Systematic Control Design and Stability Analysis Approaches for Microgrid Inverters to Enhance Overall Performance and Efficiency
Creators: Amouzegar Ashtiani, Nima
Publisher: name: University of Alberta Library
Year: 2022

<p>This thesis addresses a systematic design and implementation of control systems of power converters in microgrids to guarantee system level stability and improve performance and efficiency. Due to the high penetration of power converters in power systems, proper operation of converters in different situations is crucial to maintain the entire system stable. Therefore, depending on the role of the power converter in the system, the control system should be designed properly to address control objectives. With respect to the control objectives and converter configurations, design of the control system faces various challenges. For a single-converter system, uncertainties and practical issues may affect the system performance and its stability. For a multiple-converter system, the interaction among power converters along with its impact on the system stability, and also the overall efficiency optimization introduce other challenges for the control system. In this research, the proposed systematic control system design for a single- and multiple-converter systems are investigated, simulated and experimentally verified as summarized below. </p>

<p>First, a robust control system for a single-phase photovoltaic (PV) converter with an LCL-filter under weak grid operation is proposed. The control system guarantees system robustness against weak grid uncertainties such as grid impedance variations and voltage harmonics. Moreover, the controller compensates the system delays. The control system minimizes the bus voltage fluctuations caused by variations of PV system power generation. Furthermore, power decoupling is achieved by the control system without any auxiliary circuit.</p>

<p>For converter control systems, a systematic and optimal control design approach is proposed for control structures which involve multiple cascade, or nested, loops. The proposed approach relaxes the limiting condition that the internal loops must be much faster than the external loops. Therefore, faster yet smoother responses can be achieved using the proposed design approach. As an example, the method is also used to systematically design a cascade control system, including voltage and current control loops, for a single-phase standalone inverter.</p>

<p>To further investigate the performance of the individual inverters in a microgrid, an approach is proposed for modeling and stability analysis of a single-phase standalone microgrid containing parallel inverters. Using the approach, interactions among parallel inverters and the impact of each parameter variations on the system are studied in detail. Based on this approach, a nonlinear microgrid can be linearized, and the microgrid behaviors can be assessed using several tools available for linear systems. As an example, the model and stability of a single-phase microgrid with two parallel inverters are provided. It is shown that the proposed model predicts the system responses more accurately than the existing modeling approaches, especially, for systems with a small stability margin. </p>

<p>Finally, to improve the overall efficiency of the investigated microgrid at light loads, a communicationless modified droop strategy is introduced. The main idea is to revise the power sharing among parallel inverters such that the output power of each inverter is maintained within a proper range with respect to the inverter efficiency curve and the load demand instead of the proportional power sharing, realized through the conventional droop control strategy. Moreover, to evaluate the performance of the proposed method, a system with parallel inverters is simulated and experimentally tested. It is observed that the proposed strategy can improve the overall system efficiency by up to 14% at light loads.</p>