Abstract:Voltage source converter based high voltage direct current(VSC-HVDC) provides a new way to solve grid-connected accommodation problem of renewable energy. However, the active power fluctuation of renewable energy will be transmitted to receiving-end power grid through VSC-HVDC under no additional control. In order to solve active power fluctuation problem of renewable energy grid-connected through VSC-HVDC, an islanded large-scale renewable energy power system in Zhangjiakou area which delivered by VSC-HVDC is studied. The short-time power fluctuation characteristic of wind farms in this area is obtained by actual operation data analysis. A virtual frequency control strategy is designed on the basis of voltage frequency(VF) control in an islanded VSC station. Thus,the active regulation of power fluctuation function can be realized by proposed virtual frequency control in VSC station and primary frequency regulation(PFR) in renewable power generation. Finally, a simulation platform of large-scale wind power delivered by single terminal VSC-HVDC is built. Simulation results testify that the power fluctuation is well regulated by proposed virtual frequency control cooperated with the PFR control in terms of wind power generation operated normally.

Abstract:In order to make the new energy terminal of voltage source converter based high voltage direct current (VSC-HVDC) interconnected system with new energy access participate in the frequency adjustment of AC network as a whole, an active power control strategy for the converter station terminal with frequency-voltage dead zone is proposed. This strategy using frequency-voltage dead-zone limit and real-time control of the output power of VSC-HVDC. When the frequency of AC network changes on the system side, the kinetic energy reserve of the wind rotor increases or decreases the active output in a short time, so that the speed of the wind Turbine decreases or increases. By increasing/reducing the kinetic energy reserve in the rotor of the wind motor, the unbalanced active power at the AC system terminal can be alleviated, through the VSC-HVDC interconnected system to make frequency response to participate in frequency modulation.Finally, simulations with PSCAD/EMTDC verify that the proposed control strategy is able to enhance the frequency stability of the VSC-HVDC interconnection system with new energy access.

Abstract:In the sending system of wind-photovoltaic-thermal-bundling power grid transmitted by multi direct current(DC), the new energy and DC transmission proportion is large, system inertia level is low, carrying capacity of alternative current(AC) grid structure is not strong and stability issues of power angle, frequency and voltage are prominent. Based on summarizing new characteristics of the northwest China power grid after wind-photovoltaic-thermal bundled, problems during security operation of the northwest China power grid are analyzed, and shortcomings of existing perceiving and controlling methods are pointed out. The security and stability defense system construction scheme of wind-photovoltaic-thermal bundling power grid for DC transmission is focused. Besides, main functions of defense system are elaborated in detail, such as synchronous stability controlling, frequency controlling, full process voltage controlling and key technologies of major breakthrough. The evaluation effect shows that the defense system can effectively deal with fault impact of ultra high voltage direct current, block cascading faults, and improve security and stability level of the northwest China power grid operation. The technical scheme guides the development of controlling technology and engineering practice for power grid with wind-photovoltaic-thermal power bundling and multi-DC weak sending-end.

Abstract:Fast and effective assessment of power systems security region can provide decision-making guidance for system operation. In order to identify operational state of power system, a method of static voltage security region (SVSR) considering wind power integrated is presented. Firstly, index of voltage security margin is presented by combining continuous power flow model and condition of limit induced bifurcation, which could quantify the level of voltage security. Then, hyper-planes of SVSR boundary by blending quasi steady state equation and heuristic algorithm are calculated. The hyper-planes assist in online assessment of security state and load margin. Further, the method of wind power forecasting considering correlation is introduced, which establishes foundation for situation prediction. Finally, the correctness and effectiveness of the proposed method are demonstrated by cases studied in the IEEE 118-bus system. The simulation results show that when the permeability of wind power is high, correlation between wind power output and fluctuation range of voltage safety margin are greater.

Abstract:The technology of HVDC based on VSC develop rapidly in recent years. To ensure the safe operation of power grid, the fault-crossing capability has become a basic demand for wind farms connected by HVDC transmission. The paper researches on the fault-crossing capability of VSC-HVDC connected wind farm. In order to solve the problem that the fault of grid connection point leads to the reduction of transmission active power of converter station, which makes the power of AC system and DC system unbalanced, and causes the problem that the rapid rise of DC voltage affects the operation of the system. Based on the traditional energy-dissipating resistance circuit, a new resistance topology is proposed. When the fault occurs on the grid side, the VSC-HVDC wind power grid-connected system can smoothly cross the fault by putting in the energy-consuming resistance to absorb the power difference and coordinated control combined with wind turbines is carried out. Finally, the model of wind farm connected by VSC-HVDC is built in PSCAD/EMTDC and dynamic simulations are presented to assess the performance of the control method which is proposed.

Abstract:With the improvement of lean operation demand of power grid, continuous regulation control technologies such as direct current emergency power support, rapid power generation, and load control have received more and more attention in the current power grid operation control. In view of frequency problem under increasing scale of new energy and ultra high voltage direct current(UHVDC), control requirements for different stages of frequency after the fault are analyzed. Then coordination principle and frequency emergency coordination optimization control measures which consider response characteristics, cost, as well as quantitative index of transient frequency security margin are proposed. The cost of frequency control after fault is reduced through coordination of continuous and discrete control measures which also meets the requirement of modern power grid optimization frequency control. Based on actual UHVDC sending-network, feasibility of the proposed method is verified.

Abstract:When virtual inertia control and droop control are used simultaneously in high proportion wind power system, there will be frequency regulation power competition problems. In order to improve the frequency regulation effect of the system, it coordinates and optimizes the virtual inertia control and droop control of wind turbines. A model of coordinated control strategy is constructed for wind turbines participating in frequency regulation. The particle swarm optimization algorithm is improved:according to search algebra, inertia weight ω, self learning factor c₁ and social learning factor c₂ are dynamically adjusted;and mutation operation is added. Based on the improved particle swarm optimization algorithm, the control parameters are optimized. The coordinated control strategy of wind turbine participating in frequency regulation is obtained. Finally, a provincial power grid is taken as an example to verify the effectiveness of control strategy and improved algorithm.

Abstract:Situation awareness of smart distribution network (SASDN) is the basis of stable operation and precise scheduling. Distribution network has a broad application prospect. However, SASDN is faced with a series of time-delay uncertainties caused by communication delay, scientific calculation time, and different sub-station system response time, which cannot meet the requirements of accurate scheduling of intelligent distribution network and limits its application scope. For this reason, the time-delay uncertainty of intelligent distribution network situation awareness is analyzed and expounded from the technical level. Firstly, the research status of this problem at home and abroad is introduced. Secondly, the mechanism and influence of time-delay uncertainty of situation awareness in intelligent distribution network are analyzed . Finally, the general framework of the study on the time-delay uncertainty of situation awareness in smart distribution network is constructed to provide reference for the future research.

Abstract:In order to effectively forecast large area power grid line trip blackout caused by mountain fire, spatial-temporal characteristics of fire hazards in Guangdong power grid mountains are studied by using the data of fire spots number, the area of overfire and the tripping frequency of main network from 2008 to 2017. A model of optimized risk early warning system based on partial least square(PLS) is proposed. It is found that the east and north of Guangdong are sensitive to mountain fires, and the occurrence of mountain fires in the overhead line coverage area has an obvious increasing trend. In addition, based on historical meteorological data, vegetation data and topographic data, the PLS method is used to select the main influencing factors of mountain fire prediction, and an optimized risk early warning model of mountain fire in power grid is proposed based on the principle of applied statistics. The validity and practicability of proposed method are verified by the case test of mountain fire trip in Guangdong power grid.

Abstract:The best way and technical path of optimizing the primary energy structure is studied, so as to provide reference for China's energy optimization development and technical innovation. Based on the analysis of current situation and trend of global energy development, it has taken theoretical innovation as the breakthrough point, established the energy-brain model, put forward the concept of energy life-cycle, studied the ways to optimize the energy system, as well as analyzed and discussed the technical demand and technical path of energy optimization in China. It has revealed that the energy industry can work as a closed-loop system, pointed out that optimizing primary energy through secondary energy is the most convenient and effective way to optimize the energy system and extend the energy system life-cycle. Furthermore, electric energy is the best choice of secondary energy to optimize primary energy. It has concluded that through the technical innovation in three directions of primary energy to electric energy conversion, electric energy transmission, energy storage & Energy Internet, and the continuous and stable supply of electric energy can be ensured. Meanwhile, the energy system life-cycle can be extended, and the clean, low-carbon, safe and efficient energy system can be built more efficiently.

Abstract:When studying the interaction between the ultra high-voltage direct current(UHVDC) system and the AC system, the accurate electrical quantity changes of DC fault are important. In order to accurately simulate and analyze the characteristics of fault changes in UHVDC transmission systems, on the PSCAD/EMTDC simulation platform, taking Binkin DC transmission system as an example, a 800 kV, 8 000 MW bipolar UHVDC transmission system model is built. This model mainly details the reactive power compensation equipment, filtering device, DC line and DC control system in the DC transmission system. For the transmission and reception AC systems, the Thevenin equivalents of the actual AC systems on both sides are made at the converter bus, and a practical system transmission model with a DC system as the main and a simplified AC system is built. The commutation failure of typical faults in the DC system is simulated and analyzed. The main electrical changes in UHVDC system before and after the failure of commutation failure are analyzed. The feasibility of the model is verified by comparing with the waveform of the actual fault recorder. The changes of active power and reactive power flowing into the receiving AC system on the inverter-side converter bus are emphasized. The research conclusion provides technical support for analyzing the impact of UHVDC commutation failure faults on the actual AC system.

Abstract:Considering prediction accuracy of wind and solar power generation increasing with time scale reduction, a multi-time scale unit commitment model with wind and solar power output rotating reserve capacity is established. Meanwhile, dispatching method considering uncertainty of wind and solar power is established. Based on three time scales reduced step by step, the model updates predicted values of wind and solar power step by step to the equivalent load of wind and solar power output and rotating reserve. Then, the lower level is used to correct the upper level deviation, and the dynamic programming algorithm is used to solve the problem at different time scales to obtain the unit combination mode which approximates to maximize wind and solar output and takes economic optimization of traditional units into account. The modified IEEE 30-bus system is used as an example for simulation calculation and the results verify the validity and rationality of the proposed method.

Abstract:Optimal power flow (OPF) is generally a non-convex optimization problem. The integration of unified power flow controller(UPFC) increases the nonlinearity of the original OPF problem. As a result, it is hard to obtain a global optimal solution using the traditional interior point method. Tree growth algorithm (TGA) is proved to be efficient and robust in solving complex engineering problems. TGA can globally search for optimal solutions to solve the OPF model that considers UPFC. In this model, the power generation cost, active power loss, and voltage deviation are included in the objective function, while the secure operation constraints of the power system integrated with UPFC are considered. Finally, numerical results on the IEEE 30-bus system and an actual system of Nanjing west ring network are carried out. The optimal solutions obtained from TGA, particle swarm optimization and interior point method are compared. At the same time, different heuristic algorithms are calculated 50 times using Monte Carlo method. These results verify the effectiveness of proposed approach and better accuracy and robustness of TGA.

Abstract:In view of problems existing in multi-terminal ultra-high voltage direct current(UHVDC) transmission line, such as wide fault range and easy malfunction of protection actions, a fault diagnosis method based on wavelet transform and transient current energy difference(TCED) is proposed. Firstly, the wavelet transform is used to analyze fault current, and a mother wavelet selection method based on Pearson product-moment correlation coefficient is proposed. Then, the fault direction is determined by characteristics of TCED between two sides of the fault. The fault area is determined by fault direction information of each converter station. Finally, PSCAD / EMTDC simulation model of four terminal UHVDC transmission line is established, and performance of the proposed method is demonstrated in terms of fault location, resistance and distance. The simulation results show that the proposed method can effectively distinguish the power system transient disturbance and DC fault, accurately determine the fault area of the line, sharply improve accuracy of determination and shorten detection time.

Abstract:In order to suppress the continuous commutation failure of DC system, the main reason of the first commutation failure is the rise of DC current and the decrease of AC system voltage at the inverter side. The action characteristics of the control system and the law of the electric quantity change in the recovery phase after the first commutation failure are studied. It is concluded that during the recovery phase of the commutation failure, the fast recovery of the DC voltage at the inverter side will cause the fast rise of the DC current command value, and the inverter side will switch to the constant current control. The influence of the DC current rise on the turn off angle which is not considered in the control process results in the control loses of the extinction angle which leads to the failure of DC continuous commutation. Therefore, an optimal control strategy of DC current command value considering the turn off angle is proposed, which combines with low-voltage current limiting control to suppress the occurrence of continuous commutation failure. Finally, the effectiveness of the proposed control strategy is verified by PSCAD / EMTDC simulation software and CIGRE HVDC model simulation.

Abstract:Harmonic current tracking control in harmonic compensation is one of the key factors to measure the performance of active power filter. Aiming at the problem of adjusting linear sliding surface parameters anyway cannot make the tracking error converge to zero in a finite time for ordinary sliding mode control, harmonic current tracking control strategy based on global fast terminal sliding mode control in current loop is proposed. The method introduces nonlinear function based on a linear sliding surface, to make up for shortcomings of ordinary sliding mode control which can achieve gradual convergence of state, while improving the transient performance of the sliding mode control and eliminating the switching term, which essentially weakens the chattering. The steady-state and transient performance tests of active power filter are carried out without changing the harmonic detection link, the comparison between the global fast terminal sliding mode control proposed and the sliding mode control of the conventional exponential approach law verify the correctness and effectiveness of the control strategy proposed .

Abstract:It is particularly important to replace high-loss distribution transformers according to local conditions in the energy-saving transformation of distribution networks. Calculating distribution transformer losses under actual load conditions is one of the key points of transformer selection technology and economy. In low-voltage three-phase and four-wire system, unbalanced load often results in asymmetrical distribution and transformer three-phase current. The loss calculation model of transformer under three-phase unbalance condition is discussed at first, and then the current phase estimation method of transformer low-voltage side is put forward in view of the fact that the current phase of existing distribution transformer low-voltage side mining system data is missing. Thus, the loss calculation model based on distribution transformer current phase estimation under three-phase unbalance condition is established, which solves the problem that cannot be calculated due to missing phase data in actual engineering. Finally, the current phase estimation and loss calculation models of Yyn0 and Dyn11 commonly used transformers are verified by simulation and experiment respectively.

Abstract:For virtual power plants(VPP) composed of wind power generation(WPG) and electric vehicles(EV), uncertainty of wind power output, deviation between wind power forecast and actual output, randomness of EV and the volatility of market prices need to be considered when participating in market bidding. Considering above factors, two stage bidding model for VPP to participate day-ahead and real-time market is proposed. VPP can not only reduce the bidding deviation of wind power through EV charging and discharging, but also make bidding strategies according to the price signal to realize peak clipping and valley filling. Based on cooperative game theory, the Shapley method is used to reasonably distribute the total profit between WPG and EV according to their respective contributions. Finally, feasibility and effectiveness of proposed model are verified by an example. The results show that VPP can increase the profit and reduce the risk caused by output and price uncertainty, and provide reference for the construction of new energy participation in the spot market.

Abstract:Energy storage system is an important part of micro-grid, and ensuring the storage state of charge (SOC) of energy storage system is the key to the safe and efficient operation of energy storage system and even the whole micro-grid. A storage SOC coordinated control of AC/DC hybrid microgrid interface converter based on virtual synchronous generator (VSG) control is designed, which can improve the frequency and power stability of hybrid microgrid and the rationality of each storage SOC in the system. Firstly, the droop control mode of distributed power supply on both sides of AC-DC microgrid and subnet characteristics are analyzed. Based on this characteristic, a VSG control strategy applied to interface converter is proposed to improve the frequency and power stability of the system. In the power sharing control block, the SOC control strategy of the energy storage system is added to control the interface converter to actively exchange power between subnets in order to optimize the SOC state of each storage system. Finally, an AC/DC hybrid microgrid model is built in Matlab/Simulink to verify the effectiveness of the proposed algorithm.

Abstract:At present, the research for gas-insulated transmission line(GIL) lacks quantitative analysis of the influencing factors, especially the ventilation speed when pipe gallery is laid, the solar radiation temperature field distribution and the thermal characteristics of open-air arrangement. Therefore, the multi-field coupling analysis method is used to carry out the GIL temperature field distribution based on COMSOL. Its influencing factors are studied. The multi-physics coupling calculation model of electromagnetic field thermal field is established, and the influence of different operating environments on the thermal characteristics of GIL is analyzed. The results show that under the influence of pumping speed and solar radiation, the axisymmetric distribution of GIL temperature field is destroyed and the magnitude and position of shell temperature extremes change. The thermal properties of GIL in different operating environments are different. With the increase of pumping time and pumping speed, the difference between inlet temperature and outlet temperature gradually increases, and finally tends to a stable value; the GIL shell temperature extreme point changes with the change of sunshine direction and intensity. The influence of field distribution can not be ignored. In the case of three-phase vertical arrangement in the GIL pipe gallery, the temperature between the casings has little effect when the casing spacing is left with a suitable heat dissipation distance.

Abstract:At present, it usually relies on monitoring the surface leakage current of the insulator to realize the flashover warning. As a contact online monitoring method, it still has much room for improvement in terms of reliability and convenience. However, studies have indicated that it can be effectively monitored in a non-contact manner. Therefore, the electric field distribution characteristics of insulator space under typical surface conditions are studied. Firstly, the electrostatic field simulation model of common post insulators is established by using finite element simulation software. The magnitude of the electric field around the insulator under dry, wet and discharge conditions and the law of change are compared and analyzed. Finally, the artificial contamination test is used to verify the simulation results. The results show that the magnitude of the amplitude reflected by the insulator space in different surface states is obvious. Under the condition of surface wetting and arc creepage, the vertical component of the spatial electric field increases by nearly 50% and 20%, respectively. The research results can provide new ideas for condition monitoring of transmission and distribution external insulation.

Abstract:Two retired high-voltage cross-linked cables with service years of 16 and 32 are chosen to conduct a 180-day prequalification test to research the diversity of space charge behavior and evaluate the reliability of reusing these cables for practical operation. Space charge behavior in cross-linked polyethylene(XLPE)insulation of the samples before and after aging test is tested by the pulse electro acoustic(PEA) method, and space charge characteristics of the samples is investigated. Fourier transform infrared spectroscopy(FTIR) and X-ray diffraction(XRD) experiments are used to observe changes on micro-structure and aggregation structure of each sample. Experiments reveal the potential relationship between the structural changes and space charge behavior, including the characteristic of migration, accumulation and dissipation of space charge. The results show that the impurities causing severe accumulation of space charge inside the samples of the cable with 16 service years. After the accelerated aging test, reduction of impurities and annealing effect decreases the accumulation of space charge and speeds up dissipation process. On the contrary, the accumulation and dissipation of space charge inside the cable with 32 service years is moderate. After the accelerated aging test, the oxidative degradation and destruction of crystalline structure result in aggravation of space charge accumulation and more moderate dissipation process of space charge.

Abstract:The current research on non-technical loss(NTL) detection is mainly aim on residential customers, however, the related methods are not suit industrial and commercial customers. According to this problem, a deep learning based NTL detection method by embedding the principle of electricity measurement is proposed. Firstly, various NTL is analyzed and the phenomena show that only smart meter data is not enough for detecting NTL. Hence, smart meter data and some principles of electricity measurement are organized which describe the inherent relationship among electricity magnitudes as samples for deep learning. Secondly, an improved hybrid residual neural network is proposed to extract advanced features of NTL from massive smart meter data for detecting NTL. The experiment results show that the approach in this paper has achieved significant improvement on all metrics by comparing with the baselines.

Abstract:Since the existing monthly electricity consumption forecast has fewer influencing factors, and it is unable to comprehensively reflect the factors associated with strong electricity consumption. An elastic network electricity consumption forecasting model for high-dimensional data variable screening and high-precision prediction is proposed. The volume prediction model analyzes the monthly data of 340 variables and 96 time points for electricity consumption, economy, transportation, and meteorology. By using elastic network to screening for high-dimensional variables, and Granger causality analysis to find out the dependence of electricity consumption data and other data, the monthly electricity consumption of the whole society in a year is predicted. And the mean absolute percentage error of the prediction results is 3.07%. Compared with the VAR model, BP model and Lasso, the feasibility and effectiveness of the method are verified.

Abstract:The reliability of acquisition terminal is crucial for the safe operation of electricity information acquisition system. In this paper, the reliability evaluation model of the electricity information collection system is established, including the outage model of the acquisition terminal, the multi-dimensional feature factor association failure determination model and the risk assessment method. Sequential Monte Carlo method and non-sequential Monte Carlo method are used to evaluate the risk of the instance, and experimental results demonstrate that sequential Monte Carlo method is more suitable for the risk assessment of electricity acquisition system, since it adopts time sampling technology and can simulate the acquisition system on the time axis. Furthermore, multi-dimensional feature factor correlation model is adopted to judge whether the electricity information acquisition system is invalid and the relationship between the risk index of the electricity information acquisition system and the threshold value in the multi-dimensional characteristic factor correlation model is studied. Then the relationship curve between the risk index and the threshold value is obtained. It can be concluded from the instance that the optimal value of the threshold value is the inflection point in the curve.

Abstract:In order to study the mechanism of axial ablation failure of cable buffer layer under operating voltage, a mathematical model of axial ablation of high voltage cables is established. According to the structure and actual size of 110 kV XLPE cable, the requirement for the separation length of the metal sheath and the outer insulation shield is calculated, and the influencing factors of the axial ablation along the surface of the high voltage cable are analyzed. The results show that the main reason for the ablation of cable buffer layer and insulation shield layer is the separation of metal sheath and cable insulation shield, and the buffer layer resistivity has a significant effect on the detachment length. Reducing the buffer layer resistivity can increase the allowable detachment length. When the resistivity is 10⁵ Ω·mm, the allowable detachment length is 1 186 mm. In addition, the allowable voltage of the buffer layer and the structure size of the buffer layer are also the characteristic parameters to control the fault of the buffer layer.

Abstract:Holographic multi-scenario UHVDC converter station anti-accident deduction platform can analyze the cause of fault quickly and it is an indispensable means of fault analysis. But for the converter station, because of its model is not suitable to be built, the logic of the protection program is complex, and the simulation workload is large, so far there is no converter station anti accident deduction system. A holographic multi scene UHVDC converter station anti accident platform is developed, including PSCAD model calculation engine, simulation control system, converter station operator workstation and 3D virtual reality server. For the first time, the automatic conversion technology of converter station control and protection program is used to reduce the modeling workload while ensuring the accuracy of simulation; through the automatic test technology, various operation modes and fault types of model simulation are carried out in batches to improve the efficiency of simulation test; at the same time, the platform display system adopts the holographic three-dimensional technology to realize the visual display of the primary and secondary equipment of the whole converter station operation. In the process of anti accident simulation and verification, the earth fault of pole bus is simulated and inverted, the result is consistent with the action result of field protection device, and the accuracy is high. The platform can provide effective technical support for operators and maintenance personnel to carry out anti accident deduction, fault simulation preview and control and protection strategy analysis.

Abstract:In order to achieve the fast and reliable data interaction between HVDC control and protection system and protect information substation, an Ethernet 103 dual network communication method in HVDC based on graphical programming is proposed. The method extends the serial port 103 communication defined by IEC 60870-5-103 standard, and proposes the Ethernet 103 dual network communication scheme to improve the real-time and reliability of communication. In addition, the method separates the application program from the underlying communication procedures, and uses the standard IEC 61131 graphical design language to develop a graphical programming method for the integration of highly encapsulated underlying communication processing module and user function module, the software programming is easy and efficient, system running and debugging information can be on-line monitoring. The experimental data of project such as Xilingol League-Taizhou project and Zhalute-Qingzhou project shows that the method has characteristics of high reliability, fast development progress, convenient operation and maintenance. Also it has an important application value to achieve the fast and reliable data interaction between control and protection system and protect information substation.