| VSC-HVDC based DC grid technology provides a new solution for large scale renewable energy integration,island power supply and urban power distribution.Similarly in AC grids,The interconnections among DC girds with different voltage levels are usually realized by means of DC/DC converters(DC transformer).The high-voltage and high-power DC/DC converters are characterized by bidirectional energy flow,power flow control and fault blocking function,which is distinct from the traditional chopping wave DC/DC converters.For the time being,the research of high-voltage DC/DC converters for DC grids is still in the stages of circuit topology design,simulation analysis and prototype verification.Therefore,taking the high-voltage DC/DC converters as research object,this work focus on the circuit topology design,mathematical modelling,parameter design and control strategy design.1.Considering the design and manufacturing process issues of high capacity medium frequency transformers and filtering devices commonly-used in present topologies,a new T-type DC/DC converter is proposed based on the configuration rule of non-isolated DC/DC converters.This topology does not need any high capacity medium frequency transformers and the large inductance filter at DC sides,which makes it highly suit for engineering application.Besides,it is extensible to a multi-port DC/DC converter,which can interconnect DC grids with multiple voltage levels.Moreover,it is able to restrain DC fault currents,which provides an additional way to protection measures of DC systems.2.Considering the facts that high randomness of the switching of MMC submodules and that the switching frequency approximates the fundamental frequency,averaged modeling method based on equivalent switch period is proposed.Based on that,state space average model of T-type DC/DC converter considering the time-domain model of arm current and capacitor voltage of submodules is build.Furthermore,the dynamic phasor model and linearized small signal model of T-type DC/DC converter is derived,which mathematically contribute to the solution of stationary operation point and design of controllers.3.The calculating method for the power losses of the half-bridge-full-bridge hybrid bridge and the determination method for the number of sub-modules,the bridge arm inductances are developed.Based on that,with the objective of minimizing the power losses of the power devices in the converter,the determination of the parameters in the main circuit of T-type DC/DC converter is given by using global optimization techniques.A comparison of the different topologies with the same parameters shows that T-type DC/DC converter needs a less number of sub-modules and has a lower power loss,which makes it technological and economical.4.To cope with the issues of the high power losses and the large computation in the control systems caused by the middle/high fundamental frequency in T-type DC/DC converters,a fundamental frequency modulating strategy and dual sorting capacitor voltage balance control method are proposed.All the sub-modules are switched once and voltage balance control is runned once in a fundamental period,which ensures a reduction of the power loss and an achievement of voltage balancing of the capacitors in the submodules.5.To meet the requirments of desired DC voltage,current and power flow in DC/DC converter,the power flow control strategies contianing power balance control and common point voltage control are proposed for the T-type DC/DC converter.This strategy has the advantages of clear physical meaning and simple control configuration.By means of simulations and experiments,the effectiveness of the proposed topology and the related control strategy are validated.The proposed topology are capable of the power flow reverse,converting at multiple voltage levels and restraining DC fault currents.It also has quite good static and dynamic performances. |
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