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Publication# Data-driven Power Electronic Converter Control Design in Power System Applications

Résumé

In recent years, power systems have evolved in physical and cyber-physical layers. In the physical layer, the changes are motivated by environmental concerns resulting in the integration of new types of generation/demand/storage into the grid. These integrations offer the opportunity of forming self-sufficient microgrids. Based on the hierarchical microgrid control structure, it should provide a high-performance low-level control while damping the high-frequency oscillations. Moreover, it should share power among DGs at the primary level and restore voltage/frequency at the secondary level. However, achieving these objectives is challenging because of factors such as lack of inertia, system complexity, different line characteristics, high-frequency modes of switching converters and their filters, delay, and incomplete communication graph. With the advent of smart grid technologies, the cyber-physical layer of the grid has transformed, and more measurement data are available. These data are mostly used for monitoring, metering, and protection. This thesis tries to bridge the gap between measurement data and high-performance control design for microgrids to address their operational challenges. The scope of this thesis covers up to the secondary control level. At the lowest level, the passivity theory has been used to tackle the problem of high-frequency oscillation between converters in a decentralized way. To this end, conditions of data-driven robust passivity for a performance channel are proposed. This method is used to make the input admittance of an AC grid-connected converter passive while the tracking performance is optimized. To validate the performance of designed controllers with high fidelity, an experimental Hardware-in-the-Loop (HIL) setup is developed and extended to a Power-HIL (PHIL) setup. However, PHIL tests have a performance limitation due to their stabilization. A data-driven method is proposed to optimize their performance while robust stability is guaranteed. This PHIL setup is employed for the validation of the passivity-based converter control. In addition, inspired by railway system standards, conditions for data-driven partial positive realness over an arbitrary frequency set are developed and used for the traction converter control design. Moreover, the primary/secondary microgrid control problem is formulated as a comprehensive data-driven multivariable synthesis problem. In this method, active power-sharing and frequency/voltage restoration are optimized while the closed-loop stability with a predefined margin is guaranteed. Due to the fixed structure, the controller can be designed based on the available communication while considering its delay. Since there is no need for time-scale separation between primary and secondary, this method leads to better performance. Moreover, because of data-driven property and multivariable structure, there is no need for decoupling or any assumption on the grid impedance. This design is extended to reactive power sharing using the spare capacity of PhotoVoltaics (PVs). In addition, a data-driven Linear Parameter Varying (LPV) multivariable synthesis method is proposed and used for the microgrid control to extend the applicability in different operating points. All the proposed methods are in a data-driven framework, which is suitable for complex power system applications. As well, the design problems are in convex programming form, which can be solved efficiently.

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Design

vignette|Chaise de Charles Rennie Mackintosh, 1897.
Le design, le stylisme ou la stylique est une activité de création souvent à vocation industrielle ou commerciale, pouvant s’orient

Structure

A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buil

Robustesse (ingénierie)

En ingénierie, la robustesse d'un système se définit comme la « stabilité de sa performance ».
On distingue trois types de systèmes :

- les systèmes non-performants, qui ne remplissent pas les fonct

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An active distribution network (ADN) is an electrical-power distribution network that implements a real-time monitoring and control of the electrical resources and the grid. Effective monitoring and control is realised by deploying a large number of sensing and actuating devices and a communication network to facilitates the two-way transfer of data. The reliance of ADN operations on a large number of electronic devices and on communication networks poses a challenge in protecting the system against cyber-attacks. Identifying these challenges and commissioning appropriate solutions is of utmost importance to realize the full potential of a smart grid that seamlessly integrates distributed generation, such as renewable energy sources. As a first step, we perform a thorough threat analysis of a typical ADN. We identify potential threats against field devices, the communication infrastructure and servers at control centers. We also propose a check-list of security solutions and best practices that guarantee a distribution network's resilient operation in the presence of malicious attackers, natural disasters, and other unintended failures that could potentially lead to islanded communication zone. For the next step, we investigate the security of MPLS-TP, a technology that is mainly used for long-distance inter-domain communication in smart grid. We find that an MPLS-TP implementation in Cisco IOS has serious security vulnerabilities in two of its protocols, BFD and PSC. These two protocols control protection-switching features in MPLS-TP. In our test-bed, we demonstrate that an attacker who has physical access to the network can exploit the vulnerabilities in order to inject forged BFD or PSC messages that affect the network's availability. Third, we consider multicast source authentication for synchrophasor data communication in grid monitoring systems (GMS). Ensuring source authentication without violating the stringent real-time requirement of GMS is challenging. Through an extensive review of existing schemes, we identified a set of schemes that satisfy some desirable requirements for GMS. The identified schemes are ECDSA, TV-HORS and Incomplete- key-set. We experimentally compared these schemes using computation, communication and key management overheads as performance metrics. A tweak in ECDSA's implementation to make it use pre-generated tokens to generate signatures significantly improves the computation overhead of ECDSA, making it the preferred scheme for GMS. This finding is contrary to the generally accepted view that asymmetric cryptography is inapplicable for real-time systems. Finally, we studied a planning problem that arises when a utility wants to roll out a software patch that requires rebooting to all PMUs while maintaining system observability. The problem we address is how to find a partitioning of the set of the deployed PMUs into as few subsets as possible such that all the PMUs in one subset can be patched in one round while all the PMUs in the other subsets provide full observability. We show that the problem is NP-complete in the general case and and formulated it as binary integer linear programming (BILP) problem. We have also provided an heuristic algorithm to find an approximate solution. Furthermore, we have identified a special case of the problem where the grid is a tree and provided a polynomial-time algorithm that finds an optimal patching plan that requires only two rounds to patch the PMUs.

The scope of this thesis encompasses two main subjects: fixed-structure data-driven control design on one side, and control design in power systems on the other. The overall goal is to identify challenging and relevant problems in power systems, to express them as rigorous specifications from the viewpoint of control systems, and to solve them by developing and applying advanced methods in robust control. This work aims to combine expertise from both fields to open up a holistic perspective and bridge the gap between control and power systems.
First, the derivation of a novel fixed-structure, data-driven frequency-domain control design method for multivariable systems is described. A key feature of the method is that only the frequency response of the plant is required for the design, and no parametric model is required. The designed controllers are fully parametrized in terms of matrix polynomial functions and can take a centralized, decentralized or distributed structure. The controller performance is formulated as H_2 and H_infinity constraints on any loop transfer function. A convex formulation of the optimization problem is derived, and it is shown that the solution converges to a locally optimal solution of the original problem. The versatility of the design method is demonstrated in various simulation examples, as well as in experiments on two electromechanical setups.
Next, a frequency-domain modeling approach for power grids is discussed. A model based on dynamic phasors is developed that represents the electromagnetic and electromechanic dynamics of lines, inverters, synchronous machines and constant power loads. It also offers a modular structure that makes it straightforward to combine white-, grey- and blackbox models in a single framework.
Then, the control design method and dynamic phasor model are applied in two relevant power systems case studies. First, the design of a decentralized current controller for parallel, grid-connected voltage source inverters in a typical distribution grid is considered. It is shown how performance specifications can be formulated as frequency-domain constraints in order to attenuate the resonances introduced by the output filters and coupling effects, and to provide robustness against model uncertainties and grid topology changes. The controllers for all VSIs are designed in a single step, and stability and performance is guaranteed by design. Furthermore, an approach for plug-and-play control design is presented. The results are validated in numerical simulation as well as in power-hardware-in-the-loop experiments.
The second study concerns the design of a distributed controller that combines primary and secondary frequency and voltage control for an islanded, meshed low-voltage grid with any number of voltage source inverters and synchronous generators in a single framework. No assumption on the R/X-ratio of the lines is made, and it is shown how advanced control specifications such as proportional active power sharing, zero frequency steady-state error and decoupling can be formulated as constraints on the norm of weighted sensitivity functions. Furthermore, the communication delays of the distributed controller are considered exactly during the design. The controller is implemented in numerical simulation, and results show significantly improved performance as compared to the classical hierarchical structure.

Alireza Karimi, Seyed Sohail Madani

In this paper, reactive power sharing for Photovoltaic (PV) units in islanded microgrids has been formulated as a robust control design problem and is solved using convex optimization method. In addition to reactive power sharing, the disturbance rejection for voltage and active power have been formulated using infinity-norm constraints on the sensitivity functions and considered in the design. The proposed method uses only the measurement data of the power system with no need for a parametric model of the power grid equipment. The size of the problem is independent of the order of the plant which makes it applicable to power systems including a high number of buses and equipment such as synchronous generators, batteries and inverters. In the proposed method, the communication system can be considered in the control design process for centralized, distributed and decentralized structures. The proposed method has been validated through simulation of a microgrid encompassing synchronous generator, switching inverters and storage system. The results show that this method has successfully shared reactive power among different PV units while providing disturbance rejection for voltage and active power.