| Under the background of energy structure transition,the generation of wind power,solar power and other renewable energy are booming.Flexible DC grid provides a reliable way for large-scale renewable energy integration,transmission and consumption,and will become a supporting means of energy reform and an important direction of power grid development in the future.The DC grid,which is mainly composed of power electronic devices,has low inertia,weak damping and limited ability to withstand over current.The problem of dealing with DC fault has become one of the bottlenecks restricting the development of DC grid.Based on the National Key R&D Program of China “Basic theoretical research on fault current suppression of flexible DC grid”(No: 2018YFB0904600),this dissertation focuses on the research of flexible DC grid which takes half-bridge sub modules and DC circuit breakers as fault isolation technical route.The fault current characteristics in different scenarios are discussed and the corresponding current limiting measures are proposed.(1)The influence of control strategy on fault current and its optimization method are studied.Firstly,the control characteristics of the station-level control strategy and system-level control strategy of flexible DC grid are analyzed to establish the premise of steady-state operation of DC grid for fault analysis.Then,the influence of the control strategy on the fault current is discussed,and the conclusion that the control strategy affects the fault current by changing the power level and DC voltage level is obtained.Finally,a hierarchical cooperative adaptive droop control strategy for DC power grid is proposed.By optimizing the power flow in system control layer and droop coefficient in station control layer,the equilibrium of power level and voltage level of DC grid is improved,which helps to reduce the fault current level.(2)The fault characteristics and fault current level suppression method of symmetrical monopole flexible DC grid are studied.Firstly,a high-frequency equivalent model for analyzing pole-to-pole fault is established,and the influence of radial topology and ring topology on pole-to-pole fault current is discussed respectively.It is concluded that the grid topology has no influence.Then,an accurate calculation method of pole-to-ground fault current is proposed,and the analytical expression of fault current with respect to system parameters is obtained based on matrix exponential method.The influence of DC grid topology on pole-to-ground fault current is further studied.Then,based on the influence law,a simplified index to evaluate the fault current level is established,which can evaluate the fault current level of different topologies of DC grid simply and effectively.Finally,taking the simplified evaluation index of fault current as the fitness function,a DC grid topology optimization method based on genetic algorithm is proposed.The optimization results show that,from the point of view of fault current limiting,the mesh structure or ring structure is recommendable for DC grid.The optimization results show that,the maximum fault current can be reduced by more than 20% with the optimal mesh or ring structure.(3)The evaluation and suppression methods of fault current level in bipolar flexible DC grid are studied.Firstly,an improved high-frequency equivalent model for fault current calculation is established.The model shows that in the initial stage of fault,only the adjacent converters and the sub-adjacent converters of the fault line will feed fault currents to the fault point,and the number of sub-adjacent converters and related component parameters are the key factors affecting the fault current level.Then,according to the above mechanism,a simplified evaluation index of fault current level of symmetrical bipolar DC grid is proposed,which avoids the complicated calculation process of fault currents when the topology changes.Finally,a topology optimization method to limit the fault current level of pole-to-pole fault is proposed based on intelligent algorithm with the minimum simplified evaluation index as the objective function.The results of studied cases show that the fault current level of radial topology DC grid is the highest,while that of chain topology is the lowest.By setting constraints and reoptimizing,the optimal topology can be obtained to meet the reliability and security design requirements of DC grid,and the maximum fault current can be reduced by more than 18%.(4)Based on the analysis of impedance characteristics of converter station after the DC fault occurs,a joint current limiting control strategy to suppress both fault current and arm current is proposed.Firstly,the DC impedance model of post-fault converter is established,and the influence of the proportion of invested sub modules on DC impedance in frequency domain and on fault current in time domain is analyzed.Then,based on the above influence law,a DC fault current suppression control strategy based on virtual impedance reshaping is proposed.By adaptively changing the proportion of bypassed sub modules,the reshaping of DC impedance in high frequency region and the suppression of fault current are realized.Then,in view of the arm overcurrent phenomenon caused by the sub modules bypassing,the influence law of the control system on the arm currents is analyzed,the key influence link in the inner loop control is extracted,and the arm current suppression control strategy to eliminate the key link is proposed.Finally,combining with two current limiting control strategies and cooperating with DC circuit breaker,the selection principle of control parameters is formulated.The simulation results of PSCAD and RTDS show that the proposed control strategy can reduce DC fault current by more than 30% and arm current by more than 50%,which can not only reduce the breaking current and breaking capacity of DC circuit breaker,but also prevent the converter from being blocked due to arm over-current before DC circuit breaker breaking,which can improve the fault ride through capability of DC grid. |
