The presence of bubbles in the transformer oil is very prone to cause partial discharges in the oil, which affects the insulation characteristics. Therefore, it is of vital importance to study the severity of the electric field distortion and the dynamic characteristics of the bubble under this physical model of gas-liquid two-phase flow. In this paper, a multiscale finite element method of electric field coupled with the dynamic characteristics of the bubble is proposed. The coupling between the electrostatic field and the hydrodynamic field is established through the level set function. The level set function is an output of data from the hydrodynamic field for the position of the bubble and at the same time an input of data for the description of the interpolation function when solving for the electric field by the extended finite element method. Unlike the conventional finite element method, this algorithm eliminates the need to re-dissect the elements at each time step, and thus can greatly reduce the solution time while maintaining accuracy. © 2024 IEEE.

With the development of Low voltage DC (LVDC) distribution system, the demand of household appliances witnesses continuous growth in a decade. For meeting different requirements, the voltage levels of the appliances become more various, which challenges their integration into the building and increases the design difficulty of a household outlet. Due to most household appliances supplied with DC power, the ideal outlet in the DC household power system should be a DC outlet with multilevel voltage outputs. However, it is not user-friendly for consumers. Moreover, the outlets with high voltage DC output increase the risk of appliance malfunction and electric shock, and the extra DC-DC conversion stages in the multilevel-buses LVDC system also increases the power loss and complicates the wiring topology. A novel design of DC household outlet is proposed in this paper. The proposed DC household outlet can distinguish the operating voltage of the household appliances automatically, and then supply the appropriate operating voltage for appliances. With the embedded DC-DC buck converter, a wide range of output voltage from 5V~400V can be provided by the proposed outlet. For verifying the performance of the proposed DC household outlet, the simulations on the power supply process of smartphone and electric kettle respectively are conducted on Matlab/Simulink. The simulation results show the feasibility of the proposed outlet topology and control scheme.
© 2020 IEEE.

Owing to the demand for larger wind turbine generators, the megawatt wind energy conversion system using the modular multilevel converter (MMC) has shown great potential in this area. However, the capacitor voltage of this topology is floating, whi0ch leads to the requirement of large capacitor and the increment of project cost. This paper proposes a minimal capacitor voltage ripple (MCVR) control for the MMC-based wind energy conversion system. In the method, the second and third harmonic components of the capacitor voltage are controlled to counteract its fundamental component. With the proposed MCVR control, the capacitor voltage ripple can be reduced without influencing the maximum power point tracking of the wind energy conversion system. Finally, the effectiveness of the proposed method is validated in MATLAB/Simulink. © 2020 IEEE.

Over the past few years, wind power witnesses a continuous growing share in the conventional generation mix for its advantages in low-carbon and sustain. However, due to the randomness of wind energy, the integration of suburban wind farms into the urban power grid challenges the system operating safety. Due to the independent control ability of the active power and reactive power, modular multilevel converter based HVDC (MMC-HVDC) technology has become a good option for integrating the suburban wind farm into the urban power grid. Compared to the traditional AC transmission technology, MMC- HVDC could provide dynamic reactive power support to the urban power grid besides its excellent active power controllability, which could bring both control and stability benefit to the urban power grid. In this paper, a dynamic reactive power control scheme is proposed for the MMC-HVDC system integrating the suburban wind farm into the urban power grid. The proposed control provides different reactive power supports strategies for different operating conditions of the urban power grid. In addition, the proposed control could also realize fast and smoothly switch between different control scenarios of urban power grid dispatching instruction and emergency events so that providing effective reactive power support for the urban power grid. The small-signal model is also established in this paper for evaluating the stability of the dynamic droop coefficient. With the simulation results, the proposed control scheme is proved that this scheme can better support the voltage of the urban power grid. © 2020 IEEE.

With the increasing demand for renewable energy integration and DC power consumption, the application and investigation of residential low voltage DC (LVDC) power system have attracted more attention in recent years. The residential DC power system has advantages on distributed generation integration and conversion cost reduction, which could provide high efficiency and high power quality power supply to consumers. However, the lack of voltage level standards challenges the commercial application of the LVDC power system. In this paper, a comprehensive study on the performance of different voltage levels is proposed, in order to provide a voltage level guidance for DC power supplier and appliance manufacturer. Existing voltage selections from applications and researches are compared with each other in terms of voltage compatibility, topology, power supply capability and electrical safety. Comparative simulation is conducted in the residential DC power system model of a community with four household subsystems. Based on the simulation and investigation, the comparison result indicates that: 240V and 380V is optimal for the unipolar residential system with low power demand and ±375V bipolar DC system should be used in heavy load scenarios. © 2020 IEEE.

Sharing frequency response reserves through HVDC connections among the three Interconnections is expected to be one high value product that provides benefits to economically justify the HVDC Network. In this paper, A HVDC frequency response control scheme considered frequency response reserve allocation is developed for the VSC-HVDC links connecting asynchronous interconnections. The simulation results show that the proposed control scheme can greatly help the frequency stability of interconnection after contingencies of tripping the major generator while reducing the financial loss of disturbed interconnection during the event.
© 2019 IEEE.

As the typical clean and renewable energy, wind energy has witnessed a continuous annual increase in the last few decades. Due to the random and intermittent characteristics, the wind producers in the electric market meet serious financial losses caused by the deviation between the actual power output and forecasting result. It is difficult to increase the accuracy of forecasting in a short time. How to improve the financial income has become a major task to wind producers and academia. Being the backbone network of the offshore wind farms (OWFs), voltage source converter-based multi-terminal HVdc system (VSC-MTdc) has been regarded as one of the effective solutions to transport wind power. In this paper, an optimal distribution method is proposed for VSC-MTdc system integrating OWFs to reduce the financial loss due to wind power output deviation. The proposed method could be divided into two optimizing functions. The first optimizing function of the proposed method is to analyze the onshore external system according to the historical system operation data and adjust the droop coefficient with the analytic hierarchy process. The second optimizing function of the proposed method is to do further adjusting with the regulating price. With the case study, the proposed optimal distribution method has proved that it can bring more benefit to wind producers.