| With the development of hydroelectric power plants in the direction of intelligence and unattended,the reliability and stability of the operation of hydroelectric generator sets are becoming higher and higher.However,the continuous improvement of stand-alone capacity,the continuous increase of excitation current,and the increasing complexity of the system have kept the failure rate of the carbon brush-collector ring system at a high level.The improvement of the material of the carbon brush and collector ring and the optimization of the operating parameters cannot solve the problem of overheating,ignition,and even ring fire of the carbon brush-collector ring and the resulting toric damage.It is found that these problems are related to the current-carrying stability of the carbon brush-collector ring system,and the surface film state of the friction interface is an important reason for the current-carrying stability.However,the formation of current-carrying friction interface surface film is affected by many factors.Numerous studies have shown that suitable operating conditions help to form a surface film on the surface of the collector ring as quickly as possible.However,most of the current research work focuses on high-speed rail pantographs,motor commutators,etc.,and the surface film formation of carbon brush-collector ring components of hydroelectric generator sets has not been reported.Therefore,this paper studies the formation mechanism of the surface film of carbon brush-collector ring components of hydroelectric generator sets,analyzes the surface state of carbon brushes and collector rings by SEM,EDS,and other means,designs arc tests to study the influence of surface film on the stability of current-carrying friction,and studies the role of the surface film.On this basis,according to the working conditions of the carbon brush-collector ring system,the carbon brush-collector ring matching material and structural form used in the current hydroelectric generator are selected,and the operating state of the carbon brush-collector ring system in service in large and medium-sized hydroelectric generator sets is restored based on the principle of similarity,and the simulation test is carried out on the current-carrying friction and wear simulation test bench to study the suitable working conditions for rapid generation of surface film,and solve the problem that it is difficult for the collector ring to form a surface film.Finally,according to the results and specific data of the working condition test,combined with the changing trend of contact resistance,the formation mechanism and influencing factors of the surface film are inductively derived.The main conclusions are as follows:1)Under normal operation,the surface of the collector ring is rougher than the surface of the carbon brush,and the roughness of the carbon brush is mainly determined by the roughness of the collector ring.The main wear forms of carbon brushes are abrasive wear and oxidation wear,and the positive carbon brush is more oxidized than the negative carbon brush,and there are obvious oxidation peeling pits.The severely damaged carbon brush has Fe sputters on the surface,and the main damage comes from arc damage and the third body wear caused by it.Therefore,the surface layer of the well-run collector ring surface film does not contain Fe elements and is composed of C,O,and Si elements.2)The arc damage area when the carbon brush is connected to the positive electrode is smaller than when the carbon brush is connected to the negative electrode.A collector ring with a surface film suffers less arc damage than a collector ring without a surface film,and a positive collector ring specimen suffers less damage than a negative collector ring specimen because the carbon layer deposited on the surface of the positive collector ring is thicker,and the arc mainly removes the oxidized carbon layer on the surface.The surface of the negative collector ring is the most damaged when the carbon brush is connected to the negative electrode,and the positive and negative poles of the cable can be replaced in time to reduce the degree of damage to the surface of the collector ring.3)In the test condition,the surface square resistance under the test condition of speed 12 m/s and current 800 A has the fastest growing extreme value,the corresponding average temperature of the carbon brush is 70～75 °C,and the surface temperature range of the collector ring is 70.2～74.6 °C.The biggest factor affecting the growth rate of surface square resistance in the test is the rotational speed because the increase in speed tears the overly thick surface carbon layer and provides oxygen for the oxidation of the substrate.The current density significantly affects the temperature of the friction pair,and the current density increases and the temperature increases,which accelerates the oxidation reaction,but the excessive temperature will make it difficult for water molecules on the friction surface to adhere,and the lack of lubrication on the surface will cause serious wear.The difference in spring pressure mainly leads to the temperature difference between the negative carbon brushes,and the elastic force difference of less than 3N under the test conditions is enough to lead to a temperature difference of more than 10 °C,therefore,the actual operation must avoid mixing of springs of different specifications,and regularly check the degree of elastic attenuation,so that the spring pressure of each carbon brush is kept at the same level,which can effectively reduce the maximum temperature of the carbon brush individual and reduce the occurrence of ignition accidents.4)The main function of the surface film layer is lubrication;Improves the real contact area and increases the stability of current-carrying friction;Reduce oxidation and uneven distribution of resistance to the substrate;Reduces arc damage and protects metal substrates.The main reasons for damage to the surface film are polarity and working conditions.The material used as the anode is more susceptible to oxidation,and the carbon layer on the surface of the negative collector ring is thinner and more susceptible to damage.During the formation of the surface film,the dominant factor of contact resistance changes from shrinkage resistance to film resistance. |
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