Research On Control Of Multi-energy Complementary DC Microgrid Main Based On Tidal Energy

The researches and exploitation of the main countries on renewable energy source are increased because of the energy crisis and the worsening environment problem in all over the world.Now,solar energy,wind energy,ocean energy and biological energy are the main renewable energy source,in which the solar and wind energy have been developmented very well in many countries,however,because of the poor development conditions,the researches on ocean energy are late than other kinds of energy.Whereas,the energy crisis and the rich deposit of ocean energy such as the tidal current energy and tidal energy have accelerate the researches and utilization.Unlike the traditional fossil energy,these renewable energy aways strongly influenced by the geographic position and environment conditions.So the tradition centralized power plant can not be suit for the development and transmission of the renewable energy source,besides,with the development of power electronic technology,more and more DC loads have been used,then the DC rmicrogrid was rised in response to the proper time and condition.DC microgrid,compared with the AC grid,have numbers of advantages,so based on the tidal generation and some other renewable energy,this dissertation is mainly researched on the followeing questions about DC microgrid:In the second chapter,several key questions in tidal generation system is researched.First,the system small signal model consist of water turbine,permanent magnet synchronous generator and PWM rectifier is established.Consider of the low speed and great inertia,the control design of the tidal generation system is a little difficult.So based on the model of the system,the control system is designed using the intelligent PI controller for the control of rotor speed and the stability of system is also analyzed.Finally,according to the characteristic curve of water turbine,a kind of self-optimization step size variable maximum power point tracking(MPPT)method based on the continuous slope function is proposed in this chapter,several other MPPT methods have compared with the proposed method by simulation and experiment.In the third chapter,the design of photovoltaic power generation system and composite energy storage are researched.First,the output characteristic of PV cell is simulated in the MATLAB,and then the primary reason of the unbalanced current distribution in triple Boost DC-DC converter without current sharing control is analyzed.Therefore,in order to either tracking the maximum power point of PV or distributing the current with a balanced way,a control strategy on the basis of impedance matching principle is proposed in this chapter.Besides,the MPPT method proposed in last chapter is also utilized in the control of PV directly,simulation and experiment have conducted compared with several other MPPT method.Finally,as a part of DC microgrid,the control of composite energy storage consist of battery and ultracapacitors is analyzed and verified in MATLAB.In the fourth chapter,the negative impact of the unbalanced load interface with three-phase inverter to DC microgrid is analyzed and controlled.First,the switch cycle model of three-phase inverter has established,taking this as foundation,the mechanism and suppression principle of the unbalanced voltage is analyzed.And then,the control strategy based on symmetrical component method is designed,in which the positive and negative component are acquired with Delayed Signal Cancellation(DSC)that is fast than other method,and then the inhibition principle of the control strategy with unbalanced voltage is analyzed in the way of impedance characteristic,simulation and experiment are conducted.Finally,the influence of the 2nd-harmonic current in DC side with DC microgrid is analyzed,and a DC bus conditioner is proposed to compensate harmonic current in DC microgrid,which is act as a current source.The fifth chapter,is focused on the stability research of the DC microgrid.First,the constant power load negative resistance characteristic of common power conversion load are analyzed,and then the adverse impact of CPL on stability of DC microgrid is analyzed too,so as to evaluate the stability of the system,the impedance stability criterion is introduced.On the basis of the above analysis,the means to improve the stability margin is analyzed in the way of adjusting the source output impedance.Finally,an active damping method which is act as a virtual impedance that just damp the harmonic voltage is proposed,which can improve the stability margin of DC microgrid without increase the DC bus capacitor.Several other method which are also can improve the stability of system are compared with the proposed method in the way of simulation and experiment.