Regarding the different ownerships and autonomy of microgrids (MGs) in the distributed multi-microgrid (MMG) system, this paper establishes a multi-stage energy scheduling model based on a multi-agent system (MAS). The proposed mechanism enables a microgrid agent (MGA), a central energy management agent (CEMA), and a coordination control agent (CCA) to cooperate efficiently during various stages including prescheduling, coordinated optimization, rescheduling and participation willingness analysis. Based on the limited information sharing between agents, energy scheduling models of agents and coordinated diagrams are constructed to demonstrate the different roles of agents and their interactions within the MMG system. Distributed schemes are introduced for MG internal operations considering demand response, while centralized schemes under the control of the CCA are proposed to coordinate MGAs. Participation willingness is defined to analyze the MGA’s satisfaction degree of the matchmaking. A hierarchical optimization algorithm is applied to solve the above nonlinear problem. The upper layer establishes a mixed-integer linear programming (MILP) model to optimize the internal operation problem of each MG, and the lower layer applies the particle swarm optimization (PSO) algorithm for coordination. The simulation with a three-MG system verifies the rationality and effectiveness of the proposed model and method.

The flashover performance of insulators is the foundation of the calculation of precise lightning protection. The material and structure of an insulator affect its flashover performance but the research on the differences in the flashover performance of different insulators and the analysis of the reason for these differences have not been clear. In this paper, lightning impulse flashover tests of two types of insulators (composite and glass insulators) were carried out in the National Engineering Laboratory (Kunming) for Ultrahigh-voltage Engineering Technology. In these tests, the insulators were hung in a 110 kV double-circuit transmission tower and a standard lightning impulse (1.2/50 μs) was applied to the insulators. The discharge path and the 50% impulse flashover voltage of the insulators were recorded. At the same time, the electric fields in the vicinity of different insulators were calculated using the finite element method with COMSOL Multiphysics. The electric field distribution and the uneven coefficient were analyzed. Combined with the flashover test data and the electric field simulation, the relationship between the flashover performance (discharge path and 50% impulse flashover voltage) and the electric field (electric field distribution and uneven coefficient) is found. The simulation results are in accordance with the test data and the conclusions can provide useful references for the external insulation design of transmission lines and the optimization of insulators.

The occurrence of frequent high impulse currents during the process of switching-on AC filter banks can accelerate the aging of arresters in ultrahigh-voltage direct-current (UHVDC) and HVDC transmission systems, threatening the safe operation of converter stations. Therefore, impulse aging tests with different amplitudes were performed on zinc oxide (ZnO) varistors under a negative polarity 15/35 s impulse current to provide a basis for the aging evaluation criteria of AC filter arresters in UHVDC and HVDC transmission systems. The ratios of even-order to basic resistive harmonic components linearly increase with the number of impulse groups and suddenly decrease after complete damage occurs. The variation trends of the ratios of even-order to basic resistive harmonic components were analyzed and discussed based on the current waveform and amplitude-frequency characteristics of the resistive component and the DC V-I characteristics. Based on the results, a new criterion for the unipolar impulse aging of the ZnO varistor is proposed, i.e., the ratio of second to basic resistive harmonic components, and the application of ZnO varistor even-order harmonic characteristics in online monitoring is briefly discussed.

Based on a ±10 kV three‐terminal MMC‐MVDC (medium voltage DC distribution system based on modular multilevel converter) system, in this study, the authors mainly study on the influencing factors and protection schemes of the lightning intruding overvoltage (including direct lightning and back‐flashover lightning). Firstly, a lightning intruding overvoltage simulation model of the MMC‐MVDC distribution system using overhead lines is established based on the PSCAD/EMTDC program. The influence of steady‐state operating voltage and the number of converter stations on the simulation results are simulated and analysed. Meanwhile, the proper simulation modelling method of the lightning intruding overvoltage for multi‐terminal MMC‐MVDC system is proposed. Secondly, the lightning intruding overvoltage is simulated when the line is struck by the lightning stroke current that occurs once in 20 years and the different lightning protection schemes are proposed. Finally, based on the lightning impulse withstand voltage, lightning trip‐out rate and the cost of the protection schemes, the lightning protection schemes in different scenarios are proposed. The simulation results show that if the cost of the system is prioritised, the protection scheme of increasing the number of parallel stacks of the DL arresters can be selected, and if the power supply reliability of the system is prioritised, the protection scheme of installing the overhead ground wire as well as reducing grounding resistance can be selected.

This paper establishes a complete lightning overvoltage simulation of an offshore wind farm, considering the electromagnetic effects including the blade, the tower and the power cable based on transmission‐line theory. And the influences of grounding schemes of the power cable inside the tower, different WTs struck by lightning, lightning current amplitude and the number of blades struck by lightning on lightning overvoltages of WT are all analysed. The results demonstrate that the grounding of both ends can effectively suppress the lightning overvoltages of WT. Second, the lightning overvoltages of WTs are related to their connection modes in case of lightning strike to different WTs, because of the differences of the equivalent impedance at the outlet of the WTs. Furthermore, the lightning overvoltages of WTs caused by lightning strike to double blades are larger than that caused by lightning strike to a single blade. In the case of the lightning current with an amplitude of 200 kA striking double blades, arresters should adopt multiple parallels connection to ensure the equipment would not be damaged. The distance between the tower and the power cable should be maintained approximately 0.25 m, to prevent the damage to power cable caused by flashover.