# Hybrid Modulation Strategy to Eliminate Current Distortion for PV Grid-Tied H6 Inverter
## 1 Introduction  
Photovoltaic power generation has become one of the key research subjects in renewable integration research, attracting great interest from both industry and academia. The grid-tied inverter plays a vital role in the energy conversion and control of the PV (photovoltaic) system [1]. With the proliferation of grid-tied PV generation and other distributed generation, the contribution which such generation can make to grid stability has come into sharp focus. Consequently, many countries are raising the grid code compliance standards for the grid-tied inverters. In Germany, the latest standard VDE-AR-N4105 [2] directs that the photovoltaic grid-tied inverter should have a strong active power regulation and reactive power compensation capability that can help stabilize the grid voltage and frequency [2]. Also, the inverter needs to quickly output the corresponding reactive power to support low-voltage ride through (LVRT) [3], to aid grid recovery and avoid a chain reaction [4]. Efficiency and cost are key drivers in the development of grid tied inverters, which has motivated the move towards the use of transformerless inverters. Although eliminating the transformer generally improves cost and efficiency, it can create the problem of common-mode leakage current [5] which can reduce the security of the entire system and result in serious EMI (Electromagnetic Interference) issues [6]. For the common full bridge (H4), non-isolated topology, this common mode leakage current is not an issue if a bipolar modulation is used. However, the use of bipolar modulation requires a large inductor and is typically less efficient than unipolar modulation. Under unipolar modulation the H4 topology experiences serious leakage current problems [7; 8]. The problem of leakage current suppression when using unipolar modulation has been addressed by several approaches, typically either by modifying the inverter topology, or by introducing extra filtering to deal with it. Therecent paper by [PERSON] et al. [9] gives a good overview of different approaches. For example, the German company SMA have developed the H5 topology [10] a configuration which can suppress the leakage current, although in that work the operation of the topology under non-unity power factors was not presented. The ability of the inverter to operate under a range of power factors is an important consideration under most grid connection codes. In 2013, [PERSON] et al. [11] proposed the single-phase non-isolated H6 inverter utilizing a hybrid modulation. Experimental results validated that the leakage current issues in this H6 inverter are eliminated and the efficiency is also maintained. However, from the results presented in that paper, it appears the issue of grid current zero crossing distortion--due to a loss of control of the current in the region of the zero crossing--was not considered. As has been indicated by [PERSON] [12], the zero-crossing distortion can cause a resonant response between the grid impedance and the inverter filter capacitor which can cause voltage distortion that could cause potential disturbance to nearby electrical equipment. Also, the authors did not address the H6 inverter's non-unity power factor operation. The problem of common mode leakage can also be dealt with by the introduction of extra filtering and [PERSON] et al. [9] recently showed how the simpler H4 topology with unipolar modulation, combined with an active filter can eliminate the leakage current and provide for reactive power. However, their results still show an efficiency penalty associated with the use of the active filter. In summary, there are still improvements possible in the pursuit of a non-isolated inverter which satisfies the requirements of high efficiency, reactive power provision (free of current distortion) and elimination of common mode leakage currents.