Fault Detection Method For AC/DC Hybrid Power Grid Based On Integrity Of The Protected Element Model

Most of the energy bases are located in the northwestern,but the load center is mainly located in the east and southeast coastal areas,and the power shortage is very large.The long-distance transmission by HVDC is more advantageous than the AC transmission,with more and more operation of the DC project,the scale of AC and DC power grid is also growing.Different from the traditional AC grid,the AC/DC hybrid system presents a series of new characteristics after the line fault.The variation of the electric quantity is complicated,and a large number of undamped harmonics appear.The key to the problem lies in the AC/DC hybrid system.In the DC system part,its control system has a complicated working principle and flexible operation modes.It contains a large number of power electronic devices,which is quite different from the traditional AC system.It is also difficult to model and analyze the DC system.By finding a protection principle that is fundamentally unaffected by the DC source fault characteristics,it is possible to avoid modeling and fault characteristic analysis of the DC source,thereby the reliability and flexibility of fault detection are ensured.Based on the analysis and summary of previous research work,this paper proposes a protection method for the protected component model.The main idea is that the amplitude and phase of the electrical quantity of one-terminal or multi-teriminals power system component are constrained by the power source and the physical model of the component equivalent.The power source generates the electrical quantity,which the changes must match the physical model with equations corresponding respectively.The physical structure of the selected component is different,and the mathematical relationship of the component’s electrical quantity is different.This mathematical relationship is independent of the power supply characteristics and is only related to the physical model of the selected component.Therefore,it is possible to write a mathematical equation describing the protected component model,and bring the collected electrical quantity into the equation to verify whether the equation is valid.If it is established,the protected object is running normally,otherwise the protected object fails.The main feature of this method is that the mathematical equation has nothing to do with the power input characteristics.Therefore,when used as a protection judgment criterion,the fault characteristics of the DC system can be no longer analyzed,and good protection performance can be obtained.This paper analyzes the variation characteristics of the electric quantity after the failure of the HVDC transmission system,and analyzes the applicability of the traditional protection principle in the HVDC transmission system.The analysis results show that the performance of the traditional protection principle based on power frequency electrical quantity in DC system will be affected by many factors.Furthermore,a protection principle for the protected object model is proposed,and the general idea of fault judgment based on this principle is introduced.In the specific implementation method for the protection principle of the protected object,this paper describes the protected object by defining the operating voltage,and judges whether the protected object is intact by comparing the waveform relationship between the working voltage and the measured voltage,and then gives the fault judgment result.When the line is in normal operation or an external fault occurs,the protected component(line)model is not destroyed,and the operating voltage and the measured voltage are similar in waveform;when the line has a model internal fault,the protected component(line)The model is destroyed,and the operating voltage and measured voltage are basically opposite in the waveform.The difference between the two is large,and the fault detection can be completed accordingly.In order to verify the feasibility and effectiveness of the proposed protection method,the PSCAD simulation software was used to build a two-terminals AC power transmission system and an AC/DC hybrid transmission system.The circuit in the two systems is used as the protection object to simulate the change of the electrical quantity after the fault occurs in different positions on the transmission line.The time domain operating voltage waveform and measured voltage waveform comparison method are used to detect the fault position.The simulation results verify the effectiveness and reliability of the fault detection and judgment methods are described in this paper.