【正文】 diode and IGBT switching losses at an output current io is given by [21] ? ? I G B Tr e f OI G B Tr e f dcO F FONSWI N V I G B TSW I IV VEEfP , 2,1 1 ?? ? （ 12） drefOdrefdcSWIN V dSW I IVVfP,2,1 1?? （ 13） The total loss of the inverter is obtained as the sum of (10)–(13) ? ? I N V I GB TSWI N VdSWI N VI GB TcdI N VdcdI N V I GB Tdt PPPPP , ????? （ 14） The power loss of the conversion stage of the PMG based SWT is given by ? ? ? ?I N V I G B TdtBC I G B TdTDBdtP M Gt PPPP ?? ??? , （ 15） . Reliability analysis for a PMG based SWT Reliability is the probability that a ponent will satisfactorily perform its intended function under given operating conditions. The average time of satisfactory operation of a system is the mean time between failures (MTBF) and a higher value of MTBF refers to a higher reliable system and vice versa. As a result, engineers and designers always strive to achieve higher MTBF of the power electronic ponents for reliable design of the power electronic systems. The MTBF calculated in this paper is carried out at the ponent level and is based on the life time relationship where the failure rate is constant over time in a bathtub curve [23]. In addition, the system is considered repairable. It is assumed that the system ponents are connected in series from the reliability standpoint. The lifetime of a power semiconductor is calculated by considering junction temperature as a covariate for the expected reliability model. The junction temperature for a semiconductor device can be calculated as [24]： JAlossAJ RPTT ?? （ 16） losP is the power loss (switching and conduction loss) generated within a semiconductor device and can be found by replacing the losP from the loss analysis described in Section for each ponent. The life 12 time, ? ?JTL of a semiconductor is then described as ? ? ???????? ??JJ TBLTL e xp0 （ 17） where L0 is the quantitative normal life measurement (h) assumed to be。 Reliability Analysis of Grid Connected Small Wind Turbine Power Electronics Md. Arifujjaman, . Iqbal, . Quaicoe ABSTRACT： Grid connection of small permanent mag generator (PMG) based wind turbines requires a power conditioning system prising a bridge rectifier, a dc–dc converter and a gridtie inverter. This work presents a reliability analysis and an identification of the least reliable ponent of the power conditioning system of such grid connection arrangements. Reliability of the configuration is analyzed for the worst case scenario of maximum conversion losses at a particular wind speed. The analysis reveals that the reliability of the power conditioning system of such PMG based wind turbines is fairly low and it reduces to 84% of initial value within one year. The investigation is further enhanced by identifying the least reliable ponent within the power conditioning system and found that the inverter has the dominant effect on the system reliability, while the dc–dc converter has the least significant effect. The reliability analysis demonstrates that a permanent mag generator based wind energy conversion system is not the best option from the point of view of power conditioning system reliability. The analysis also reveals that new research is required to determine a robust power electronics configuration for small wind turbine conversion systems. Keywords:Renewable energy,Wind energy, Power electronics,Gridtie inverter,Permanent mag generator, Small wind turbines,Switching losses,Reliability Mean time between failures 1. Introduction Small wind energy conversion systems (WECSs) have evolved rapidly along with the large WECS for generation of electricity in either ongrid or offgrid applications. WECS are considered as plex systems prising mechanical subsystems (rotor, hub, and gearbox) and electrical subsystems (converter/inverter, rectifier, and control) and loads. Failures in any of the subsystems can cause substantial financial loss. The problem bees more severe if the system is offgrid leading to unavailability of power. In light of this, there is a need for reliability evaluation of small WECS in order to determine a configuration that is efficient and reliable. Almost all mercially available small wind turbines are based on PMGs. The power conditioning systems (PCSs) for grid connection of the PMG based configuration requires a rectifier,boost converter, and a gridtie inverter. The reliability analysis of such PCS is greatly influenced by the operating conditions, ., covariates and therefore it is desirable to investigate the magnitude of their effects on the system reliability. Reliability calculation consider the voltage or current as a covariate for an electromechanical system [1], while the reliability of power electronic ponents is strongly influenced by the ponent temperature and variations [2]. Knowledge of the reliability of power electronic ponents is a key concern when differentiating between topologies. However, recent research intermittently endeavors to determine the reliability and advancement of the inverter rather than the PCS [2–4]. Most of the reliability calculations are 8 based on the accessible data provided by the military handbook for reliability prediction of electronic equipment which is criticized for being obsolete and pessimistic [5,6]. A parative reliability analysis of different converter systems has been carried out based on the military handbook by Aten et al. [6]。但是系統(tǒng)的平均無故障時間 MTBF 沒有顯著的變化。但是目前的較低的開關(guān)頻率的連鎖反應(yīng)是巨大的，會有一個相當(dāng)大的開通損耗和關(guān)斷損耗。此外，一個可靠的 PCS系統(tǒng)要使整體系統(tǒng)效率損耗很小。標準電網(wǎng)能夠最好的反映風(fēng)力發(fā)電機組最佳運行效果。在確定了每個組件使用的失效率（ 19）后綜合了總的失效率（ 20）來確定系統(tǒng)的故障率。這樣，工程師和設(shè)計師總是努力設(shè)計具有更高 MTBF的電力電子元件來提