Abstract:The penetration of high-proportion renewable energy brings significant volatility and uncertainty to power grids. Thus the receiving end of the power system faces the problem of insufficient adjustable resource capacity. It requires the full utilization of a large number of flexible energy resources to participate in power system dispatch and provide auxiliary services. However, the vast number, wide distribution, and diverse characteristics of these flexible energy resources pose significant challenges to centralized dispatch and control. It is necessary to manage and dispatch these flexible energy resources in a clustered way. To accurately assess the role of clusters in the power system dispatch and control, it is essential to aggregate these flexible energy resources and evaluate their performance by using appropriate quantitative metrics based on application requirements. According to the application demands of flexible resource clusters in power system dispatch and electricity market, a quantitative metric system of flexibility for evaluating the adjustable capacity of flexible energy resource clusters is proposed. Based on their physical characteristics, these resources are categorized into three types: generator-like characteristics, storage-like characteristics, and common characteristics. For quantitative evaluation of the cluster flexibility metrics across various application scenarios, an aggregation reference model comprising an equivalent generator and an equivalent energy storage is proposed. This aggregation reference model has a clear physical interpretation, exhibits nested properties, and therefore is suitable for various time scales and energy forms. It can be used to calculate the quantitative metrics for specific application scenarios and widely applied in scenarios such as asynchronous dispatch of distributed energy resource clusters, joint optimization dispatch of integrated energy systems, and participation of virtual power plant in electricity market bidding.

Abstract:Aiming at the current carbon emission reduction tasks and the safe and stable operation requirements of the power system, the low-carbon safe operation technology of the power system is studied. The existing energy flow security region model makes security monitoring and control more scientific and effective, but it ignores the potential “high carbon” risk during system operation. Based on this, a model and calculation method for the energy and carbon coupled security region (referred to as the energy-carbon security region) considering carbon flow constraints are proposed, aiming to ensure system security while reducing emissions and improving efficiency. To fully characterize the low-carbon operation capability limit of the power system, the total supply capability curve and total accommodation capability curve models of the power system considering carbon flow constraints are proposed from the perspectives of load and source ends, respectively. Based on the working point of the total supply capability, the simulation fitting calculation method of the energy-carbon safety boundary is proposed, and the dimension reduction observation of the energy-carbon security region is realized. Finally, the validity of the proposed model is verified by combining the cases.

Abstract:Under the strategic goal of carbon emission peak and carbon neutrality, in order to promote the wind and photovoltaic (PV) accommodation and energy power low-carbon transformation, and improve the energy utilization rate of coal mine, this paper proposes a low-carbon economic scheduling model of coal mine integrated energy system (CMIES) with associated energy and the coupling of carbon capture and power to gas. Firstly, considering the utilization of associated energy in the coal mine such as gas, ventilation and water gushing, the basic model of CMIES is established, and the carbon capture and power-to-gas devices are used as coupling units to promote energy conservation, emission reduction and renewable energy accommodation. Secondly, the reward and punishment ladder-type carbon trading mechanism is introduced, and the collaborative optimal scheduling model of CMIES is established with the goal of minimizing the operation cost of the CMIES. Finally, taking a large coal mine in Yunnan, China as a case, the simulation analysis is carried out by setting different scenarios. The results show that the proposed model can promote the low-carbon economic operation of the CMIES and improve the wind and PV accommodation rate.

Abstract:With the extensive access of the renewable energy equipment and energy storage devices on user sides, how to effectively promote the local consumption of renewable energy and reduce the operation costs of energy storage become crucial. Aiming at sharing electric power and energy storage resources among multiple regions, a sharing operation mode for multi-regional electric power and energy storage based on master-slave-evolutionary hybrid game is proposed. Firstly, the master-slave game model and the evolutionary game model are established for the optimal decision. The operator, as a leader, sets the intra-regional tariffs according to the balance of supply and demand in each region. The users, as followers, adjust their own electricity consumption and region selection probability according to the regional tariffs and transmission cost information of inter-regional trades based on the evolutionary game model. Secondly, considering the different interest demands of each entity, the revenue and cost models of operators and users are established, and the pricing strategy of operators, the allocation and charging/discharging strategies of shared energy storages, and the final revenue of each entity are obtained by taking the maximization of the revenue of both parties and the minimization of the electricity quantities interacting with the power grid as the objective functions. Finally, the validity of the proposed operation mode is verified by case study.

Abstract:With the rapid growth of distributed photovoltaic (PV) installed capacity, the impact of power prediction for distributed PV clusters on grid dispatch is becoming increasingly significant. There are three technical routes for power prediction of distributed PV clusters, i.e., accumulation before prediction, prediction before accumulation and clustering before prediction. Through experiments on a dataset of over 600 distributed PV sites, the spatial complementarity is found in the power prediction for distributed PV clusters, whereby the prediction errors of the three cluster prediction routes are lower than the average prediction error of a single site. To explore the generation mechanism and influencing factors of the spatial complementarity, this paper first categorizes it into two types of spatial complementarity characteristics, namely power curve complementarity and prediction error complementarity, based on the mechanism analysis. Secondly, the concept of spatial complementarity coefficient is proposed to quantitatively evaluate the complementary effect. Finally, the effects of cluster scale, distribution range, weather type and number of clusters on the spatial complementarity characteristics are explored. The results show that the two types of spatial complementarity characteristics have significant effects on improving the short-term power prediction accuracy of distributed PV clusters, with prediction error complementarity superior to power curve complementarity. The research results can provide a basis for the division of distributed PV clusters and contribute to achieving more efficient and accurate power prediction.

Abstract:This paper proposes a reliability evaluation method for the distribution network incorporating cyber-physical-social factors to analyze the influence of advanced technology in cyber systems and the emergency conservation measures in social systems in the major social events on the reliability of the distribution network. First, in order to analyze the interaction of cyber-physical-social systems (CPSS), this paper constructs a CPSS interactive framework for distribution network reliability. Then, by constructing the equipment developmental and destructive fault state transition models, it quantifies the relationship between the defect diagnosis and action period of risk warning technology in the cyber system as well as the technical level of overhauling personnel and the equipment fault rate in the social system. By constructing the equipment fault recovery time model, it quantifies the relationship between the accuracy of fault location technology in the cyber system and the traffic road condition, the sufficiency of emergency supplies and the equipment fault recovery time in the social system. Finally, to verify the effectiveness of the framework and models, this paper establishes a test system containing the basic information of CPSS. The results validate the role of cyber and social systems in enhancing the reliability of the distribution physical systems.

Abstract:Aiming at the phenomenon of large-area power failure in the distribution network caused by extreme events, in the background of traffic-grid coupling, this paper proposes a post-disaster multi-source cooperative islanding operation strategy for the distribution network considering mobile energy storage system dispatching. First, based on the post-disaster operation framework in the background of traffic-grid coupling, with the objective of minimizing the weighted load shedding, and considering the influence of traffic flow changes on the dispatching process of mobile energy storage system, a multi-source cooperative islanding operation model is established. Then, according to the characteristics of the proposed model and the decision-making requirements, the decision-making process of islanding operation is established based on the two-stage optimization framework to ensure the coordination of load restoration between the island area after power loss and the area with local power supply. Finally, a case of IEEE 33-bus distribution network coupled with 29-bus traffic network is used to verify the effectiveness of the proposed strategy. The results show that the proposed islanding operation strategy can give full play to the collaborative ability of all power sources in the post-disaster distribution network restoration, achieve the optimal distribution and transfer of limited electricity, and effectively improve the post-disaster restoration effect of distribution network.

Abstract:To address the problem of inertia insufficiency in microgrids due to the integration of renewable energy sources, an inertial power control strategy for charging stations in collaboration with renewable energy generation is proposed. First, considering that electric vehicles (EVs) have good flexible regulation potential, they are treated as the generalized energy storage at the charging station level to provide power response capability for the controller. Then, the control strategy of virtual-inertial power compensation is proposed in reference to the rotor operation characteristics of virtual synchronous generator (VSG), which can cooperate with renewable energy generation to provide frequency support. Finally, the corresponding simulation model is built on MATLAB/Simulink for verification. The simulation results show that the proposed control strategy of virtual-inertial power compensation can effectively make use of the regulation potential of charging stations to provide inertia support power to the system, so as to improve the frequency stability of the system.

Abstract:Transient harmonics caused by commutation failure, excitation inrush current and impulsive loads entering the power grid can easily lead to serious consequences such as converter overcurrent and disconnection as well as unit stability damage in the doubly-fed induction generator (DFIG). To improve the operating performance of DFIG in the power grid with transient harmonic and improve its power quality fed into the power grid, the mathematical model of DFIG in the power grid with transient harmonic is firstly established. Secondly, a control strategy based on repetitive control regulator is proposed, which achieves the control objectives of rotor current balance and sine as well as stator current balance and sine under transient harmonic impact, respectively. Afterwards, the control performance of the proposed control strategy is analyzed, including harmonic voltage anti-interference performance and stability performance after switching the control target. Finally, based on the hardware-in-loop simulation platform, the effectiveness of the proposed control strategy is experimentally verified.

Abstract:Focusing on the islanded microgrid with uncertain modelling error and renewable energy integration, an integrated load frequency control (LFC) method combining the bounded L₂-gain baseline control and disturbance compensation is proposed. First, the full-actuated model of microgrid is derived, and the influence of modelling error and renewable energy is summarized into an integrated disturbance. To improve the system robustness, the integrated disturbance is defined as the opponent of the control signal. Further, the bounded L₂-gain problem regarding the disturbance suppression is equivalent to solving the Nash equilibrium of zero-sum game between the integrated disturbance and the control signal. Thus the bounded L₂-gain baseline controller is derived. To improve the secondary frequency regulation, a disturbance compensation control based on the fifth-order generalized integral observer is designed. And a new anti-GRC measure for generation rate constraint (GRC) is introduced on this basis. Finally, a 16 MW renewable energy-diesel hybrid microgrid simulation model is constructed, and the proposed integrated LFC method is tested under various operation conditions to verify its effectiveness.

Abstract:The operation mode of large-scale power grid with high proportion of renewable energy presents strong uncertainty and nonlinearity, and the risk of mismatching of emergency control strategy formulated by using single state quantity as mode word (such as sectional power) is increasing. Based on the extended equal area criterion quantitative analysis theory, the mechanism of dominant mode stability change and emergency control strategy mismatch caused by node power injection space change is analyzed, and a mining method for dominant characteristics influencing strategy mismatch obtained from the massive node power injection space is proposed. Based on the transient stability participation factors of the dominant characteristic, the extended mode word determination method of the emergency control strategy based on the multi-state quantity characteristics is proposed through the cluster aggregation of the dominant characteristics. By expanding the mode words of the emergency control strategy, the risk of offline emergency control strategy mismatch is reduced and the adaptability of the emergency control strategy is improved. Finally, the effectiveness of the proposed method is verified by a practical grid example.

Abstract:With the rapid development of renewable energy, the types of power system equipment are increasing, and the oscillation characteristics are more complex, which puts forward higher requirements on accuracy and efficiency of the electromagnetic transient simulation. A fine-grained modeling method for the power system with renewable energy is proposed based on the latency insertion method (LIM) which is used in the design of large-scale integrated circuits. Combined with the resource advantages of the graphics processor unit (GPU), the parallel solution of the algorithm is realized. The proposed method can decouple the traditional power grid and the power electronic equipment. The step sizes of sub-systems can be obtained according to the mixed numerical stability criterion and the local truncation error. Then, the multi-rate simulation of the power system with renewable energy is realized by interpolation. Finally, based on the GPU hardware platform, an improved 39-node system with renewable energy integration is taken as a case to verify the accuracy of the proposed method, and its advantages of simulation efficiency are verified by the simulation of systems with different scales and different combinations of step sizes.

Abstract:Before the active distribution network reclosing, the distributed energy resource (DER) must be disconnected from the grid. However, in practice, many substations do not have line-side voltage transformers installed, resulting in the inability to implement a voltage-free reclosing scheme. This paper proposes a time-limited adaptive reclosing method in coordination with DER anti-islanding protection. The operation time of DER anti-islanding protection connected to the line is inferred by the system-side bus voltage before the protection trip, and the operation time of distribution line reclosing is automatically adjusted to achieve the timing coordination of reclosing and anti-islanding protection. The proposed method does not require the installation of line-side voltage transformers and does not rely on communication networks. It considers the requirements of low voltage ride-through of DERs while achieving fast reclosing of distribution lines, providing support for the stability of power system operation and power supply reliability. The effectiveness of this method has been verified by simulation and experimental results.

Abstract:In view of the possible problem of wrong user-transformer relationships in the topology file of the low-voltage distribution station area, this paper proposes a user-transformer relationship identification method based on multi-feature symbolic aggregate approximation (MF-SAX) and hierarchical clustering. First, the symbolic aggregate approximation expression method is used to convert the user voltage time series into string series, and two additional parameters, i.e., the voltage fluctuation coefficient and the voltage change trend, are introduced to strengthen its feature expression. Then, the similarity matrix of the user voltage curves is generated based on the edit distance, and the hierarchical clustering algorithm is combined to realize the identification of the user-transformer relationships. Finally, the results of the practical case show that, compared with some existing methods, the proposed method achieves higher accuracy and fewer false alarms, which can directly respond to missing data situations, and has higher efficiency.

Abstract:For the capacitive wireless power transfer (CWPT) system, the accurate estimation of the pickup detuning degree is the premise for the system to maintain resonance. However, because the induced voltage at the pickup loop of the system cannot be directly observed, it is difficult to estimate the detuning degree of the pickup loop by measuring the phase difference between the induced voltage and the loop current. A CWPT system for indirectly measuring the phase information of the induced voltage at the pickup loop is presented and the detuning degree of the pickup loop is evaluated. Based on the Z parameters, the mathematical model and equivalent circuit of the three-port CWPT system are established. Based on the model, a secondary-side decoupled capacitive coupler is proposed. The errors in the measurement of induced voltage and the estimation of detuning degree are analyzed, and the correction method for the detuning degree of the pickup loop is proposed. A prototype of the proposed CWPT system with secondary-side decoupled coupler is constructed. Experimental results indicate that the estimation error of the detuning degree shall not exceed 1 Ω when the pickup fully tunes, and the estimation error of the detuning degree under the inductive detuning and capacitive detuning conditions shall be less than 2% and 5%, respectively.

Abstract:The flexible DC transmission system is susceptible to oscillations with different frequencies under various operation conditions due to the impact of the characteristics of power electronic equipment and control systems. As a determining factor for achieving equipment safety and stable operation of the power grid, the protection system should achieve accurate monitoring of system harmonics and reliable operation. Based on the cumulative effect of current energy during the high-frequency oscillation, a current-type harmonic protection algorithm based on inverse time action is proposed. Based on the characteristics of high-frequency harmonic voltage, the zero-crossing monitoring protection algorithm, the harmonic distortion rate protection algorithm, and the high-frequency harmonic protection algorithm are provided. Each protection algorithm cooperates with each other to achieve comprehensive protection for different voltage oscillation types. Finally, the high-frequency harmonic protection configured in domestic flexible DC engineering practice is presented, and the operation characteristics of the system after high-frequency oscillation are analyzed based on the protection principle, verifying the effectiveness and accuracy of high-frequency harmonic protection.

Abstract:Establishing the thermal stability control limit is a significant work for the power dispatching and control center and an important means to guarantee the safe operation of power grids. In order to address the problem that it is gradually difficult to establish thermal stability control limit due to the high uncertainty of power flows after the large-scale grid-connection of renewable energy, the power flow transfer relations are calculated and the power grid security constraints are established based on contingency analysis. Then, a three-stage thermal stability control limit calculation method for power grids based on the formulaic control limit and maximization of minimum safety margin is proposed to achieve the automatic calculation of thermal stability control limit. In the first stage, control limits of transmission channels are initialized. In the second stage, formulaic control limit requirements are established for those channels that are prone to overload after a contingency happens according to the criteria that power flow of the bottleneck equipment does not exceed its short-term current-carrying capacity after a contingency happens. In the third stage, the channel control limit is calculated successively according to the maximization principle of minimum safety margin for the set of channels with coupled transfer relation constraints. Both formulaic control limit and channel control limit requirements for the channels with tight transfer capacity can reduce the conservatism and fully release the transfer capacity of the channel. The control requirements can be reduced and simplified by establishing only channel control limit requirements for the remaining channels. Taking the summer peak operation mode of a regional power grid in 2023 as a case, the automatic limit calculation results are in good agreement with the stability regulations, which verifies the effectiveness of the proposed method.