Dynamic Transitional Droops for Seamless Line-Switching in Islanded Microgrids

Abstract

Line-switching operations introduce several concerns to microgrid operators since they trigger reliability and power quality issues in addition to increasing the wear and tear cost of switching devices. This paper proposes a novel methodology for achieving seamless line-switching in low-inertia islanded microgrids. The proposed methodology aims at mitigating the detrimental line-switching impacts by minimizing the power flow in the switched lines at the moments of executing the switching operations. The minimization of the power flow is realized by optimizing the droop control parameters of the DGs during the transition from one network topology to another. Besides, during executing a series of line-switching operations, the sequence of switching is also optimized to minimize the total switched power. While optimizing the line-switching execution sequence, the transitional droops are optimized for each line-switching operation within that sequence. A dynamic mixed-integer nonlinear programming problem is formulated to find both the optimized dynamic transitional droops and line-switching sequence simultaneously. Simulation results demonstrate the effectiveness of the proposed seamless line-switching methodology where the switched power can be reduced by more than 80% for a single line-switching operation and 54% considering a series of four switch exchanges in addition to achieving several instances of zero-current switching. As a result, a significant decrease in the peak of voltage transients has been recorded for some cases.

Robust Secondary Controller for Islanded Microgrids with Unexpected Electrical Partitions Under Fault Conditions

“…Du et al [12] introduced a distributed secondary controller for networks MGs employing also smart switches, aiming at reducing the transient voltages during MGs reconfiguration. Similar approaches for achieving seamless reconfiguration of MGs were also proposed in [13] and [14], using single-and three-phase mathematical formulations, respectively. Despite their sophisticated contribution in several applications, none of the controllers in [7]- [14] handle the frequency problems occurring at the SMGs after unexpected network partitions, in the events of faults.…”

Bumpless Topology Transition

“…For microgrids, similar topology transition problems exist to facilitate network reconfiguration. El-Sayed et al [10] developed a nonconvex mixed-integer nonlinear program (MINLP) which reduces the negative impacts of topology transition on transient voltage by simultaneous optimization of the lineswitching sequence and droops of distributed generators. The power flow through the switched lines was used as an indicator of the peak of voltage transients, yielding a tractable objective function.…”

“…The power flow through the switched lines was used as an indicator of the peak of voltage transients, yielding a tractable objective function. Following the paradigm in [10], a sensitivity-based method was developed in [11] to improve the computational efficiency. More system details such as three-phase unbalance were considered while the line-switching sequence was fixed.…”

“…(1) The static and dynamic factors associated with topology transition should be both considered. Firstly, a comprehensive evaluation of transition processes require the metrics capturing both static and transient performance, rather than those with only one of them considered [6], [10]. Secondly, as a basic requirement, the transition methodology should contain mechanisms to ensure stability of the entire transition process, which however, is neglected in existing works.…”

“…The topology transition problem where lineswitching sequence is optimized essentially features a nonconvex MINLP. Unlike for small-sized microgrids in [10], such a program is computationally intractable for real transmission networks and thus requires particular study. Moreover, the transient components in performance metrics potentially further complicate the solution method.…”