Research On Ride Through Technology Of Load-side Converter Under Symmetrical Fault On Load For Stand-alone Brushless Doubly-fed Generation System

Due to the cancellation of the brush and slip ring,the brushless doubly-fed machine(BDFM)is more reliable than the doubly-fed induction machine and it has broad prospects in the field of variable speed constant frequency power generation.For stand-alone brushless doubly-fed power generation system(BDFG),similar to grid-connected operation,when the load is symmetrically short-circuited,it is a low-voltage ride-through problem.When a short-circuit fault is removed,it is a high voltage ride-through problem.However,due to the strong coupling of motors,converters,and loads,there are some new features and requirements that are different from grid-connected operations.Therefore,it is necessary to study the control strategy for the load-side converter of the stand-alone brushless doubly-fed generator system under symmetrical short-circuit faults.Based on the basic structure and operating principle of brushless doubly-fed machine,the control strategies of the machine-side converter and the load-side converter of BDFG are introduced.Based on the equivalent circuit of the BDFM,the active power through the machine-side converter is calculated under the consideration of the internal loss of the motor,and the active power feed forward control strategy is adopted to improve the dynamic performance of the load side converter.The AC current characteristics of the load side converter after load symmetrical short-circuit fault have been analyzed.The use of feedforward of the common connection point(PCC)voltage into the output voltage of the converter can effectively suppress the rapid increase of the inductor current at the moment of voltage drop at the common connection point and will not cause overcurrent of the AC current.The boundary condition that the bus voltage can maintain stability is analyzed.The new characteristics of the system's power flow after taking into account the internal loss of the motor have been deduced,and the mechanism that the bus voltage can't maintain and will fall after a deep short circuit is deduced.For the condition that the DC bus voltage can be maintained,a closed-loop plus feedforward control method consistent with normal operating conditions is proposed,and with the reactive power control strategy.For the case that the DC bus voltage cannot be maintained and dropped,three control strategies are proposed.By analyzing and comparing these three strategies,it is recommended to block the pulse of the load side converter.The mechanism of the voltage surge at the PCC during the fault removal and the influence on the control performance of the load side converter are analyzed.For the situation that the bus voltage can be maintained,the control strategy of switching directly to the normal operating mode is adopted.For the situation that the bus voltage cannot be maintained,the recovery control strategy adopts voltage slow start,and the effect of different slow start steps on the recovery effect are compared.A simple and feasible current limiting strategy is proposed,which can effectively reduce the current charged by the load side converter to the capacitor at the moment of fault recovery.The relevant control strategies are verified through simulation and experiments.