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In this paper, a simple digital hysteresis current control (DHCC) with quasi-constant switching frequency (SF) for the current control of a single-phase grid-connected inverter (SPGI) is proposed. The hysteresis band is cancelled, and the SF is fixed by online adjusting the sampling period. The principle of the DHCC with constant sampling period is analyzed, and the distribution of the switching period (SP) corresponding to the phase angle of the grid is derived. The rule of online adjusting the sampling period to fix SF is then determined according to the former theoretical analysis results. Furthermore, the effect of variation of the filter inductor on the SP is analyzed. An online estimation of the filter inductor based on the linear model of the current increment within one switching cycle is developed to eliminate this kind of effect. The proposed scheme shows the features of simple control, easy implementation in a digital way, robust to the filter inductor, and excellent steady and dynamic performance. The detailed simulation and experimental results verify its accuracy and feasibility.

In this paper, the dynamic expressions of the amplitude and frequency estimated by the standard enhanced phase-locked loop (EPLL) and the ones with input DC offset estimation integrator (DCEI) are derived originally and reveal that, DCEI enlarges the amplitude of the periodic ripples caused by the dynamic disturbances and prolongs the dynamic process. To achieve correct estimation when the input signal contains DC offset and harmonics while without deteriorating the dynamic performance, an improved EPLL combined with delayed signal cancellation (DSC) is proposed. A DSC operator is employed to the input of EPLL to eliminate DC offset and even order harmonics. A cascaded DSC (CDSC) module is applied in both frequency and amplitude loops to remove the effect of most of residual odd order harmonics. The structure of the CDSC module and delay coefficient are designed in detail. Experiment results of all the three PLLs are presented and compared to validate the theoretical analysis results and the proposed EPLL.

In this brief, a new two-phase stationary-framebased enhanced phase-locked loop (TPSF-EPLL) is proposed for detecting the amplitude, phase angle, and frequency of a threephase grid. The differential equations of the proposed TPSF-EPLL are originally derived based on minimizing the cost function with a gradient descent algorithm. Moreover, the system structure, the design guidelines of each parameter in both frequency and amplitude loops, and the stability analysis of the TPSF-EPLL are presented. The main advantage of the TPSF-EPLL is that it requires less calculation since transformation into a rotating reference frame is not involved, and all the calculation is performed in a two-phase stationary frame. Experiment results of steady state and dynamic performance of TPSF-EPLL are presented and compared with some classical PLLs to verify the validity of the theoretical analysis and its advantages.Index Terms-Cost function, enhanced phase-locked loop (EPLL), parameter design, two-phase stationary frame.

This study proposes a simple unipolar maximum switching frequency limited hysteresis current control (UMSFL-HCC) strategy without band suitable for the single-phase grid-connected inverter. At the starting moment of each sampling cycle, the actual grid-connected current is sampled and compared with the sine reference one without band offset and the control signals of various power switches are obtained directly according to the comparison results. The distribution law of the switching period corresponding to the phase angle of the current and other system parameters within one line cycle is derived. The design rule of the filter inductor considering the various requirements is presented as well. By selecting proper sampling period and filter value, the UMSFL-HCC achieves excellent control performance. At the same time, the ripple of the switching frequency is limited within a controllable range. The proposed strategy is very compact and the switching loss is lower. The simulation and experimental results verify its accuracy and feasibility.

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