Secondary Frequency Automatic Control Method for Micro-grid Inverters

Micro-grid is as an effective technology for controlling the influx of distributed generations and improving the percentage of renewable energy generation, both of which are researched all over the world. The traditional linear droop control is important for micro-grids in order to realize the plug and play characteristics. According to the defects of traditional linear droop control technology applied in DGs dealing with a large load demand fluctuation with the result of a larger frequency deviation problem, a new droop control method was presented. The new droop curve design is based on secondary frequency operations of synchronous generator trajectory, and the total frequency range was divided into segments, which consist of qualified and unqualified frequency segments by only primary frequency control. Through segmented design, the traditional primary frequency droop control is still used in the qualified frequency segment to keep the advantage of droop control. While in the unqualified frequency segment, the mathematical mapping function is built to project from the defect zone controlled by primary frequency operation to the aim projection zone controlled by secondary frequency operation. The mapping function builds the negative feedback relation between the source output power and frequency deviation to implement the automatic operation of secondary control. By means of the location set of projection points and the known droop coefficient value, the new nominal operation point can be located, which can be changed according to the load demand fluctuation in real time. The test in MATLAB/Simulink environments was carried out, and the results verified the validity of the auto-secondary control strategy, which deals with large load demand. Keywords-microgrid; droop control; secondary automatic frequency control; mapping function; segment design