Research On Networked Hierarchical Cooperative Control Strategy For AC Microgrid Systems

In recent years,energy crisis attracts great attention in worldwide.The cooperative control of microgrid systems with multiple distributed generations paralleled is one of the research hotspots in renewable energy technology field.Microgrid is considered as a main part of smart grid,which control networking is a development trend of smart grid.The cooperative control among micro sources,power quality control and cooperative optimization of system level are the key to the stable operation of microgrid system.This thesis focuses on the voltage and frequency stability control,the reactive power sharing,the voltage quality control,and the multi-objective reactive power optimization in AC microgrid.The main research works are as follows:1)A networked hierarchical cooperative control architecture for microgrid is established.Based on the networked control technology and hierarchical control theory,the architecture of networked hierarchical coordination control is established for low voltage AC microgrid system.The primary control layer is combined with the secondary control layer of the architecture,which adopts distributed control method.Then the centralized control method is adopted in the tertiary optimized control layer.The establishment of the control architecture provides a theoretical basis for subsequent research.2)A distributed secondary control approach for microgrid and its stability analysis.Aiming at the shortage of traditional droop control method to realize the cooperative control for distributed generation(DG)and the centralized control is excessively dependent the central controller,a distributed secondary control(DSC)method is proposed.The full state-space equations of microgrid system are derived,and the small signal model is established.The distributed secondary control parameters are determined according to the eigenvalues analysis.A microgrid based on distributed secondary control method was designed.The simulation experiments results show distributed secondary control method has better dynamic performance.Compared with the centralized secondary control approach,the DSC method is more robust and makes the system more reliable.3)Research on networked hierarchical coordination control strategy for voltage and reactive control in island microgrid.For dealing with voltage amplitude deviations and the inaccurate sharing of reactive power caused by droop control in microgrid with multiple inverters paralled,a networked distributed hierarchical coordination control strategy is presented in this thesis.The local control layer adopts droop control,virtual impedance control,proportional resonant(PR)voltage and current control.In the secondary control,the discrete consensus algorithm is used to design the observer so as to get global average values and restore the frequency and voltage amplitude deviations,and improve the accuracy of reactive power sharing.The simulation experiment platform is built based on Matlab/Simulink software and d SPACE controller.Simulation and experiment results indicate that the proposed algorithm is very robust with respect to time delays and has plug and play capability.The validity and feasibility of the proposed method has been further verified by experiment results.4)Research on networked hierarchical cooperative control strategy for voltage quality control in island microgrid.To solve the problem of the three-phase voltage unbalanced and harmonic at the point of common coupling(PCC),and the negative sequence current cannot be well shared because of the influence of line impedance in the island microgrid,a networked distributed hierarchical cooperative control strategy is used to achieve voltage compensation,which considering negative sequence current or harmonic current sharing control.This method is proposed by modeling and analysis of the unbalanced system.In the distributed secondary control level,real-time data exchange between the neighboring distributed generations using distributed sparse communication network.The consensus algorithm is used to evaluate the global average voltage and negative sequence current or harmonic current,and adaptively regulate the voltage reference value and voltage unbalance compensation reference vector of the droop control to improve voltage quality and negative sequence current or harmonic current sharing proportionally in DGs.The simulation and experiment results show that the proposed strategy can not only realize voltage unbalance and harmonic compensation in PCC,but also realize negative sequence current or harmonic current sharing control.5)Multi-objective reactive power optimization strategy for microgrid considering randomness.In the microgrid tertiary optimized control level,to solve the problem of the uncertainty and randomness of intermittent distributed generation active power output affect the reactive power optimize control,a scenario generate method that using Wasserstein distance metric and K-means cluster scenario reduction technique to generate optimal scenario is proposed in this thesis.So the stochastic optimization problem is transformed into a deterministic optimization problem.The multi-scenario tree models of wind-photovoltaic-load are established.A multi-objective reactive power optimization mathematical model of microgrid containing intermittent DGs is built,which includes objectives that are the total active power losses and the voltage deviations of the bus,and energy storage states of charge characteristic constraints.Also,the modified artificial bee colony algorithm(MABC)is used to solve the optimization problems.The simulation results show that the optimal scenarios based on the Wasserstein distance indicators and K-means cluster technology simulate the randomness of distributed generation active power output more exactly.Finally,the simulation analysis of IEEE33-bus distribution test system is carried out to verify the effectiveness and feasibility of the proposed multi-objective optimization strategy.