Microgrid Frequency Control Using Multiple Battery

Micro grids being a new developing technology operate in two distinct modes of operation, namely, the utility-grid-connected mode and the autonomous (or islanded) mode. In transition from the utility-grid-connected mode to the autonomous mode, the frequency of a micro grid can be seriously affected due to unbalance between power generation and power demand and at that moment, micro-sources may respond slowly to eliminate this unbalancing situation. To solve this interruption and to operate the micro grid steadily, some rapid response storage systems, namely Battery Energy Storage Systems (BESSs) are needed [1]. In this study, several parallel inverters control schemes with BESSs based on droop characteristics is proposed to control the frequency of the micro grid upon isolating from the utility grid. Several parallel inverters based BESSs have been implemented to deliver desired real power to the micro grid by state feedback control system. This system is used as closed loop control system to direct inverter power in the micro grid. The real power balancing in the system is achieved by controlling frequency based on the droop control method. In this thesis, multiple inverters with BESS based on angle droop controllers are proposed to share real power in the isolated micro grid system consisting of inertia based DG and variable load. The angle of output voltage in inverter (or voltage source converter, VSC) can be changed instantaneously to control real power and this changing angle is beneficial for quick attainment of steady state. Thus with converter based BESS, load mismatch is minimized by drooping the angle droop control method for better dynamic response and also real power sharing in micro grid [2]. For accurate minimizing load unbalance, the droop gain is considered in such that they can share the load in proportion to their rating. Therefore, the choice of droop gains is often trade-off between power balancing and stability. The proposed angle droop control method helps to balance the supply and demand in the micro grid autonomous mode through charging or discharging of the BESSs and at that moment the output power of inverters increase or decrease in such a way to keep the state of charge (SOC) of the storage device within the safe condition and also to balance of power generation and load demand.