Control Strategy And Research For Smooth Switching Of Microgrid

For the purpose of low carbon energy, microgrid is considered as a main part of the low-loss, high reliable and flexible smart grid. Sustainable running both under connected and islanding modes of microgrid are the key technologies to achieve technical and economic goals. The smooth transition is an important technology to guarantee stable and safe transitions between these two modes.According to the characteristics of microgrid in both grid-connected and islanding operation modes, control strategies are proposed to achieve smooth transition between these two modes. Under grid-connected mode, the fluctuation of power in microgrid is balanced by utility power grid. PQ control is adapted to balance the power and manage the energy for the micropower, and the energy storage equipment is at the status of charging standby. When the utility power grid failed or active micro-grid solution with the column, islanding detection device detects that and the Microgrid runs into the islanding operation mode, the improved droop V/f control strategy is employed to battery, mainly to compensate the power shortage of the whole system, and at the same time support the voltage and frequency of the microgrid system. Super capacitor uses V/f control, mainly to provide for the system voltage and frequency when the Microgrid switching modes. DG maintains adapting PQ control to shorten the transition time of the voltage and the frequency. When microgrid reconnect to utility power grid, caused by the voltage deviation of microgrid to utility power grid, directly reclosing to grid may cause huge impact current, so a three phase software phase-locked loop (SPLL) based pre-synchronizing unit is designed to track the utility grid voltage by controlling the inverter output. This unit can simultaneously track the amplitude and the phase angle of voltage. Involving this unit, the impact of the reconnecting process is weakened, which makes the process of reconnecting to grid stable and safe. In the whole process, the optimal distribution and reliable protection of the energy storage is well considered to meet control targets of smaller capacity and high efficiency, which can reduce the pollution and emissions levels.Simulations are done by MATLAB/SIMULINK software packages. Simulation results confirm that the smooth switch of the control system is effective. By controlling the energy storage and DG, the operation voltage and frequency under grid-connected mode, islanding mode and transitions between them are stable. Furthermore, the stability of the output power of the DGs is also achieved and actively reduces the renewable energy generation intermittent and randomness of output power.