Research On The Key Technology And Performance Of Modular UPS Based On Three-level Converter

With the development of communication technology,information technology,the supply capacity of uninterrupted power supply(UPS)has been put forward.Modular UPS is the rise in recent years in the UPS echnology,it is a high frequency,high power density,small size and N+X is characterized with online expansion,easy maintenance features.Based on these characteristics as a starting point,this paper carried out from the main circuit topology to control the key technology research,including the ability to achieve high power density,small loss of PFC and inverter topology,the non ideal power grid adaptability,high quality output voltage waveform control,multi module parallel technology research.This paper mainly has carried on the following several aspects of research:(1)Three-phase PFC converter is prerequisite for high power UPS,for which a variety of circuit topologies has been developed so far.This PHD research discusses the three levels Boost converter composed of three-phase PFC converters.The main advantages of the three-level power factor correction converter(TLPFCC)are low voltage stress and switching power losses in switching devices,high reliability.On the basis of analysis of the TLPFCC operation principle and mathematical model,thediscrete average current control was designed,and the single zero and dual poles compensation current control was studied which can increase the current open-loop gain in the low frequency band and effectively suppress the high-frequency disturbance and oscillation.Secondly,the novel calcualtion method of the reference current for controlling the dual switch three level of three-phase PFC converter was studied.In orde to acquire the phase of the PFC current reference,the improved second order generalized integrator(SOGI)was proposed,throuht which frequency and phase of the grid voltage positive sequence component can be quickly and accurately attained whatever the grid is in the three phase voltage umbalance state or contains harmonics.Then,in view of the neutral voltage balancing three level converter,is proposed based on zero sequence current injection of equalizing ring neutral voltage control method,the output voltage offset stack equalizing ring to the power grid voltage,the equivalent of injecting zero sequence current in the current benchmark,so as to realize the control of the line current,and realized the control of the neutral voltage.The simulation and experiment validated the control strategies mentioned above are feasible.(2)According to the demand for the configurable output as single or three phases,the three-phase four-wire three-level inverter topology is selected in the research,in which each phase voltage of the three is controlled independently,and the three level inverter is able to satisy the demand of 100%umbalance load.Firstly the operating principle of single-phase inverter is analyzed.In order to achieve the logic function for the four switching devices on each leg,On the basis of analysis the conclusion is made that only the single polarity pulse width modulation(PWM)is suitable for the three lever inverter topology,and the alternative phase opposition disposition PWM is adopted.Secondly,the inverter continuous and discrete models are established,and the influence of the parasitic resistor in the LC filter inductor and capacitor on the inverter system is analyzed,respectively.Finally,three factors affecting the single phse inverter waveform quality are analyzed in detail,including dead time,SPWM overmodulation,delay of digital control.And the control strategy is proposed based on the dual voltage sampled from the front and back terminal of the filter inductor,in which strategy the differential factor was introduced.The simulation and experiment validated the control strategy mentioned above is feasible.(3)The dead-time effect of driving signal and the time delay effect of digital control seriously worsen the inverter voltage static and dynamic performance,the static and dynamic performance of diode clamp three-level single phase inverter was studied in the paper.The dead-time effect and time delay effect of digital control are viewed as a periodic disturbance and the proposed improved lead repetitive control can improve the accuracy of output voltage and reduce voltage distortion caused by dead-time effect.The control method combined the voltage dual closed-loop with the repetitive control is proposed to enchance the inverter dynamic response.The proposed approach does not need to detect the inductor current polarity and it saves the current sensors which are used in traditional current closed loop control.The design method of repetitive control filter and phase lead compensation was studied,and the derivation method was proposed to determine the lead compensation element k value according to the differential equations in the repetitive control system.Finally,the validity of proposed control strategy has been verified by simulation and experiment.(4)The circulating current characteristics of the paralleling inverter system and the relation between closed-loop control and circulating current are studied.The power sharing control strategy that is classify as distributed-logic-control is adopted for the parallel operation inverters,the current sharing strategy is studied mainly considering the resistive component of inverter output impedance,furthermore the current sharing strategy is deduced theoretically and simulation result is analyzed,and the synchronization control based on SPI communication and sine-cosine table to calculate power are studied and the current sharing control of three phases inverters are realized.The simulation and experiment validated the control strategy proposed above is feasible.