Operational Control and Analysis of a Hybrid AC/DC Microgrid

Abstract

In light of the growing demand for electrical power around the globe, the need to increase electrical power generation in order to diminish total carbon emissions has led to the installation of renewable resources to replace traditional generators. Most of today’s microgrids are AC microgrids, whose advantages and shortcomings with respect to control techniques and stability assessment have been demonstrated through extensive studies reported in the literature. These considerations have led to the recent proposal and investigation of DC microgrids, accompanied by the introduction of the hybrid AC/DC microgrid as a means of combining the advantages and benefits of both types of microgrid. However, since a hybrid microgrid is viewed as a weak system with low inertia, controlling and assessing the performance of a hybrid microgrid constitutes a high-priority issue that requires further investigation. The lack of inertia of power electronics converters, especially in an islanded hybrid microgrid, poses a threat to stability and control. For these reasons, effective stability analysis has become a necessity with respect to the implementation of hybrid microgrids. Because of these challenges, the emulation of synchronous machine (SM) inertia and damping is now viewed as necessary for enhancing the effect of a VSC on an active distribution system and for facilitating its participation in voltage and frequency support. Improving the stability and performance of a hybrid microgrid therefore requires the introduction of a form of inertia into a hybrid microgrid. This research first proposes the incorporation of a novel form of virtual inertia into a hybrid microgrid using virtual synchronous machine (VSM) control of the intertying converter (IC) controller. The second proposal of this research is to employ the VSM control to establish autonomous control of the IC. A first research component, a novel control strategy for the Intertying converter in hybrid AC/DC microgrid has been proposed to ensure the benefit of a virtual synchronous machine (VSM) control algorithm in the hybrid AC/DC microgrid. The VSM controller application in hybrid AC/DC microgrid is capable to enable an IC converter to support the AC-side voltage and frequency as well as the DC-side voltage. The proposed control application of the VSM is chosen based on a comprehensive assessment of VSM control algorithms that are exist in the literature. Moreover, proposing an autonomous operation control of the VSM intertying converter based on dual droop characteristics which is quite different compared to using only current controller. The autonomous operation of the intertying converter based on dual droop control is modified and proposed to be capable to feed the VSM controller (swing equation) to ensure accurate power exchange management between the AC and DC sub-subsystems. The most important portion for the hybrid microgrid system is the stability study due to that fact that the behavior of the system when it is subjected to a temporary disturbance is the main concern. In hybrid microgrid, the disturbances take place continuously because of the load changing endlessly. Satisfying the hybrid microgrid operation during the disturbances conditions must be achieved in order to supply the demand. Therefore, the second part of the research introduces a generic small-signal state space model of the hybrid AC/DC microgrid system, and built to carry out the stability analysis. The development of the small-signal state-space model for the entire hybrid AC/DC microgrid was developed to investigate the overall system stability under different operating points. The final part of this thesis reveals three serious issues of operating hybrid AC/DC microgrid; some of these issues are temporary take a place based on the system operating conditions. In hybrid AC/DC microgrid, an Intertying converter (IC) becomes harmonics voltage source due to the antiparallel diodes and the shunt capacitor at its DC side. The nonlinearity behavior of ICs introduces another operation issue that is circulating current in case of parallel ICs. Reconnecting an IC after abnormal operation condition or schedule maintenance requires an extra challenging synchronization control due the variation of the AC subgrid voltages and frequency; which is the third issue. This part proposes a solution for all these issues by developing a new control strategy that combines the VSM control concept with a dual based droop control. The developed VSM controller on the IC solves these issues. The test system used in this research, which is simulated in a PSCAD/EMTDC environment, consisted of simulated voltage source converters with two AC voltage levels; while the stability analysis is conducted in MATLAB environment.