Research On The Reliability Of DC-link Capacitor In The Multiple Adjustable Speed Drives System

The DC side of the adjustable speed drive system needs a filter to realize the power decoupling of the front and rear end.Common passive DC side filters include LC filter composed of the aluminum electrolytic capacitor and C filter composed of a large number of the film capacitors.The capacitor is one of the most vulnerable devices in power electronic converters.Its reliability evaluation is of great significance for the safe and reliable operation of the system.The reliability of DC-link capacitor in a single adjustable speed drive system has been discussed in the literature.In most applications,the multiple drives are often connected in parallel with the power grid.In the multiple drives system,a large number of harmonic distortion generated by nonlinear rectifier will transfer and couple between the grid and the drive,thus changing the DC-link capacitor stress.Therefore,the reliability evaluation of DC-link capacitor in multiple drives needs to be further studied,and various adjustable speed drive structures and working conditions need to be considered comprehensively.Firstly,this thesis proposes an equivalent circuit model and a DC-link capacitor current analytical model to obtain the continuous DC-link current in the multiple drives system by decoupling the interaction between the parallel drives.Then,by analyzing the failure mechanism of capacitor,the reliability evaluation method of capacitor is studied.The thermal model and the lifetime model of the capacitor are established.The influence of parameter variations in the lifetime prediction model is considered based on the Monte Carlo simulation.By using the life prediction method,the influence of the number of drives,the condition of grid impedance,the type of drives,and the condition of grid voltage on the reliability of DC-link capacitors in multiple adjustable speed drives system is quantified.Finally,the parameters of DC-link capacitor are optimized from the perspective of reliability.In this thesis,the equivalent circuit model,the analytical model of capacitor current and the thermal model of capacitance are verified by case study.The results show that the models used in this thesis have high accuracy.For a multiple drives system,such as hundreds of drives,the proposed equivalent circuit model can shorten the simulation time and reduce the complexity.The results of capacitor reliability evaluation show that: a)For a multiple LC drives system,with the increase of the number of drives and grid impedance,the reliability of DC-link capacitor is higher.However,for the multiple C drives system,this rule is not followed;b)Compared with single type of the multiple C drives system,the reliability of DC-link capacitor in the hybrid drives system is higher;c)The unbalance of grid voltage will greatly reduce the reliability of DC-link capacitor in adjustable speed drives.The results of DC-link capacitor reliability evaluation can provide reference for the design of DC-link capacitor in the multiple drives system.