Yuan Fu

Interconnection of wind farms with grid using a MTDC network

In the light of the practical project experience, the multi-terminal DC (MTDC) is regarded as one of the preferable solutions to solve the grid interconnection issue of wind generation. This paper mainly focuses on the application of the voltage source converter(VSC) based MTDC technology to integrate large scale wind farms to the electric power grid. A radial MTDC system is explored as the best choice for wind power integration, due to that it can mitigate the fluctuation of the aggregated wind power. Based on the analysis of the VSC model and control, the coordinated control strategy for the proposed MTDC system is designed. The operation performance of a four-terminal MTDC system connecting two DFIG-based wind farms, the local and remote grids is also studied, and the proposed control strategy is adopted to achieve a constant power for long distant transmission to the load center under wind speed variations and faults on DC line.

Pole-to-ground fault analysis in transmission line of DC grids based on VSC

The DC transmission lines have become one of the components with high fault probability as DC transmission technology based on VSC is often used for long distance transmission in which system most of faults are pole-to-ground fault. Therefore, with pole-to-ground fault of DC lines being a typical fault of DC grids transmission system, a detailed analysis of the fault characteristics is of great practical significance for fault detecting and protection design. According to the characteristics of the fault current circuit when the pole-to-ground fault occurs, the three stages of fault process were presented in detail. Firstly, the voltage of capacitor and fault current expressions of DC-side capacitor discharge stage were derived. Secondly, the state equation of grid-side current feeding stage was listed. Thirdly, the distribution of DC-side capacitor voltage in voltage recovery stage was analyzed. Moreover, for different grounding resistance, the fault characteristics of the system in each stage were analyzed, which was divided into under-damping and over-damping. Finally, the accuracy and effectiveness of the fault analysis was validated through a two-terminal DC transmission system which was established in Simulink.

Research on power coordinated control strategy of wind turbine-based DC microgrid under various modes

Power balance of a DC microgrid is an important guarantee for the safety and stability of power network. To have an integrative consideration of DC microgrid and capacity of converters, the operation of DC microgrid is divided into four modes, which contains the full rated converter induction generator, storage batteries and AC/DC loads. The four modes include grid-connected operation, current-limited operation, short-time failure operation and island operation. According to the four modes, a power coordination control strategy based on multivariable is proposed. The working operation of each converter can be automatically coordinated by the proposed control strategy, according to the DC voltage variation, the state of charge of battery and the current of grid connected converter. Then, the active power of DC microgrid can be balanced under any working condition to maintain the voltage stability of DC bus. Finally, MATLAB/Simulink simulation results are presented to validate the effectiveness and feasibility of the proposed control strategy.

A new calculation method for fault transient expression of ring DC systems

When the fault occurs in the ring DC system, the fault process can be divided into the current discharge stage, the diode freewheeling stage and the steady-state phase under the AC power supply. This paper mainly studies the capacitor discharge stage. Firstly, the multi-terminal DC system is decomposed into n single-ended systems. Secondly, write the state equation of the single-ended system and solve the expression of fault current and voltage. Thirdly, get the fault analysis expression of the multi-terminal system by the relationship expressions for each transient value between single-ended system and multi-terminal system. The simulation are carried out by MATLAB / Simulink to provide the basis for fault location and isolation.