Accurate Output Power Control For Energy Storages- Based Converters In Microgrids

A microgrid is such a miniature intelligent power supply network that is composed by a variety of distributed generation units, energy storage units, loads, control and protection devices connected by power electronic interface devices. The microgrid makes up the shortcomings of renewable energy generation units, such as discontinuity and fluctuation, improves the reliability and availability of distributed generation units, and represents the development direction of the smart grid.The controllers in a microgrid based on energy storage units play an indispensable role in the power balance and voltage stability of the microgrid. The traditional control algorithm for the microgrid controller has three problems. First, the proportion of load distribution is affected by the line impedance at islanding mode. Second, the current or power overshooting during modes switched from islanding mode to grid-connected mode. Third, when the grid voltage drops, the traditional control algorithm uses a number of coordinate transformations, phase sequence separarion and trigonometric calculations, which not only causes the error accumulation of calculation, but also easily leads to control delay.In order to solve the above problems of the conventional control algorithm for the microgrid controller, the improved control algorithms for normal and fault conditions of the grid are proposed, the main contents and contributions are shown as follows:1) Under the normal conditions of the grid, the conventional droop control metnod is improved by adding a dual virtual impedance and power compensation conductance, so as to elimitate the impact of line impedance on the proportional distribution of the load among multiple inverters in parallel.2) Through the adaptive voltage compensation link, the output voltage drop caused by the droop control and virtual impedance is eliminated, and the seamless and automatic transition between islanding and grid-connected modes is realized. The sampling signal required for the algorithm is obtained locally, which is of benefit to the modular design of the controller. The proposed control algorithm is suitable for all types of line impedance.3) Under the fault conditons of the grid, three different control objectives for different application occations is summed up, namely, output symmetrical three phase current, no fluctuation in active power and no fluctuation in reactive power. A new control algorithm for microgrid controller in two phase stationary coordinate system under the conditions of grid voltage unbalance is proposed, which is more simplified and reliable, and effectively reducing the number of sequence decomposition and coordinate transformation, thereby reducing the execution time of the control algorithm and the cumulative error.4) The correctiveness and effectiveness of the proposed method are proved by the simulation and experiment, and the microgrid controller could realize the control objectives under normal and fault condition of the grid.