A novel method for frequency observation applied to droop control on grid-connected inverter

Droop control is an important control scheme applied to inverter interfaced distributed generators accessing to distribution network. As to the phase locking and frequency measurement issues involved in control process, this paper proposed a novel method based on phase sequence separation and least square method (LSM). To get the values in two phase static frame, use Clarke transformation to the three-phase voltage in the first step; then differential operations had been done to establish linear equations to extract the positive-sequence component, as negative-sequence component and 2nd harmonic would be eliminated by solving these equations; since other higher harmonics would be suppressed by using low-pass filter in rotating reference frame, the real-time phase angle can be obtained. Then frequency can be acquired through the improved LSM, which achieves decoupling through time displacement, and thus reduces the computation load. Besides, phase error caused by filter can also be compensated by the improved LSM. Experimental results testified that grids' phase and frequency can be observed accurately and steadily even in distorted condition, and that droop control based on the proposed method can apply to grid-connected inverter.