Research On Control Technology Of Flexible HVDC Transmission System Based On MLVR-VSC

With the rapid development of the national economy,the demand for energy continues to rise,and large-capacity,long-distance power transmission has become the development trend of my country’s future power transmission technology.Flexible DC transmission technology is widely used in the field of power transmission due to its many technical advantages,such as no commutation failure,independent regulation of active and reactive power,and ability to connect to weak AC power grids and passive power grids.Voltage source converter is the core device of flexible DC transmission,and its performance directly affects the performance of the entire transmission system.To this end,this thesis constructs a multi-level voltage reinjection voltage source converter(MLVR-VSC)based on the principle of voltage reinjection conversion,and applies it to the two-terminal flexible direct current transmission system.The following research work is carried out around the operating performance of the converter and the control strategy of the transmission system under normal and fault conditions of the AC grid:Firstly,a single MLVR-VSC(Single MLVR-VSC,S-MLVR-VSC)and its switching modulation strategy are designed based on the principle of voltage reinjection conversion.Through the working principle of the converter and the theoretical waveform analysis of the AC side and DC side,it can be obtained: when the voltage on the DC side is fixed,the amplitude of the output voltage on the AC side is a certain value,so the flexibility of the reactive power control at the transmitting and receiving ends in the application of the DC transmission system is limited.To this end,this thesis uses two S-MLVR-VSCs connected in parallel on the DC side and connected in series on the AC side to form a double MLVRVSC(Double MLVR-VSC,D-MLVR-VSC).In the D-MLVR-VSC topology,the phase angle of the output voltage of the upper and lower two converters can be freely controlled,so that the amplitude and phase of the composite voltage on the AC side can be controlled.The simulation verified that D-MLVR-VSC can control the amplitude and phase of the output voltage on the AC side freely and has good output performances in rectification and inverter modes,which provides a prerequisite for the realization of flexible control of reactive power at the sending terminal and receiving terminal.Secondly,based on D-MLVR-VSC,a two-terminal DC transmission system is constructed,its basic working principle is analyzed,and the mathematical model of the AC and DC side of the system is established.The idea of hierarchical design is adopted for the control of the transmission system,with emphasis on the design of converter station-level control at the sending and receiving terminals.Based on the deduced mathematical model and combined with D-MLVR-VSC’s control method of AC output voltage amplitude and phase,a dual-loop control structure of power(voltage)outer loop and current inner loop is established,which to realize to independently control active power and reactive power at the sending and receiving terminals.The simulation results show that the independent adjustment of active and reactive power can be achieved by adjusting the amplitude and phase of the AC output voltage at the sending and receiving terminals;D-MLVR-VSC has good dynamic and static response performances under different working conditions,which can ensure the stable and reliable operation of the transmission system.Finally,when a symmetrical fault occurs in AC grid of the sending terminal,in view of the phenomenon that the conventional control strategy will lead to voltage sag and current overcurrent in AC side of the sending terminal,this thesis adopts a reactive power priority control strategy.This can provide a certain amount of reactive power support for the recovery of AC voltage in the event of a fault and suppress system overcurrent.Aiming at the occurrence of asymmetrical faults in the AC grid at the sending terminal,a mathematical model of the system under asymmetrical faults is established.The delay method is used to separate the positive and negative sequence components of the voltage and current,and the negative sequence voltage feed forward control idea is adopted to establish control strategy for effective suppression negative sequence current.The corresponding simulation model is established and the correctness and effectiveness of the designed control strategy are verified through simulation.