Abstract:Aiming at the problems of insufficient operational flexibility and low economic benefits caused by the shortage of adjustment resources of the park-level integrated energy system (PIES), this paper coordinates and considers two measures of residential users participation in integrated demand response (IDR) and the addition of energy storage and conversion equipment, and proposes an economic optimization method for PIES. First of all, a IDR model including price-type and incentive-type loads on the basis of considering the adjustment capabilities of equipment such as air-conditioning and gas heating is established. Secondly, the behavioral characteristics of residential users participating in demand response is analyzed. Then a correlation model between room temperature deviation and user participation. Finally, an economic optimization model of PIES is constructed that takes into account the IDR and equipment expansion. The analysis of an example shows that the proposed economic optimization method fully considers the participation of residential users in comprehensive demand response, adjusts resources and optimizes the allocation of additional equipment capacity, which effectively improves the economy of PIES.

Abstract:This paper takes the multi-energy complementary energy internet economic operation as the research purpose, considers the cooperative operation, constraints and time-of-use electricity price factors among multi-energy flow equipment, and takes the economic and environmental comprehensive benefits as the multi-objective function, and proposes three different operational optimization control strategies. In this paper, the mathematical model including wind and solar power generation unit, combined cooling, heating and power unit, cooling/heating unit and electricity storage unit is studied. The model parameters are obtained by optimization using the adaptive niche genetic algorithm. According to the second-level comparison principle, the comprehensive benefits under three different operation modes are compared based on the model. Finally, the system scheduling optimization model is applied to a typical summer day in an industrial park. The simulation results show that the comprehensive benefits of the park are optimal in the state of grid-connected electricity sales.

Abstract:In order to improve the balance of energy supply and demand and reduce energy consumption, the design of digital twin system of energy Internet under the background of "double carbon" is proposed. The physical perception layer, transmission interaction layer, data sharing layer, application service layer, virtual space and security protection constitute the overall framework of the digital twin system of energy Internet. Starting from the two objectives of carbon dioxide emissions and capacity investment cost, the objective function is constructed. Based on relevant constraints, the harmonic search algorithm is applied to solve the objective function, so as to realize the planning of the energy Internet and complete the design of the digital twin system of the energy Internet. The experimental results show that the gas consumption, water consumption and electricity consumption of the designed system are low, and the storage battery capacity is high, and the energy supply and demand are balanced.

Abstract:For the purpose to reduce the cost on energy storage for smoothing wind power fluctuation, this paper proposes a new method which selects a hybrid energy storage with certain confidence level to smooth wind power fluctuations. Firstly, comparing the smoothing effect of mixed energy storage for the fluctuation of typical daily wind power output with different time constants, and the reference power of energy storage is obtained. Then, the empirical mode decomposition (EMD) is utilized to decompose the energy storage reference power into a series of intrinsic mode function (IMF). According to the least alias in the instantaneous frequency-time curve, the energy storage reference power is divided into a power type high-frequency energy storage configuration and an energy-type low-frequency energy storage configuration. A reasonable energy storage type is then chosen based on energy storage cost on this basis. Finally, the actual operation condition of a wind farm in Hunan is simulated. It shown that the capacity and power required for energy storage to smooth wind power fluctuations is significantly reduced after considering the confidence level.

Abstract:The integration of a large number of single-phase loads in distribution networks could lead to the imbalance of three-phase power flow. In order to eliminate a series of hazards caused by the unbalanced three-phase loads in distribution networks, an automatic balance control method is proposed to mitigate the degree of the three-phase unbalance of the distribution transformer. Subsequently, the structure of the commutation type unbalanced load automatic balance control system is systematically analyzed, and the commutation control flow and commutation strategy are further designed. The control flow of the circuit breaker and thyristor of the commutation unit based on the composite switch is elaborated. Finally, a three-phase unbalanced simulation model of the distribution transformer is established to verify the effectiveness of the proposed control strategy and control flow.

Abstract:Multiple energy storage units in DC microgrid may encounter some problems during the progress of supplementing or absorbing the power of a system, such as the overcharging and over-discharging problems due to their different initial state of charge (SOC), and the traditional droop control method cannot reasonably allocate load current of the energy storage system in real-time. A new droop control method based on the battery SOC exponent is developed in response to the above problems. The difference from the traditional method is that the new method can change the droop coefficient in real-time according to the change of the SOC exponent so that the battery SOC and the load current reach an equilibrium state. In the paper, the SOC control characteristics are analyzed, and then the reasonable parameters n=2 and p=7 of the SOC exponent control algorithm are selected. The The automatic charging and discharging process under battery charging, discharging, and load fluctuations is simulated on Matlab. It is shown that the proposed method can effectively balance the load current and SOC of the two battery groups. In addition, the voltage fluctuation amplitude is controlled within the allowable range.

Abstract:When a fault occurs on a DC microgrid line, the rapid surge of fault current is extremely detrimental to the reliability of the system power supply. Therefore, a quick fault detects and remove is important for the reliable operation of power supply. The traditional current differential protection has good quick rapidity and selectivity. However, its sensitivity is greatly affected by the short circuit impedance and may lead to the malfunction of protection under the high resistance short circuit fault. Aiming at the radial topology of DC micro-grids, according to the current variation characteristics at both sides of the fault point, a low braking coefficient is utilized to realize the detection of high resistance faults, and the protection scheme is simulated and verified by building a DC micro-grid model in PSCAD/EMTDC. The simulation results indicate that the improved protection scheme can accurately identify and remove the faults when different short-circuit impedance faults occur in the protected DC lines.

Abstract:The typical scene generation method has problems such as failing to fully consider the coupling relationship between uncertain factorslike wind, light and load, traditional clustering methods perform poorly on high-dimensional data sets, and the extracted typical scenes cannot well reflect the characteristics of the original data. To solve these problems,combining with Bayesian formula and multivariate Gaussian distribution, this paperfirstly uses Gaussian process regression (GPR) to model the coupling relationship of various uncertain factors in the power systemand generate the simulation operation data on the basis of improving the kernel function. Secondly,using the time series segmented typical scene extraction method, the total scheduling interval is divided into several sub-intervals. The center points are clustered respectively. Sub-interval weighted typical scenes are obtained and connected by Cartesian productto generate typical scene set of the full scheduling interval. Then, a method based on land movement distance (EMD) is applied to evaluate the extraction effect of typical scenes.Finally, it is verified that the extracted typical scenes can better retain the probability distribution characteristics of the original basic scene setand fully reflect the coupling relationship between uncertain factors in the original data set. The resultsshow that the typical scenes extracted by the method can be better reflect the characteristics of the original data.

Abstract:During the process of the commutation failure in power grid, a large amount of dynamic reactive power are consumed. Once a commutation failure occurs, it affects the safe operation of the grideasily. Under the background, anreservereactive poweroptimization for AC-DC hybrid power gridsisproposed to reduce the risk of accidental DC commutation failure propagation. Firstly, a differential algebraic equation for the AC / DC hybrid system is established. The multi-order trajectory sensitivity of the system is obtainedon the basis of the first-order trajectory sensitivity. According to the current operating conditions of the power system, a dynamic reactive power reserve optimization model including a generator and a reactor is then established, and the optimal solution of the dynamic reactive power reserve optimization is solved. Finally, a practical system ismodeledto verify the feasibility and effectiveness of the method.

Abstract:In China, most of the receiving systems with the UHVDC access are in the situation of "strong DC and weak AC" in present. After the DC blocking fault occurs, the system will undergo the large-scale power flow transfer, which may cause overloads in some lines. Under the circumstance, a thyristor-controlled series capacitor (TCSC) device is added to the receiving system, and a bilevel planning model is established for fixed-capacity and site selection of TCSC devices which takes N-1 security constraints into account. The upper-level planning model aims to minimize the investment cost of installing a TCSC device. The lower-level planning model aims to optimize the power flow under the constraint of N-1. Afterwards, the lower layer selects the N-1 fault that has a serious impact on the system operation through the optimal power flow solution. The lower layer results are returned to the upper layer to add a new constraint and the final solution is given by iterating. The planning model proposed in this paper are verified in the modified IEEE 39-node system.

Abstract:The existing methods ignore the clustering optimization process when predicting the short-term load of electricity, which leads to a long prediction time and a low accuracy of short-term load prediction. Therefore, a short-term load forecasting method based on improved K-means algorithm is proposed. This method uses the improved K-means algorithm to cluster the big data of power load, uses the training samples obtained after clustering to construct the RNN topology structure of the recurrent neural network. Then the optimal weights are set for the RNN neural network model to realize short-term forecast of the power load. The experimental results show that the proposed method has high forecasting efficiency and high short-term load forecasting accuracy.

Abstract:In order to promote building energy conservation and realize the coordinated control and management of distributed energy in different buildings, this paper firstly builds a building virtual power plant framework including wind power, photovoltaics, energy storage equipment, electric vehicles and flexible loads, and expounds the flow of energy and information in the virtual power plant. Then, taking the lowest net cost as the objective function, a coordinated and optimized scheduling model of building virtual power plant is constructed, which also considers the optimal sharing of electric energy between different buildings. Finally, taking a building virtual power plant including a residential building, a commercial building and an office building as an example, three scenarios, i.e., not constructing a virtual power plant, building virtual power plant scheduling each unit in the building and building virtual power plant scheduling units in a building and power sharing between buildings, are analyzed. The scheduling results in the three scenarios are compared to verify the effectiveness of the above models. The results show that the model can realize the coordinated optimization of electric energy within the building and among different buildings, and effectively improve the economy and load of the virtual power plant.

Abstract:The topology of distribution networks can be instantly modified to handle faults, but the operation ranges of the protection devices will be affected. In this paper, an adaptive protection method for distribution networks based on the dynamic topology is proposed to realize the optimal configuration of protection devices under different topologies. A dynamic topology tracking technology of node switch is built based on the traditional adaptive protection method. According to the system structure changes, this technology also amends the topology adaptively. During this step, the topology matrix is modified, and the system protection parameters are updated adaptively. Finally, the proposed method is compared with the traditional method in a test case. The results show that the proposed method improves the setting range of adaptive protection and the protection performance.

Abstract:The fast and accurate diagnosis of fault components is one important precondition to realize the on-line regulation and operation of power grids. For the sake of its understandably modeling and precise logic, the Petri net fault diagnosis method are widely studied. However, most current methods only consider the digital information rather than the analogue information, despite that the analogue information is more accurate, fault tolerant and so on. Thus, a fault diagnosis method of power grid based on Petri net is proposed by considering both the analogue and digital information. Firstly, the fault current of the components is extracted by Hilbert-Huang transform, and the fault probability of the components is obtained through fuzzy logic analysis and fuzzy logic Petri net reasoning. Secondly, based on the digital information, the Petri net model of the components is established, and the constraint of fault sequence information is probed, and the fault probability is obtained by fuzzy reasoning calculation. Finally, D-S evidence theory is utilized to explore the fault probability level to obtain the final failure probability of components. Several fault cases of the New England 10 machine 39 bus system are tested. The results show that the method can quickly diagnose fault components, improve the limitations of the previous single fault source and single diagnosis method, and hence effectively improve the accuracy and fault tolerance of fault diagnosis.

Abstract:In order to solve the difficulty when identifying the high-resistance ground faults in resonant grounded distribution networks, this paper proposes an identification method for based on multi-frequency admittance measurements. An equivalent circuit for injecting characteristic signals is established. By injecting multi-frequency current signals into the distribution network, and measuring the return voltage signal, the parameters of multi-frequency admittance and damping rate in the distribution networks can be obtained. The incremental damping rate generated by the fault transition resistance is superimposed to effectively amplify the fault characteristic signals, and thus the influence of interference can be reduced so as to effectively identify the high-resistance ground faults. A typical 10kV distribution network is established via the PSCAD / EMTDC to simulate the ground faults under different operating conditions. Simulation results show that the method can explore ground fault characteristics by superimposing the system's incremental damping rate under multi-band characteristic signals. This validates the high anti-noise ability of this method when identifying high resistance ground faults.

Abstract:The electricity-to-hydrogen technology provides a new idea for solving the problem of abandoning wind and light. Aiming at the issue of large-scale environmental protection of hydrogen energy, this paper proposes an idea for constructing an electricity-hydrogen integrated station. Based on the function settings of peak shaving and valley filling, renewable energy consumption and hydrogen supply, the power purchase strategies of the integrated station in the main energy and new energy markets are analyzed. Then, taking the minimum total operating cost of the integrated station as the objective function and considering the power balance and constraints of the power-hydrogen system in the integrated station, an optimal operation model of the power-hydrogen integrated station considering the market environment is established. The model can be solved by linear planning. Finally, numerical example results verify that the integrated station proposed in this paper can effectively improve the peak shaving and valley filling capacity of the power grid and the absorbing capacity of renewable energy.

Abstract:With the advancement of distributed energy and power electronics technology, traditional rigid substations can no longer meet the needs of users. AC and DC power distribution technologies based on flexible substations have emerged and developed rapidly. As the node of energy transmission, flexible substations can improve the safe and stable operation level of power grids. In order to explore the technical and economic evaluation of AC/DC distribution power systems considering the flexible substation, this paper considers the technical, economic, and social aspects of AC-DC distribution network operation, through constructing a comprehensive evaluation index system firstly. Then the index selection model is proposed based on the gray correlation clustering for analysis. Secondly, the preference ambiguity and objective information of decision makers are taken into account to establish a comprehensive index weighting model based on the fuzzy analytic hierarchy process and entropy weight method. In addition, the TOPSIS method improved by the gray correlation is utilized to construct a transaction comprehensive evaluation model of DC distribution networks. Finally, different distribution network operation scenarios are considered analysis. It is shown that the integrated evaluation model established in this paper can better reflect the true level of the evaluation scheme. On the other hand, the AC/DC distribution network involving flexible substations for distributed power sources such as the wind and solar power has greater technical and economic advantages than traditional distribution networks.

Abstract:Effective management of voltage sag is beneficial to users, power grids and the region where it is located. Comparing the investment and financing economic evaluation of various modes of voltage sag treatment has practical application value. This paper firstly proposes an optimal configuration model aiming at the highest cost-effectiveness for users in the user self-pay governance model. Secondly, a rental model is presented with multiple considerations in the service fee governance model.Then, the cost control factor is introduced into the PPPO&M model to optimize the rent model.The PPP model based on the introduction of government investment is incorporated into the regional economic benefit and regional environmental benefit optimization investment model. Finally, based on the four models, the economic evaluation of different governance models is discussed. Simulation is carried out in the IEEE 30 system to calculate the net present value, internal rate of return and investment return period of all parties in each model and compare the economics of multiply models under the same conditions.

Abstract:In recent years, power grid companies continue to increase their investment in distribution network. However there is a lack of a scientific and reasonable investment benefit evaluation system for distribution network projects. In order to solve this problem, this paper firstly constructs a two-level distribution network project investment benefit evaluation index system from the aspects of equipment level, grid structure, power supply capacity, and newly added power supply. Secondly, the analytic hierarchy process and entropy method are used to calculate the weight of each index in the evaluation system. Then the game theory method is used to synthesize the two weights to form the final weight of each index. Finally, the principal components of the evaluation index data of each project are extracted by the nuclear principal component analysis method, and the investment benefits of each project are sorted based on the comprehensive score of the principal components and the investment amount. Numerical simulation examples show this method can scientifically and reasonably evaluate the investment benefits of distribution network projects and effectively guide the construction and transformation of distribution network.

Abstract:XLPE insulation aging affects the operation of the power system. Based on the insulation state detection project, this paper proposes a PLS aging time prediction model based on multiple feature detection quantities. Aiming at the small data collected and the multi-collinearity problem in the model, the least squares support vector machine (LSSVR) is introduced to optimize the model principal component score vector. Then, the LSSVR-PLS aging time model is established utilizing the new score vector. Finally, the nonlinear processing ability is compared and tested by a T test and the 110 kV XLPE cable samples in a certain area of Hangzhou is considered. It is shown that the improved model is suitable for the processing of small sample data of cable detection, which can eliminate the multi-collinearity problem existing in the original model and achieve a higher prediction accuracy. The proposed research provides an important guiding significance for the cable operation and maintenance and the transformation of power grid.

Abstract:With the development of city and its power supply demand increase, the overhead-cable hybrid line have become more and more widely application due to its flexible structure. The overvoltage caused by the closing operation of the hybrid line has become an important factor affecting the safe and reliable operation of the power system. In this paper, PSCAD simulation software is employed to study the closing overvoltage characteristics of 220 kV extra high voltage (EHV) overhead-cable hybrid lines in a certain area. An overvoltage simulation model is established, and the closing overvoltage characteristics generated during the closing operation of the hybrid line are simulated and analyzed. Finally, the affecting factors of the closing overvoltage are discussed. The results show that, the overvoltage amplitude of three-phase non-synchronous closing is 10%~20% larger than that of three-phase synchronous closing. The closing overvoltage is related to the closing phase angle, and the closing overvoltage amplitude is inversely proportional to the closing resistance. Through the coordination of the closing time, the closing resistors are input in stages to release energy systematically. Thereby, it is found that the application of multi-level closing resistance can limit the operating overvoltage more effectively. The proposed research provides a reference for the design of EHV overhead-cable hybrid line.

Abstract:To study the variation law of drag coefficient for the bundled conductor with the wind attack angle and splitting distance, the CFD numerical simulation method is employed to study the drag coefficient variation law with the wind direction angle of drag reduced conductor under different splitting numbers and spacing. A model test is carried out in the boundary-layer wind tunnel for a 90-cm conductor. Under different wind attack angles, the drag coefficient variation of the drag reduced conductor, concerning Reynolds number and interference effect, are obtained and then compared with numerical simulation results. Research results show that the interference effect of the multi-bundled conductor drag coefficient is noticeable, and the average drag coefficient can be reduced by more than 20% with the increase of splits. The multi-bundled conductor drag coefficient varies obviously with the wind attack angle. The peak value of the wind drag coefficient will appear in the case of windage yaw. The peak value of 2-bundled conductor appears at a wind attack angle of 18°~24°, and the peak value of 4-bundled conductor appears at a wind attack angle of 22.5°~30°. The wind load design of bundled conductors should consider the adverse influence of windage yaw and the interference effect.

Abstract:The wireless power transfer technology has been applied to the robot charging system based on its safety and reliability. In order to improve the coupling performance of the system and reduce the electromagnetic radiation around the coil, a magnetic coupling resonance of a novel semi-enclosed magnetic core structure is proposed. Firstly, the magnetic circuit model is established with a disc core, and the expressions about coupling coefficient are derived, which provide the basis for designing magnetic core structures. The reluctance of mutual-coupling area is reduced by adding a certain height magnetic ring on the outer edge of the transformer with disc core. Meanwhile, a certain inner diameter of the core is removed from the bottom core to increase the reluctance of self-coupling area. Secondly, the magnetic circuit model of the new Semi-enclosed magnetic core is established, and the coupling coefficient expression of the new magnetic core system is derived. Three structural parameters (magnetic ring wall thickness, magnetic ring height, core inner diameter) affecting the system performance are obtained. Based on Ansys Maxwell simulation, three parameters impact is analyzed for the coupling coefficient and electromagnetic radiation, and the magnetic core structure parameters are optimized by parameter scanning. Finally, the simulation results are verified by setting up an experimental platform. Compared with the system with a disc core, the new Semi-enclosed core structure has a 5.1% improvement in the coupling coefficient, and the new core offers 1.46 times improvement in the shielding effect under the coil.

Abstract:In order to solve the general lightning problems of 35 kV transmission lines, a new arc-extinguishing and lightning protection gap (NALPG) is proposed in this article. So as to verify the feasibility of the device, the principle of arc extinction of the device is discussed, and the mechanism of arc development and extinction is studied. Meanwhile, the electromagnetic transient calculation software is applied to build a 35 kV transmission line model of lightning strike to analyze the transient process of the device after arc quenching. Simulation results show that NALPG has the ability to reduce lightning overvoltage and harmonic components. Then, experimental circuit is constructed to conduct power frequency arc cut off test. The test results show that the entire arc extinction time is only 4.0 ms and the arc does not reignite for a period of time. Until now, the actual lightning protection effect of NALPG shows excellent results in line operation.

Abstract:In recent years, the misoperation of gas protection caused by the external short-circuit impact occurs frequently, which threaten the operation efficiency of the transformer. Under the background, in order to explore oil flow characteristics in the pipeline when the external short-circuit fault occurs, this paper establishes a transient oil flow measurement system on the power transformer and carries out an external short-circuit fault test. The test results show that the transformer winding deformation and vibration caused by the short-circuit current will lead the oil flow through the pipeline. Due to the inertia of the liquid, the time delay of oil flow measured in this paper is up to hundreds of milliseconds. In the actual operation, the time of gas protection misoperation lags behind the external fault for hundreds of milliseconds, which keeps consistent with the delay phenomenon of oil flow action. The test also shows that the oil flow surge is affected by the severity and location of the fault. In addition, the results of the oil flow action delay also provide an essential reference for further study on the mechanism of oil flow shock and the misoperation of gas protection.