportion, Chinese energy transformation strategy is strongly supported. However, due to the poor network-related protection performance of renewable energy units in the DC sending power grid where renewable energy is centrally connected, AC and DC faults can easily result in large-scale disconnection of renewable energy, which brings great challenge for power system transient frequency security. The failure process of large-scale disconnection of renewable energy from the UHVDC receiving grid is firstly analyzed. Then, the existing measures to deal with disconnection of renewable energy are discussed. The emergency control technical scheme using the information of DC commutation failure is designed according to the characteristics of fault process. Finally, a simulation example of the northwest power grid is built to verify that the proposed frequency emergency control strategy can effectively deal with the problem of large-scale renewable energy off-grid caused by AC and DC faults, and greatly improve the UHVDC transmission capacity and renewable energy generation of the power grid.

Abstract:There is active power coupling between AC system and DC system in AC/DC interconnected power grid, and modular multilevel converter based HVDC(MMC-HVDC)system could not completely isolate the fault from AC system to DC system or DC system to AC system. In order to achieve the effect of independent decoupling operation of HVDC system and make full use of firewall function of the MMC-HVDC system, the battery energy storage system(BESS)based MMC-HVDC system is a preferred choice. For the BESS based MMC-HVDC system, the basic structure and operation principal is analyzied firstly. Then, based on numerical integration and nested fast and simultaneous simulation approach, the Thévenin equivalent circuit of BESS based SMs and BESS based MMC is derived. The case studies are performed on a ±500 kV 2-ternimal MMC-HVDC system. The time-domain simulation results show that the AC side active power and the DC side power of the BESS based MMC-HVDC system can be decoupled from each other, and the transmitting power during and after the AC or DC fault can be kept.

Abstract:With the construction and development of power system, the structure of power grid is becoming more and more complex. Due to the long-term operation of power lines and changes in the surrounding environment, deviations exist between the original line parameters model and the actual line parameters, which affect the real-time monitoring and optimal operation of power system. Considering the sufficient measurement of data acquisition and monitoring system(SCADA) in power system transmission lines, a line parameter identification model is presented based on multi-innovation least squares(MILS) algorithm in order to achieve accurate identification and correction of the whole network lines. IEEE 39-bus power system simulation model is built on real-time digital simulation(RTDS) platform to obtain power flow operation data. Then, parameter identification is carried out in Matlab environment, and the identification results are compared with the line parameters on RTDS platform. The results show that the parameters identification results based on MILS algorithm have high estimation accuracy, and can be used as the basis for judging suspicious lines in power system.

Abstract:With large-scale and high penetration level wind integration, it is difficult to meet power grid need when only conventional units participate in primary frequency modulation, so the participation of wind power is urgently needed. Based on the research of wind power frequency modulation control, a unit commitment model considering the wind turbines participating in primary frequency modulation is proposed. By systematically adding the constraints of steady-state frequency, minimum frequency and rate of change-of-frequency, the model ensures that enough spare capacity is reserved to support primary frequency modulation. The big M method is used to transform the nonlinear model into a mixed integer linear programming model, and then the CPLEX package of Matlab is used to solve the problem. Finally, the validity of the proposed model is verified on IEEE 39-bus system. The simulation results show that the proposed model can improve the capacity of the system to withstand load disturbance, so as to meet the standby requirement of primary frequency modulation under larger load disturbance.

Abstract:As more and more devices sensitive to voltage fluctuation are widely connected to the power grid, the direct and indirect economic losses caused by the faults in the distribution network become more and more serious, which puts forward higher requirement for power supply quality. Accurate location of fault sources is an indispensable step in the treatment of power quality problems. A fault source location method for distribution network based on positive sequence voltage is proposed for fault-type disturbance. Based on the distribution characteristics of positive sequence voltage at each monitoring point when a fault occurs, the information based on monitoring points and the fault distance location model of each branch are established. The concept of similarity index is defined based on the correlation coefficient between Mahalanobis distance and Pearson correlation coefficient, which serves as the objective function to determine the location of the fault source. The genetic algorithm is used to find the optimal fault distance as the location result. The effectiveness and accuracy of the proposed method are verified by PSCAD simulation based on IEEE 14-bus network.

Abstract:In view of the problem of the redundancy of indicators and the poor accuracy of the evaluation results in the traditional transmission efficiency evaluation system of wind farms, the correlation analysis method is first cited to remove the more relevant indicators, and then the combined weight method is applied. Reasonable empowerment highlights the dominant factors, so as to screen and streamline a reasonable evaluation index system with 10 representative indicators, and construct a fuzzy neural network comprehensive evaluation system. The fuzzy neural network uses the GD method to train parameters with slow convergence rate and easy to fall into the local optimal solution. Therefore, the convergence speed and accuracy of GD, PSO and NPSO algorithms are compared and analyzed, so that the NPSO algorithm with optimal convergence performance is selected to solve the most. The parameters are updated and updated into the neural network. The test data shows that the actual output and the predicted output have a high degree of fit. Finally, the efficiency of transmission grid operation in city L, city T and city B of Gansu Province from 2011 to 2016 is evaluated and analyzed.

Abstract:The development of green and clean energy represented by wind and photovoltaic energy has become the trend of the times. At the same time, as the country deepens the process of coal de-capacity, a large number of coal mines have been abandoned, and research on waste mine management has received much attention. To this end, this paper firstly design the basic structure of the abandoned mine pumped storage station, and mathematically model the operating power and different operating conditions of the abandoned mine pumped storage station. On this basis, aiming at reducing the investment cost of abandoned mine pumped storage station, the operation and maintenance cost of mine distribution network system, and the penalty cost of abandoned wind and photovoltaic shortage and load shortage, the optimal configuration model of abandoned mine pumped storage station is constructed.The optimized configuration model is solved by a hybrid particle swarm and a simulated annealing algorithm.Based on the improved IEEE 14-node distribution network system, the test cases are established.The effects of the installed capacity of the waste mine and the access node on the objective function in different scenarios are compared.The effectiveness of the optimized configuration model and algorithm is analyzed and verified. Relevant parameters such as installed capacity and site selection of abandoned mine pumped storage station is determined.

Abstract:In view of the common high energy cost problem of the enterprise optical storage system, considering the influence of the correlation of photovoltaic output, load demand and peak valley time-sharing price on the optical storage capacity allocation in different periods, based on the peak valley time-sharing price and storage state information, the system operation strategy and capacity allocation principle are determined based on the actual size of photovoltaic output and load. Based on the cost and benefit of each component, capacity allocation model with the lowest energy cost as the objective is established. The genetic algorithm is used to solve the optimization problem, and the optimal allocation capacity and operation strategy of optical storage are obtained. The validity of the model is validated by an example. In addition, the sensitivity analysis of the system shows that the future optical storage system planning can also select the appropriate capacity optical storage combination according to the change of time-sharing price and energy storage investment cost, which has practical engineering value.

Abstract:Faced with the difference in dispatch characteristics of controllable resources and the strong coupling of active and reactive power, the distribution network is facing many challenges in how to realize the coordinated operation of many resources and the efficient solution of the model. To solve the above problems, an ADN active-reactive coordination operation and efficient solution method considering the multi-period operation characteristics and reactive power adjustment ability of adjustable resources are proposed. Firstly, based on the second-order cone relaxation and linearization modeling technology, the optimization model is transformed into MISOCP. Further, based on the "ε-relaxation" method, the second-order cone is relaxed by polyhedron, and the model is transformed into a 0-1 MILP that can be efficiently solved. Finally, based on the IEEE 33-node power distribution system, it is verified that the proposed method can balance the accuracy and efficiency of the model. Meanwhile, on the basis of realizing the full consumption of WT and PV, the purpose of saving energy and reducing voltage levels is achieved.

Abstract:Considering large-scale electric vehicle(EV) use is limited by transportation convenience and distribution network acceptance capacity, a location and capacity optimization planning method of EV charging stations is proposed. Firstly, the Monte-Carlo method is applied to analyze the load demand characteristics of EV connected to distribution network under two different scenarios of orderly charging and disordered charging. Secondly, a location and capacity optimization model of EV charging station is proposed, considering the transportation convenience and capacity of distribution network, and the particle swarm and Voronoi method are used to solve the model. The location selection of EV charging stations is determined, and the number of charging piles in each EV charging station is optimized. Finally, a case of the northern urban area verifies the rationality of the proposed optimization planning model, so as to provide reference for the planning and construction of EV charging stations in China.

Abstract:In order to improve state diagnosis accuracy of on-load tap changer(OLTC)for converter transformer, the research work of OLTC vibration signal preprocessing method is carried out. Based on the analysis of OLTC working principle and on-site noise, the OLTC vibration signal denoising method based on Wiener filter and the OLTC vibration signal stage division method based on Teager energy operator-double threshold method are proposed. In terms of noise reduction, the noise segment and the noisy signal segment are divided by the motor current signal, and the Wiener filter is constructed based on the spectral features of pure noise and noisy signal.By comparing the denoise result with original signal and result of other noise reduction methods, the effectiveness of proposed method is verified. As for division, the difference between vibration pulse and low frequency noise is amplified by the Teager energy operator. The validity of the method is verified by comparison between result of observation and proposed method. Based on the vibration signal pre-processing method proposed in this paper, high signal-to-noise ratio vibration signal and vibration stage information for OLTC state diagnosis can be provided.

Abstract:The continuous induced current at the fault point during the internal ground fault of the double circuit line GIL pipe section may cause the pipe gallery to break down. The mechanism and hazards of induced current and voltage in the GIL section of the double circuit line are analyzed. Combining the protection configuration features of check zone and differential zone in the installation of the double-line GIL, the automatic control closing strategy of GIL induced current quick release device is proposed. The method realizes the automatic closing of delayed high speed grounding switch after the circuit breaker cuts off the internal fault of the GIL pipe gallery by discriminating the operation of the line differential protection at the GIL, the operating state of the line and the GIL and the release device position, thereby avoiding manual operation. Aiming at the unsuccessful closing of the release device, the phase selection action results of the check zone protection of the fault line are collected to determine the faulty closing failure condition. The closing failure warning strategy and the closing failure inter-tripping strategy of the healthy line are proposed to reduce risks of GIL breakdown caused by the induced current. And it means to improve the efficiency of the on-site GIL operation and maintenance. The closing failure alarm strategy and the closing failure action strategy of the normal operating line based on the closing failure fault are proposed. According to the proposed closing control strategy and closing failure strategy, automatic control devices are developed and RTDS test is performed. The test results show that the control system can reliably operate under different conditions, and minimize the risk of the induced current breaking through the pipe gallery.

Abstract:In order to clarify the distortion effect of space charge on the electric field intensity of insulation paperboard and the effect mechanism of degradation degree of paperboard and space charge, the bipolar carrier transport model is adopted to simulate characteristics of space charge in insulation paperboard with different degradation degree. When DC voltage is applied in the insulation paperboard with the same aging condition, the mono-polar charge accumulates in the paperboard, and the distribution range of positive polarity charge is significantly narrower than negative polarity. When the applied voltage time increases, the electric field distortion rate in paperboard also increases. When the degradation degree of insulation paperboard deepened, both the trap density and trap depth increase, resulting in a large amount of space charge accumulate in the paperboard adjacent both electrode sides, which further weakens the electric field intensity at the boundary of the paperboard and the electrode, and intensifies the field intensity distortion inside the paperboard. Moreover, the more serious the deterioration of insulation paperboard, the more prominent the distortion effect of space charge on the electric field. The degradation of paperboard is accelerated. On the other hand, the breakdown fault of paperboard is prone to be induced. The conclusions provide reference for the design of oil-paper insulation.

Abstract:Temperature-dependent stress has become one of the main factors that impair the structural safety of the supporting tube bus system of substation. Based on the structural stress constraints of the supporting tube bus insulator, the influence of the installation temperature of the tubular bus system on the allowable range of operating temperature change is studied in this paper. Based on ABAQUS, the finite element simulation model of the supporting tubular bus system of substation is built. The temperature-dependent structural stress simulation of the tube bus system is carried out in high-temperature installation and low-temperature installation scenarios separately with the traditional 1/2 installation method. A mathematical model of quantitative fitting between temperature range and stress is constructed. Based on the conclusion that high temperature and low temperature installation are not conducive to the allowable temperature change of the tubular bus-bar, a calculation method and engineering countermeasures are proposed of the installation position of the sliding fittings considering the installation temperature. The research results can provide guidance for the engineering installation and safe operation of the substation hard tubular bus system, and have engineering application value.

Abstract:Fairness of data rates among nodes within a LoRaWAN cell is studied. The reasons why the LoRaWAN data extraction rate is unfair are:firstly, the data rate allocated to the nodes is unreasonable; secondly, because of the distance problem, LoRa / LoRaWAN shows a capture effect, where only the stronger signal in the collision signals is extracted. In order to solve the above problems, a fair adaptive data rate allocation and power control scheme is proposed. By deploying the fairest ratio of each data rate and controlling the transmission power in a cell, the capture effect is reduced, and fair data extraction is possible regardless of the distance between them and the gateway. Numerical simulation results show that compared with the latest technology, the method in this paper achieves higher fairness in data extraction rate. At the same time, the proposed method can maintain the life cycle of the node without excessive transmission power, thus reducing the overall energy consumption of the entire transmission system, in line with the concept of green communication.

Abstract:In the current integration configuration process of smart substation, integrators manually configure the virtual terminator connection in SCD file according to the virtual terminator connection table provided by the design institute, which is not only heavy workload, time-consuming, but also prone to connection errors. For this reason, by analyzing the structure of Substation element in SCD file and the virtual terminator connection of typical bay, a typical bay virtual terminator connection template is defined, in which the IED is represented by secondary device type. In the process of automatic configuration, the bay in the primary wiring diagram is mapped to the typical bay, and then the IED associated with the primary device in the bay is mapped to the secondary device type in the typical bay. Finally, the virtual terminator connection is configured for IED according to the virtual terminator connection between the secondary device types. This method can be used to develop the virtual terminator automatic connection function of the system configuration tool, which can simulate the virtual terminator automatic connection of the bay in the new station and the expansion project, and help to shorten the virtual terminator connection configuration time.

Abstract:As an important index reflecting the production efficiency and energy saving of power plants, the authenticity and accuracy of the reported data of power plant power consumption rate are very important. Therefore, a method of abnormal data identification based on predictive model is proposed. Firstly, the NARX neural network is improved by Adaboost to build the forecasting model of the power consumption rate. Then, the present power consumption rate is forecasted in a dynamic way by continuously introducing the reported value. When the time series of the power consumption rate suddenly changes, the residual time series obviously increase or decrease. Then the outliers of each residual vector group are obtained by using the isolation forest algorithm. Finally, the method is used to identify the actual generation data injected with false data, and the effectiveness of the proposed method is verified.

Abstract:For frequency fluctuation of AC power grid caused by the continuous increasing of distributed generation permeability, a design method of cascaded energy storage device based on isolated DC/DC converter is proposed. Combined with the maximum level condition of carrier phase-shift modulation and the power transmission limit of isolated converter, the design method of bi-directional power regulation region is described in detail. The general model of maximum discharge time is established, and the discharge time characteristics of energy storage device based on double active bridge and double half bridge isolated converter are analyzed in turn. The relationship between discharge time and converter type, transformation ratio, leakage inductance and supercapacitor quantity is obtained, which shows that the isolated converter optimization benefits to leakage inductance integration, easier manufacture of isolation transformer and reduce energy storage cost under the fixed discharge time index. Finally, the simulation results show that the proposed method has high control accuracy when the size, type and direction of power instructions change suddenly, which is helpful for the output power of device to follow the AGC instructions in real time.

Abstract:The accurate identification of the faulty phase is of great importance for active interference arc suppression device. When mis-selection occurs, more serious inter-phase fault occurs. In order to improve the accuracy of fault phase selection, method of phase selection using additional resistance of neutral point is presented. The method calculates the parameters of ground in real time by switching in/off the additional resistance on the neutral point, and the equivalent impedance of the network can be estimated. Then the fault phase can be identified by using the difference of parameters between normal and fault states. Simulation results show that this method performed better than the traditional phase voltage amplitude comparison method. The accuracy for faulty phase selection is high even under arcing or high resistance grounding fault scenarios.

Abstract:Predicting the error of electronic current transformers is significance for tackling the long-term stability problem of electronic current transformers’ error in time and ensuring the validity of electric power trade. The error model for electronic current transformers is established, where the error of electronic transformers is taken as a theoretical model with one input and multiple outputs, and the input and output variables are determined. Since the relationship between the input and output variables is indistinct, a forecasting method for electronic transformers based on clustering RBF neural network is proposed. The data are pre-processed using Z-score normalization method to avoid the problem of different variable magnitude and unit. The input variables are analyzed by K-means clustering method to simplify the neural network. The numerical example suggests that the predicting error of ratio error is less than 0.05% and the predicting error of phase error is less than 10'. The method provides an effective approach to analyzing the development state of electronic current transformers in operation and to managing the instruments actively, as a result, the risk of electric energy trade can be alleviated.

Abstract:In order to solve the problems of long demagnetization time,high required voltage and large equipment required of traditional AC demagnetization equipment, a low-voltage AC degaussing method based on sinusoidal pulse width modulation(SPWM) technology is proposed based on the analysis of the residual magnetization of the converter transformer and the traditional demagnetization principle. Using high stability SPWM digital power supply technology and high computing performance microcontroller, the AC voltage amplitude and frequency is output from SPWM by digitally controlling to achieve the purpose of obtaining a large AC demagnetization current under low voltage conditions and being able to accurately adjust the current amplitude, while the low voltage conditions can greatly reduce the size and weight of the equipment and shorten the demagnetization time. Using quasi-synchronous sampling technology without phase-locked loop and without strict synchronization requirements, high-precision measurement of the effective value of AC voltage and current in a wide frequency range is realized. A degaussing and verification device for the converter transformer is developed and a degaussing test is carried out for the converter transformer with residual magnetism. The results show that the demagnetizing inrush current after demagnetization is significantly reduced, reflecting the significant degaussing effect of the device. A degaussing effect verification method is proposed by connecting the low-voltage AC test voltage to the converter transformer and comparing the no-load current of the transformer before and after degaussing. The test results show that the verification method is effective and feasible.

Abstract:According to the maintenance cost and long-term benefits of electric equipment, a predictive maintenance decision method based on the service ageis proposed. Firstly, the failure rate is calculated before maintenance based on the equipment status. Secondly, the failure rate after overhauling is predicted based on equipment service age reduction. Thirdly, according to the failure rate, the degree of EENS, the change of LCC and the cost-effectiveness ratio of maintenance are calculated, respectively. Finally, the best maintenance plan is chosen on the basis of the principle of reliability priority, economy priority and cost-effectiveness priority. The results show that the decision-making algorithm of elastic period and elastic state based on the analysis of the age reductionconsidering uncertain factors such as repair efficiency and equipment status of different maintenance strategiesis in line with long-term benefits.

Abstract:As the key equipment of high voltage direct current(HVDC) flexible transmission system, the reliability of modular multilevel converter sub-module operation directly affects the safety and stability of the whole system. A sub-module bypass scheme is fully proposed. An active bypass scheme based on a permanent magnetic dual-drive bypass switch is designed. A passive bypass scheme based on breakover bidirectional thyristor is designed. Considering the economics of active bypass and the reliability of passive bypass, a hierarchical bypass scheme is designed. The test results show that fault sub-module can be reliable bypassed by command from the active bypass scheme. Meanwhile, in the case of the failure of active bypass scheme, the passive bypass scheme can reliably bypass the fault sub-module. The hierarchical bypass solves the problem that a single faulty sub-module causes the transfer system tripping. Design guidance for the implementation of HVDC-flexible valve is provided.

Abstract:Based on the prevailing problems of the Roots water-ring vaccum pump combination extraction system for steam turbines that the inappropriate configuration of vaccum pumps leads to the increase of condenser vaccum degree, this paper put forward a Roots water-ring pump frequency conversion control extraction system. Based on the mathematical model of vaccum pumps, a simulation experimental platform for the extraction system is established. Aimed at increasing the pumping capacity and reducing energy-consuming, the optimal operation plan is beterminede, the control strategy of the frequency conversion system is developed, the condenser step tests are performed. The results show that the control strategy of the pumping system is reliable and the system runs smoothly. And the frequency conversion system has obvious energy saving effect compared to the fixed speed system.