Research On Fast Algorithm Of Neutral Point Potential Balance Of Three-level NPC Inverter Based On Carrier-based PWM

Compared with two-level inverter,the three-level NPC inverter has the advantages of high output power,low voltage drop and low distortion.Therefore,it has been widely used in the field of medium and high voltage.But it also has the problem of neutral point potential imbalance,as a result it will lead to switch overvoltage and AC harmonic increase,which will has a negative impact on the performance and safety of the inverter.The commonly used Neutral point potential balance control methods are Neutral point potential balance method based on SVPWM and Neutral point potential balance method based on carrier-based PWM.The method of neutral point potential balance based on SVPWM is intuitive,but the calculation is complex.The method of neutral point potential balance based on carrier-based PWM is simple,however,the principle of zero sequence component selection is abstract and not perfect present.When the switching sequence is greater than 8,the equivalence between three-level SVPWM and carrier-based PWM is imperfect.In this paper,based on double modulation wave carrier-based PWM theory,the equivalence between three level SVPWM and double modulation wave carrier-based PWM is demonstrated.The fluctuation of neutral point potential is analyzed from the perspective of space vector.It is proved that the neutral point potential has a triple frequency fluctuation,the fluctuation amplitude is proportional to the load current and inversely proportional to capacitance and output frequency.The influence of active power and reactive power on neutral point potential is analyzed.Under the premise that modulation voltage is not over modulation when a zero sequence component voltage was injected.The variation of the neutral point current with zero sequence component is analyzed under different conditions,the variation of the neutral point current with a zero sequence component is analyzed under different conditions.Compare the actual current in the middle point of the capacitor when the zero sequence component was injected with the target current which can make the neutral point potential balanced in a carrier cycle.The optimal zero sequence component was chosen to make the neutral point potential reach balance quickly and the switching loss is small,as a result the fast algorithm of neutral point potential balance of three-level NPC inverter based on carrier-based PWM was abtained.Fast algorithm of neutral point potential balance was used to simulate the neutral point potential fluctuation in three-level NPC inverter when the modulation ratio and power factor angle are different.The normalized amplitude of the neutral point potential with different modulation ratio and power factor angle,can be obtained by normalizing the fluctuation amplitude of the neutral point potential.When the range of fluctuation,DC voltage,output frequency and power factor angle are given,the capacitance can be selected by using the normalized amplitude of the neutral point potential.In this paper,the fast neutral point potential balance algorithm and the "predictor check"neutral point potential balance algorithm were used to simulate the neutral point potential balance of three-level NPC inverter,when the power factor angle is different,the capacitance voltage has large disturbance,the capacitance value is not equal,and the grid current content negative sequence component.Compared with the "predictor check" neutral point potential balance algorithm,the neutral point potential fast balance algorithm can make the neutral point potential reach balance more quickly.