Optimal control of three-phase embedded power grids

Abstract

Abstract-This paper presents an automated and scalable approach for the tuning of power converters control systems in embedded power grids. These are composed by different power converter interconnected to each other and are increasingly adopted in a range of applications among which micro-grids and more electric aircrafts. The interaction between the grid components may lead to instability, especially in presence of small passive filters. A structured state feedback optimal control approach is proposed to jointly design all power converters controllers in a coordinated way to maximize the performance of the grid and avoid instability due to converters interaction.

Decentralised Optimal Controller Design of Variable Frequency Three-Phase Power Electronic Networks Accounting for Sub-System Interactions

“…Thus, if V P cd = V cd and V P cq = V cq , then any error between these values can be attributed directly to θ e . Assuming the error between the frames remains small, one can therefore translate (12) and (13) in terms of angle error between the two dq frames, such that: (16) where the term ω 0 represents the nominal grid frequency, which for aircraft is typically set to 400Hz. This is only necessary during the implementation phase of the controller, and as such, can be dropped for the remainder of the analysis.…”

“…This is only necessary during the implementation phase of the controller, and as such, can be dropped for the remainder of the analysis. At this point, equations (15) and (16) can be linearised about the known steady-state equilibrium point of θ * e = 0, V * cq = 0, and V * cd set to the VSI V cd reference voltage, providing the final control equation in state-space form:…”

“…However, such optimization approaches are computationally heavy since they require online optimisation, and are thus not easily scalable to expanding systems. In light of such issues, a H 2 decentralized optimal controller was selected for this study since the H 2 controller has been popular offline optimization method for embedded grid applications in recent years in order to optimize individual converter controllers all whilst keeping in consideration dynamic interactions between sub-systems [16], [28].…”

“…Though good at actively damping against harmonic effects due to the filters [14], counteracting against sub-system interactions requires measurements of converters outside the local scope of each decentralised controller, thus requiring additional communication. Therefore, such controllers, in a fully decentralised form can only be optimal on the local level, and not at the global level as required for interaction mitigation [16]. Other papers investigated the use of H ∞ optimal control [17], [18], however, due to the conservativeness of these kinds of control methods, it can be very hard to develop block diagonal structured decentralised [19].…”

Hardware-in-the-Loop Methods for Real-Time Frequency-Response Measurements of on-Board Power Distribution Systems

“…Due to interactions among the converter feedback loops through dc bus interconnections, converters that are standalone stable may exhibit different dynamic behavior when interconnected and small-signal stability may be compromised [3]. Because of reduced grid size, the interactions between the converters are often strong, leading to undesirable operation and even to instability [4]. The major challenges of such systems include the power flow regulation, the voltage stability, and the varying dynamic characteristics of the grid [5].…”