Translated Abstract
In order to alleviate the crisis of large investment, weak ability of fighting natural calamities and severe pollution of the power grid, microgrid, as the effective carrier of distributed energy resources, has increasingly attracted the attention of researchers around the world. It is depend on the effective implementation of the control strategy of microgrid-connected converters that the microgrid can achieve smooth switching with the power grid and some large distributed energy resources like the DFIG can achieve the grid connected operation reliably and efficiently. The control strategy of grid-connected converter of microgrid studied in this paper mainly includes the control strategy of constant voltage source grid-connected converter, converter system of offshore DFIG with its transmission system and the modular multilevel converter, which are significant for improving power quality and guaranteeing the safe operation of microgrid. The specific works done are as follows:
Firstly, the typical control strategy of constant voltage source grid-connected converter of microgrid is studied, including the master-slave control and the peer-to-peer control. It is found that the subordinate sources rely heavily on the principal source in the master-slave control, and the stability of frequency and voltage in the microgrid is poor in the peer-to-peer control. The traditional control strategy is improved in this paper, and the smooth switching between the running states of grid connected operation and grid isolated operation is validated under the Digsilent platform, which shows that the proposed control strategy can suffer the influence of DG’s switching between grid connected operation and grid isolated operation. Furthermore, the safety and reliability of microgrid are improved efficiency.
Secondly, the wind power system is one of the most important part of the microgrid, a topology structure of converter system of DFIG and its transmission system based on the flexible DC transmission technology is studied in this paper. The mathematical models and control strategy of the converter system and its transmission system is stated in detail. And under the Digsilent platform, the simulation model of the whole system is established, which validates the feasibility and stability of the system.
Thirdly, as some distributed energy resources in the microgrid output high level of voltage and power, the topology of modular multilevel is usually adopted in the grid-connected converter, and the suitable control strategy should be adopted to ensure the safety and reliability of the system. Each submodules’ capacitor voltage is selected as object and the hierarchical control is also studied in this paper. The carrier phase-shifting modulation is adopted in each submodules, which can reduce the harmonic contents of output voltage in AC side on the premise of ensuring the balance of submodules’ capacitor voltage. Lastly, the simulation model of single-ended MMC-HVDC system is established under Simulink software, which verifies the validity of the control strategy. The bidirectional power transmission of the system can also be verified through the regulation of the command current.
Finally, the bottom power module is designed under the simulink simulation, the MOSFET and MOSFET drive delay time are measured at ns level, which satisfy the requirement of actual project and verify the reliability of this power module. Furthermore, the AC to DC and software phase-locked loop experiments are finished in the control board, which offer the accurate frequency and phase to the realization of the future control strategy.
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