\chapter{Conclusions and Future Research Direction}

Dynamic modeling, stability studies through eigenvalue analysis and time domain simulations of series compensated power systems with weak interconnections have been conducted to analyze bifurcations and stability domains with respect to series compensation. The results of these studies have been presented for various network topologies and the following conclusions are presented in the following section.

\section{Conclusions}
\begin{enumerate}[label={[\arabic*]}]
    \item Systems with SCIG-WTs integrated with the grid through series compensated radial feeder lose stability through a Hopf bifurcation due to SSR. Stability domains with respect to series compensation become smaller as the interconnection grows weak due to feeders of high impedance. A saddle-focus equilibrium is possible at high wind speeds in systems where the generator terminal voltage is unregulated due to the usage of only fixed shunt capacitors for reactive power compensation.
    \item Systems with SCIG-WTs integrated to the grid by means of non-radial interconnections may have a bigger stability domain. This is due to the fact that in a non-radial interconnection, the X/R ratio of the network may smaller resulting in a higher stability domain with respect to series compensation. However, the critical series compensation level which determines the size of the stability domain depends on the parameters of the SCIG and the X/R ratio of the interconnection. Wind speed may not play a big role in determining the critical series compensation level at which SSR occurs.  
    \item A Static Var Compensator (SVC) used in the place of a fixed shunt capacitor to provide reactive power compensation is able to improve the stability domain of the radially connected system by a small margin. However, delays introduced due to the constants of measurement transducers used in the voltage regulator has an adverse effect on the stability domain. When the SVC is further equipped with a supplementary damping controller using generator slip or real power as it's input signals, the stability domain becomes bigger by a wide margin under various operating conditions.
    \item Induction motors interconnected to substations by series compensated feeders may experience instability through subcritical Hopf bifurcations due to SSR resulting in motor stalling and excessive voltage and current. This is exacerbated by higher feeder impedances resulting in a weaker system. Stability domains due to series compensation also decrease by a small margin under higher loading conditions in the motor. Systems with high X/R ratios may experience a reversal in the direction of rotation of the induction motors in addition to stalling causing excessive voltages and currents. 
    \item As the dynamics of synchronous generators connected to substations which supply induction motor loads are taken into account, the stability domain with respect to series compensation is affected by the generator voltage control system parameters in addition to the strength of the grid and the load on the motor. Systems with generators using AVRs of high gains have smaller stability domains. The system may be susceptible to reverse Hopf bifurcations when high gain AVRs are used. 
    \item Values of time constants of the AVR and measurement transducer determines if the system may become unstable due to a subcritical Hopf bifurcation due to SSR or a supercritical Hopf bifurcation due to the exciter mode destabilization. Stability domain of the relatively stronger system is larger if the upper limit of series compensation is determined by the level at which SSR occurs. However, the weaker system has a larger stability domain if the upper limit of series compensation is determined by the critical level at which the exciter mode becomes unstable due to a Hopf bifurcation. 
\end{enumerate}

\section{Future research direction}  

Stability domain analysis enhancement of weak power systems can be explored in the context of systems with multiple generators of the same type or different types. Utilization of energy storage systems to mitigate SSR is another area of research which can be an offshoot of this work. Using other signals in the system in the supplementary damping controller to eliminate SSR is another potential area of research. Identifying optimal locations of controllers to provide maximum improvement in system stability could be another extension of this research. With regards to the impact of dynamic loads on the stability of series compensated systems, modifying the already existing voltage controller and power system stabilizer design to achieve stability domain enhancement is a way to extend the work done in this dissertation.
