Optimal voltage control processes in active distribution networks

Konstantina Christakou, Jean-Yves Le Boudec, Mario Paolone, Dan-Cristian Tomozei
The Institution of Engineering and Technology - IET, 2015
Chapitre de livre

Résumé

Typical optimal controls of power systems, such as scheduling of generators, voltage control, losses reduction, have been so far commonly investigated in the domain of high-voltage transmission networks. However, during the past years, the increased connection of distributed energy resources (DERs) in power distribution systems results in frequent violations of operational constraints in these networks and has raised the importance of developing optimal control strategies specifically applied to these systems. In particular, two of the most important control functionalities that have not yet been deployed in active distribution networks (ADNs) are voltage control and lines congestion management. Usually, this category of problems has been treated in the literature by means of linear approaches applied to the dependency between voltages and power flows as a function of the power injections. On the one hand, recent progress in information and communication technologies, the introduction of new advanced metering devices such as phasor measurement units and the development of real-time state estimation algorithms present new opportunities and will, eventually, enable the deployment of processes for optimal voltage control and lines congestion management in distribution networks. On the other hand, ADNs exhibit specific peculiarities that render the design of such controls compelling. In particular, it is worth noting that the solution of optimal problems becomes of interest only if it meets the stringent time constraints required by real-time controls and imposed by the stochasticity of DERs, in particular photovoltaic units (PVs), largely present in these networks. Moreover, control schemes are meaningful for implementation in real-time controllers only when convergence to an optimal solution is guaranteed. Finally, control processes for ADNs need to take into account the inherent multi-phase and unbalanced nature of these networks, as well as the non-negligible R/X ratio of longitudinal parameters of the medium and low-voltage lines, together with the influence of transverse capacitances. Taking into consideration the aforementioned requirements, the distribution management systems (DMSs) need to be updated accordingly in order to incorporate optimization processes for the scheduling of the DERs. This chapter starts with a general description of a centralized DMS architecture that includes voltage control and lines congestion management functionalities. Then, the formulation of the corresponding optimal control problems is described, based on a linearized approach linking control variables, e.g., power injections, transformers tap positions, and controlled quantities, e.g., voltages, current flows, by means of sensitivity coefficients. Computation processes for these sensitivity coefficients are presented in Sections 8.2 and 8.3. Finally, in Section 8.4, we provide case studies of optimal voltage control and lines congestion management targeting IEEE distribution reference networks suitably modified to integrate distributed generation.

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Konstantina Christakou, Jean-Yves Le Boudec, Mario Paolone, Dan-Cristian Tomozei
The Institution of Engineering and Technology - IET, 2015
Chapitre de livre

Résumé

Official source

https://infoscience.epfl.ch/record/220195?ln=fr

À propos de ce résultat

Concepts associés (35)

Concepts associés

Chargement

Publications associées (53)

Publications associées

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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.

EPFL2017

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The optimal power-flow problem (OPF) has always played a key role in the planning and operation of power systems. Due to the non-linear nature of the AC power-flow equations, the OPF problem is known to be non-convex, therefore hard to solve. During the last few years several methods for solving the OPF have been proposed. The majority of them rely on approximations, often applied to the network model, aiming at making OPF convex and yielding inexact solutions. Others, kept the non-convex nature of the OPF with consequent increase of the computational complexity, inadequateness for real time control applications and sub-optimality of the identified solution. Recently, Farivar and Low proposed a method that is claimed to be exact for the case of radial distribution systems under specific assumptions, despite no apparent approximations. In our work, we show that it is, in fact, not exact. On one hand, there is a misinterpretation of the physical network model related to the ampacity constraint of the lines’ current flows. On the other hand, the proof of the exactness of the proposed relaxation requires unrealistic assumptions and, in particular, (i) full controllability of loads and generation in the network and (ii) no upper-bound on the controllable loads. We also show that the extension of this approach to account for exact line models might provide physically infeasible solutions. In addition to the aforementioned convexification method, recently several contributions have proposed OPF algorithms that rely on the use of the alternating direction method of multipliers (ADMM). However, as we show in this work, there are cases for which the ADMM-based solution of the non-relaxed OPF problem fails to converge. To overcome the aforementioned limitations, we propose a specific algorithm for the solution of a non-approximated, non-convex OPF problem in radial distribution systems. In view of the complexity of the contribution, this work is divided in two parts. In this first part, we specifically discuss the limitations of both BFM and ADMM to solve the OPF problem.

Elsevier2017

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The problem of optimal control of power distribution systems is becoming increasingly compelling due to the progressive penetration of distributed energy resources in this specific layer of the electrical infrastructure. Distribution systems are, indeed, experiencing significant changes in terms of operation philosophies that are often based on optimal control strategies relying on the computation of linearized dependencies between controlled (e.g. voltages) and control variables (e.g. power injections). As the implementation of these strategies in real-time controllers imposes stringent time constraints, the derivation of analytical dependency between controlled and control variables becomes a non-trivial task to be solved. With reference to optimal voltage and power flow controls, this paper aims at providing an analytical derivation of node voltage and line current flows as a function of the nodal power injections. Compared to other approaches presented in the literature, the one proposed here is based on the use of [Y] compound matrix of a generic multi-phase unbalanced network. In order to estimate the computational benefits of the proposed approach, the relevant improvements are also quantified versus traditional methods. The validation of the proposed method is carried out by using both IEEE 13 and 34 node test feeders. The paper finally shows the use of the proposed method for the problem of optimal voltage control applied to the IEEE 34 node test feeder.

2012

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Elsevier2017

EPFL2017