Research On Fault Detection And Location Method Of DC Distribution Line And Solid State Circuit Breaker

DC distribution network is based on flexible DC transmission technology,which can realize independent power regulation and bidirectional energy flow.With its multiple advantages in technology and economy,it will become an important carrie r of the future energy Internet.Nevertheless,DC grid is a low inertia system,when a fault occurs,the fault current amplitude is extremely high and the development process is extremely fast,about 1 ? 2ms;DC distribution network line fault is global,and its local fault will threaten t he security and stability of the whole power grid.Traditional protection algorithms and DC circuit breakers(DCCBs)have inherent delay response characteristics,which lead to high current stress of DC equipment during the fault.Fault location is helpful to quickly locate the fault point and eliminate the line fault in time,but the performance of existing methods is easily affected by many factors,and the practical effect is not ideal.In addition,the operation speed of existing DCCB is difficult to match the deterioration process of the line fault,so it needs to have faster response ability.Therefore,the research on fault detection,location and isolation technology of DC distribution line is of great theoretical and practical significance to guarantee the secure and reliable running of DC grid.This paper focuses on the key scientific issues such as fast detection method,fault location method and high-speed isolation method of the DC distribution network line fault.The main research contents and innovations are as follows:(1)Aiming at the problem of fast identification of DC line fault segment,a single-ended fault detection method based on boundary inductance is presented.Typical line faults are introduced,and the discharge process of voltage source converter(VSC)under different fault types is described quantitatively.In order to quickly identify the fault zone of the DC line,the boundary in ductance is added at the end of the DC line,and the concept of pseudo voltage(PSDV)is proposed and defined.By analyzing the fault characteristics of DC line with the boundary inductance in detail,it is found that the PSDV has significantly difference between inside and outside of the line fault segment.The PSDV attenuation of the fault in the line segment is large,while the PSDV of the fault outside the line segment has no obvious change.Therefore,combined with the characteristics of the boundary inductance and the PSDV,the detection and discrimination scheme of the line fault segment is derived.Simulation results prove that the single-ended detection scheme can quickly identify the fault zone and fault pole of the DC line.(2)Aiming at the problem of fast location of the DC line fault,an off-line location scheme is presented.Firstly,an active signal-injection-type location module is designed,and the equivalent circuit between the module and the fault line segment is established,and the location voltage signal is injected into the fau lt loop after power loss by the module.Secondly,according to the energy transformation relationship during the discharging process of the location module,a location algorithm based on the principle of energy conservation is presented.Then,in order to further improve the efficiency of the off-line location method and reduce the response time,a passive energy-absorption-type location module is designed,and the equivalent circuit between the module and the fault line segment is established.By studying the attenuation characteristics of the residual current of the DC line after tripping,the functional relationships among the discharging process of the residual current of the DC line,the fault distance and the fault resistance are revealed,and the off-line location method based on the residual energy of the line is presented.Finally,the parameter design methods of the two location modules are studied,and the performance of the two location algorithms in different application scenarios is discussed.Experiment results indicate that the schemes are able to locate the fault point correctly.(3)In view of the shortcomings of the off-line fault location method in location accuracy,anti-resistance capability,etc.,an on-line location method based on the principle of voltage similar triangle is proposed.Based on the concept of the PSDV,this paper analyzes the distribution mechanism of two terminal PSDVs on the DC line,and reveals the linear relationship between the PSDV at different positions and the line distance.In this paper,the distribution curves of the two terminal PSDVs of the DC line are mapped to rectangular coordinate system at the same time,and the graph formed by the distribution curve forms two c losed similar triangles.According to the similarity ratio principle,the on-line location algorithm is deduced,which can eliminate the influence of the fault resistance in theory.This paper analyzes the location error caused by the weak synchronization of two terminal data,puts forward an effective solution,and discusses the performance of the algorithm under the conditions of noise,distributed capacitance and arcing fault,etc.Simulation analysis and experimental results prove that the on-line location scheme has advantages in efficiency,precision and anti-interference ability,and so on.(4)Aiming at the problem that the action speed of DCCB cannot match the deterioration process of the line fault,a new bidirectional solid-state DCCB with fault detection and current limiting ability is proposed.The DCCB is the executive unit of cutting off fault current,and its cutting speed and maximum cutting capacity are its important indexes.The topology and control method of the DCCB designed in this paper are given,and its working principle is described in detail.This topology is composed of thyristor,diode,coupled inductor,and so on.The primary coil of the coupled inductor softly shuts off the thyristor of main circuit by coupling the fault current of secondary coil.In addition,the cutting speed of the DCCB reaches the level of microseconds,which can quickly suppress the rise of the fault current.The topology can adjust the fault detection threshold by changing the turn ratio of the coupled coil,which improves the flexibility of the fault decision-making.The circuit design process and parameters of the proposed DCCB are given.Simulation analysis and hardware test results show the practicability of the DCCB scheme.(5)Aiming at the loss problem of semiconductor power devices in DCCB in high power,high voltage and high current applications,a low-loss active DCCB is proposed.By deeply studying the working mechanism of the Z-source DCCB,the circuit topology of the DCCB is optimized and improv ed,and the number of semiconductor devices in the main current path is reduced,so as to effectively reduce the loss of the DCCB.The working principle of the DCCB is analyzed,and the circuit design process and parameters are given.The efficiency and lo ss of the DCCB are quantitatively analyzed and calculated,and the actual measurement results are given.Compared with other topology schemes,the DCCB scheme has some advantages in power loss and volume.Simulation and hardware test results show the effectiveness of the DCCB scheme.This paper takes the key scientific and technical problems of DC distribution network relay protection as the research object,and deeply studies the relevant theories and methods of DC distribution network line fault protectio n from many aspects,including fault analysis,detection,location,SSCB isolation technology,and so on.Based on the above research,this paper puts forward the single terminal fault detection method,the off-line and on-line location method,as well as the design scheme of the new DCCB,forming a more perfect DC distribution network line fault treatment scheme.The theoretical analysis method,design idea and the research results of the paper can provide guidance and reference for the development of DC distribution network and and its relay protection technology.