Interruption Method Of Cascading Tripping In Power Grid Based On The Electrothermal Effects Of Transmission Lines

With the rapid development of the AC/DC hybrid power grid,power flow transfer caused by large-capacity DC blocking or AC lines excision have been increasing in frequency and intensity.As a result,the overload problem of AC transmission lines is increasingly prominent.The unreasonable action of overload protection promotes the development of large-scale cascading trip,which threatens the safe and stable operation of power grids.Therefore,in order to ensure the electrothermal safety of equipments,it is necessary to enhance the ability of overload protection to respond to power flow transfer and implement timely and effective safety control strategy,which is of great significance for preventing the blackout in the AC/DC hybrid power grid and ensuring the safe operation of power grid.This work conducted an in-depth study on the electrothermal safety tolerance of electrical equipments when short-time power flow transfer occurs,established a thermal network model to describe the electrothermal characteristics of transmission lines accurately,and proposed an overload thermal protection strategy and an overload control method to respond to power flow transfer for interdiction of cascading trip.The main contributions of this work are as follows:1)Analysis of electrothermal safety tolerance of AC equipments under power flow transfer and construction of a temperature rise response test platform.The power flow transfer is worse in the AC/DC power grid than the traditional AC power grid,thereby affecting the electrothermal safety of AC equipments and the action characteristics of overload protection.To address such problem,this work,adopting theory analysis and test study,studied the temperature rise and overload tolerance time characteristics of equipments in the AC transmission channel,such as transmission line,circuit breakers,and transformers.The overload current–tolerance time characteristic curves of equipments are plotted,and the results showed that conductor is the weak link of equipment electrothermal safety on the whole transmission channel by comparison between different curves.A conductor temperature rise test platform was built.The existence of conductor radial temperature distribution,and the difference of temperature rise response under the step current of conductor different parts were further verified by conductor radial temperature distribution tests.Finally,the comparison between the test results and existing standard calculations showed that standard calculation models have difficulty in reflecting the abovementioned law of conductor cross-section temperature distribution and temperature rise response characteristics.Constructing a refined electrothermal calculation model of the transmission line is necessary to characterize the electrothermal safety tolerance of transmission lines accurately and provide a foundation for appropriate protection and control strategies.2)Research on the electrothermal characteristic model and temperature calculation method of transmission lines based on thermal network.Accurately understanding the electrothermal safety tolerance of the conductor under the coupling effect of time-varying meteorological environment and power flow transfer is the basis for construction of overload thermal protection and overload optimization control.The accuracy of electrothermal models and calculation methods of transmission lines must be improved.To address this problem,this work established a thermal network model for transmission lines considering the influence of forced convection heat transfer.Two temperature calculation methods based on nonlinear iteration and parameter identification were proposed.Finally,the experimental platform verified that the accuracy of the proposed model is higher than IEEE and CIGRE standards.The thermal network model reveals the heat transfer mechanism of transmission lines under the coupling effect of external forced convection and internal current,effectively reflecting the radial and circumferential temperature distribution characteristics of the conductor under power flow transfer,and provides a theoretical basis for accurate electrothermal calculation of transmission lines.3)The overload thermal protection strategy based on the electrothermal safety tolerance model of transmission lines.The existing overload protection boosts the development of cascading trip because its principle and setting method are inapplicable to power flow transfer.The ability of overload protection to respond to power flow transfer could be improved,and the integrity of the power grid could be maintained by introducing electrothermal safety tolerance of transmission lines.To solve this problem,this work proposed an overload thermal protection strategy based on the electrothermal safety tolerance model of transmission lines.Firstly,considering the influence of the current shape difference on electrothermal safety of transmission lines during power flow transfer in practical operation,the electrothermal safety tolerance model of transmission lines under different current variation modes was established by correcting the value of AC resistance and Joule heating parameters.Secondly,the wavelet packet energy entropy was adopted to realize current shape recognition and select a corresponding electrothermal safety tolerance model.Finally,the overall overload thermal protection strategy of transmission lines was designed.The effectiveness of the proposed protection strategy was verified by simulation analysis under various disturbances.The proposed protection strategy could provide reference for coping with power flow transfer,effectively avoiding cascading trip in power grids.4)An overload control strategy for the AC/DC hybrid grid considering the electrothermal characteristics of transmission lines.When a large power shortage occurs in the AC/DC hybrid power grid,the proposed overload protection could avoid the faulty removal of the transmission line and maintain the integrity of power grid.However,the short-term overload withstand capability of the transmission lines is limited,and further overload control is needed to achieve smooth transition.This work introduced the dynamic electrothermal characteristic of conductor as the lines safety constraint and coordinated with various methods,such as DC transmission power adjustment,unit active power adjustment,and load shedding,to establish a dynamic optimization control model associated with time.The proposed optimization model utilized the minimum total control cost as the objective function and considered the impact of postaccident load volatility and meteorological variation of lines in different meteorological zones.The safe economic overload control within relatively short period could be achieved via variable optimization selection and electrothermal coupling simplification,to block the occurrence of cascading trip in the hybrid grid effectively.Compared with control strategies with different line safety constraints and optimization variables,the effectiveness and superiority of this strategy were verified by the AC/DC hybrid test system.The proposed thermal network model of transmission lines in this thesis revealed the electrothermal coupling mechanism of the conductor during short-term power flow transfer and overload.The overload thermal protection and overload control strategy make full use of electrothermal characteristics of transmission lines,achieving interdiction of cascading trip effectively.Research on the interruption method of cascading trip realized the optimal combination of power equipment and system safety,which can provide a certain reference against power grid blackouts.