Two-Stage Transformerless Dual-Buck PV Grid-Connected Inverters with High Efficiency

A semi-two-stage common-ground-type transformerless dual-buck-based grid-connected inverter is proposed in this paper.The common-ground-type topology can eliminate common mode(CM)leakage current by connecting the negative terminal of the photovoltaic(PV)directly to the neutral point of the grid,which bypasses the PV array’s stray capacitance.The dual-buck-based topology guarantees increased robustness since the dc-link cannot be short-circuited by a shoot-through event.The semi-two-stage topology features multi-level characteristic,which has a lower forward-voltage drop and smaller dv/dt.Compared with the conventional two-stage inverter,the proposed topology achieves higher efficiency and higher reliability.Experimental results of a 1.5kW prototype show that the proposed inverter is able to achieve high efficiency and low leakage currents.

An Accurate Common-Mode Current Prediction Method for Transformerless Photovoltaic Inverters Based on Graph Theory

The common-mode current is an important indicator with transformerless photovoltaic inverters.However,up to now,there is not an accurate method to predict common-mode current in the inverter design process,resulting from inappropriate device selection or exceeded the expected common-mode current.In order to solve this problem,this paper proposes an accurate common-mode current prediction method based on graph theory for transformerless photovoltaic inverters.In this paper,the mathematic model of the common-mode current is derived using graph theory analysis method in the full-bridge topology,and it is used to predict common-mode current.The validity and correctness of the proposed prediction method are validated by simulation and experiment.The oscillation frequency and amplitude can be predicted by the proposed common-mode prediction method,whereas the traditional common-mode analysis method cannot.This paper provides a novel way to predict and analyze common-mode current in the transformerless photovoltaic inverters.

Analysis and Validation for an Inverter-side Current Controller in LCL Grid-connected Power Systems

Transformerless grid-connected inverters offer greater efficiencies when transferring power from renewable energy sources to the electrical grid.If the grid-inverter connection is done with an LCL filter,high attenuation of switching harmonics is achieved while preserving a small-size output filter.However,damping must be included in the controller to assure closed-loop stability.This paper proposes a reference computation methodology for the inverter-side current feedback in a photovoltaic(PV)generation system connected to the grid through an LCL filter.Theoretical analysis of the closed-loop system stability and of the steady-state performance are presented as well as experimental validation of the closed-loop performance.The feedback controller includes active damping and relies on a resonant control structure which improves the ability of dealing with grid harmonic distortion.The controller uses a reduced set of measurements,which requires the inverter-side current and grid voltage only,and assures a power factor close to unity.