Clean and renewable energy resources such as wind power and photovoltaics, along with advanced environmentally-friendly loads like electric vehicles, will be significant components of future grids. These resources are rapidly increasing, but they have an inherent tendency to degrade system performance due to their intermittent generation, complicated system dynamics, and distributed nature with an increasing penetration level. This research focuses mainly on studying and analyzing voltage and frequency dynamics in grids with these resources contributing to frequency and/or voltage stabilization. The following presents the highlights of the contributions of the research.
The use of wind generators for frequency regulation is becoming an essential objective in power grids. However, the conventional control applied to wind turbines and their generators does not allow them to participate in frequency regulation. A major part of this study is focused on forcing wind generation to mimic conventional generators in frequency regulation, with minimal possible modifications in their structure and controller being applied. The system stability and sensitivity of the proposed control were measured, and various implementation methods were compared using the linear control systems theory. The proposed solutions consider practical system constraints, including generator fatigue. Moreover, wind generators will be required to remain connected to power systems during grid faults in high penetration of this source. In this work, the behavior of one of the most common types of wind generators is studied and enhanced during faults.
In some offshore wind plants, connecting power-electronic-interfaced generation to long transmission lines with very high-impedances can be challenging. A conventional voltage source converter (VSC) is incapable of injecting its maximum theoretical active power in such grids. Benefiting from a comprehensive analytical model, a detailed analysis of the VSC dynamics is presented and showed how the assumptions which are usually made for designing VSC regulators in conventional grids are not valid in grids with high impedances. Two solutions which enabled the VSC to operate at its maximum theoretical active power by making minimal modifications in the widely accepted control method are proposed and compared.
The studies on asymmetrical distributed single-phase sources and loads such as photovoltaics and electric vehicles show that a central controller may result in delays and that a distributed control may lead to asymmetry. Both of these controllers may destabilize the system. In the presence of such loads/generations, the classic assumption of the decoupling between frequency and voltage stability is not valid. In this research, both linear and nonlinear control system theories were used and practical solutions suggested. This study shows the disadvantages of independent utilization of wind generators and electric vehicles for frequency regulation and come up with a coordinated control method in which these two sources compensate for each other weakness. Linear control theory has provided proper tools to consider the practical constraints of each resource and assure the effectiveness of the method.
In summary, this thesis shows that neglecting the impact of emerging power system components on system stability will lead to large and complicated problems in the near future. The research work then evolved to develop new mechanisms to mitigate the adverse impacts of the new components on system stability, thereby contributing to the creation of a sustainable future.