Research On Reliable Operation Of NPC Grid Connected Inverter Under Unbalanced Grid Conditions

Neutral point clamped three-level inverter has the advantages of less EMI,better efficiency and voltage / current distortion.Therefore,the application of neutral point clamped three-level inverter requires more and more strict power quality,such as medium and high voltage industrial power.A large number of scientific research scholars and institutions are more optimistic about the natural advantages of three-level inverter,and the inverters are applied in a wide range of areas.However,the quality of power supply will be affected when the distributed generation is connected with the low-voltage distribution network,especially when the voltage imbalance,DC component and high-order harmonics exist in the power grid,it is easy to damage the working electrical appliances,Therefore,the reliable PLL technology is researched to assure the secure operation of the power grid.From the economic point of view,the integration of distributed generation into the grid will change the voltage and current of the grid,and the voltage and current will vary,which will change the loss of power devices.Power devices will break down,because the loss of power devices will increase,resulting in the increase of grid connected investment,From the angle of stable operation of inverter,it should assure the secure and reliable work of PLL during connected with grid.Due to the high failure rate of three-level inverter power devices,especially when the power devices are in working state,the internal loss will increase,which is more likely to cause damage to the power devices,which will increase the cost of integrating the distributed generation into the grid.Therefore,it is necessary to study the inverter loss.In this paper,NPC three-level inverter is used as the research object,and the junction temperature of three-level inverter under unbalanced load and the grid connected PLL of three-level inverter under unbalance grid are studied.The main contents of this paper are as follows:Firstly,the differences between three-level inverters with different structures and the changes of working characteristics caused by the differences are described.Secondly,several modulation strategies used with higher frequency(SPWM,SVPWM,SHEPWM)are described respectively.Secondly,the loss model of NPC three-level inverter under unbalanced load is deduced.The modulation strategy of SHEPWM based on quantum genetic algorithm is simulated,and the loss is compared under different power factors.Thirdly,The NPC three-level inverter is linked to the grid when the grid is unbalanced,the DC component cannot be eliminated by second-order generalized integrator in the unbalanced power grid.In view of this shortcoming,the traditional SOGI is modified.the extracted positive sequences by the modified PLL can balance when the power grid voltage is unbalanced,the power grid voltage contains DC component and high-order harmonics,and track the phase-locked angle accurately.Finally,Build the experimental platform of NPC three-level inverter,measure the balance of positive sequence components extracted under unbalance power grid,so as to achieve accurate phase-locked.