The Research On The Digital Controlled SPWM Inverter And Its Parallel Technique

Digital technology and parallel technology are the two hot technologies of research andapplication on SPWM inverter.Digital SPWM inverter has become the mainstream of thedevelopment of inverter.Though digital control possesses many advantages compared withanalog control,the traditional analog control realized by digital method can not acquiregood effect as the analog control.This paper focused on the research on the digital effect of SPWM inverter systemperformance from two aspects of zero-order-hold and one-step-delay.Firstly,the samplingperiod was limited to make the research effective,then the analog control inverter systemwas analyzed,and discrete system with no-step-delay control was compared with discretesystem with one-step-delay control in the stability from proportion closed-loop control toproportional-integral control.The clear conclusions were obtained on the effect of thedigital process on inverter performance which can provide guidance to the analysis anddesign of the inverter.Under the condition of system stability,the upper limit of the naturalfrequency ofω_n was used as constraints of the design parameters on the inverter filter.Theabnormal phenomenon is found that one-step-delay control has much wider scope thanno-step-delay control when sampling period,T_s＞0.25T_d.So the introduction of predictivecontrol can not improve the performance of inverter in any sampling frequency.Predictivecontrol can upgrade the performance of inverter when T_s＜0.25T_d while predictive controlcan downgrade the performance of inverter when T_s＞0.25T_d on the contrary.It is first timethat the essential reason was revealed why the performance of digital control inverter wasdegraded compared with analog control inverter even if the sample frequency is ten timesgreater than the closed-loop bandwidth according to the digital control system theory.In this paper,time domain simulation waveforms were done to verify the conclusionsabout the effect of digital control on the stability of the SPWM inverter system.On thisbasis,the analyses of frequency domain and root locus were carried out,too.The change ofsample period,damping ratio,natural frequency,load,one-step-delay effect on theperformance of digital control inverter can be sum up as the change of stable margin in frequency domain.The zero-pole position changes with sample period,and it results in thechange of root locus.By the analysis of frequency domain and root locus,the uniquephenomenon that the stable range of one-step-delay control system is larger than that ofno-step-delay control system in the low sample frequency no matter whether digital Pcontrol or PI control is adopted is explained.It is relative that no-load inverter is thetwo-order weak damp system with resonant peak by the analysis of frequency domain andthe root locus becomes closer to inner unit circle with sample frequency decreasing for theeffect of pole at origin by the analysis of root locus.Digital control improves the control flexibility of inverter system and realizes high-performance of inverter parallel system more easily.An effective strategy of parallel controlcan break the restriction of the power grade,and compose the parallel redundant system toenhance the reliability.As an independent parallel control method,distributed parallel logiccontrol strategy can realize redundant parallel in deed.This paper studies the circulatingcurrent restraining strategy of PID instantaneous value control based on distributedlogic-control.A new digital current sharing technology is introduced based on simplifiedpower detection and the experiment results are given.The control scheme is simple andpractical.At the same time,the method based on power regulation to restrain the circulatingcurrent is affected by the character of parallel impedance and the disadvantages such as thelow response speed of sharing current were pointed out.So the circulating currentsuppression strategy based on tracing instant average current is adopted in the paper and theprinciple of restraining the circulating current was researched by analysis of impedance.But this strategy has some disadvantages such as the limited ability of current-sharingregulator,etc.The new strategy is combined with tracking instant average currentfeed-forward control and simplified power detection strategy for improve the effect ofsharing current.