Abstract:In recent years, the DC distribution network based on MMC is developing rapidly. However, DC side fault protection has become an urgent problem because of the sharply rising overcurrent generated by short circuit and the poor overcurrent capacity of power electronics. Considering the two different topologies of half-bridge and full-bridge MMC, the fault characteristics are analyzed and the analytical expressions of fault voltage and current are derived respectively when a bipolar short circuit occurs on medium voltage DC lines. For the half-bridge MMC, a protection strategy based on bridge arm current-limiting module and DC circuit breaker is proposed, which solves the problem of slow fault current attenuation and excessive steady current, and thus reduces the requirement for the breaking capacity of the DC circuit breaker. Based on the fault current self-clearing capability of the full-bridge MMC sub-module, an improved fast recovery protection strategy is proposed to reduce the system outage time by unlocking the MMC. Finally, the PSCAD/EMTDC simulation results prove the effectiveness and practicability of the protection strategy based on the actual engineering parameters.

Abstract:With large-scale distributed generation access to distribution network, the uncertainty of network operation increases, active distribution network requires high observability and controllability of the network, AC/DC hybrid distribution network’s operation is more flexible and reliable, it’s the development trend of distribution network in the future. In this paper, network reconfiguration and DG reactive power compensation of active distribution network are considered.The exchange power evaluation model between AC/DC distribution network and transmission network based on second-order cone programming is established. So the exchange power flexibility range between distribution network and transmission network is solved. This range can provide a certain technical reference for the operation and dispatch of the distribution network and transmission network. Solution of 94-node example verifies the validity of the model. The results show that the AC/DC hybrid power distribution network has a larger flexible range than AC distribution network, VSC capacity and DG output level are related to the flexible range.

Abstract:DC distribution network is the development trend of power distribution system in the future, in order to achieve the targeted improvement of DC power quality problems and promote the in-depth development of DC power distribution technology, it is necessary to propose an identification method suitable for DC power quality disturbance characteristics. In this paper, the formation mechanism and disturbance characteristics of four types of power quality problems in DC distribution network are deeply analyzed, and five characteristic indicators are proposed for the characteristics of various problems, which are used as the characteristic elements to identify DC power quality problems. The intra-class aggregation and inter-class separation of the proposed feature set were proved by k-means cluster analysis. Finally, the PSO-SVM classifier is used to accurately identify the DC power quality problem. Simulation examples verify the accuracy and effectiveness of the proposed method.

Abstract:The reactive voltage control of AC/DC hybrid distribution network is the key problem to be solved to ensure its efficient and reliable operation. For this reason, a distributed reactive power and voltage interactive control strategy based on limited time control is proposed, which realizes the interactive reactive power and voltage support between AC subnet and DC subnet under the framework of no centralized controller. This method constructs a distributed decentralized cooperative control architecture. Achieve distributed consistency cooperation between multiple inverters with limited time control. At the same time, it can ensure the accurate division of reactive power increment, avoid the reactive power circulation problem between multiple drooping control inverters, and further improve the voltage quality. Compared with the traditional consistency strategy, the proposed strategy has faster convergence speed, and can adapt to various disturbances in system. In order to verify the cooperative interactive control effect of the control strategy, a simulation model of AC/DC hybrid distribution network is established in PSCAD/EMTDC. The effectiveness and adaptability of distributed interactive support and reactive power sharing strategy between AC subnet and DC subnet are verified under various operating conditions.

Abstract:While the active distribution network has fully utilized the advantages of various clean energy sources, it has to face the dilemma of increasing power quality problems. The unified power quality controller can comprehensively deal with the voltage quality and current quality problems in the active distribution network. Firstly, the UPQC system is decoupled and controlled, and the mathematical model under dq coordinates is converted to the αβ coordinating system to achieve full decoupling. The transfer functions of the series side and parallel side converters are obtained. Then the controllers on the series side and the parallel side are designed respectively. Finally, the simulation model of the UPQC system is established.The simulation results show that the controller designed in this paper has better compensation effect under the condition of the voltage drop or suspension, load current distortion.

Abstract:Aiming at the significant differences between operating characteristics, reactive voltage characteristics and traditional distribution networks after the introduction of a large number of distributed power sources in the distribution network, an automatic voltage control system architecture of active distribution network based on distribution automation system is proposed. Based on the real-time topology connection of feeders, an autonomous control region division method with reactive power adjustable device as the control object is proposed, including: voltage control partition strategy, voltage over-limit control strategy, feeder reactive over-limit control strategy, coordinated control strategy with other systems and safety blocking strategy. Then, a voltage and reactive power control strategy based on autonomous control region is designed, and the overall flow of the automatic voltage control system for distribution network is given. Finally, combined with the actual case, the active distribution network automatic voltage control system architecture described in this paper is a useful exploration of voltage reactive power control at the level of distribution automation system.

Abstract:In order to control air pollution and protect the environment, more and more power energy alternatives covering electric heating and electric vehicles, emerge in distribution networks. Power supply capability of distribution network receives a significant effect and the traditional distribution network operation optimization method needs to be improved. According to the randomness characteristics of electric heating and electric vehicles, the methods of Latin super cube sampling, Cholesky decomposition and synchronous back generation reduction are adopted to realize the rapid generation of probability multiple scenes. Based on the probability multiple scenes, combining robust optimization and stochastic programming method, the scenario analysis method is accepted. With satisfying robust constraints, taking the optimal expectation values of running cost and load balancing degree as objectives, robust network reconfiguration model in distribution network with power energy alternatives integration is built. Ant colony algorithm is used to solve the model. Finally, a case study is given to verify that the proposed method can effectively improve the power supply capacity of the distribution network with large scale power energy alternatives integration.

Abstract:The metallic return lines of ±500 kV Zhangbei VSC-HVDC grid have no voltage during normal situation, and there are two kinds of line forms, including single-loop line and double-loop line. At the connection point of single-loop line and double-loop lines, the wave impedance is discontinuous, thus traditional methods are not suitable for the fault monitoring of the metallic return lines. The wave impedance and traveling wave characteristics are analyzed, and a fault monitoring system and waveform comparison analysis method are presented, which are based on differential mode Time Domain Reflection (TDR) method. The system applies differential mode pulse to monitor the fault, and uses waveform comparison method to eliminate the wave reflection caused by inherent impedance discontinuity of the line and highlight the fault characteristics. The simulation and field tests show that the system and method can adapt to the operating condition and wave impedance discontinuity of the metallic return lines of Zhangbei VSC-HVDC grid. The method has high reliability and sensitivity for fault monitoring, and the location accuracy is better than 300 m.

Abstract:Spectrum monitoring system of power wireless private network is of great significance to guarantee the service quality and improve the management level of the power wireless private network. Scientific and reasonable plan for the deployment and effectiveness evaluation are conducive to reducing the cost of system construction and improving the monitoring performance of the system. In view of the application scenario of power wireless private network, considering the characteristics of service distribution and monitoring requirements, this paper designs the planning process of monitoring system based on the characteristics of power service distribution. Considering the factors of monitoring capacity, construction cost and utilization efficiency, a multi-dimensional evaluation index system of planning effectiveness is constructed, and a calculation method of deployment effectiveness evaluation based on composite weights is proposed and analyzed. The results show that the proposed evaluation method can comprehensively evaluate the adaptability of different planning algorithms in system deployment, which is conducive to forming a scientific deployment plan, promoting the optimal utilization of spectrum monitoring resources and guiding the construction of spectrum monitoring system.

Abstract:Traditional thermal power plants and hydro-power plants are difficult to deal with the problem of frequency stability caused by the rapid development of power system and the generation and parallelling in grid of renewable energy, while large-scale energy storage is applying itself in frequency regulation in power system for its characteristics of swiftness and accuracy. Energy storage system (ESS) has advantages of fast response speed, high following precision and easy-changing adjust direction, which has disadvantage of limit capacity, either. Considering the characteristics of ESS, control strategy of ESS participating in AGC is studied. Firstly, power grid AGC system model and ESS model including state-of-charging(SOC) is established. Then, with comprehensively considering the characteristics of ESS and conventional power resources, a coordination strategy between fast and slow regulation sources is proposed. Finally, three different scenarios is designed, and the efficiency of the proposed strategy is demonstrated through the sets of simulation tests.

Abstract:The structure of distribution network is becoming more and more complex.In this paper, based on the research on the distribution characteristics of the zero sequence current in the selected frequency band (SFB), a new method of single phase grounding faultyline selection based on the difference of the transient zero sequence current waveform is proposed. The method introduces the zero sequence currentcharacteristic component of feeder based on SFB by utilizing wavelet packet transform. By defining the absolute sums of the mutual deviations of the zerosequence currentcharacteristic components filtered out by sign function as the waveform difference coefficient of the zero-sequence current of each feeder, the faulty feeder is selected according to the maximum waveform difference coefficient of the zero sequence current of faulty feeder, which highlights the difference between faulty feeder and healthy feeder. A lot of PSCAD simulations show that the proposed method has high accuracy and reliability, and is not affected by fault resistance, fault distance, neutral grounding mode, arc and noise.

Abstract:The traditional detection method for the defects of the pin on the power connection fitting in the aerial survey of the drone is dependent on manual marking. Aiming at this problem, the deep learning algorithm RetinaNet is used to automatically extract the features of normal and defective samples and complete the fusion of low-level features and top-level features to achieve automatic labeling of defects. Considering the fact that the number of defective samples is much smaller than the normal number of samples, Firstly, the influence of the category imbalance caused by the deficiency of the defect sample on the recognition result is analyzed. The results show that the trained model in this case will make a large number of defective samples be mistakenly recognized as normal classes. Therefore, at the data level of RetinaNet, class balanced sampling is proposed to ensure that each category has the same opportunity to participate in training. The experimental results show that the proposed method can significantly improve the average precision of defect categories under the premise of maintaining the high recognition rate of normal categories.

Abstract:Currently, the cross-section optimal design of overhead transmission lines for wind farms is on the basis of typical environmental parameters, which neglects the dynamic changes of weather conditions (like wind speed) and still has potentials to dig out transmission capability. In order to furtherly optimize the cross-section, this paper establishes mathematical model to characterize the optimal cross-section design process of overhead transmission lines for onshore wind farms and proposes an method which can be applied to optimize the cross-section design of overhead transmission lines. In which, the cross-section of transmission lines is designed and selected optimally considering the match between dynamic ampacity increasing of overhead transmission lines and wind turbine output characteristics on the basis of reginal historical meteorological data. Finally, the proposed method is verified by case study. Results indicates that the proposed method efficiently decreases the cross-section of transmission line, saves wire material and improves economic performance of electrical design for land wind farms.

Abstract:Based on the user′s response mechanism and price elasticity theory in the peak-to-valley time-of-use price, the user will adjust the power consumption with the change of the electricity price. Therefore, the user will voluntarily adjust the power consumption plan through the electricity price change, thereby reducing the load curve. The peak-valleydifference reduces the cost of each side of "source", "grid"and "load" and increases the income. In this paper, the fuzzy clustering algorithm is used to classify the daily load, and the real-time electricity price model is established according to the peak-middle-valley membership degree of each hour. Based on the relevant policies after the new electricchange, the mathematical model of the power supply side and the grid side is established. The "source", "grid", and "load" sides of the revenue are the optimization model with the largest gain on the grid side and the minimum peak-valley difference as the objective functions, and the load side user electricity cost does not rise as the constraint. The particle swarm optimization algorithm is used. Solve and verify the model′s narrow peak-valley difference and increase the effectiveness of the gain on the Matlab simulation platform.

Abstract:When the AC system occurs breaker failure fault or dead zone fault in Ultra high voltage AC/DC hybrid system, the fault will be isolated after a long time, it can lead to commutation failures and blocking in DC transmission systems. In order to solve this problem, the fault isolating time components as well as the optimized links of breaker failure and dead zone protection in AC system by 3/2 connection are analyzed, and optimization techniques of breaker failure and dead zone protection is researched. It presents one kind of distributed fast failure and dead zone protection system by string integrated. By optimizing the logic of circuit breaker failure and dead zone protection, simplifying the circuit, shortening the transmission time of remote trip command and improving the logic of remote trip protection, reducing the redundant delay of multi-link, it can reduce the breaker failure and dead zone fault isolating time to less than 200 ms. Results of simulation and prototype experiment verify the effectiveness of the proposed theory and control strategy.

Abstract:There are many zero injection nodes with zero injection power in the actual power grids, and the injection power of zero injection nodes is measured as absolute accurate measurement, but these measurements are not fully utilized. In the state estimation, the result of the first step linear process of the bilinear WLAV state estimation is modified by the zero injection constraint, which not only improves the estimation accuracy, but also does not increase the order of the coefficient matrix, and the improved algorithm still has a higher computational efficiency. Simulation results based on the IEEE standard systems and a real provincial network verify the effectiveness of the proposed method in improving the computation accuracy and computing efficiency.

Abstract:In view of the failure of continuous commutation of hybrid HVDC in the experiment, the of commutation process and continuous commutation failure of HVDC are analyzed firstly. Under this test condition, harmonic is one of the reasons for the continuous commutation failure of HVDC during restoration. Then the control strategy to suppress hybrid HVDC continuous commutation failure by harmonic voltage compensation is proposed. The harmonic voltage compensation component is added to the link to improve the reactive power support of the MMC-HVDC to the AC system during recovery. The reactive power control on the inverter side of the MMC-HVDC can help reduce the influence of harmonics, thereby suppressing the continuous commutation failure, the reliability and stability of MIDC Transmission System with MMC-HVDC is enhanced. At last, the effectiveness of coordinated control strategy is verified on PSCAD/EMTDC under the same test condition.

Abstract:With the large-scale use of sensitive loads in oil fields, the voltage sag problem has become increasingly prominent. In recent years, oilfield distribution networks have promoted information construction, providing data support for the realization of voltage sag source segmentation. Based on the structural characteristics and load moment model of the mainline distribution network, the constraint relationship between the voltage monitoring value and the electrical distance of the line is established based on the similarity of the load moments. The information monitoring point suitable for the distribution characteristics of the mainline network load is constructed. Determine the method, and characterize the steady-state voltage distribution of the line by the linear relationship between the distance coefficient and the voltage loss, and use the relationship between the voltage increment and the electrical distance between the monitoring points to locate the voltage sag source segment. Simulation results demonstrate the effectiveness of the proposed method.

Abstract:As more and more devices sensitive to voltage sag are widely connected to the power grid, the direct and indirect economic losses caused by voltage sag interference become more and more serious, which puts forward higher requirements for power supply quality. Accurate identification of sag sources is an essential step in the treatment of voltage sag.It analyzes the types of voltage sags caused by various short-circuit faults and the changes of voltage sags waveforms after transformer transmission, and establishes the standard sample waveforms of voltage sags according to theoretical analysis. A voltage sag waveform matching method based on mutual approximation entropy principle is proposed. The measured waveforms are compared with the sample waveforms and the similarity matching is carried out directly. Finally, the effect of identifying voltage sag disturbance source is achieved. The simulation results show that the method has high accuracy in identifying the measured voltage sag waveforms in power grid, and the feasibility and effectiveness of the method are verified.

Abstract:Aiming at the problems of large voltage fluctuation and high local voltage caused by reverse power flow due to large-scale access of wind and solar distributed renewable energy sources to distribution network, basing on the traditional risk assessment theory of distribution network operation, the probability model of distributed generation systems, electric vehicle and load are combined, the risk of high and low voltage are described by using severity function, and a severity function is proposed for high and low nodal voltage. The accident risk assessment in traditional risk assessment theory is replaced by contditional value of risk (CVaR) model. A calculation method of accident impact and loss considering the uncertainty of electric vehicle and load is proposed. The CVaR risk index of voltage fluctuation is established. The estimation method of special functions of distributed generation, charging and discharging power of electric vehicle is proposed. Taking IEEE33 system as an example, the CVAR risk values of voltage fluctuation in normal operation and anticipated fault cases are calculated, which verifies the feasibility and applicability of this method.

Abstract:Interface defect can be commonly found between the rod and the sheath of the composite insulator. The defect will distort the electric field nearby, which have a bad effect on the electricaland mechanical properties of the insulator.In order to study the electric field distributionof insulators under different fault conditions, a 3-D model of AC 110 kV composite insulator is established using the finite element analysis software COMSOL. The influences of the interface gap filled with air or water on the internal and the axial electric field distribution of the insulator are discussed. The results show that the electric field magnitude at the air gap increases significantly compared with the normal case. The maximum value of field is correlated positively with the gap span and thickness, and negatively with the gap length. Theoretical analysis based on the air gap equivalent arc cylinder model is given, and the deficiency of the equivalent cylinder model is corrected. Water permeation reduces the air gap field magnitude, meanwhile severely distorts axial electric field of the insulator. The field magnitude goes up linearly as the sheath damps, which may lead to corona discharge, causing flashover and other accidents, when the damping reaches a certain degree.

Abstract:In the HVDC transmission project, the problem of water leakage in the valve cooling system of the converter station will seriously affect the stability of the HVDC transmission. To study the problems like above, it will consider the factor of electric field strength around the cooling system in valve tower, and the method of finite element analysis is used to construct the three-dimensional model of the cooling system in the valve tower and a detailed model of S-type pipe. The electric field simulation is performed according to the operating conditions of the valve tower. In the end, an experiment is designed to verify the FEA model. The results of simulation show that the electric field strength of some parts of cooling water pipe is greater than the air breakdown field strength and the grading wire will reflect on the electric field around cooling water pipes. The experiment proves that there will be obvious discharge between the electric grading wire and the cooling water pipe under certain conditions. The research provides a reference for the analysis of the cause of water leakage accidents in valve towers.

Abstract:Early insulation defects and hidden dangers in gas insulated switchgear (GIS) can be found by partial discharge (PD) detection of GIS, and then the insulation accidents can be prevented. In this paper, the complex wavelet transform (CWT) is used to process the ultra-high frequency partial discharge (UHF PD) signal in GIS at different scales. The trend curves of CWT energy entropy (CWT-EE) under different decomposition scales are analyzed, and it is found that the PD feature information mainly distributed in the scales, in which the gradient of CWT-EE are big. Besides, The CWT-EE characteristics and their scales are extracted to the structure characteristic pairs for PD type identification, which contained not only the PD signals energy feature information, but also the wavelet scale information of UHF PD signals. Finally, the support vector machine (SVM) method is used to classify four typical defects UHF PD signals in GIS. The recognition results show that the characteristic pair can effectively identify four typical defects in GIS and obviously reduce the decomposition scales of UHF PD and the feature dimension.

Abstract:On-line monitoring system for MOA has been adopted in more and more applications in intelligent substations, but also encountered some problems during the application process, such as insufficient reliability , data accuracy, low integration and high rate of system failure. In order to solve the above mentioned problems, the design for the on-line monitoring system for MOA is improved. Integral structure, design of the sensor, installation method and the design of the intelligent electronic device are proposed, and a high measurement accuracy, flexible and adaptable on-line monitoring system for MOA with high reliability is provided, which also fully supports fiber-based GOOSE, SMV and station level MMS to better meet the application requirements in intelligent substations.

Abstract:The energy feedback device can feed regenerative braking energy of motor from dc bus of inverter to the power grid. Due to the parallel structure between the diode rectifier of inverter and energy feedback device, circulating current is easy to happen in the system when the energy feedback device is running. As a result, the inverter power loss will increase and efficiency of energy feedback system will reduce. Based on the analysis of the equivalent circuit within the condition of circulating current, it is found that the magnitude of circulating current is mainly related to the zero-switching vector. And then a dual-carried wave SVPWM modulation strategy is proposed. With this strategy, the switching state of energy feedback device is changed to suppress the circulating current and it will not increase additional hardware cost. The simulation results show the effectiveness of the proposed method.

Abstract:Because the power system has many uncertainties, the deterministic method can not evaluate the risk of the shaft of the generator suffer from the transient amplification accurately, this paper presents a risk evaluation method. It takes the influence of fault clearing time on the transient torque into account, and introduces probabilistic collocation method to solve the probability density function of the transient torque. Meanwhile, it also defines the severity function of the transient torque amplification. On this basis, it determines the study area of the shaft of generators suffer from damage, reduces the scope of risk assessment, and thus more comprehensive and accurate risk assessment results is obtained. Finally, The results of calculation example of typical IEEE 16-machine 68-bus system show that the proposed assessment strategy is effective and practical.