| The mine power supply system mainly relies on the mining power cable lines to transmit power,and densely laying a large number of cables in the limited space underground will increase the risk of internal-caused fire of the cables.The root cause of the fire in the mining cable is the flammability of the power cable and the electric heating under the fault state,and its very early feature is the abnormal temperature rise of the core conductor.Therefore,accurate prediction of the transient temperature of the core conductor is the key to the early warning technology of internal-caused fire in the mining cable.In order to solve the problem that the temperature of the core conductor cannot be directly measured during the live operation of the cable,and the abnormal temperature rise of the cable core cannot be used to realize the early warning of internal-caused fire of cable,the steady state and transient state of the three-core mining cable are constructed.The thermal circuit model achieves the purpose of calculating the temperature of the cable core from the surface temperature of the outer sheath;through the finite element temperature rise simulation and the internal-caused fire experiment,the variation law of temperature of the cable outer sheath surface and the the cable core under the effect of electric-heat coupling is revealed.The data foundation for thermal circuit model research has been laid;a new transient thermal circuit model is proposed,which reduces the complexity of the modeling of the transient thermal circuit model for three-core mining cable;through the method of hierarchical optimization,the calculation accuracy of the new transient thermal circuit model is improved;through the Levenberg-Marquardt algorithm,the inversion and prediction of the cable core temperature is realized;finally,the core temperature inversion and the internal-caused fire warning technology of the three-core mining high-voltage cable are successfully applied in practice,and good results are obtained.The main results are as follows:(1)The steady-state and transient thermal circuit models that conform to the heat transfer characteristics of the radial cross-section of the three-core cable are established,and the parameters in the thermal circuit model are calculated,so that the core temperature can be calculated from the surface temperature of the outer sheath of the cable.Among them,the steady-state thermal circuit model of the three-core mining cable is suitable for the calculation of the core temperature when the cable is thermally stable,and the transient thermal circuit model of the three-core mining cable is suitable for the calculation of the core temperature when the cable temperature field changes with time.Aiming at the problem that harmonics will cause extra temperature rise of cable,the actual state of harmonic of underground power grid current is measured.Based on the measurement data,it is calculated that the extra temperature rise of the conductor of the cable under steady harmonic in that period is about 6℃.(2)Taking my JV22-6/6KV-3 ×50 three-core mining high-voltage cable as the research object,the simulation of cable temperature rise and internal-caused fire experiment were carried out to reveal the temperature change rule of cable outer sheath surface and cable core under the action of electrothermal coupling.Main results: 1)The simulation proves that the steady-state temperature of the cable core increases significantly with the increase of the number of harmonics when the current value remains unchanged,indicating that the excessive content of harmonic current is easy to cause abnormal heating of the cable,which leads to the internal-caused fire of the cable.2)The experiment found that the temperature rise on the surface of the cable sheath is obviously hysteresis relative to the cable core,and the temperature rise rate is less than the cable core.The greater the cable current,the greater the temperature rise rate and the greater the rate difference.3)The time required for the cable to reach 137℃ from steady-state temperature will decrease significantly with the increase of ambient temperature.4)The traditional 1min differential temperature cable fire detection and alarm criterion does not meet the needs of cable internal-caused fire warning.5)The temperature rise rate of the cable core and the surface of the outer sheath is positively correlated with the number of overload cores.The more overload cores are under the same current,the shorter the time for the cable core to reach 137℃ from the steady state.6)It is confirmed that the calculation accuracy of the steady-state and transient thermal circuit models of the three-core mining cable can meet the needs of the cable internal-caused fire warning.(3)A new transient thermal circuit model is proposed to reduce the complexity of modeling the transient thermal circuit model of the three-core mining cable;The calculation accuracy of the new transient thermal circuit model is improved by the layered optimization method.The calculation method of internal-caused fire warning and response time of three-core mining cable is proposed,and the calculation accuracy of warning and response time of new transient thermal circuit model is verified.Main achievements: 1)A new transient thermal circuit model suitable for temperature calculation of core conductor of three-core mining cable is constructed,which reduces the complexity of modeling the transient thermal circuit model of three-core mining cable on the basis of ensuring the calculation accuracy.2)A layering optimization technical scheme of the new transient thermal circuit model is proposed,and the layering method and the optimal layering number are determined.The influence of the temperature gradient inside the cable material on the calculation accuracy of the new transient thermal circuit model is reduced,and the calculation accuracy of the model is improved.In the thesis,the insulation layer,inner sheath and outer sheath of cable are all layered with equal thickness.When the number of layers is 50,the maximum relative error of the calculated values of the model is less than 5%,and the average relative error is 3.5%,which meets the needs of cable internal-caused fire warning.3)The calculation method of the early warning and response time of cable internal-caused fire is proposed,and the study of the early warning and response time of the new transient thermal circuit model is carried out,and the calculation accuracy of the early warning time of the new transient thermal circuit model meets the needs of the early warning of cable internal-caused fire.(4)On the basis of the research of thermal circuit model,the factors affecting cable core temperature are analyzed,and the inversion prediction of cable core temperature is realized by using Levenberg-Marquardt algorithm,which solves the problem that discrete data cannot effectively reflect the continuous change law of cable core temperature.Six cables in four different cable laying scenarios are selected as the application objects,and the core temperature inversion and internal-caused fire warning technology of three-core high-voltage cable for mining are successfully applied in practice,and good results are achieved. |
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