Deadbeat predictive current control (DPCC) can track the current command with a two-sample step response. However, voltage insufficiency is common during abrupt torque changes, and the current command tracking performance is inevitably degraded. To address this issue, an optimized overmodulation method is developed in this paper. By predicting the current vector hexagon in the subsequent switching period, the crossing point between the predicted current hexagon boundary and the expected current trajectory is determined as an optimized current reference. The purpose is to mitigate the current trajectory drift in the overmodulation zone, thus ensuring optimal dynamic trajectory tracking performance and voltage utilization. The solution of the crossing point is solved by the dichotomy due to the randomness of the expected trajectory. By combining DPCC with the proposed method, dynamic current reference optimization and deadbeat response in the overmodulation zone can be achieved, and accurate current trajectory tracking can be realized. Taking $i_{d}=0$ and $i_{d}=-i_{q}$ trajectory tracking as validation examples, the proposed method is compared with the traditional methods. Both simulation and experimental results have verified the effectiveness of the proposed method.
The modular multilevel converter(MMC)topology has been widely used in medium/high voltage high-power transmission and distribution and motor drive fields.The full-bridge submodule(FB-SM)MMC topology with fault handling capability is currently receiving increasing attention and application.However,in order to suppress the SM capacitor voltage ripple,the usage of SM capacitors with larger capacitance significantly increases the hardware cost and volume of the system.In this paper,a modified FB-SM with active power decoupling circuit(APD-SM)is introduced,it combines the ability of capacitor voltage ripple suppression and DC fault traversal through device reuse without changing the external output characteristics of the topology.Compared to traditional MMC,the SM capacitor voltage ripple of this topology can be suppressed significantly in the full power factor range.The deduction rules,operating principles,modulation methods,and control strategies of this topology are introduced in this artical.In addition,key parameters in the topology structure are designed and the topology is compared with traditional FB-SM topology from multiple aspects.Finally,the simulation and experimental platform of MMC with APD-SM and FB-SM(Abbreviated as APD-MMC and FB-MMC,respectively)are built,and experimental verification was conducted based on the prototype model.Simulations and experimental results verify the validity of the APD-MMC topology and control strategy.
Power system is an important field to achieve the goal of carbon neutrality in the future.With the aggravation of source and load uncertainties,the low-carbon economic dispatching method based on the deterministic model cannot accurately describe the impact of uncertain factors on carbon emissions.In view of the above problems,a low-carbon robust optimization model considering uncertainty is constructed at the level of power system,in which,the uncertainties of source and load are modeled using the probability model,and the significance level of the target meeting the expectation is described by chance constraint.The robust dispatching scheme under the risk aversion strategy can be obtained by maximizing the confidence level of the uncertainty.Then,an event-driven low-carbon response model is constructed at the level of power users.According to the calculation results of the level of power system,the excess carbon emission event is defined by setting the carbon emission threshold,and the low-carbon power consumption behavior of users is guided in the form of price.Finally,through the case analysis,the results show that the proposed model can effectively quantify the uncertainty level of the low-carbon economic dispatching results,give full play to the carbon reduction ability of the user side,and realize the coordination of the carbon reduction goals of both sides of the source and load.
Supercritical carbon dioxide (sCO2) is a promising working medium for coal-fired power plants, high-temperature solar power systems, nuclear reactor systems, and fuel cells owing to its high efficiency and inertness. In these cases, the sCO2 is in a round tube under high temperature and pressure, far from the pseudo-critical point. Here, the thermophysical properties of sCO2 changes, resulting in large prediction errors. In this study, a conjugate heat transfer model with the k-ε turbulent model was created to analyze the tube performance of the sCO2, and the effects of turbulent Prandtl number (Prt) on the sCO2 turbulent flow with high heat flux were examined through experimental and numerical investigations. Prt had a considerable effect on the performance of the sCO2 turbulent flow far from the pseudo-critical point. The mean relative errors of the inner wall temperature and convective heat transfer coefficient were reduced by approximately 13% and 27%, respectively, when Prt was decreased from 1 to 0.55. For the sCO2 turbulent flow with a high heat flux, constant Prt values of 0.55–0.6 were applicable. The findings afford key insights for the development of sCO2 turbulent flow with high heat flux.
The large-scale integration of renewable energy into the power grid leads to the decrease of system voltage support strength,which is usually characterized by its short circuit ratio(SCR).This results in stability issues such as static voltage stability problems and sub/super synchronous oscillation.Grid-forming(GFM)converters have effective voltage support capability,and thus equipping renewable energy stations with a certain proportion of GFM converters can improve the stability of the system.However,the analytical relationship between the capacity of GFM converters and the stability margin is unclear,which makes it difficult to estimate the required proportions of GFM converters theoretically.To fill this gap,this paper discusses the estimation method for the required capacity ratios between GFM converters and grid-following(GFL)converters to maintain small signal stability.First,based on the voltage-source equivalent analysis of GFM converters,the influence of the capacity ratios on the system strength and stability margin is analyzed through the generalized short circuit ratio(gSCR)index.Then,considering the practical engineering scenario of updating some wind turbines to GFM control or installing new GFM converters,the typical values of the capacity ratios are discussed based on typical parameters of step-up transformers and the related industry standards as boundary conditions.The validity of the conclusion is verified by simulation results of a typical multi-wind-farm system.
