低压直流断路器双金属片性能退化特性及机理分析

Degradation Characteristics and Mechanism Analysis of Bimetal Sheet of Low Voltage DC Circuit Breaker

针对低压断路器热脱扣器用双金属片长期服役过程中的性能退化问题,该文首先建立热脱扣器电热结构耦合有限元模型,分析不同电流条件下双金属片表面温度和挠度的分布特性,结合正交试验确定比弯曲为双金属片性能退化关键特征;然后在考虑失效机理不变的前提下设计110、130、150℃三种温度应力下的加速寿命试验,以比弯曲作为特征量,分析双金属片性能与比弯曲的关联关系,对老化前后双金属片主被动层界面处元素分布进行定性分析,揭示了界面间金属原子运动行为,对双金属片比弯曲下降做出微观解释;建立双金属片分子动力学模型,分析主被动层界面原子扩散行为,得到被动层Ni原子与Fe原子向主动层扩散导致双金属片热膨胀系数改变从而影响其宏观动作特性的结论,从微观到宏观阐明双金属片性能退化机理。该文研究有助于揭示低压断路器过载保护系统性能退化规律,为双金属片性能优化提升提供理论基础。

A low-voltage DC circuit breaker plays a role in power distribution,control,and protection in low-voltage DC systems.As the core component of overload protection of low-voltage DC circuit breakers,the reliability of thermal release directly affects the safe and stable operation of the DC system.However,during service,the thermal release is subjected to large temperature stress fluctuations,the degradation mechanism is not clear,and the reliability evaluation is difficult.Therefore,the simulation model of thermal release is established to determine the key parameters of thermal release performance degradation.The degradation mechanism of thermal release is revealed by the accelerated life test,and the degradation mechanism of the bimetal sheet is explained from the perspective of molecular dynamics.Firstly,a three-dimensional finite element model of thermal release is established to explore the distribution mechanism of temperature,stress,and deflection of the bimetal sheet.An experimental platform is established,and the active layer expansion coefficient is determined as the key parameter of bimetal sheet performance degradation through orthogonal tests.Secondly,the accelerated stress is temperature,and the accelerated life model is the Arrhenius model.The accelerated life test under 110℃,130℃,and 150℃is designed,and the specific bending K is used as the degradation parameter.With the increase in temperature stress,the K decreases by 7.01×10^(−6)/℃at 150℃and 3.21×10^(−6)/℃at 130℃.The e overall decrease in the specific bending K at 110℃is about 1.03×10^(−6)/℃.The distribution of elements at the interface of the bimetal sheet is observed by SEM.Finally,the molecular dynamics model of the bimetal sheet is established.The diffusion behavior of atoms in the passive layer is more intense than in the active layer.The diffusion behavior of atoms at the interface is further explained by analyzing the mean orientation shift,diffusion activation energy,and diffusion factor of different atoms.The results show that the diffusion probability of all atoms was almost the same.Due to the low diffusion activity of the passive layer Ni atom,the diffusion is more intense.The performance degradation mechanism of the overload protection element of low-voltage DC circuit breakers is revealed from a microscopic point of view.The following conclusions can be drawn.(1)The active layer expansion coefficient is a key parameter affecting the degradation of bimetal sheets,so specific bending K can be used as a characteristic parameter characterizing the degradation of bimetal sheets.(2)The degradation rate of specific bending K is almost the same at the same temperature,and the decrease rate of specific bending K is greater with the increase of temperature.(3)The molecular dynamics analysis shows that the diffusion of atoms at the interface changes active and passive expansion coefficients,and the diffusion of atoms in the passive layer is intense.The degradation mechanism of the bimetal sheet is explained from the microscopic view.