Inter-Pole Power Balancing Strategy For DC Power Generation System Of Renewable Energy

With the great reliance on and use of energy,the problem of energy shortage and the environmental pollution of traditional energy is becoming more and more serious.The issue of energy is a very prominent issue in twenty-first century.As a result,the utilization of renewable energy has also attracted more and more attention and development.Renewable energy,such as wind energy,solar energy and so on has the advantages of clean,inexhaustible and so on,making it a hot spot in theory and technology research.This article is based on the bipolar three-wire DC distribution system background to start the study and discussion.The bipolar DC distribution system has many advantages such as providing many voltage level,high reliability,small insulation stress and good safety.Wind,solar power generation have notable features of strong randomness,strong volatility,weak support,weak survivability and so on.A large number of direct current wind farms and photovoltaic units constitute renewable energy DC power generation system.Due to external natural conditions or internal failures,the power imbalance between positive and negative poles will occur.In order to keep the stable operation of the external DC distribution system,it is necessary to introduce the power balance circuit between the poles.It is proposed that the front end current balancer can effectively balance the power between positive and negative poles,and has good control effect.Firstly,the DC distribution system research status,advantages,web form and key technologies are described,and then bipolar DC distribution system background is introduced.Due to the advantages of bipolar DC power distribution system,it can be predicted that as the advantageous technical scheme,bipolar DC power distribution system will be widely used in the future regional DC distribution system construction.In bipolar DC distribution system,voltage balancer is a key component.It constructs a bipolar three wire DC distribution system by drawing a middle line,and makes voltage balance between positive and negative.Then the analysis and research of the incorporated structure voltage balancer and the embedded structure voltage balancer are carried out.The topology,control strategy,working principle and small signal model of various voltage balancer are emphatically analyzed.In addition,it is also concluded that the incorporated structure voltage balancer can be regarded as the conversion of bidirectional input and output DC converter with opposite polarity,so that a new voltage balancer can be generated correspondingly.Then,the embedded voltage balancer and the parallel voltage balancer are compared and analyzed.Considering the need of system expansion and "N+1" redundancy design,parallel operation of voltage balancers in the bipolar system can be expected.First,the types of voltage unbalance in bipolar DC distribution system are discussed.Based on that,a control method of distribution voltage balancers is introduced.In the control strategy,the first layer control is droop control,the second layer control is mainly used to eliminate the voltage unbalance under critical load,and the third layer control is used to minimize the system loss.The improved layered control structure has been verified by simulation and has good control effect.Unlike the typical P-N bipolar DC distribution system,the concept of P-Z-N bipolar DC distribution system is proposed in this paper.The two bipolar systems are compared in terms of cost,efficiency and reliability and the relevant conclusions are obtained.Moreover,the concept of voltage balancer is extended to "interlinking converter".A two level control strategy for energy balance management by interlinking converter(IC)between PZ sub-microgrid and ZN sub-microgrid is proposed,and the simulation is carried out.Based on the bipolar three wire DC distribution system background,the related formulas are calculated,and the causes and performance of the imbalance between the positive and negative poles are explained by the formula.It is concluded that under the condition of unbalanced load and unbalanced distribution power supply,only voltage balancer can not guarantee the balanced operation of bipolar DC distribution system.In order to ensure the overall balance of voltage,current and power in bipolar DC distribution system between positive and negative poles,a front current balancer needs to be introduced on this basis.The physical essence of the front-end current balancer can be seen as the establishment of an energy transfer channel between both positive and negative poles,which is consistent with the working mechanism of the voltage balancer.It can also be considered as an interlinking converter to realize the energy management between positive and negative poles.Therefore,the front end current balancer can draw lessons from the existing topology of the voltage balancer.In this paper,a front-end current balancer topology is proposed and a new closed loop control strategy is introduced.On the basis of theoretical analysis,simulation and analysis are carried out under different light intensity and different working conditions to verify the effectiveness and feasibility of front-end current balancer of controlling positive and negative pole power balance.The final result is that the current in the neutral line is about zero,and the control effect is good.The simulation results show the correctness of the theoretical analysis and the proposed front-end current balancer can effectively make power between positive and negative poles balance.