Research On Topology And Control Of High-voltage Large-capacity DC/DC Converters For Hvdc Grids

With the breakthroughs in the capacity of power electronic devices and equipment,VSC-HVDC technology has been rapidly developed in recent years.The HVDC grid formed on this basis is attracting attention and is considered to be a key technology to solve large-scale renewable energy consumption.High-voltage and large-capacity DC/DC converters are the core equipment for interconnecting HVDC lines of different voltage levels and constructing HVDC power grids.Existing MMC based DC/DC topologies are unable to avoid injecting AC voltage into the arms,which result in problems such as a large number of semiconductors,high costs,low efficiency,and bulky magnetic components such as filter inductors and transformers.In order to overcome the above problems,this dissertation starts from the basic structure and commutation method of capacitive energy transfer DC/DC converter(CETDC),summarizes the CETDC topology derivation and combination method,and expands the CETDC topology to HVDC grid applications such as monopolar symmetrical HVDC and DC power flow control.The control methods of the converters are designed according to the CETDC topologies and their applications.This dissertation refers to the energy transfer process of the classic low-voltage Buck-Boost converter,uses the sub-module based storage arm as an energy buffer element,and switches the charge and discharge state of the energy storage capacitor in sub-modules by commutation valves based on semiconductors in series.In this way,the capacitive energy can be transferred.On this basis,the basic structure of CETDC topology is proposed,and its commutation principle,power regulation principle and energy balance principle are revealed.A CETDC topology is proposed,in which the commutation valves are composed of thyristors and diodes and the storage arms are constructed by half-bridge sub-modules.In consideration of commutation waveform and reliable turn-off,ZVS turn-on,and du/dt limit of the thyristors,the commutation process and commutation control method of the thyristor/diode commutation valves are designed.The proposed topology and control method have passed the verification of simulation and experiment.The comparative analysis with traditional MMC based DC/DC topology proves the advantages of CETDC in terms of cost,efficiency,and volume and weight.According to the basic structure and operating principle of the CETDC topology,the structural constraints of the storage arm and the commutation valve are respectively summarized,and three types of CETDC topologies of Buck-Boost,Buck and Boost are derivated.Topologies suitable for power unidirectional transmission are further obtain.On this basis,the Buck-Boost and Boost topologies with unidirectional power transmission are combined.Through the switching of mechanical switches,a CETDC topology with bidirectional power transmission capability is constructed,whose commutation valves are only composed of low-cost and low-loss diodes.By using voltage and current control ability of stor age arm,the switching control method of mechanical disconnectors are designed,which can create conditions for zero-voltage zero-current switching(ZVZCS)of mechanical disconnectors.The proposed topology and control method are verified by simulation and experiment,and the advantages of the diode commutation valve in the combined topology in terms of cost and efficiency are proved.The application range of CETDC topology is extended to monopolar symmetrical HVDC,and the storage arm connection mode that connects the positive and negative polars at the same time is proposed.Under the premise of meeting the structural constraints of the storage arm,the storage arm interpolar reuse is realized.On this basis,a monopolar symmetrical CETDC topology is constructed,which can save two phase circuits and reduce the number of sub-modules and series components in commutation valves.Aiming at the problem of unequal charging and discharging power caused by the interpolar reuse of storage arm,a method for energy balance is proposed by adjusting the charging and discharging time to realize storage arm energy balance.During the switching process of the commutation valves,ZVZCS is realized by reasonably controlling the voltage of the storage arm.The operation control method of monopolar symmetrical CETDC phases interleaved with 90° is designed to ensure the continuous input and output current.The proposed topology and control method have been verified by simulation and experiment.Through comparative analysis,it is proved that monopolar symmetrical CETDC can further enhance the advantages of CETDC topology in terms of cost,efficiency,and volume and weight.This dissertation further applies CETDC to interline DC power flow controller.The relationship between the line current and the port voltage of the interline DC power flow controller is analyzed,and the principle of line current regulation by the interline DC power flow controller is revealed.In order to ensure the current control ability under any current direction working condition,the interline DC power flow controller based on CET principle is proposed,in which the storage arms adopt the full-bridge sub-modules,the commutation valves adopt the bidirectional thyristors.Since the interline DC power flow controller can only change the distribution ratio of the two line currents,a port voltage common mode and differential mode decomposition control method is proposed,in which the differential mode component is used to regulate the line current,and the co mmon mode component is used to realize the energy balance of storage arm,and the two components do not affect each other.The topology and control method of the CET interline power flow controller are verified through simulation and experiment.Compared w ith the traditional modular interline DC power flow controller,CET interline DC power flow controller has advantages in cost and efficiency,and provides a solution for the DC power flow control of the HVDC grid in the future.