光伏逆變器

1 Introduction

Renewable energy technologies are becoming less expensive and more efficient， which have made it an attractive solution of recent energy crises ［1， 2］。 Furthermore， renewable energy sources have the advantage that the power is produced in close proximity to where it is consumed. This way the losses because of transmission lines are not present. Among a variety of renewable energy sources， photovoltaic （PV） is predicted to have biggest generation， up to 60% of the total energy by the end of this century ［3， 4］， because the energy which converted into electrical energy， is the light from the sun is free， available almost everywhere and will still be present for millions of years long after all non-renewable energy sources have been depleted ［3， 5］。

The PV generates direct voltage; thus， it requires a converter to convert into a voltage of corresponding amplitude at main frequency for feeding it into utility grid. However， the problem can arises because of the hazardous voltage that can be avoided by providing galvanic isolation between the PV module and the grid through a transformer ［6， 7］。 Nevertheless， the use of a transformer leads to additional drawbacks such as less efficiency， bulky， more expansive and less durability. In order to overcome these drawbacks， transformerless inverter has been introduced which has the benefits such as lower cost， higher efficiency， smaller size and weight ［6， 8］。 Owing to the missing galvanic separation， large voltage fluctuation both at main frequency and high frequency that depends on the topology structure and control scheme， resulted in leakage current flow from the PV module to the system through the inevitable parasitic capacitance with respect to ground potential ［9， 10］。 This ground leakage current increases the grid current harmonics and system losses and also creates a strong conducted and radiated electromagnetic interference ［11–13］。 Accordingly， some standards have been established to fix a maximum allowable leakage current such as the German DIN VDE 0126-1-1 standard which states that the grid must be disconnected within 0.3 s if the root-mean-square （RMS） value of leakage current is more than 30 mA ［14］。 The RMS values of the fault or leakage current and their corresponding disconnection times are presented in Table 1.

The half-bridge inverter family can eliminate the difficulties of leakage current and injection of DC current into the utility grid having the necessity of high input voltage （700 V） corresponds to 230 V AC application. On the other hand， the problem of leakage current and high input voltage can be solved by using the bipolar sinusoidal pulse-width modulation （SPWM） full-bridge inverter. However， the conversion efficiency of bipolar SPWM inverter is lower because of the high switching losses and magnetic inductor losses. Therefore to solve the problem of leakage current and low efficiency， many DC–AC inverter topologies based on full-bridge inverter have been proposed ［6， 8， 15–25］。 Gonzalez et al. ［8］ proposed full-bridge with DC bypass topology， in which two switches and two diodes are added with a full-bridge inverter. It exhibits low leakage current and high efficiency compared with the full-bridge inverter with bipolar modulation. Another topology with DC bypass is proposed in ［21］， referred as H5 topology. This topology is patented by SMA Solar Technology AG. Schmidt et al. ［26］ proposed a highly efficient and reliable inverter concept （HERIC） topology by adding two extra switches in the AC side of a full-bridge inverter. Two extended HERIC topologies are proposed in ［16， 27］。 Although these topologies can achieve high efficiency and low leakage current， they have not yet been analysed from the point of view of reactive power handling capacity. In this study， a new transformerless grid-tied PV inverter topology is proposed based on the conventional full-bridge inverter with two additional power switches， which ensures the DC decoupling at the freewheeling mode. As a result， leakage current is minimised to safe level. The proposed topology is also capable to inject reactive power into utility grid; therefore， it can satisfy the requirement of the standard VDE-AR-N 4105. Finally， to verify the accuracy of theoretical analysis， a prototype inverter rated at 1 kW has been built and tested. This study is prepared as follows： topology relationship among existing topologies and their reactive power control capability are analysed in Section 2. The proposed circuit structure， detail operation principle with reactive power flow and differential mode （DM） characteristics of the proposed inverter are investigated in Section 3. Simulation and experimental results are depicted in Sections 4 and 5， respectively， and Section 6 concludes the study.