Investigating the effect of sinusoidal pulse width modulation and vector space pulse width modulation in three-phase inverters on common mode voltage

Renewable energy sources ,particulary resources that are the source of photovoltaic have had ,A lot of advancement have had in recent years that is mainly because of increase in temperature and privileyes are given to governments for this type of technology is. Power processing of renewable energy sources done by power converters which the issues of efficiency and cost as key factors ,along with their. In particular status ,grid –connected PV inverters ,Most topologys of power converter is used the transformer which works in a low or high frequency and This isolation creates galvanic between the photovoltaic panel and the power grid . Low-frequency transformers are large ,heavy and expensive and they are entered additional losses into the system .Isolating transformer size with using two –level topology which in that transformer works in high frequency ,can be reduced to a large extent. This method reduces efficiency,because at least two power converter of the type cascaded is required. As a result , a large number inverter with transformerless topology have been proposed in recent years which Leading to generat power processing systems, is more compact ,cheaper and more efficient .In addition when using of the inverters without transformer,some measurement techniques of isolation reactance and remanent flow should be used.It makes the inverters without transformers even safer than transformers with transformer. Given the size of power inverters connected to the network ,changing pattern has been observed in recent year. Large central inverters with power above 100 kw by power inverters in small size which supplied much energy by a string or a small group of strings can provide ,have been replaced. Following this method ,tracking point of maximum power of panels groups of large photovoltaic can be improved ,because they can be placed under very different levels of solar radiation . In this context ,using of single –phase inverters up to 5 kw is paramout importance. For the reasons mentioned a significant number of single –power topology to implement single and three phase transformerless inverters connect to grid ,have been proposed. In this type of converters ,there isnot galvanic isolation between photovoltaic panels and network. So that problems can arise that require special attention such as common mode voltages and leakage currents across the photovoltaic panels ,which this is due to this fact that there is a non –negligible parasitic capacitance between photovoltaic cells and isolation ground and under specific operating conditions(for example moisture,dirt and mode installed) can reach very high values.Normally values of this capacitance between 50 to 150 for silicon crystalline cells and for film cells reach up to values 1uf/kw_p. Common mode current leakage increases The harmonic of zinc in the system ,Reduces the connection quality of network current and directing rupture and electromagnetic radiation interference is caused and lead to problems in personal safety. In solar cells that are connected to the network via the transformer,Electrical isolation of transformer windings and high frequency of common –mode voltage and current are not any where to flow and therefore doesnot produced virtually certain common mode current .Therefore, type of inverter arrangement and switching style hasnot so much effect on this problem. But in transformerless arrangement should be discover away to prevent the transmission of leakage current produced by common mode voltage to network. In figure(a) common mode current and parasitic capacitance and in figure (b) photovoltaic system model is shown

Keywords:

sinusoidal pulse width modulation , vector space pulse width modulation , common mode voltage