Research On Fault Analysis And Protection Strategies Of MMC-HVDC

Modular multilevel converter based high voltage direct current(MMC-HVDC)has been widely researched and applied in large-scale renewable sources and long-distance and large-capacity power transmission as its advantages of expandable,low harmonic,flexibly controlled,etc.and is regarded as the most promising topology of DC transmission.Different from AC grid,dc fault especially a short-circuit fault of MMC-HVDC transfers very fast as its 'weak inertia' which will cause serious overcurrent threatening system operation and equipment safety.MMC is composed of large numbers of sub-modules(SMs)which raise the failure rate of MMC.Introduction of electronic devices bring more non-linearity to the system which makes it impossible to form an accurate analytical formula of most electrical quantities and difficult for power system protection.In view of the above problems,this paper studies fault diagnosis and redundant operation strategy of sub-modules,short-circuit current calculation,over-current suppression and fault protection methods based on data-driven artificial intelligence.The main work is as follows:(1)In this paper,short-circuit fault characteristics of IGBT in sub-module are analyzed,the evolution law of capacitance voltage is studied,the calculation method of capacitance voltage is deduced,and an improved differential under-voltage protection method based on the calculation value of capacitance voltage is proposed.This method can detect the sub-module of short-circuit fault quickly and accurately.Combining with the switch function of sub-module,the fault IGBT location is carried out based on wavelet transform.The effect of fault location is compared under the different scale factors and noise conditions.The simulation results show that the IGBT short-circuit fault diagnosis method proposed in this paper is effective.(2)The operation mode called "intermittent backup”of redundant sub-modules is proposed,which can ensure that the redundant sub-modules maintain a high voltage level and avoid frequent switching.It not only ensures that the system returns to normal operation quickly after the redundant sub-module is switched in,but also prolongs the service life of the redundant sub-module.The scheduling trigger strategy considering redundant sub-modules is optimized,which greatly reduces the complexity of scheduling process,reduces the switching frequency and improves the system operation efficiency.(3)The characteristics of single-pole grounding and pole-to-pole short-circuit are analyzed,and the evolution law of short-circuit current is studied.For pole-to-pole short circuit faults,the calculation method of short circuit current based on calibration coefficient is deduced and the influence of different operating conditions of the system on the calibration coefficient is also explored.A method of calculating short-circuit current based on "mesh current method" is proposed.By dividing the fault network into different meshes,the state equation of each mesh is established.Finally,the fault current is calculated.This method can also be extended to more complex network structure.The simulation results show that the short circuit current calculation method proposed in this paper can accurately calculate the short-circuit current of pole-to-pole short-circuit fault of transmission line and provide reference for protection design and equipment selection.(4)Overcurrent limitation strategies for different stages of pole-to-pole short circuit fault are proposed.When the fault occurs,an auxiliary structure based on current limiting module and branch transfer module is proposed.On one hand,it restrains the three-phase AC short-circuit current and changes the current path to avoid the short circuit current flowing through the sub-module components.On the other hand,it accelerates the decay speed of inductance continuous current and DC current,making fault isolation easier.A method of current limiting resistance configuration is proposed,which should consider the current limiting effect and equipment safety.In the post-fault reclosing stage,the strategy of configuring current-limiting circuit at DC outlet is put forward,which can limit the overcurrent of bridge arm to the allowable range in reclosing process.The method of configuring current-limiting resistance is designed to avoid the problem of repeated block of converter station during reclosing process under permanent fault and improves the system security and shortens the time of fault recovery.(5)A method of MMC-HVDC line fault protection based on data-driven artificial intelligence algorithm is proposed.Convolution Neural Network(CNN)is taken as an example to matrix sequence data,select samples,enhance data,adjust model parameters and evaluate model.Based on the accuracy and recall rate,the excellent performance of CNN algorithm in line fault protection of MMC-HVDC is verified,which provides a reference for the application of artificial intelligence algorithm in MMC-HVDC fault protection.