2021, 45(18):1-8. DOI: 10.7500/AEPS20210426006
Abstract:The goals of achieving carbon peak and carbon neutrality will drive the energy structure transition, and the large-scale integration of distributed energy resource (DER) will become a prominent problem. In order to ensure the safe and stable operation of large power grids and support the sustainable accommodation of renewable energy, the form of DER participating in the security and stability control of large power grids is discussed, and an interaction mechanism based on virtual power plants (VPPs) is proposed. Firstly, the evolution of power systems driven by the dual carbon goals is analyzed, and the challenges faced by its safe and stable operation are analyzed. Secondly, the definition, composition structure and functional characteristics of the VPP are elaborated, and the connotation of integrating massive heterogeneous DERs to interact friendly with large power grids is revealed. Then, the hierarchical dispatch and control architecture of the VPP is built based on the multi-agent technology. And the mechanism of the inner operation and the participation in the security and stability control of large power grids of the VPP is proposed. Finally, it is pointed out that VPP is a new technology form that accommodates a high proportion of renewable energy in the new power system, and there are several technical issues in this field that need to be focused on.
2021, 45(18):9-17. DOI: 10.7500/AEPS20200407004
Abstract:At present, the research on energy storage demand mostly focuses on the configuration of energy storage devices, such as electrochemical cells with short discharging time in specific application scenarios, and there is a lack of research on the planning method of system-level energy storage demand and the impact on the overall cost. Aiming at the characteristics of two different types of short-time and long-time energy storage, a quantitative analysis model and a method with the optimization goal of the lowest system comprehensive cost are established. The system-level energy storage demand considering three key factors, namely, the capacity, the structure, and the cost is quantitatively analyzed. The energy storage planning is solved jointly with the power supply planning problem through the mixed integer optimization. Taking Europe, the world and China as examples, the energy storage type, installed capacity and acceptable cost of energy storage to support the clean energy transition of power system are analyzed. The results show that the short-time energy storage which is only represented by the lithium battery cannot meet all the system requirements of the scenarios with high proportion of renewable energy in the future, and the long-time energy storage will gradually play a significant role in the deep clean transition after 2035.
2021, 45(18):18-27. DOI: 10.7500/AEPS20200616007
Abstract:Installing different types of energy storage devices in wind farms can effectively smooth wind power fluctuations and reduce the impact of wind power uncertainty on the secure operation of power systems. For the hybrid wind-storage system composed of wind farms and hybrid energy storage systems (HESSs), this paper proposes a probabilistic forecasting based stochastic optimal dispatch and control method of HESS for smoothing wind power fluctuations. Firstly, the wind power probabilistic forecasting is combined with multivariate Copula function to generate the temporally correlated wind power scenario set. Then, an adaptive variational mode decomposition method is proposed to calculate the pre-dispatched target of grid-connected wind power and charging/discharging power of HESS. After that, based on the pre-dispatched power, a stochastic optimal dispatch and control model of HESS for smoothing wind power fluctuations is constructed and solved, which considers the lifecycle constraint of energy storage charging/discharging and the chance constraint of stored energy. Finally, the effectiveness and economy of the proposed stochastic optimal dispatch and control method of HESS for smoothing wind power fluctuations are illustrated through case analysis.
2021, 45(18):28-34. DOI: 10.7500/AEPS20210121003
Abstract:Aiming at the impact of the low-probability and high-loss extreme disasters on the safe and stable operation of distribution networks, this paper proposes a coordinated optimization framework of multi-timescale restoration strategies for resilience enhancement of distribution networks. Firstly, a fault restoration model considering time uncertainty in extreme disasters is established for the problem of maintainers staff dispatch. Secondly, the load restoration model combining distributed generators and contact switches for network reconfiguration is constructed under the three-phase unbalanced operation condition of the distribution network. Then, a coordinated optimization framework is constructed based on the above two models with different time scales using the repair state of fault component as the coupling variable, and a two-stage robust optimization model is established using a column-and-constraint generation method for iterative solution. Finally, different scenarios are constructed considering the time uncertainty of maintainers staff, the correlation of multiple recovery strategies and the solution time step, and the effectiveness of the proposed model and solution method is verified by examples.
2021, 45(18):35-44. DOI: 10.7500/AEPS20200901004
Abstract:With the extensive development and utilization of distributed energy resource (DER) represented by photovoltaics, its disturbance to distribution networks, subsidy gap and other problems lead to the widespread application of the distributed energy sharing mode among users in local areas. In this paper, the interest problem of various related participants is analyzed from the overall perspective, including the distribution companies, sharing coordinators, distributed photovoltaic owners and sharers in shared mode, and a system dynamics model for coupling modules is built, including the distributed photovoltaic power sharing module, distribution grid company revenue module, and transmission and distribution tariff module. The changes in sharing value with different distributed photovoltaic installation ratios and feed-in tariffs are simulated, and a forward-looking analysis of its impact on the distribution company interest and the transmission and distribution tariff is conducted. The case analysis results show that there is a peak of sharing value with the change of distributed photovoltaic installation ratios in a local area. Therefore, the reasonable planning of the distributed photovoltaic access level in a local area is conducive to the strategic adjustment of each participant and the promotion of the coordination of multi-participants’ benefits.
2021, 45(18):45-51. DOI: 10.7500/AEPS20210220003
Abstract:Current research on security region of distribution networks generally focuses on AC distribution networks. This paper proposes the convex hull based security region (CHSR) model for AC/DC hybrid distribution networks. Firstly, a nonlinear security region model for AC/DC distribution networks is constructed by taking into account the voltage constraints, feeder capacity constraints and critical equipment output constraints. Secondly, the security boundary points of the AC/DC hybrid distribution network are solved based on the nonlinear programming model. Finally, the convex hull fitting approach is adopted to generate the observable security region space. The results of the example verify the rationality and superiority of the CHSR. Meanwhile, the impact of the control modes of the converters, the reactive compensation of the converters and the lower limit of the system voltage on the constructed security region are investigated.
2021, 45(18):52-60. DOI: 10.7500/AEPS20210112007
Abstract:First, this paper uses the Markov and velocity-flow models to predict the space-time load of fast-charging vehicles and electric buses. The charging and discharging power of fast-charging stations, switching stations and cascade energy storage stations are combined to construct the charging-swapping-storage integrated station (CSSIS) model. Then, a power flow model of AC/DC with CSSIS is established, and a multi-objective siting model that takes into account travel time of electric vehicles (EVs) to the station, queuing time and voltage deviation of AC/DC distribution networks is constructed. The dynamic weights combined with binary genetic algorithm are used to solve the model. Further, the Voronoi diagram is used to delineate the CSSIS service area. An optimization model is established to determine the final station site with the objective of minimizing the comprehensive investment cost. The number of chargers is optimized by the queuing theory, and the system bus voltage and CSSIS power balance are used as constraints to determine the station equipment capacity. Finally, the effectiveness of the proposed model and method is verified by examples.
2021, 45(18):61-70. DOI: 10.7500/AEPS20201210011
Abstract:Traditional probability modeling of electric vehicle charging load is usually based on the Monte Carlo simulation method, which faces the problems such as a large number of coupling parameters and long time computing. To this end, this paper proposes a numerical calculation method for the probability distribution of electric vehicle charging load based on probability distribution characteristics of the combined state of charge (CSOC) for vehicle collection. Firstly, multiple trips of the same vehicle are disassembled into independent single trips, and origin destination (OD) analysis is carried out on single trips to reduce the parameter coupling error in the modeling of multiple trips. Secondly, the CSOC dynamic probability model considering the probability characteristics of travelling of vehicles is established to determine the probability density function of the initial state of charge (SOC) for single trips. Then, combining the law of large numbers, the spatial-temporal probability distribution function of electric vehicle charging load is established. Finally, a 12-bus road network case is used to calculate the spatial-temporal probability distribution of the charging load. The results show that compared with the traditional Monte Carlo simulation method, the proposed method does not have the problem of coupling error, and greatly improves the calculation efficiency on the premise of ensuring the calculation accuracy.
2021, 45(18):71-77. DOI: 10.7500/AEPS20201123001
Abstract:Aiming at the problem of the missing or abnormal household-transformer relationship in the topology of low-voltage distribution stations, this paper proposes an identification method of household-transformer relationship based on the derivative dynamic time warping (DDTW) algorithm and the density-based spatial clustering of application with noise (DBSCAN) algorithm. Firstly, the DDTW algorithm is used to analyze the similarity of the time series of the voltages between the low-voltage side of the substation transformers and the users. Secondly, the probabilistic results of the household-transformer relationship are obtained by clustering the substation transformers and the users according to the DDTW distance. The influence of the parameters of the clustering algorithm on the clustering results is reduced. The method is able to analyze voltage time series with different time intervals and unequal lengths. It is insensitive to missing or abnormal voltage data and does not require artificially threshold setting, and has high accuracy in identifying household-transformer relationships. Finally, the effectiveness of the proposed method is verified through case analysis.
2021, 45(18):78-84. DOI: 10.7500/AEPS20201207003
Abstract:In weak power grids, the inherent nonlinear characteristics of phase-locked loop (PLL) will affect the transient stability of grid-connected converter systems under the condition of power grid disturbances. Aiming at the operation condition of frequency disturbances of power grids in the grid-connected converter system, the second-order differential equation for the output phase angle of PLL is established. Based on the multi-scale method of nonlinear dynamics, the model is analytically solved in time domain, and the influence of power grid disturbances on the output phase angle of PLL is quantitatively analyzed. Furthermore, the transient stability boundary is given. Considering the operation conditions of the grid frequency, current and impedance disturbances, the analytical solutions of the established model are compared with the numerical solutions and simulation results, and the analytical analysis method is also compared with the traditional phase-diagram analysis method, which verify the correctness of the proposed transient analytical model of PLL and the analytical method. The effectiveness of the transient stability boundary is further verified by experiments.
2021, 45(18):85-93. DOI: 10.7500/AEPS20210105004
Abstract:There is a typical negative feedback link in the voltage control of the power system, which makes the system Jacobian matrix present the sign pattern. Such structural features and system characteristics are worthy of in-depth analysis, because they affect the robust behavior and determine the response law of voltage dynamics. The response mechanism of voltage dynamics considering the excitation system is studied. According to the sign pattern of the Jacobian matrix in the voltage model, the mixed monotone dynamic system theory is used to analyze the voltage response law. It is pointed out that the network connection mode and the synchronous machine voltage regulation mechanism are the internal reasons for the better voltage stability in the power grid. The influence of regulator proportional coefficient and time constant on the voltage stability is also investigated. Firstly, this paper sorts out the part of monotone dynamic system and the negative feedback link in the mathematical model to clarify the structural feature of the equation and the sign pattern of the Jacobian matrix. Then, the mixed monotone system theory is applied to exchange the information on the feedback channel to quickly construct an augmented monotone system, which can carry out the bilateral estimation of voltage response. Finally, combining the sufficient conditions for the bounded solution trajectory of the augmented monotone system, a stable region estimation result is obtained to judge whether the voltage response is bounded after a large disturbance. The result is helpful to understand the robust behavior of voltage dynamics.
2021, 45(18):94-102. DOI: 10.7500/AEPS20201215001
Abstract:Large-scale integration of renewable energy has brought great challenges to the peak regulation of the power systems. Under the background that the power supply side is gradually running out of the peak regulation capacity, an efficient market mechanism is urgently required to guide the controllable load of the demand side to actively participate in the renewable energy accommodation. This paper proposes a peak regulation auxiliary service market framework for end-users and thermal power units with the participation of the load aggregator (LA). The market framework can be described as a two-stage market scheduling model to achieve peak regulation capacity supply by end-users, while fully considering the flexible peak-regulation capacity of thermal power units. LAs evaluate the response quantity and corresponding cost of their signed end-users, and then bid by their marginal utility prices. Meanwhile, thermal power units evaluate their peak regulation capacity, and then bid by their marginal cost price. After the bidding is done, the system operator uses a real-time peak regulation ancillary service market model based on a chance-constrained stochastic programming method to achieve market clearing. In addition, this paper further analyzes the key issues such as market implementation steps, market clearing rules and resource dispatch methods, etc. Case studies verify the the effectiveness of the proposed market framework.
2021, 45(18):103-112. DOI: 10.7500/AEPS20201106002
Abstract:With the rapid development of renewable energy in China and the promotion of market-oriented reform of power systems, the demand for greater accommodation of renewable energy and the construction of the national unified electricity market bring new challenges to the development of cross-provincial and cross-regional electricity trading. Combined with China’s cross-provincial and cross-regional incremental spot trading rules and based on the “high-low matching” trading, an incremental spot trading model considering trading paths and transmission loss is established. In order to reflect the nodal type of the sending- and receiving-ends of the trading path, a seller/buyer node and a virtual seller/buyer node are set at the sending-end and receiving-end of the trading path respectively. The model is used to analyze the impact of the available capacity of the transmission channel, feed-in tariffs of renewable energy and declared amount of abandoned renewable energy on renewable energy accommodation. Through the simulation calculation of the actual trading data in the “Three North” region of China, the effectiveness of the model is verified, and the key influencing factors of renewable energy accommodation in the cross-provincial and cross-regional incremental spot market are analyzed.
2021, 45(18):113-121. DOI: 10.7500/AEPS20210311003
Abstract:The close cyber-physical coupling makes network security problems a major challenge to the safe and stable operation of power systems, and it also provides new potential for the cyber-physical coordinated perception and security defense. Focusing on the security and stability control business, aiming at the network security problems brought by cyber-physical integration, this paper excavates the potential of cyber-physical coordination, puts forward the framework of cyber-physical coordinated defense, and improves the ability of security and stability control systems to defend against malicious attacks. Firstly, the impacts of malicious attacks on power security and stability control systems and power primary systems are analyzed through experiments. On this basis, the cyber-physical coordinated defense system and framework for network security are proposed from the time and spatial dimensions, respectively. Combined with the traditional network security technology and security and stability control business logic, the network security identification and protection schemes including device-side- and master-station-side schemes are proposed, respectively, and the network security monitoring and analysis application framework of the security and stability control system is given, so as to improve the ability of the security and stability control system to deal with malicious attacks.
2021, 45(18):122-130. DOI: 10.7500/AEPS20210106001
Abstract:Affected by environmental factors and characteristics of arc ignition and extinction, arc duration of intermittent grounding faults is generally shorter than the protection operate time. Therefore, it is difficult for traditional protection to operate effectively. In this paper, characteristics of grounding faults in low resistance neutral grounded systems are analyzed. Then, the relationships between zero-sequence current of the feeder lines and the neutral point are compared. Based on the thermal stability principle that grounding resistors generate heat during faults and dissipate heat after faults, a protection method is proposed for intermittent grounding faults in low resistance neutral grounded systems. In this method, the characteristic energy of thermal accumulation during the faults is constructed using zero sequence current, and the characteristic energy of thermal dissipation after the faults is constructed using the difference of heat. The protection operates when characteristic energy of thermal accumulation exceeds the operation setting value. It is verified by simulation and field record data that this method is not likely affected by fault interval time and transition resistance, which can be used as backup protection for feeder lines in low resistance neutral grounded systems to improve system operation reliability.
2021, 45(18):131-140. DOI: 10.7500/AEPS20201222003
Abstract:This paper proposes a frequency-division and hierarchical control strategy for three-phase flexible low-frequency transmission system based on the full-bridge modular multilevel matrix converter (M3C). The operation principle of M3C is analyzed from the mechanism of dual-frequency power coupling. The control of fundamental-frequency and low-frequency side of M3C is realized in the rotating coordinate system. On the premise of maintaining the total active power balance of the system, the balance of capacitor voltage of power modules among three phases is realized by injecting the negative sequence current into the fundamental-frequency side. And the balance of capacitor voltage of power modules inside each phase is realized by the fundamental-frequency circulating current control inside the bridge arm in the static coordinate system. The proposed hierarchical control can realize the stable operation of the flexible low-frequency transmission system based on M3C under symmetrical and asymmetrical conditions. According to the analysis of the instantaneous power of each phase unit, it is proposed to use only two kinds of sliding filters with different frequencies to ensure the transient stability in the capacitor voltage deviation control of power modules. Finally, a simulation model of the dual-terminal flexible low-frequency transmission system is built to verify the effectiveness and the feasibility of the proposed control strategy.
2021, 45(18):141-150. DOI: 10.7500/AEPS20210207001
Abstract:In order to solve the problem of deviations in the transmitted power of the interlinking converter (IC) due to line impedance voltage drop, this paper proposes a control strategy based on the line impedance compensation. First, harmonics of a certain frequency are injected into the IC side with the help of a converter at the AC bus. The harmonic frequency is a function of the bus voltage at the time of injection and is obtained on the IC side through a filter and a phase-locked loop. Then, the line impedance is obtained by combining the local power and voltage on the IC side. Based on the line impedance detection, the line voltage drop can be compensated in the control of the IC, so as to achieve accurate power transmission without interconnected communication. Finally, a hybrid AC/DC microgrid model is built on MATLAB and StarSim real-time simulation platform to verify the effectiveness of the proposed method.
2021, 45(18):151-161. DOI: 10.7500/AEPS20200917001
Abstract:High-frequency resonance happened in both Luxi back-to-back high voltage direct current (HVDC) project and Chongqing-Hubei back-to-back HVDC project of China. The link delay of the modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system is the key factor leading to high-frequency resonance. For the medium- and high-frequency resonance problems of the MMC-HVDC transmission system, this paper establishes a detailed impedance model from the AC side port of the MMC-HVDC transmission system based on the harmonic linearization method. And the reason why the impedance of MMC presents negative damping characteristics in the medium- and high-frequency bands is analyzed. Then, the improvement of impedance characteristics of MMC and the harmonic suppression effect by using the low-pass filter, the band-stop filter and the nonlinear filter in the voltage feedforward link are compared and analyzed. Meanwhile, three kinds of additional damping controllers are proposed, and their effects on improving the impedance characteristics of MMC are analyzed. Finally, taking the model for Guangxi side of Luxi back-to-back HVDC project as an example, the correctness of the detailed impedance model and the effectiveness of the additional damping control are verified by the electromagnetic transient simulation on PSCAD.
2021, 45(18):162-169. DOI: 10.7500/AEPS20210128004
Abstract:Aiming at the problems of low accuracy and slow speed in identifying the load and mutual inductance of the magnetic coupling resonant wireless power transfer (MCR-WPT) system, a TensorFlow neural network based identification method for the load and mutual inductance of the double-side LCC type MCR-WPT system is proposed. This method is based on the TensorFlow deep learning framework and adopts a neural network model. The load and mutual inductance identification problem of the MCR-WPT system is equivalent to the problem of solving nonlinear equations, which is then transformed into a deep learning nonlinear fitting problem. The training method of the model is given and the TensorFlow neural network based identification model for the load and mutual inductance of MCR-WPT system is finally obtained. The identification model for the load and mutual inductance is trained offline and then the trained model is imported into the micro controller. It only needs to collect the input current value and transmission distance of the system to realize the online simultaneous identification of the load and mutual inductance. The identification speed is fast and the accuracy is high, which is conducive to the real-time control of the system. And the model is low in cost and easy to implement, which is conducive to the engineering popularization and application.
2021, 45(18):170-178. DOI: 10.7500/AEPS20201209001
Abstract:With the wide application of DC power grid technology, as a key protection device, the DC circuit breaker has become the research focus in the related fields. This paper presents a multi-port current limiting DC circuit breaker based on the voltage clamping principle, which has the advantages of low on-state loss, good economy and fast reclosing speed. Firstly, the topology and operation strategy of the new circuit breaker are proposed, in which the fault is removed by the voltage clamping principle and then the branch thyristors are turned off by LC oscillation. Secondly, the bus and line faults are analyzed and deduced, respectively, and then the parameters are designed according to the shutdown process. Finally, the effectiveness and applicability of the proposed circuit breaker are verified by three-terminal DC network model in PSCAD/EMTDC, and two kinds of fault simulation are analyzed. Through the comparative analysis of fault current, system voltage and branch voltage, it is verified that the proposed circuit breaker can replace several conventional circuit breakers, which can reduce the demand for main circuit breakers.
2021, 45(18):179-191. DOI: 10.7500/AEPS20200827001
Abstract:The realization of fast and effective fault current limitation is one of the key technologies for the control and protection in DC systems. The DC superconducting fault current limiter (SFCL) developed from superconducting characteristics has ideal current-limiting performance, which has been widely studied and continuously developed in recent years. The main types and operation principles of the existing high-temperature SFCL are summarized based on the current-limiting effect presented by the superconducting module. The influence of superconducting materials on the current-limiting performance and applicability of SFCL is described. The connection scheme of SFCL in DC systems is illustrated. Also, based on the cooperative relationship among DC SFCL, converters and DC circuit breakers, the performance requirements of DC SFCL applied in DC systems are refined. The development pattern of DC SFCL is concluded to indicate the future application trends of DC SFCL.