Coordinated Control Strategy Of Micro-grid Based On Consensus Algorithm In Island Model

For distributed power generation techniques,favorable control strategies are required when numerous distributed power sources of different kinds in micro-grid operate in parallel.Micro-grid,as an autonomous system,should be provided with the ability of working independently without main grids.Although droop control technology could implement peer-to-peer control in microgrids with distributed method currently,the higher requirements of frequency and voltage could not be sustained.To solve the defect problem of frequency offset and reactive power expectations in traditional droop control,this thesis proposed a new cooperative control strategy in microgrids to improve and utilize consensus algorithm in droop control,which enables secondary regulation of frequency and power expectations distribution.Firstly,this thesis considered the basic theory of consensus algorithm and droop control with the double-layer structure of information layer and power layer,along with analysis of the limitations of microgrids applying traditional droop control in island mode.In conclusion,implementing control of output active power and reactive power at the cost of frequency could not achieve the expected distribution,which leads to the problem of frequency offset and reactive power expectations in mirco-grid.Secondly,there is a problem in the regulation process of active power-frequency in droop control which is not conductive to secondary regulation of frequency: the frequencies of all nodes tend to be the same in steady-state but different in transient-state.In order to implement microgrids secondary regulation of frequency in island mode,this thesis set a leader node and improved the consensus algorithm by making the data of other nodes tend to be the same with the leader node.When the improved consensus algorithm is applied in the droop P-f control,the leader node would measure the difference between itself and standard frequency and use it as frequency adjustment value.As the frequency adjustment value of other nodes will tend to be the same with the leader node,all nodes would use the same frequency adjustment value to compensate the output frequency,which makes the output frequencies of all nodes restore and maintain at standard frequency and P-f curve move evenly.Then it comes to the problem that different equivalent output impedance of distributed power sources lead to different local output voltage in reactive power regulation process of droop control,which results in the reactive power cannot reach expected distribution.This thesis further improved the consensus algorithm using the topology of undirected connected graph.The improved algorithm could calculate the status value of all nodes(i.e.the average value of all nodes)in real-time,then come up with the concept of virtual voltage in droop Q-U control,which is the product of reactive droop coefficient and reactive power).Every node calculates its own virtual voltage,then acquire the global uniform virtual voltage average in real-time by the virtual voltage of adjacent nodes.By tracking the global virtual voltage average,the local virtual voltage of all nodes tend to be the same,so that we could ensure reactive power reach expected distribution by setting different reactive droop coefficient.Finally,this thesis build a system simulation model based on Matlab and Simulink,simulated the actual operation under island mode,the analysis of simulation result is as below:(1)Translating the droop control P-f curve of all nodes could achieve frequency restoration,the consensus control strategy in information layer could reduce communication expense,and the frequency compensation in power layer could achieve secondary regulation of frequency,ensuring the mirco-grid system working under standard frequency.(2)By tracking from nodes in power layer to the global virtual voltage average and the improved consensus strategy and droop control strategy,nodes in power layer only need virtual voltages of adjacent nodes to achieve the uniform virtual voltage average.Then setting different droop coefficient could ensure output reactive power reaching expected distribution,ignoring the affection of line impedance.(3)This thesis achieves reasonable distribution of power,secondary regulation of frequency and peer-to-peer control under the disturbance of load switching and power failure by building a 6-nodes 2-layered microgrids simulation model,realizing real “plug and play”...