The whole process of conventional DC thyristor valve commutation is analyzed comprehensively, including the commutation overlapping process during the forward conduction current being zero, and the recovery process of the reverse blocking ability. Especially, the thyristor’s commutation overlapping process is reinterpreted in terms of commutation reactance energy dissipation, and the thyristor’s recovery process of the reverse blocking ability is analyzed in terms of power charge. On the basis of these, the influence of the reverse recovery power charge, carrier lifetime and reverse turn-off time of the thyristor valve on the commutation process are studied. Finally, the theoretical basis of commutation failure criterion ‘γ<7°’ is analyzed and deduced.

To reduce the probability of continuous commutation failure in HVDC power transmission system, the occurrence mechanism and influencing factors of continuous commutation failure are analyzed theoretically. A suppressing method of continuous commutation failure based on the limit of maximum short circuit current is proposed, which can be triggered by the signal of first commutation failure, together with voltage dependent current order limiter (VDCOL), and revises the DC current setting value during the faults, so as to mitigate continuous commutation failure. Based on the CIGRE benchmark test model of HVDC, the proposed control method is fulfilled in PSCAD/EMTDC. Simulation results show that the proposed optimal control strategy can mitigate the continuous commutation failure under single-phase grounding and three-phase grounding fault conditions effectively, and realize the smooth recovery of the electrical parameters of the system, and improve the fault recovery performances of system effectively.

The floor area of a vertical grounding electrode is much smaller than that of a conventional horizontal grounding electrode, which is an effective way to solve the difficulty of site-selecting of DC grounding electrodes. The vertical grounding electrode of Pu’er is the first vertical grounding electrode of UHVDC in China. Starting from the problem of uneven distribution of inner and outer electrodes of Pu’er electrode, it is found that the shielding effect of the inner electrode is higher than that of the outer electrode, and the idea of unequal parameter design for inner and outer electrodes is put forward. Then, based on the boundary element simulation software, it is found that changing the length of the electrode can make the shunting of the inner and outer electrodes more uniform, the influences of the length and number of electrodes on the length matching of inner and outer electrodes are studied. The fitting formula for the optimum length of the outer electrode is given, and an algorithm for optimal length of outer electrodes for horizontal layered soil structures is proposed. Finally, a typical double circular vertical grounding electrode optimization design case is used to verify the feasibility of the algorithm.

Due to the lack of generators at sending end in the early operation stage of the Dianxibei UHVDC project, if it is passively entered into island mode, the system will encounter harmonic instability and will cause a great risk to the steady operation. In previous projects, the strategy is to disconnect AC filters and block bipolar DC with a certain time delay when receiving island operation signal. However HVDC system may block valves, cut generation capacity or shed power by error when receiving false island operation signal. Therefore, the locking strategy at low power under passive island mode is optimized. In Dianxibei UHVDC project, the power on AC tie lines is used as criteria of island operation to prevent from miscarriage. The logic and realization of the control strategy are illustrated, then RTDS simulations are carried out and it is verified that by judging the power on AC tie lines, the system can effectively prevent entering island mode by error. Finally, the field test proved that the proposed optimized strategy will not cause rejecting of the existing “island locking logic”. The proposed strategy can be a reference to other DC projects.

In this paper, a model of high-frequency excitation de-icing large power source for transmission lines is designed. The multiple PWM controlled rectifier and the carrier phase shift control technology for power unit input are adapted at the input side to realize the control of the input harmonic and power factor. Phase shift SPWM technology is used to overcome the difficulty of output high frequency voltage with low switching frequency devices. The control strategy of the input voltage and output current equalization sharing is carried out. Each unit communicates through equal voltage bus and equal current bus, thus the circulation current problem caused by power unit parallel output is solved. The results of simulation experiment of de-icing large power source show that the output frequency of the de-icing source is up to 40 kHz, and this solution has characteristics of steady output voltage with low harmonic, easy to control, which provides a technical reference for de-icing equipment development of transmission lines.

At present there is few researches on shed parameters of transformer bushing at home and abroad. In addition, most of the existing studies on flashover characteristics of the bushing use hollow bushing as experiment object without considering the influence of the internal conductive tube on the insulation electric field. In this paper, a 500 kV transformer high-voltage bushing equivalent model is established by finite element software COMSOL in order to study the effect of shed spacing and overhang on bushing external electric field under rain condition. Meanwhile simplified model without conductive tube and capacitance core is established to study the effect of conductive tube and condenser on bushing external electric field. The results show that the increase of shed spacing leads to obvious decline of air gap field relative range, and also leads to subtle increase of air gap field strength. The average electric field along the surface path decreases linearly with the increasing of shed overhang, and the relative range decreases with the increasing of shed overhang when big shed overhang is in range of 85~100 mm. Without conductive tube, the electric field intensity increases slightly at the shed depressions while decreases at the shed edges. And bushing external electric field is similar to the equivalent model with condutive tube.

The differences of laying way and soil environment along the way result in the current-carrying bottleneck of cable line, which determines the load capacity of the total cable line. A method for identifying the bottleneck of cable line is proposed based on the evaluation of cable conductor temperature. In view of the actual situation of 220 kV Qilin-Tianhe Line I & II of Guangzhou, the current-carrying capacity bottleneck is identified in typical scenes, and the time-space characteristics of current-carrying capacity bottleneck are analyzed, the load capicity of the line is evaluated. The results show that the capacity bottleneck of line is related to the magnitude of load and season at continuous load, and the capacity bottleneck of line is related to the magnitude of initial load current, the length of the emergency time, the magnitude of emergency current at emergency load. The research results can provide technical support for load dispatching of complicatedly laid transmission cable line, and improve the safety of cable operation at peak load.

For their outstanding resistance to the contamination flashover, composite insulators have been widely used in the EHV/UHV transmission lines. In this paper, an abnormal heating defect in 500 kV AC composite insulator operated in Shenzhen has been studied. With the visual test, AC voltage withstanding test, water diffusion test, absorption test and FTIR test, it is confirmed that water absorbed in the sheath and polar group generated from the aging process are the main reasons of the heating of composite insulators, and the heating process will accelerate the aging process at the same time. The test results also show that during the performance evaluation of newly accepted composite insulators, the saturation absorption water of silicone rubber should be considered besides the interfacial resistance to hydrolysis performance.

This paper proposes a wind turbine virtual inertia control strategy for high frequency problems in power systems based on virtual inertia technology. According to the strategy, the wind turbines under moderate or low wind speed zone are available to engage in primary frequency control for high frequency problems. Based on proportion-differential(PD) virtual inertia method, this strategy recovers MPPT control process of wind turbines in frequency control period when the rotor speeds of the turbines reach their limits. It also demands the wind turbines exit inertia control when the system frequency reaches the highest point, and sets the limitation of the electromagnetic power corresponding with the reference value of electromagnetic torque. Applying this strategy, system simulation is carried out for the directly-driven wind turbines under different wind speeds ranged in moderate or low wind speed zone which encountering a frequency ascend fault. The results show that the strategy can effectively decrease the highest level of the system frequency in power systems penetrated by wind powers, and the strategy is available for wind turbines under different wind speeds ranged in moderate or low wind speed zone to engage in primary frequency control.

Large-scale electric vehicles (EVs) disorderly charging will bring strong impact to the grid and threaten the security and economic operation of it. A bi-layer scheduling strategy considering temporal and spatial dimensions is proposed for the charging load optimal scheduling problem in the context of the fast charging mode of EVs. In the upper layer mode, the strategy obtains the optimal allocation of the EVs charging load under temporal domain by using a time-of-use (TOU) electricity price to minimizing operators’ cost. In the lower layer mode, according to the congestion level of charging station, a regional time-of-use (RTOU) charging electricity price is formulated and the EVs’ total charging cost and charging time are minimimum. And the optimization is solved by MCSO (modified cat swarm optimization) containing nonlinear decreasing inertia MR. Finally the results of examples demonstrate effectiveness and applicability of the proposed model. And the strategy can satisfy the interests of power grid, operators and EV users while realizing the friendly access of large-scale EVs fast charging.

Aiming at the calculation of the total supply capability (TSC) of large-scale renewable energy in the form of distributed generation access to the distribution network, considering the typical output scenarios of new energy and the probability of corresponding scenarios, the model is built with the objective function of maximizing the expected value of load magnification under new energy typical scenarios. The model takes into account the issue of locating the distributed generation and the network reconfiguration, and uses the second-order cone relaxation technique to relax the nonconvex and nonlinear power flow constraints. The model can be transformed into a mixed integer second-order cone programming problem in which the solution space is a convex feasible region and CPLEX is used for effective solution. The 94-bus test system example verifies the correctness of the model. The simulation results show that the total supply capability of the distribution network can be improved by taking into account the distributed generation’s access, and the capacity, location and access mode of the distributed generation have a certain influence on the total supply capability of the network.