Dissertation Announcement for Sushil Silwal — 10/09/2019 at 1:00 PM
September 25, 2019
Dear Faculty, Graduate and Undergraduate Students,
You are cordially invited to my Ph.D. dissertation defense.
Dissertation Title: Enhanced controllers for voltage-sourced converters interfaced with weak ac power grids
Date and time: Wednesday, October 09, 2019, 01:00 pm
Venue: Simrall-228 (Conference Room)
Candidate: Sushil Silwal
Degree: Ph.D., Electrical and Computer Engineering
Committee:
Dr . Masoud Karimi-Ghartemani
Associate Professor of Electrical and Computer Engineering
(Major Professor)
Dr. Randolph Follett
Associate Professor of Electrical and Computer Engineering
(Committee Member)
Dr. Yong Fu
Associate Professor of Electrical and Computer Engineering
(Committee Member)
Dr. Chanyeop Park
Assistant Professor of Electrical and Computer Engineering
(Committee Member)
Abstract: “Distributed energy resource (DER) technologies such as wind and solar are interfaced with the ac grid through power electronic converters, e.g. voltage-sourced converters (VSCs). They must comply with the standards set by the utilities related to power quality, islanding detection, and operation under grid faults. Weak grid conditions present challenges to satisfy the desired operation of such interfaces. An ac grid is considered weak (from the standpoint of a given DER) if its voltage and frequency at the point of DER coupling experience fluctuations due to interactions with the DER. It is often the result of high integration of distributed and renewable energy systems to different locations of the distribution lines combined with high impedance lines. A micro-grid which includes multiple small generation and consuming units can be an example of a weak grid.
Most of the renewable resources are remotely located and thus they are often interfaced at low or medium voltage levels. Therefore, they can cause a weak-grid situation. A power converter designed to operate in normal grid conditions may not perform satisfactorily to meet the requirements during weak and distorted grid conditions. Hence, considering the full dynamics of the system during the weak-grid conditions in the design of converter control is indispensable to ensure the stability of the grid. For instance, the phase-locked loop (PLL) has been identified as a critical component of the VSC controllers that can compromise the DER performance during weak-grid conditions and it has to be fully integrated in the controller.
This dissertation investigates and enhances the performances of inverters connected to weak and polluted grids. It will primarily present a novel approach of enhancing the inverter current controller by including the PLL state variables among the entire system state and use them to optimally generate the control input for the VSC in the vector current control scheme. This reduces the loop interactions between the PLL and other control loops resulting in mitigation of the oscillations that could cause system instabilities. The procedure is done using the recently developed linear time invariant (LTI) model of the enhanced PLL (EPLL) for single-phase applications. To carry the task for three-phase applications, an alternative model for the three-phase PLL is also developed and used in this dissertation.
Extensive simulation and experimental results are presented to evaluate and validate the performances of the proposed control approaches in various operating conditions. Full practical models of all system components are considered for simulation studies. The experimental tests are carried on a practical inverter connected to the utility grid. Significant improvement of the inverter performance in weak-grid conditions is confirmed. The dissertation offers a systematic way of integrating and designing the PLL and controller in a VSC to achieve a robust performance.”
Thank you,
Sushil Silwal