| Aiming at the problems existing in the traditional LCC-HVDC(Line Commutated Converter Based HVDC)power transmission system,the converter transformer is noisy and the inverter side is prone to commutation failure.The research group innovatively proposed the LCL-HVDC(LCL Structure Converter)power transmission system and applied for a patent for invention.For this new type of DC transmission technology to be applied in engineering,there are a series of theories and technologies that need to be studied.The converter is the core component of the HVDC power transmission system and an important hub to realize the AC-DC conversion function.Therefore,this paper only studies the inverter fault characteristics of the LCLHVDC system,and compares the theory and simulation with the LCC-HVDC system.The topology of the LCL-structured HVDC transmission system is based on the topology of the LCC-HVDC transmission system.A shunt capacitor and a series current limiting reactor are added to the connection between the converter and the valve-side winding of the converter transformer.It also has an inductance,so it is called an LCL type structure.The system also cancels the AC filter and reactive power compensation equipment on the AC side.The parallel capacitor can not only provide the reactive power required for commutation,absorb the high-order harmonics generated by the converter,but also make the commutation faster and more stable,while the series inductance can limit the current rise rate di/dt at the valve can withstand In the range.Firstly,the mechanism of different fault types of the converter is analyzed,including converter valve short circuit,DC side outlet short circuit,DC side ground short circuit,AC side phase-to-phase short circuit,and AC side single-phase ground short circuit.Among them,the detailed theory deduces the valve short-circuit current and fault line voltage of the valve short-circuit,DC 12-pulse outlet short-circuit and AC-side single-phase-toground short-circuit faults that occur inside and outside the converter area and have the most serious impact on the system in the LCL-HVDC system.Secondly,based on the CIGRE simulation standard model in PSCAD/EMTIC simulation software,the LCL-HVDC system simulation model is built.The selected control mode of the rectifier and inverter side is introduced,and the parameter values of new components are designed based on the 12-pulse DC quasi-steady state model of the system.In the PSCAD/EMTIC simulation software,the steady-state simulation of the LCLHVDC system is carried out according to the determined main circuit parameters,and the steady-state simulation of the LCC-HVDC system is compared to ensure the correctness and feasibility of the new system model.Finally,various fault simulations are simulated in the LCL-HVDC and LCC-HVDC transmission system models.The theoretical calculation and simulation results of the three most representative typical faults,such as the converter valve short circuit of the LCL-HVDC system,are approximately equal,which verifies the correctness of the theoretical analysis of the new system and the establishment of the model.On this basis,a total of 8 kinds of faults including short circuit of converter valve,12-pulse DC and 6-pulse high and low voltage outlet,DC high voltage and midpoint-to-ground short circuit,AC single-phase-to-ground short-circuit,and AC two-phase short-circuit are simulated and analyzed.The fault simulation results of the LCL-HVDC system and the LCC-HVDC system are compared and analyzed.Under different fault conditions,the maximum value of the valve current in the LCL and LCC commutation modes are approximately the same,and the valve current stress is equivalent;when the LCL-HVDC system fails,the maximum value of the valve current is approximately equal.The converter and the converter transformer bear a larger voltage.When the DC 12-pulse outlet is shortcircuited,the line voltage on the valve side of the converter is 29.23%higher than that of the LCC converter system,and the line voltage on the valve side of the valve side winding of the converter transformer is increased by 29.17%.Therefore,compared with LCC-HVDC,LCL-HVDC must improve the withstand voltage of the valve side windings of the converter and converter transformer. |
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