Optimal voltage control processes in active distribution networks

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.