Performance Optimization Control Strategies Of Islanding Mode Microgrid

ABSTRACT:Microgrid is a small electric power system with distributed generations, load, energy storage, monitoring and protecting devices. It can operate in both grid-connected and islanded mode, which can enhance the reliability of electrical energy supply. Distributed genetations are always connected to grid through power electronic devices. The power electronic devieces have bad overload ability and small inertia character, which pose difficulty to the control of microgrid. The flexible and controllable charcter of power electronics devices provide convenience to micro-grid power quality improving. Based on the characteristics of power electronic devices, this thesis concentrates on the stability enhancement and power quality improvement of microgrid islanded mode. The main jobs of the thesis are as the following.Firstly, small signal stability analysis of islanding mode microgrid is investigated. The small signal modeling and stability judging method is discribed based on Lyapunov first method. State equation of single droop-control supporting micro source (DSMS) is derived from its control character, and the small signal model is also caculated. The small signal modeling method of microgrid with multiple DSMS and feeding micro source (FMS) is proposed based on the particular system example. The droop parameters and load changing eigenvalue are calculated. Key factors related to the example system stability are given, and the theoretical results are verified by time domain simulations.Secondly, the effect of virtual impedance on DSMS control performance has been discussed. The analysis of relationship between different line impedance and DSMS power decoupling performance is carried out. Key parameters caused DSMS power coupling are determined. Virtual impedance method for power decoupling-is proposed. DSMS output power is calculated, and the mechanism of power decoupling is also described. Virtual impedance selection principle based on different power coupling level is determined. Based on virtual impedance implementation method and linearization model, the effect of virtual impedance on DSMS output voltage and stability is analyzed.Thirdly, the power quality improvement control strategy of multiple DSMS and FMS networking system are investigated. The equivalent circuit of FMS system is calculated, and key parameters that influence the FMS system resonance pointequivalent impedance are determined. The achievement method of active damping control of FMS system is given. The expression of harmonic output voltage in the DSMS system is calculated, and the influence of DSMS output impedance to harmonic output voltage is confirmed. Then harmonic reduction control method of DSMS output voltage is presented. The fundamental negative sequence of output voltage relationship between DSMS and point of common coupling (PCC) is calculated, and the fundamental negative sequence of output voltage difference between-DSMS and PCC is investigated. Based on the secondary control, the control strategy of unbalanced voltage reduction on PCC is proposed, and the influence of secondary control delay on system stability is analyzed.Fourthly, the load sharingcontrol strategy of multiple DSMS microgrid system is investigated. The equivalent circuit of multiple DSMS microgrid system with centralized load is calculated, and the load sharing principles of DSMS with linear unbalance load and nonlinear load are analyzed. Then the effect on line impedance to the load sharing performance of DSMS is clarified. The principle on the control of virtual negative sequence admittance to achieve reasonable load sharing is analyzed, and then control strategy to improve load sharing performance of multiple DSMS microgrid system is presented. The expressions of fundamental negative sequence and harmonic output voltage are calculated, and the influence of DSMS load sharing control strategy on fundamental negative sequence and harmonic output voltage is also analyzed.