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Research On Improved Droop Control Strategy Based On Virtual Impedance

Posted on:2019-02-10Degree:MasterType:Thesis
Country:ChinaCandidate:H M WangFull Text:PDF
GTID:2382330548494364Subject:Electrical engineering
Abstract/Summary:PDF Full Text Request
In recent years,clean,pollution-free and renewable new energy has attracted more and more attention.Distributed generation(DG)is used for the development and utilization of renewable energy,but because of its randomness and non-continuity,the stability and safety of the system are degraded when distributed power is connected to the grid.Therefore,the concept of microgrid came into being.Microgrids can operate in isolated islands and in parallel.The control of the microgrid is essentially the control of the distributed power interface inverters.Since the capacity of a single distributed power supply is not sufficient during the operation,multiple parallel operations are usually performed.Droop control is applicable in both islanding and grid-connected states,enabling effective control of inverters in parallell.Firstly,the basic principles and characteristics of traditional droop control are analyzed,the mathematical model of the three-phase voltage source inverter is established and the filter is designed rationally.Then analyze the power module,droop characteristic module and voltage and current double closed loop module in the microgrid droop controller.The conditions satisfied by the power distribution between distributed power sources are described.The principle of pre-synchronization control,phase-locked loop technology and analysis of grid-connected operation of inverter are analyzed.Then,adding a virtual impedance to the traditional droop control strategy and analyzing its principle,comparison of the output characteristics of the system before and after adding the virtual impedance in the island mode according to the transfer function and the Bode plot,analyze the problem of voltage drop after adding virtual impedance.Based on the virtual impedance,an improved droop control method with voltage feedback and frequency compensation is proposed.In the droop control method,the power reference value is changed to achieve stable operation after the grid connection.Finally,through simulation and verification,the proposed improved method can effectively improve the equalization effect of reactive power in island mode,greatly increase the amplitude and frequency of bus voltage,and improve the quality of power supply.Through the simulation of pre-synchronization,it is verified that the voltage amplitude,frequency and phase of the microgrid and the large power grid can be synchronized,and the inverter completes the island-to-grid switching;through the simulation of the grid-connected operation control strategy,it was verified that while periodically changing the power reference value at the common connection point(PCC),the output power of the inverter is changed,and the power flow fluctuation between the large grid and the microgrid bus is suppressed,the change of load power is compensated by the output power of the inverter,the output power of the inverter can be equally divided,the bus voltage remains unchanged and the system frequency is maintained at 50 Hz.
Keywords/Search Tags:microgrid, droop control, virtual Impedance, voltage feedback, frequency compensation
PDF Full Text Request
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