Research Of Active Neutral-Point-Clamped(ANPC) Three-Phase Three-Level Converter And Its Modulation Strategies

Active NPC(ANPC)converter has redundant switching states,which can be used to redistribute loss and improve efficiency.The great promotion in capacity and efficiency makes it gradually applied in the industrial fields of high-power photovoltaic sysyem,power conversion system(PCS),electric vehicle fast charging sysyem,railway transportation,and etc.In order to further improve the power density and efficiency of the converter,the wide band gap semiconductor with higher switching frequency is mostly adopted.At this moment,the deadtime effect will be prominent.The terminal voltage is distored,as well as the corresponding low-frequency current harmonics through the small filter inductance.On the other hand,as the increasing penetration of power electronic converters in industrial occasions,more strict requirements for their reliability are also put forward.Moreover,in some occasions with high stagnation cost and safety considerations,it is required to maintain rated or derated power within the tolerable range after failure.Based on the above problems,the following research is carried out for the high-capacity,high-efficiency and high-reliability operation of ANPC converter:1)A compensation method of deadtime effect considering current ripple was proposed.Existence of dead time that makes the ultimate terminal voltage being inconsistent with the modulation voltage,is recognized as the main reason for leading output voltage distortion.The distortion becomes particularly severe in high power-density high efficiency applications where the wide bandgap semiconductors are gradually being adopted,providing higher switching frequency with small junction capacitance.In order to explain this phenomenon,the mechanism of three-phase current distortion was thorough studied by vector analysis in the space-vector plane.The relevant factors that lead to the degradation of current quality were also summarized.Improved compensation strategy has been proposed to ameliorate the current distortion.Compared with conventional method,an additional item for current ripple compensation is introduced.This simple scheme avoids the compensation error caused by current ripple,and significantly reduces the current THD value under the case of small inductance.2)A hybrid control method of neutral-point(NP)voltage balance considering the current ripple and switching loss characteristics was proposed.NP voltage fluctuation is an inherent problem in three-level inverters.Without any control measures,the NP voltage will deviate.Therefore,the stress of power devices and capacitors increases,which will affect their service life,and even lead to their damage,thus endangering the safety of the system.NP voltage balance control can easily solve the above problems,and further reduce the bus capacitance to enhance the reliability of the system.This paper presented a highperformance hybrid control method,which it combines zero-sequence voltage regulation with the extended discontinuous PWM(EDPWM)organically.Zero-sequence voltage is given the priority,and EDPWM is employed for the natural transition where zero-sequence voltage fails to control the NP voltage balanced.Compared with the existing hybrid control methods,it can further improve the current ripple and switching loss characteristics.3)Loss equalization and efficiency optimization strategy of ANPC inverter was proposed.For active NPC(ANPC)inverter,loss estimation model of power devices was built based on the principle of switching loss distribution,and an online adjustable loss distribution method was proposed to minimize the loss difference between every power device,by means of reasonably allocating different switching devices to undertake the switching loss and regulating the starting time and duration time of each mode.Moreover,the driving delay control was introduced to realize the independent configuration of switching loss and conduction loss,so that the two zero-level circuits can be connected in parallel for freewheeling,which can effectively reduce the conduction loss and further improve the efficiency of the converter.Compared with the traditional single objective of loss equalization or efficiency optimization,the proposed method can achieve them both.4)Decoupling detection algorithm and fault-tolerant operation strategy for multiple switch open failure of ANPC inverter was proposed.For ANPC topology that contains many active switching devices,there may exist the case where multiple switch open failure happens.At this time,the coupling relationship appears under various fault conditions,leading the indistinguishable in them with utilizing the current value of fault phase.To solve this problem,this paper proposed a time-sharing decoupling detection algorithm,which contains multiple detection submodes through the combination of different driving allocation methods and modulation strategies.The output current in each mode is made successfully only related to a single switch state,and the final judgement is equal to the synthesis of each submode.Furthermore,the fault-tolerant operation ability of ANPC converter was analyzed,and a unified clamping strategy was proposed to simplify the implementation of fault-tolerant operation strategy to the greatest extent.