Research On Control Strategy Of Capacitor Voltage Fast Balance For The Neutral Point Clamped Three-level Inverter

With the rapid development of power electronics technology,power converters have been widely used.At power generation side,it serves as renewable energy and energy storage grid interface.For power transmission and distribution,high power converter is the key part for high voltage DC(HVDC)transmission and flexible AC transmission system(FACTS).In the power consumption areas,it serves as motor drive for pump,fan compressor,conveyor,grinder or propulsion system in various industrial and transportation applications.With the decreased volume and weight,higher efficiency,more intelligent control and better performance,the power converters play the key role in future smart grid.In recently,multi-level inverters have been widely used in high voltage and high power applications.Compared to conventional two-level inverter,their output ac voltage provides lower dv/dt values and reduced total harmonic distortion(THD).Neutral point clamped(NPC)three-level inverters are widely used due to their simple circuit topology and low cost.The topology of the NPC three-level inverter cause the two capacitors voltages on the DC side to be unbalanced during operation,that is the problem of neutral point unbalanced.This unbalance will affect the work of the inverter and generate many hazards.In response to this problem,this article has carried out the following research work:Firstly,the essential reason of the neutral point unbalance of the NPC three-level inverter is explored.It is known that the fluctuation of the midpoint potential is closely related to the working state of the inverter.And these states are classified according to different influences on the midpoint potential.Water can carry a boat and can also overturn.These states can not only make the midpoint potential unbalanced,but also can reasonably adjust the neutral point potential from an unbalanced state to an balanced state.Using these characteristics,this paper proposes two control strategies for adjusting the midpoint potential balance.The first control strategy is the four-vector midpoint potential balance control method.This method improves traditional vector vector modulation to control the midpoint potential balance.It synthesizes the reference voltage through four vectors and controls the midpoint potential balance.The use all of the basic vector resources of the inverter improves the response speed of the neutral point potential balance.In fact,the virtual space vector and the variable virtual space vector derived from the most recent three-vector principle a essentially use four vectors to synthesis vector and balance midpoint potential.But these methods all fix the proportion of vectors,and are a qualitative control of the midpoint potential within the majority of the modulation ratio.So they limit its ability to adjust the midpoint potential.This paper proposes a four-vector neutral point potential balance control method that can quickly adjust the midpoint potential within the full modulation ratio range.Compared with the simulation results of traditional methods,the midpoint potential response of this control strategy has been significantly improved.The second method is optimize the optimized virtual voltage vector control of NPC three-level Inverter.The method firstly constructs a continuous virtual voltage vector based on the finite discrete voltage vectors of the three-level inverter.Then,a cost function is defined for the control target by using the reference voltage and the midpoint potential balance.This cost function can flexibly and conveniently control the overall performance of the inverter.The virtual voltage vector that minimizes the value of the cost function is solved by the optimization method.This is the optimal solution for controlling the reference voltage and neutral point potential balance.This method is to find the optimal solution in the global scope,not only can guarantee the great output voltage,but also has a strong midpoint potential balance abilit.Finally,the two methods proposed in this paper are compared by the experimental platform.The experimental results show that both the optimized virtual vector algorithm and the four vector midpoint potential balance algorithm have good performance in the balance of midpoint potential.Among them,the former has better midpoint potential balance and is more suitable for high power and high current applications.The latter performs better in the sine of the output voltage and current waveforms,and is suitable for applications where the sine of the waveform is required.