Research On Topology And Control Strategy Of Resonant Converter For HVDC Electrical Collection Systems

In order to promote the realization of the "dual-carbon" target,it is necessary to reduce the carbon emissions caused by fossil energy consumption,and to adopt a new energy generation method is an effective way to achieve the "dual-carbon" target.The new energy generation represented by solar power generation and wind power generation has the characteristics of strong randomness and volatility.It is difficult to use AC convergence or AC power transmission in practical projects.In the form of one-stage step-up full direct current offshore wind power,using DC-DC converter with high power density and high step-up ratio instead of power frequency transformer can effectively solve the problems of AC convergence or AC power transmission.Therefore,DC-DC converters with high power density and high step-up ratio are the most important core components in the applications of full DC offshore wind power and other HVDC convergence applications.The DC-DC resonant converter topology with ultra-high step-up ratio proposed in this paper mainly consists of three levels:high frequency step-up transformer,resonant network and voltage doubling rectifier circuit.The step-up capability of CLLLC resonant network is used to improve the total boost ratio of the system.The total step-up ratio of the proposed converter is the product of the three step-up ratios,which can reach 160 times.At the same time,the converter has the ability of two-way energy flow and can be used for black-start of offshore wind farms.The forward and reverse operation of the system are analyzed in detail,and the working principle of the system was explained.The equivalent circuit of the switch network and the modified equivalent resistance of the voltage doubling rectifier circuit with intermittent current are obtained through the analysis of the steady-state characteristics.For the resonant network,the steady-state step-up ratio of the resonant network is obtained approximately.Under different inductance and quality factor values,the relationship curves between the voltage gain of the resonant network and the unit switching frequency are obtained,and the influence of the related parameters on the voltage gain is analyzed.In this paper,the phase-shift variable-frequency control strategy is used,and the hardware and software implementation of the control strategy are given.The relations between step-up gain and phase shift duty ratio,impedance angle and switch angle frequency are deduced,and their relationship curves are plotted respectively.The feasibility of phase-shift variablefrequency control strategy is analyzed in theory.The constraints for ZVS implementation are analyzed,and the minimum dead time constraints are derived considering the parasitic capacitance of the switch,as well as the constraints between the phase-shifting conduction angle and the impedance angle.The problem of narrow range of zero-voltage-switch under phaseshifting control strategy and wide range of switch frequency under variable-frequency control strategy are solved.The extended description function method is used to model the small signal of the converter,and the dynamic characteristics of the system are analyzed.The large signal model of the resonant converter can be obtained by linearizing the nonlinear equations and harmonic approximation and harmonic balance.On the basis of the large signal model,small signal disturbance is applied to obtain the small signal model at a steady state equilibrium point.Based on the small signal model,the state space equation of the linearized system can be listed and the correlation transfer function can be obtained.By analyzing the amplitude and frequency characteristics of the system,the system is corrected and the controller is designed.A digital simulation model of the system is built using PSIM,and the feasibility of the proposed topology and control strategy is preliminarily verified.Then,the related theory of the system is analyzed.On the basis of the obtained mathematical model,the electronic circuit is built by operational amplifier to simulate the mathematical relationship between various parts of the system.For the simplified main circuit,a set of simulation experiment platform is designed.On the platform of simulation experiment,it is convenient to debug the DSP program.This is conducive to the implementation of the experimental prototype,and improves the safety and reliability of the experiment.The control strategy is validated by simulation experiments.The system can accurately control the output voltage and ZVS angle.Finally,the hardware design and software design of the DC-DC resonant converter system with ultra-high step-up ratio are given,and the topology and control strategy are verified experimentally.The experimental results show that the proposed topology has the ability to increase the input voltage by 160 times,and the control strategy can achieve accurate control of the output voltage and ZVS angle,so that the output voltage is always within the control target range,and the switch is in the soft switching state.The rectifier diode in the voltage doubling rectifier circuit flows intermittent current,which can naturally realize ZCS.