Research

We have developed several phase-locked loop (PLL) structures that form one main aspect of our achievements. Our enhanced PLL (ePLL) resolved the historical problem with the PLL which was the generation of double-frequency errors. Its extension to three-phase systems resolved a major problem concerning the analysis, control, and protection of power systems, i.e., the errors caused by system unbalance. Major applications include grid synchronization and control for almost all sorts of electronically-coupled equipment such as renewable energy sources, rectifiers, custom power controllers, protection relays, and flexible AC transmission systems. We have published a monograph on the ePLL which has been widely recognized by many students, researchers, and industries.

     Two of our ePLL modules have been integrated into the PSIM software of Powersim Technologies which is a leading company for developing power system and power electronics simulation software platforms (this company was recently acquired by Altair). PSCAD power system software has also recently incorporated our ePLLs within its library. Canadian EMTP power system software company uses one graphical version of ePLL. Swiss Plexim power electronics software company has included one single-phase version of ePLL in PLECS software. The ePLLs are adopted by some utility companies as well, such as Statnett, a major power system operator in Norway, in their AUTODIG (AUTOmated DIAGnosis tool) used in protection relays for fault analysis.

     We have for the first time developed quasi-linear models for the PLLs. These models have played a significant role in 1) the analysis and design of PLL parameters, 2) paving the way for introducing further extensions to the PLLs and removing their limitations, and 3) enabling seamless and full integration of the PLL in the converter control systems and addressing the performance deteriorations due to weak grid conditions. 

Another main component of our research achievements is a new family of enhanced grid-forming (eGFM) controllers for inverter applications.  The idea of eGFM was neatly derived from our past research on ePLL. The GFM inverters have the potential to build an autonomous power grid without relying on conventional synchronous generators. Thus, they are promising options for future power grids with a high level of inverter-based resources. Our eGFM controllers enjoy 1) a neat yet strong damping approach, 2) natural inertial extraction from DC capacitors, 3) compact structures with minimal dynamics, and 4) closed-form quasi-linear mathematical models inspired by the ePLL models. Particularly, we have shown the possibility of deriving quasi-linear GFM controllers using these models which greatly facilitate analysis, stability, and design aspects.

     Several aspects of GFM inverters must be fully investigated before their readiness for large-scale industry adoption. These are the areas identified by the IEEE, DOE, and other industry and grant agencies as the pressing research challenges towards achieving the high-level integration of DERs and addressing the energy problems of the 21st century. We are investing in eGFM approaches and believe that they can play a crucial role in this domain. 

Another component of our research results is the development of a new set of controller architectures and design algorithms which are particularly advantageous for power electronic converters and energy applications. These controllers are informed by the famous linear quadratic regulator (LQR) of optimal control theory. They, however, neatly improve the LQR by addressing the command following and disturbance rejection properties as well. We have coined them as linear quadratic tracker (LQT) solutions. We have already applied the LQT to several converter applications successfully. Indeed, all the simulation examples in my recently published textbook (which includes over 50 professionally built real-world simulation examples) are based on utilizing the LQT approach.

     The LQT approach enjoys 1) a linear full-state feedback control with systematic design, 2) guaranteed tracking and disturbance rejection, 3) fast yet smooth dynamic responses, and 4) good robustness properties. We have received an EAGER funding from NSF to fully develop this method in the context of grid-following and grid-forming grid-connected inverter applications.