Abstract:Under the goals of “carbon emission peak and carbon neutrality”, China is building a new power system with wind, photovoltaic and other renewable energy as dominant sources. With the change of energy structure and system form, the uncertainty on both sides of supply and demand of the power systems will be significantly enhanced, and system risks such as supply and demand mismatch will become more prominent. Accurate assessment of system risk level is of great significance to the planning and operation of the new power system. The typical risk characteristics of the new power system are analyzed, and a risk assessment framework for the new power system is illustrated. First, a three-level risk index system is sorted out to describe the systematic risks, which covers the upstream and downstream risks of supply and demand mismatch and risks related to carbon emissions. Then, a risk assessment method that reflects the risk characteristics of the new power system is investigated, in which the multi-source risk factors are characterized in the system state generation phase and the multi-level risk indices are calculated in the state analysis phase. Finally, through organically integrating with the risk assessment, an identification method for the weak link of the new power system is discussed to provide quantitative decision-making basis for the risk pre-control.

Abstract:In order to improve the energy utilization, reduce the carbon emission of transportation systems and promote the integration of integrated energy systems and transportation network in the background of building a new power system, the optimal dispatching model of traffic flow of fuel cell vehicles and hydrogen refueling behavior is constructed by considering the spatial and temporal distribution characteristics of load under the influence of hydrogen refueling behavior of fuel cell vehicles. Firstly, the origination-destination pair is used to portray the traffic flow model of fuel cell vehicles, and the Dijkstra algorithm is used to build the shortest hydrogen refueling path model for fuel cell vehicles to simulate the travel of fuel cell vehicle. Then, aiming at the uncertainties of renewable energy output, the fuzzy set of predicted errors for wind power is developed based on Wasserstein distance. And considering the constraints of power distribution networks and transportation networks, the distributionally robust optimization model considering the optimal dispatching of hydrogen refueling stations is developed. Finally, the distributionally robust optimization model is transformed into a mixed-integer linear programming model by using strong pairwise theory, and the scheduling method is validated by using the improved 33-bus distribution network and 24-bus traffic network. The results show that the total cost of the proposed method is reduced compared with the robust optimization system, and the scheduling scheme of hydrogen refueling stations achieves the balance between conservativeness and optimism, and realizes the positive interaction between hydrogen refueling stations and the higher-level network.

Abstract:Aiming at the problems of uneven measurement and insufficient accuracy of measurement in distribution networks, a super-resolution measurement generation method based on graph attention networks is proposed. The method can improve the spatio-temporal resolution of distribution network states, and has the ability of topology generalization. It can adapt to the topology reconfiguration conditions of distribution network to minimize measurement acquisition and achieve high-precision state awareness of distribution networks. The proposed method uses the attention mechanism to learn the correlation between electrical state variables of adjacent buses, adds power flow constraints to model training, and designs the minimum topology set required for model training, so as to avoid overfitting training sample data and improve the generalization ability of the proposed model. The effectiveness of the proposed method is verified by IEEE 33-bus and IEEE 123-bus standard distribution network cases.

Abstract:Under the background of new power system, load fluctuation at the end of the distribution network is complex, and load curve clustering is an effective method to simplify the analysis of load fluctuation characteristics. For the loads at the end of the distribution network, the mainstream single-day load curve clustering ignores the difference of daytime fluctuation, but the multi-day load curve is difficult to achieve the ideal clustering effect. In view of this, this paper proposes a new clustering method for multi-day load curve based on the concept of network structure reduction. Firstly, the multilayer associated load time series network of clustered users is generated by horizontal visibility graph method. Secondly, the structural redundancy of the associated multilayer network is evaluated and the similar layers are aggregated. Finally, the interpretability of the multi-day load curve clustering results is analyzed from three aspects: trend effect, fluctuation trend and social attribute. The experimental results show that, compared with the mainstream clustering methods, this method can efficiently deal with the clustering problem of multi-day load curve, and has the advantages of free parameter tuning in the clustering process and strong interpretability of the clustering results.

Abstract:The grid integration of high proportion of renewable energy makes the distribution network increasingly dependent on the communication system. To reduce the influence of communication equipment fault on the operation of the distributed control system, an optimal control method for distributed generator (DG) groups losing communication in the distribution network considering multiple communication faults is proposed. Firstly, the communication link matrix of the distributed communication network is established, and the typical fault scenarios are obtained based on the communication link search. Secondly, considering the uncertainty of communication fault scenarios, source and load, a double-layer optimization model of DG groups losing communication is established to minimize the comprehensive risk. The outer layer of the model optimizes the multi-scenario control strategy of DG losing communication, and the inner layer of the model optimizes the control strategy of other DGs. Finally, the improved particle swarm optimization algorithm is used to solve the optimal control strategy of the control nodes for the DG group losing communication. The effectiveness of the proposed control method is verified through an IEEE 33-bus case system. The results show that the proposed method can effectively reduce the system operation risk and improve the power supply reliability.

Abstract:Aiming at the problem of insufficient power supply capacity of urban distribution networks, this paper designs a basic architecture of centralized-distributed hybrid compressed air energy storage (CAES) station. This architecture relies on the centralized advanced adiabatic compressed air energy storage (AA-CAES) station to establish distributed liquid air energy storage stations, which can use the advantages of a large-scale storage chamber of the AA-CAES station more effectively and extend the functionality of the AA-CAES station. Then, the operation model of the centralized-distributed hybrid CAES station is established. The model can accurately describe the energy circulation, conversion, storage and release mechanism in the process of air compression, liquefaction, storage, transportation, gasification, expansion and power generation in a hybrid CAES station. Based on the operation model of the hybrid CAES stations, combined with the theory of life cycle cost, this paper establishes the optimal planning and operation model of the hybrid CAES stations. The model also contains the constraints on the options such as the expansion of transmission lines, additional gas turbines and the installation of battery energy storage, so as to compare the economic benefits of installing hybrid CAES stations and other options. Finally, the effectiveness of the optimal planning and operation model is verified by numerical case simulation.

Abstract:Aiming at the inconsistency of the state of health (SOH) and the state of charge (SOC) of energy storage units caused by irregular charging and discharging of a large battery energy storage system (BESS) composed of multiple energy storage units, the relationship between SOH difference in energy storage units and SOC consistency is studied. Combining the prioritization of charging and discharging and adaptive mutation particle swarm optimization (AMPSO) algorithm, a BESS power allocation strategy considering the consistency of SOH and SOC of energy storage units is proposed. Based on the measured data of the grid-connected demonstration platform of centralized microgrid with common DC-bus including the BESS, wind/photovoltaic power generation, electric vehicle and conventional loads, the proposed power distribution control strategy is compared with the traditional distribution control strategies. The simulation results show that the proposed control strategy can effectively improve the SOC consistency, prolong the service life and reduce the operation loss of the energy storage unit, as well as enhance the bidirectional regulation capability of the BESS.

Abstract:Heterogeneous energy coupling is a distinctive feature that differentiates an integrated energy system (IES) from a single energy system such as a power distribution system. This paper proposes a measurement of the energy coupling degree in the IES, designs an observation method, and reveals new laws and mechanisms. First, this paper proposes the energy coupling indices for coupling units and entire IES. Secondly, this paper establishes the operation region model of the IES, which describes the maximum allowable operation range of the system in normal operation constraints. Finally, the proposed coupling degree is verified by a typical electricity-gas-coupled IES. This paper observes the variation law of the operation region of a electricity-gas-coupled IES in different coupling degrees, in which two critical coupling degrees and three types of phenomena are discovered. The analysis reveals the influence mechanism of coupling degree on the operation domain.

Abstract:Typhoons are prone to cause large-scale power outages that result in large economic losses. This paper investigates the power grid resilience assessment and differentiated planning methods against typhoon disasters. First, based on Batts wind field simulation, a fault recovery model for transmission lines is constructed, and a resilience assessment index is proposed considering the supply of critical load. Then, a two-stage optimization model for differentiated planning is constructed, in which the first stage optimizes the line reinforcement strategy based on the differentiated planning scheme. In the second stage, the resilience enhancement effect of the planning scheme is further verified by optimizing the load recovery strategy supported by energy storage. Finally, the solution is performed by the particle swarm optimation algorithm with elliptic function modified inertia weights. The effectiveness of the proposed method is verified by simulation results.

Abstract:The ever-increasing uncertainty on both sides of power source and load brings greater challenges to cascading failure risk assessment. Based on the stochastic response surface method and deep forest, a rapid assessment method of cascading failure risk is proposed considering the uncertainty of power source and load. Firstly, an improved stochastic response surface method combining distributed factor (SRSM-DF) is proposed to quickly calculate the probabilistic load flow in the process of cascading line outages. Then, the calculation error judgment method of SRSM-DF is proposed based on the deep forest, which takes into account the speed and accuracy of the probabilistic power flow calculation. Finally, a two-stage high-risk cascading failure screening strategy is constructed. Further considering the load shedding and DC power loss caused by failures, and the linear programming model for obtaining the cascading failure consequences is established to reduce the computational complexity of cascading failure screening. The simulation results of real-life power grid show that the proposed method can quickly screen high-risk cascading failures under uncertain conditions.

Abstract:With the integration of various energy such as heat and natural gas, the types of decision-making entities participating in the integrated energy market transactions are becoming more and more complex and diverse. However, the traditional optimization theory and most classical game theories can no longer solve the above increasingly complex behavioral decision-making problems of multiple entities. Aiming at how integrated energy retailers select strategies to compete in retailing energy sales, using the emerging evolutionary game theory, an evolutionary game model of retail-side market is established under the integrated energy competitive agency model, which takes the integrated energy retailers with limited rationality and information access as the research objects. Through theoretical derivation, the ranges of optimal competition strategies are obtained. Solved by the iterative algorithm based on the forward Euler formula, the case analyzes the evolutionary results, evolutionary time of integrated energy retailers and their influencing factors. It verifies the effectiveness of the evolutionary game theory in the behavioral analysis of market entities, and provides suggestions for the policy formulation of government and the behavioral decision-making of entities in the integrated energy market.

Abstract:With the continuous pursuing of electric power system reform, reliability pricing based on the differentiated demand of users has become an important part of the improvement of the electricity price system. In this paper, a reliability pricing method of distribution network is proposed based on the improved long-run incremental cost method, which comprehensively considers the system operation modes and the interruptible response factor. Combined with the proposed economic investment strategy, the influence of the unit power injection on the economic investment horizon is analyzed in different operation modes with the proposed method, the incremental reliability costs are deduced according to the smaller investment horizon in the different operation modes, and the regional reliability price with different interruptible response factors is calculated according to the incremental reliability costs. Finally, considering the differentiated energy consumption demand of users, the formulation method of the incremental reliability price is developed based on the incremental reliability cost per unit. Case analysis results show that the pricing system based on the combination of regional reliability price and incremental price can reasonably take into account the power supply reliability of power supply zones and the differentiated demand of users.

Abstract:It is of great significance to study the system frequency response characteristics of the modular multilevel converter based multi-terminal direct current (MMC-MTDC) transmission system integrated with wind farms. However, the time-domain simulation modeling process is complicated and is not suitable for theoretical analysis. Thus, it is urgent to develop a complete dynamic characteristic analysis model of frequency, active power and DC voltage for wind farms and the MMC-MTDC system. Therefore, taking advantage of the decoupling characteristics of the AC and DC sides in the average model of MMC, the AC side of the grid side converter (GSC) and the generators are independently modeled. Considering the losses of back-to-back converters, AC collection lines and the wind farm converter (WFC), the intermediated links of direct-drive wind turbine and WFC are simplified according to the power transfer law. Based on the equivalent models of the GSC and WFC, the DC lines are used as the interactive ports, and the small-signal model of active components of the MMC-MTDC system integrated with wind farms is established by the modular modeling method. The simulation results show that the general frequency response model can accurately simulate the dynamic process after the small frequency disturbance, and can be used for stability analysis, which provides a reference for parameter tuning of the control system.

Abstract:The occurrence of low-frequency and ultra-low-frequency oscillations in power systems will threaten the secure and stable operation of power grids. The parameter setting of the power system controllers directly affects the small-signal stability of low-frequency and ultra-low-frequency oscillations. To analyze the influence of controller parameters on stability, a small-signal stability analysis approach based on the minimum characteristic locus from a frequency-domain perspective is proposed. First, a compact form of the Heffron-Phillips model of the rotor circuit is established, and the stability margin expression in the frequency domain is obtained according to the characteristic locus. Then, the sensitivity analysis of the stability margin is implemented, and the quantitative analysis of the influence of parameters on the stability is realized. In addition, the exciter mode is analyzed and the reason why the PSS gain has the upper bound is explained from the theory aspect based on this method, and the solution equations of the frequency of exciter mode and the critical gain of the power system stabilizer are given. Finally, a controller parameter tuning method is proposed, and the effectiveness and practicability of the proposed method are verified by case studies.

Abstract:With the application of wide-area measurement systems in the transient stability control, the random time delay of wide-area information during the control process may cause the uncertainty of power system state during control. Moreover, the dimension of discrete decision variables for machine tripping and load shedding is extremely high, and the online emergency control decision-making of the power grid is facing challenge. Therefore, the transient stability emergency control problem is modeled as a Markov decision problem, and an decision-making method combining the deep Q-learning network (DQN) reinforcement learning and transient energy function is proposed, which can deal with the time-delay uncertainty of emergency control through the multi-step sequential decision-making process. The reward function is composed of a short-term reward function considering the control objectives and constraints, and a long-term reward function considering the stability. The potential energy index of the transient energy function is introduced into the reward function to improve the learning efficiency. With the objective of maximizing the cumulative rewards, the optimal emergency control strategy is learned in the discrete action space by DQN algorithm to solve the transient stability emergency control problem. The effectiveness of the proposed method in the emergency control decision-making is verified by an IEEE 39-bus system.

Abstract:As an important implementation form of energy router, solid-state transformer (SST) has broad application prospects in AC/DC distribution networks, energy distribution and control of microgrids, etc. In order to improve the operation stability of the SST and overcome the problem of “multi-device” state monitoring and “multi-switch” open-circuit fault location of the SST, the digital twin method of small SST state monitoring and single-tube open circuit fault diagnosis is proposed by using the high fidelity and interoperability of digital twins. This method uses electromagnetic transient program to build digital twins that can operate under open-circuit fault conditions. Combined with the data collected by the physical system, the quantum genetic algorithm is used to realize the parameter synchronization and state monitoring of the key components of the digital twins. By comparing the electrical quantities of digital twins and physical systems, a single-tube open-circuit fault diagnosis algorithm is proposed to accurately locate the open-circuit faults of switching device transistors and reverse parallel diodes. The simulation results of the case show that the proposed method can realize accurate state monitoring and fast open-circuit fault diagnosis for devices.

Abstract:Flexible DC distribution system is an important technical mean to connect renewable energy and promote the development of distributed energy. Its overcurrent calculation is the basis of current limiting scheme design, equipment selection, protection strategy, etc. The accurate theoretical calculation method with universality can simplify and guide a large number of electromagnetic transient simulations of overcurrent. Therefore, a systematic analysis on the error causes, influencing factors and application scopes of the typical theoretical calculation methods of overcurrent in radial and circular flexible DC distribution systems under pole-to-pole short circuit faults is carried out in this paper. Firstly, the analytical calculation method of overcurrent in radial flexible DC distribution systems and the numerical calculation method of overcurrent in circular flexible DC distribution systems are analyzed, respectively. It is proposed that the fundamental causes of errors in theoretical calculation methods are line model errors and public line coupling. Then, with different topological structures, the influence of current limiting reactors, line type and line length on the theoretical calculation error of overcurrent is analyzed. Finally, for different operation conditions, the applicability of the theoretical calculation methods for overcurrent in flexible DC distribution systems with different topologies is proposed.

Abstract:After the single-phase high-resistance grounding fault occurs in the flexible grounded system, the input of the parallel low resistor will lead to the reduction of the steady-state fault component. At the same time, the variation of the steady-state amplitude of the zero-sequence current of each feeder is small, so it is difficult to select the fault line. The transient fault characteristics of the flexible grounded system after the parallel low resistor is connected are analyzed; the inherent relationship between the feeder transient zero-sequence current and the bus transient zero-sequence voltage and their derivatives is obtained; and the relationship between the time when the low resistor is connected and the transient fault characteristic amplitude is analyzed. Thus a transient fault line selection method for the flexible grounded system based on the transition resistance evaluation is proposed. This method evaluates the transition resistance through the low-frequency transient voltage and current generated after the connection of the low resistor, so as to realize the fault line selection, which is not affected by the initial phase angle of the fault, has strong resistance to the transition resistance, and has low requirements for the accuracy of the transformer. At the same time, the maximum regulation of transient fault characteristics can be realized by selecting the connection time of low resistor, which can provide a new idea for flexible grounded system line selection and protection methods. Finally, the correctness of the proposed method is verified by simulation and real time digital simulation (RTDS) experiments.

Abstract:Aiming at the problems of abnormal operation condition monitoring for environmental protection in polluting enterprises at present, such as difficult implementation, large identification errors and easy tampering with the results, this paper proposes an identification method of abnormal operation conditions for environmental protection based on power quality monitoring data. The multi-dimensional power quality data obtained from non-intrustive load monitoring at the public power entrance of enterprise equipment are used to train the model of condition classification, to realize abnormal condition identification, which is different from the existing scheme of power consumption monitoring with separate meters installed for each device. First, the time series change-point detection and the clustering calculation for the characteristic data representing the production conditions are carried out to realize the division of production operation conditions of enterprises. Then, combined with the operation of environmental protection equipment, the categories of environmental protection operation conditions for classification are obtained. Furthermore, the operation condition scenarios related to environmental protection are classified and learned by the Stacking learning model. Finally, the trained classification model is used to identify the abnormal operation conditions for environmental protection in the enterprise. The effectiveness of the proposed method is verified by the simulation test data and the actual enterprise data.

Abstract:The access of high proportion of renewable energy sources and high proportion of power electronic equipment generates a lot of inter-harmonic signals which caused a series of wide-frequency oscillation and affect the safety operation of power grids. The exiting measurement devices focus on power frequency measurement and cannot monitor the wide-frequency oscillation. The paper takes the opportunity of release of “Technology specification of wide-frequency measurement device of power system”, introduces the proposal, function and terminology of wide-frequency measurement technology. The function and performance of wide-frequency measurement devices are discussed from the aspects of wide-frequency sampling, measurement function, monitoring of wide-frequency oscillation, data transmission and data recording. Then, the application schemes, scenarios and prospect of the devices are discussed further. The existing problems of this specification and the future research direction are presented. It can provide guide and reference to the application of wide-frequency measurement devices.

Abstract:Sub-synchronous oscillations caused by the interaction between large-scale wind power clusters and the power system seriously threaten the safety of power grid operation. This paper proposes an implementation framework and calculation method for sub-synchronous oscillation control based on system impedance identification. The system impedance is obtained by identifying the grid-side impedance and the impedance of wind farm branches. Based on the change of the real part of the system impedance, the effectiveness of the stability control system in cutting different wind farm branches is determined. Considering the influence of different wind farms on the system impedance, the control strategy for the sub-synchronous oscillation is formulated based on the principle of minimizing cut-off quantity. Aiming at the difficulty in identifying the grid-side impedance and the impedance of wind farm branches in the actual power grid, the methods for predicting the limit value of grid-side impedance and online measurement for the impedance of wind farm branches are proposed. Based on the PSCAD/EMTDC simulation platform, the actual power grid model in Guyuan area of China is constructed, which verifies the effectiveness of the proposed control strategy for sub-synchronous oscillation.