Research On The Optimal Operation And Control Method Of AC/DC Hybrid System Oriented To The Energy Internet

The increasingly severe problems of resource shortage,environmental pollution and climate change have made human sustainable development seriously challenged,and energy transformation is imminent.Energy Internet,as a new generation energy system,has become the key to promoting energy transformation.The architecture system centered with power system which can realize the interconnection of various energy networks,the full access of renewable energy and the plug-and-play of distributed energy has put forward higher requirements for grid operation and control.The traditional grid has been unable to adapt to the development of energy internet.And the network topology of AC/DC hybrid and the unit form of power electronics will become the most prominent features of the new generation power systems oriented to the energy internet.In view of the above changes,this paper focuses on the energy management method of the AC/DC hybrid system from the aspects of upper-level optimal operation and lower-level control strategy.Aiming at the upper-level optimal operation strategy,two time-scale optimization models of the near real-time optimal power flow and the day-ahead unit combination are constructed from the wide-area and regional level respectively to ensure the economical and efficient operation of the system.For lower-level control strategy,two energy routers with different topologies and the corresponding control strategies are studied.The specific research contents are as follows:(1)In order to realize the optimal allocation of resources in a wide area and the interconnection of regional energy networks,the optimal power flow of modular multilevel converter multi-terminal DC transmission system for wide-area energy grid is studied and a multi-objective optimal power flow approach based on improved vector estimation particle swarm optimization is proposed.Firstly,the system hierarchical control and optimization system is established.The station level adopts slope control method to stabilize the DC voltage and balance the active power,while the system level establishes a multi-objective power flow optimization model considering the line loss and voltage imbalance.N-1 security constraint of the converter station is added as well as taking into account the system stability and power balance constraint,and the reference value of control target is obtained by multi-objective optimal power flow to realize the optimal operation of the system.For the equality constraints and inequality constraints,a method based on dynamic adjustment of penalty function is proposed to improve the convergence of the algorithm.Finally,the effectiveness of the proposed algorithm is verified b simulation example.(2)In order to fully exploit the flexible and interactive resources in the regional energy grid and improve the system operation efficiency,the unit commitment strategy of the AC/DC hybrid system considering the integration of high proportion of electric vehicles(EVs)and wind power is studied.Aiming at the problem that the conventional quantitative model is difficult to solve the computational inefficiency problem caused by large-scale decision variables when the high proportion of EVs connecting to grid,a bottom-up approach is proposed to describe the flexibility of the EV cluster.Firstly,taking into account the behavior of the owner and the physical characteristics of the EV battery,the unit EV power feasible domain is described by characterizing the power and energy boundaries.Then,a virtual battery model is established to describe the power feasible domain of the EV cluster by adopting the Minkowski sum of the boundaries.Finally,the proposed model is applied to the unit commitment problem.The simulation results verify the effectiveness of the proposed model and prove that the unit commitment strategy considering cooperated dispatch of high proportion of EVs and wind power can effectively promote wind power consumption and reduce system operating costs.(3)The modular multilevel converter energy router topology and its virtual inertial control strategy suitable for the local subnets are studied.Firstly,the topology of the proposed energy router are analyzed and the mathematical models for input stage,isolation stage and output stage are established respectively.The input stage adopts the virtual synchronous machine control strategy which can effectively reduce the impact of the energy router on the upper power grid when power fluctuations and enhance the inertia characteristic of the system.Then the inertial control principle of DC system is analyzed.A virtual inertial control strategy based on improved droop control is also proposed taking the output stage DC port connected to EV as an example.By dynamic control of charging and discharging power,it can provide inertial support in response to the changes in the DC bus voltage and improve the voltage stability of the DC bus which can makes the local subnet appear flexible.(4)To overcome the shortcomings of the topology of existing energy routers,a novel structure applicable for multi-voltage AC/DC hybrid system is proposed for the energy router based on the previous chapter.The topology of the proposed energy router is analyzed,and the corresponding control methods are applied for different parts.The input stage adopts the modular multilevel converter structure and the virtual synchronous machine control is applied in order to enhance the inertia of the system.A combined mode of Input-Series Output-Series and Input-Series Output-Parallel is applied for the DAB modules of the isolation stage in order to realize network interconnection and electrical isolation of different voltage levels AC/DC grids.The power of the output port is adjusted flexibly to enable the inferior power grid to provide certain power support to the superior grid and participate in the primary frequency regulation.Then,by establishing the energy router average model,a power coordination control method is also proposed to meet the power requirement of each port of the output stage and ensure the stable operation of the system.The reliability and efficiency of the proposed topology and control strategy are finally verified via simulation.