| The flexible dc grid is considered as an important trend of power system development in the future,with the advantages in large-scale renewable energy access,new urban power supply and so on.However,there are still many challenges for flexible dc grid to isolate dc faults.Compared with the conventional HVDC transmission system,the dc fault propagation speed of the flexible dc grid is much faster,which will damage the whole system in a few milliseconds.In addition,in multi-terminal system,only the fault line is expected to be cut off,for the reliability consideration.Therefore,the fast and selective fault protection and isolation are required to ensure the security and reliability of the multi-terminal flexible dc grid.And thus,this thesis analyzed the dc fault characteristics differences between the internal faults and the external faults based on the fault developing process and the fault traveling wave propagation characteristics.On this basis,the fast,selectived and reliable protection schemes for the dc line in multi-termianl flexible dc grid were proposed.The specific research contents of this thesis are as follows:(1)The fast dc fault identification is the promise and guarantee for the fast dc fault isolation,so the dc protection for line must rely on the transient process of the fault to ensure the operation speed.Therefore,this paper analyzes the dc fault developing process in multi-terminal grid,and on this basis,the equivalent circuits of the key equipments,such as the converter,dc reactor,dc circuit breaker and dc transmission line,are alalyzed,which can provide theoretical basis for the research and design of the dc line protection.(2)To overcome the problem of the low reliability and sensitivity for the traditional protection,this thesis proposes a novel single-ended transient-voltage based protection strategy for flexible dc grid.Firstly,the frequency-domain traveling wave boundary characteristics after the internal and external faults are analyzed.On this basis,a novel single-ended transient-voltage based protection strategy for flexible dc grid is proposed,which mainly consists of the fault identification criterion based on the high frequency transient voltage energy and the direction criterion based on the high frequency transient voltage amplitudes ratio on two sides of the dc reactor.And the protection scheme uses wavelet transform to extract transient voltage energy.In order to eliminate the influences of the wavelet transform boundary effect and converter blocking,the principle to select the proper data window is proposed.Compared with the existing traveling-wave based dc protection,the proposed method has a higher reliability and stronger ability to endure high transition resistance,which was verified by lots of simulation cases.(3)To overcome the problem that the traditional direction protection cannot be directly applied to the flexible dc grid due to the influence of the distribution characteristic and frequency-dependent characteristic of the transmission line parameters,an improved transient traveling-wave based direction criterion,which is not based on the boundary components,is proposed for use in multi-terminal HVDC grid.As the wide-band distribution of dc fault characteristics,the acting sensitivity and reliability are reduced by the parameter frequency-dependent characteristics of the transmission line.Because the wave impedance of the transmission line under high-frequency band remains almost constant,an improved traveling wave based direction criterion,which uses high-frequency transient traveling wave,is proposed in this thesis.The case studies validate that the improved transient traveling wave based direction criterion has much higher acting sensitivity and reliability.(4)To overcome the problem that the sensitivity and reliability of the traveling wave based direction criterion are significantly affected by the fault transition resistance and line distribution capacitors,the frequency characteristic of the fault traveling wave is analyzed.And on this basis,the direction protection based on time-domain traveling wave is proposed,which is immune to the frequency-dependent characteristics of the line parameters.According to the analysis,the magnitude and phase angle of the line-mode wave impedance both only significantly change in the low frequency range.With the frequency increasing,they both quickly stabilize at a fixed value.In addition,at the initial stage of dc fault,there is no dc component or low-frequency components in the fault traveling wave.Therefore,in the designed direction protection,the fault traveling wave is calculated according to the time-domain measured voltage and current,as well as the wave impedance at high frequency range(almost a fixed value).The theoretical analysis and case studies validate that,the operation sensitivity and reliability of the designed traveling-wave based direction protection is much higher than the traditional method.In addition,the time-domain measured voltage and current are used directly in the designed protection,which means the high-frequency-signal extraction algorithm is not required anymore and the implementation complexity is reduced significantly.(5)For the multi-terminal flexible dc grid without line boundary,the single-ended protection cannot protect the full length of the line.Therefore,a fast dc fault handling method based on the cooperation of the line-protection and control was proposed,which can realize fast dc fault identification based on the local signals and fast recovery of the system.In the proposed scheme,the dc reactor installed at the dc outlet of the converter is utilized as the dc line protection boundary,and the dc line fault is identified and isolated by the reasonable configuration and cooperation of the line protections.And on this basis,the healthy system can be restarted reliably through the sequential restart of each converter.Finally,the simulations based on PSCAD/EMTDC simulation platform are carried out to verify the feasibility and superiority of the proposed scheme. |
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