| At present,many phenomena of DC power quality have not been clearly solved or studied.Due to the lack of actual data in DC distribution projects,the research on DC power quality standards is still in its infancy.The topological structure of AC-DC hybrid microgrid provides a good engineering background for the simultaneous study of AC and DC power quality.Based on the specific technological project of "Power quality analysis and control of AC-DC hybrid microgrid" of Zhejiang Electric Power Research Institute of State Grid,Zhejiang Electric Power Company,and certain physical demonstration project of AC-DC hybrid microgrid,this thesis focuses on the power quality,power quality control theory and technology,and stability of AC-DC hybrid microgrid through the combination of simulation analysis and on-site measurement of actual operation data of microgrid.The results of this study not only have referential significance for the construction of AC-DC hybrid micro-grid,but also for the formulation and improvement of power quality standards for DC distribution system under development.This thesis introduces the topology structure,key equipment,parameter selection method and operation mode strategy of the AC-DC hybrid microgrid,analyses the working mechanism and characteristics of the distributed power generators,and then establishes relevant mathematical models.A two-layer droop control based on virtual impedance is proposed based on the special topology of the microgrid studied.At the same time,aiming at the economic goal under the "grid-connected operation mode" and the reliability goal under the "off-grid operation mode",the control strategies of multiple distributed generators and converters in the micro-grid are established under the two operation modes respectively.The simulation model of microgrid is built in EMTP-RV to verify that when the load power changes suddenly and photovoltaic output power changes,the converter control units in the microgrid respond quickly,the bus voltage fluctuates slightly and recovers quickly,and restores to the rated value within a short time,which proves the good dynamic response characteristics of the proposed control strategy.Next,three DC power quality indicators:DC voltage deviation,DC voltage fluctuation and DC voltage harmonics,which are closely related to the safe and reliable operation of DC power grid and the normal operation of DC load,meanwhile can be measured and quantified by experiments,are proposed.Their definitions and quantitative calculation methods are given.Combined with the actual on-site measurement data,the AC and DC power quality is analyzed.It is pointed out that the odd harmonic distortion level in the system exceeds the standard and the compensation effect of active power filter is not satisfying enough.Then,a simulation model of shunt active power filter is built,whose parameter selection and design method are as well studied.A DC voltage control method based on fuzzy logic is proposed.Compared with the traditional PI controller,the speed of APF dynamic response,tracking performance and robustness of the system all outperform.By studying the harmonic compensation characteristics of shunt active power filter,it is concluded that the amplification effect of shunt APF on harmonic current of voltage-type non-linear load is the main reason for the unsatisfactory APF filtering effect in the studied AC-DC hybrid microgrid.In view of this problem,several solutions are put forward.According to the transient resonance instability of the microgrid recorded by the measured data,the specific expression of the output impedance of the energy storage battery is obtained by combining the measured data with theoretical derivation.Based on the Nyquist stability judgment,the specific measures to improve the stability of the system are given. |
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