Research On The Protection For HVDC Transmission Line Based On Frequency Spectrum Information

High voltage direct current(HVDC)transmission system has many advantages,such as long distance transmission,large transmission capacity and low line cost.Therefore,HVDC transmission system is widely used in long-distance,large-scale,trans-regional power transmission,power system asynchronous networking,new energy paralleling in the grid and other fields.In recent years,with the increasing awareness of environmental protection and the gradual adjustment of the energy structure,clean energy such as wind power,solar power and hydropower has been rapidly developed in China,and the installed capacity of China’s clean energy has reached the world’s first place.Due to issues such as the power transmission passageway and power supply mismatch,the wind,solar and hydro curtailment phenomenon in the western region is widespread.HVDC transmission system is the key of clean energy delivery.The reliable operation of HVDC system is of great significance for energy conservation,emission reduction,power source structure adjustment and power system security and stability.Based on the characteristics of the line boundary and the frequency spectrum of fault component,novel protection schemes for HVDC transmission line are researched in this dissertation.The main contributions are as follows:1.According to the impedance-frequency characteristic of DC filters,the impedance of DC filters at the tuned frequency and resonance frequency is analyzed.Considering the feature of the smoothing reactor,the boundary characteristic of the HVDC transmission line is researched.In view of the long transmission line of the UHVDC system and the split arrangement of the smoothing reactor,the effect of fault location,fault resistance and line-side-smoothing reactor on the magnitudes of the fault components are analyzed.This section provides a theoretical basis for the following protection schemes.2.In view of the current within the tuned frequency band,a non-unit transient current protection scheme is proposed.Based on the superposition theorem and the superimposed network of HVDC transmission system,the effect of line boundary on the magnitudes of the fault current components is analyzed.For different frequency bands,the intensity of attenuation is different.The current component within the tuned frequency band is chosen as the characteristic current,then a non-unit transient current protection scheme is proposed.Simulation results demonstrate that the protection scheme is able to discriminate internal and external faults quickly and accurately.3.In view of the fault DC component,a calculated DC resistance pilot protection scheme is proposed.The calculated DC resistance is defined as the absolute value of the ratio of the fault voltage DC component to the fault current DC component.Based on the superimposed network of HVDC system,the equivalent resistance of the converter,the equivalent resistance of AC system and the line’s resistance are analyzed.The magnitude of calculated DC resistance is analyzed during forward and reverse faults,and this feature is applied to construct the calculated DC resistance-based protection scheme.The protection scheme can remove the effect of the DC line distributed capacitive current.Simulation results show that the proposed method has good reliability.4.In view of the particular AC component,a boundary energy pilot protection scheme is proposed.The boundary energy is defined as the product of voltage,current and data window.The DC filters provide an additional path for fault current.The smoothing reactor provides an impedance path for transient component,and this leads to a difference in the transient voltages at both sides of the smoothing reactor.This characteristic is applied to construct the protection scheme.According to the impedance-frequency characteristic of DC filters,the harmonic voltage source feature of converters,the energy-frequency characteristic of the fault component and the electromagnetic coupling feature between two poles,the frequency range of 50–550Hz is chosen as the characteristic frequency band of boundary energy.Simulation results show that the proposed method has good selectivity and high reliability under different fault conditions with high fault resistance.5.A resonance frequency-based pilot protection scheme for HVDC transmission lines is proposed.The impedances of the converter,the DC filters and the smoothing reactor at resonance frequency are analyzed.The phase difference between the resonance frequency voltage and the resonance frequency current is analyzed during internal and external faults,then a resonance frequency-based pilot protection scheme for HVDC transmission lines is proposed.What’s more,the performance of the proposed scheme under various disturbance factors such as power fluctuation,switching and measurement noise is verified by simulation.Finally,all the achievements are summarized,and the further research directions are suggested.