Research On Fault Current Limitation And DC Line Protection For Multi-Terminal Flexible DC Grids

With the aggravation of environmental problems and the depletion of traditional fossil energy,clean energy represented by wind and solar energy is being developed and utilized at an accelerated pace.These natural properties of clean energy and their grid-connected characteristics have a significant impact on the traditional ac power grid,thus dc power transmission and dc power grid emerge at the historic moment.Compared with ac power transmission and traditional dc power transmission,the flexible dc power transmission control system has no commutation failure and reactive power compensation problems.The development of multi-terminal flexible dc grid has more advantages in new energy grid-connection,transmission and distribution.However,flexible dc transmission also has its inherent defects.Due to the characteristics of voltage source,when short-circuit fault occurs on the dc side,the converter station rapidly discharges towards the fault point,resulting in the fault current rapidly rising to a very high amplitude.In order to avoid the damage of the fragile semiconductor devices in the converter station,the converter elements will adopt blocking measures,which will undoubtedly reduce the power supply reliability of the dc grid.In addition,the removal difficulty of large dc current is significantly higher than that of ac current due to the lack of natural zero crossing.The above problems make the protection system of multi-terminal flexible dc grid face great challenges and then restrict its further development.Aiming at the dc side fault characteristics of multi-terminal flexible dc grid and the problems faced by the protection system,on the one hand,it is necessary to study and put forward effective fault current limiting measures,so as to delay or avoid the blocking of the converter station,and to provide a longer time for the protection system.On the other hand,it is necessary to study the sensitive and reliable main and backup protection scheme of flexible dc line,and put forward the principle of single terminal quantity protection and fast double terminal quantity protection.Through the innovative research of fault current limiting method and protection principle,the safe and stable operation of flexible dc grid is guaranteed.The main work and achievements of the thesis are as follows:(1)A fault current limiting method for dc side of multi-terminal flexible dc grid is presented.Firstly,the current limiting circuit topology including thyristor,current limiting resistance and comnutation capacitance is designed.Secondly,the operating principle of fault current limiter is analyzed,and the starting criterion of fault current limiter is given.In addition,the parameters of each component in the current limiter are selected to avoid the blocking of the converter when the dc side fault occurs.Finally,based on the parameters of Zhangbei±500kV four-terminal flexible dc power grid,the simulation software PSCAD/EMTDC is used to build the flexible dc grid model and the current limiter model,and the current-limiting effect of the fault current limiter is simulated and verified.Simulation results show that the designed current limiter can effectively reduce the fault current on the dc side,avoid the blocking of the converter station,and improve the transmission reliability of the flexible dc network.(2)A high-speed protection principle of multi-terminal flexible dc grid based on single-terminal quantity is presented.Firstly,according to the structure and fault transient characteristics of multi-terminal flexible dc grid,the transient equivalent model is constructed,and the analytical expressions of transient voltage are given when faults in the dc side occur in the protection region,in the forward external region and in the reverse external region.Based on this,a fault identification criterion based on transient first-wave time of voltage is constructed,and a fault pole selection criterion based on the characteristics of positive and negative current is proposed Finally,the single-terminal protection is tested by using the simulation model of Zhangbei four-terminal flexible dc grid.The simulation results show that the proposed protection principle operates quickly and is basically unaffected by fault location and transition resistance.In addition,the threshold value of the protection criterion is calculated and set according to the line parameters,which has a strict theoretical basis and is more adaptable.(3)A fast protection principle of multi-terminal flexible dc grid based on double terminal quantity is presented.Theoretical analysis shows that the voltage polarity of dc current limiting inductor at both ends of the dc line is the same when the fault occurs in the protection region,while the voltage polarity of dc current limiting inductor at both ends of the dc line is opposite when the fault occurs outside the protection region.Based on the above facts,this paper constructs a pilot protection method based on the voltage polarity of the current-limiting inductor at both ends of the dc line.In case of single-pole-to-ground fault,in order to avoid the protection misoperation of non-fault pole caused by coupling effect between lines,the two-terminal protection method integrates the voltage of current-limiting inductor,and determines the polarity of the voltage of current-limiting inductor by comparing the integral value with the threshold value.The test results of the simulation model of zhangbei four-terminal flexible dc grid show that the proposed protection method has high selectivity even in the circumstances of large fault resistance,besides,it can maintain the high protection speed and high selectivity at the same time.(4)The dc line protection prototype is produced and the closed loop test based on RTDS platform is carried out.Based on the protection principle of single-terminal/double-terminal quantity,the protection algorithm software program is written respectively.A dc line protection prototype was produced by using the dc monitoring terminal hardware platform of Tianjin Keyvia Electric Co.,Ltd.The RTDS platform is used to build a flexible dc system model of two converters,and a model-prototype-model closed-loop test system is constructed.The protection principle and the performance of the prototype are tested and analyzed.The experimental results show that the developed protection prototype can quickly and accurately identify all kinds of fault situations in and out of the protection area,which verifies the effectiveness of the protection principle and the feasibility of the prototype.