Research On Transformless Single-phase Photovoltaic Inverters And SiC Device Application

The development and utilization of solar energy is an important way to alleviate the global energy shortage.Thus,Photovoltaic(PV)power generation has become one of the research hotspots for people to use renewable energy to achieve sustainable development.As an interface connecting photovoltaic panels and power grids,photovoltaic inverters are the core of energy conversion and control in photovoltaic power generation systems.Its performance not only affects and determines whether the photovoltaic power generation system can operate efficiently,reliably,stably and safely,but also it is a major factor affecting the lifetime of the entire photovoltaic power generation system.Based on the problems of waveform distortion,leakage current,voltage rise and Si IGBT’s(Silicon Insulated Gate Bipolar Translator)high switching loss in the single-phase inverters,this thesis has done some researches on the modulation strategy improvements,the power limitation model modification and developing a new soft switching method with SiC(Silicon Carbide)Device for the transformerless inverter.The improvement of dead time effect and common mode leakage current suppression are two important aspects of transformerless photovoltaic inverter optimization research.This thesis proposes an improved modulation strategy based on hybrid single-phase transformerless H6 PV inverter with unite power factor.In this modulation,the output voltage of the inverter is modulated by the two decoupling switches on the DC side,while the H-bridge changes the output voltage polarity.There are several advantages for the modulation.First,there are only two high-frequency switches in the modulation strategy,and SiC MOSFET(Silicon Carbide Metal-Oxide-Semiconductor Field Effect Transistor)with fast switching speed and low switching loss are chosen as the high-frequency switches.The body diodes of the SiC MOSFETs don’t conduct in the whole period which eliminates the high reverse recovery loss.Compared with the pure IGBT inverter,the H6 inverter with the hybrid switches can improve the efficiency and power density.Secondly,the two high-frequency switches of the inverter do not need to add any dead time,which can effectively improve the output waveform quality of the inverter.In the experiment,compared with the traditional inverter,the hybrid inverter reduced 2.04% of the total harmonic distortion(THD)of the current output waveform.Finally,due to the symmetry of the freewheeling circuit,the inverter can reduce the common mode voltage fluctuation naturally and effectively without paralleling any additional capacitor to the switch,which is helpful for the common mode leakage current elimination.Compared to conventional inverters,the experimental result shows 75% of the common mode voltage fluctuation is cut down in the hybrid inverter.Dealing with the voltage problems caused by photovoltaic inverters,the research on the reactive power output capability and capacity is also essential.First,the inverter should have the reactive power output capability to provide reactive power support for the grid.This thesis proposes a hybrid modulation strategy for the hybrid single-phase transformerless H6 PV inverter.By adopting different modulation strategies in different power regions,the inverter not only can maintain high output waveform quality and low common mode voltage fluctuations,but also has the reactive power output capability.In the experiment,the inverter with hybrid modulation strategy can work under none-unite power factor condition.Furthermore,compared with the conventional inverter,the voltage THD and common mode voltage fluctuations of the hybrid inverter is reduced 1.69% and 85% respectively.When the PV inverter participates in the reactive power support of the power grid,the accurate maximum reactive power capacity and the output efficiency become important parameters of the power grid reference.However,the current research has not fully discussed the reactive power output capacity of photovoltaic inverters based on the power device physical limitations.In fact,due to the difference in semiconductor operation principle when injecting active or reactive power,the output power of reactive power output capacity is different from the rated value.This thesis analysis three different typical photovoltaic inverters’ active or reactive power output capacities,which show that the inverters’ active or reactive power output capacities are different.According the results,an improved power constraint model is proposed.The simulation analysis of the single-phase H6 photovoltaic inverter’ reactive power capacity is also given.It can be seen that the inverter have 14.29% output reactive power capacity more than the rated value,which mean the inverter actually can provide more reactive power to the grid.The modified power constraint model can help the grid to make a better reactive power dispatching and compensation strategies for photovoltaic inverters’ reactive power output.The low switching speed and high switching loss IGBT is one of the key reasons,which limit the improvement of the inverters’ efficiency and power density.To get the better understanding of soft switching,a detail IGBT soft switching classification is given in this thesis.It is not only decided by if the voltage or current is zero before the switching device is turned on or off,but also take the states of the rising or falling edges of voltage and current into consideration.To improve the efficiency and keep low cost of the inverter at the same time,a soft switching method with SiC device is proposed for the inverter.To reduce the dynamic switching loss,the turn-off signals are given by a SiC MOSFET to realize the Zero-voltage hard-current current ZVHC)soft turn-on and zero current hard voltage(ZCHV)soft turn-off conditions of the IGBT on the main circuit.In the experiment,the 90% turn-on loss and the 57% turn-off loss of IGBT are reduced by using the soft switching method.In order to reduce the influence of stored carriers in the IGBT during the turn-off period,this thesis not only optimizes the modulation parameters of the soft-switching method,but also proposes an improved modulation strategy for the SiC MOSFET in the soft-switching method.The turn-off loss can be further reduced by slowing the voltage rise edge.The soft-switching method proposed in this thesis is simple to design.When the method applies to a voltage-source inverter,only the topology is need to redesign depends on the principle of the inverter,while the modulation strategy of the inverter keep unchanged.With the less IGBT switching loss and the higher operating frequency,the efficiency of the soft-switching inverter can be increased more.At the end of the thesis,two application examples of H6 soft-switching inverters are proposed.When 70% of the IGBT switching loss is reduced and the inverter is operated at 100 kHz,the efficiency of the soft-switching inverter can be increased at least 1.4%.