Optimization Research Of Transient Thermal Circuit Of High Voltage Single Core XLPE Cable

The conductor temperature of the cable is the main factor limiting the ampacity of the cable,and the accurate calculation of the conductor temperature is of great significance to the dispatching operation of the cable line.The thermal circuit method is a common method for calculating the ampacity and conductor temperature,and the transient thermal circuit calculation is affected by many factors.Comprehensive analysis of various factors and the optimization method for the entire thermal circuit highlight its necessity.This paper first constructs the traditional transient thermal circuit model of the cable,taking the cable surface temperature as the boundary,and calculating the conductor temperature based on the cable surface temperature and the cable internal parameters.To explore the rationality of the thermal circuit models of different cable structure layers and the effectiveness of parameter selection,analyze the sources of calculating error,and then gradually optimize each impact factor through the control variable method to analyze the effect and rationality of optimization.The main research work includes:(1)Build a traditional transient thermal circuit model,comprehensively analyze model assumptions,model construction,and selection of thermal circuit parameters,etc.,explore the shortcomings of the traditional thermal circuit,and point out the main sources of model calculation errors,including: single-layer insulation modeling and 3-layer modeling of coating are unreasonable,the calculated thermal parameters of the insulating layer and the coating are very different from the actual thermal parameters,and the measurement error of the surface temperature is large.(2)Using the method of equal thickness layering,the cable insulation layer is divided into several layers to construct a thermal circuit model,and the effectiveness of the insulation layering to improve the calculation accuracy of the model is verified;according to the variation of the calculation error with the number of insulation layers,the calculation method of the optimal number of insulation layers is proposed,which not only meets the engineering needs,but also simplifies the calculation.(3)Investigate the change rule of the thermal parameters of cable insulation and the main influencing factors,put forward a method of combining experimental measurement with calculation,measure and calculate the thermal parameters of different types of cable insulation in real time.According to the experimental measurement results,the volume heat capacity of different types of cable insulation layers is not much different,but there is a significant difference in thermal resistance.The thermal resistance of the cable measured in the experiment fluctuates within a relatively large range,and it is different from the recommended value of IEC standard.(4)The lapped covering,air layer and aluminum sheath are encapsulated into a black box,which is collectively called the coating,and the irrational three-layer cascading modeling of the cladding layer is optimized.The thermal parameters of the coating are calculated by means of measurement and normalized backstepping.The method solves the problem that it is difficult to calculate the parameters of the lapped covering and the air layer independently.Experimental comparison analysis shows that the optimized thermal circuit model can significantly improve the calculation accuracy of the transient thermal circuit.(5)Compare the existing temperature measurement methods,and analyze the influence of the thermocouple and other spot measurement sensors commonly used in the laboratory on the cable surface temperature measurement.Using a thermocouple to measure the surface temperature of the cable will bring a negative error,and the addition of silver silicone grease and other thermal grease between the thermocouple contact and the temperature measurement surface can increase the measurement temperature and effectively improve the measurement accuracy.Studies have shown that if the surface measurement temperature is superimposed with a negative error,the error will be further amplified when calculating the conductor temperature,and the ratio of the conductor temperature calculation error to the surface temperature measurement error will stabilize at a value greater than 1.