Study On Control Scheme For Micro-Grid Bi-mode Operation

Electrical loads are continually growing while the requirements for power quality and reliability are increasingly high. The aging power equipments, bottlenecks of energy efficiency and environment problems are all becoming great challenges to the world's power industry. Microgrid has drawn a lot of attention for it has good potential to achieve the full utilization of renewable energy, and improve the quality of power supply. Operation and control is one of the key technologies of microgrid, good control strategy ensures the advantages of microgrid.This thesis focuses on the control strategies for both islanded and grid connected operation of microgrid. On the level of distributed generation control, DGs are divided into two categories, Grid-Forming DG (GFDG) and Power Injection DG (PIDG), GF DG forms the microgrid while the PIDG usually implements MPPT control, the operation characteristic of microgrid is dominated by the GFDGs. The direct-drive wind power generation system is modeled as a typical PIDG, and the control scheme of the back to back PWM converter is investigated, the generator side converter achieves maximum wind power tracking while the grid-side converter achieves PQ decoupled control. As for the GFDG, a simplified model was implemented using an ideal DC voltage source instead of the combination of prime mover and storage, the focus was on the control of the grid-tied inverters. The control scheme of GFDG was based on droop control, and it is improved by adding the line voltage drop compensation into the Q-V droop characteristic to achieve the precise sharing of reactive power. A pre-synchronizing control unit was designed base on the droop controller to achieve the smooth connection between the GFDGs and the main grid. Considering the operation features of grid connected operation mode of microgrid, a droop setting adjustment loop was added to the droop controller to implement PQ control of the GFDG. The control mode could be changed by switch this loop. The control scheme has a strong continuity between the 2 modes, which is favorable for the smooth operation mode transfer.At the system operation level of the microgrid, a hierarchical control structure was implemented. The local control of GFDG and PIDG was the basis of microgrid control and the whole network was regulated by the Microgrid Manager, a frequency restoration algorithm implemented to improve the frequency quality of the islanded microgrid. under the hierarchical control system, the microgrid can achieve a variety of operation modes and flexibility to choose it according to need, and can switch between the 2 operation modes smoothly. The switching process guarantees the uninterrupted supply of critical loads. In case of communication failures, the underlying control can maintain the stability of microgrid operation as a backup.Finally, a simulation platform of microgrid was built base on Matlab Simulink software. Several operation conditions were simulated in both islanded and grid connected modes as well as the transferring of operation mode to verify the control strategies, the simulation results was discussed in detail.