PlaNets: scheduling activities at Power Utilities

The Problem

The exploitation of a network for electricity distribution necessarily implies the planning and scheduling of activities to be carried out on it, within a certain time span. These activities basically fall into 4 categories:

• preventive maintenance
• modifications of the nominal topology
• reparation of malfunctions
• re-establishment of the nominal situation

The planning of these activities is not trivial: e.g, the area of the network involved needs to be isolated from the rest of the distribution network and connected to the ground. To continue supplying energy to other installations and consumers in the area, it is necessary to select and re-configure one of the alternative supply routings through the network, through opening and/or closing switches, without overloading any branch. In addition, various maintenance tasks would be scheduled, and certain switching operations might be shared among tasks. In some cases two tasks can never be carried out simultaneously if there are no viable network reconfiguration. Finally, activities require various resources of limited availability (manpower, vehicles, etc.), and are subject to due dates while task priorities must be respected.

PlaNets - the scheduler, a functional description

Given a collection of maintenance activities to be carried out within the weekly time span, characterized by their network location, due date and resources consumption, given the network topology and the resource availability, PlaNets finds an optimal schedule with respect to the operation cost of realizing the necessary reconfigurations in every time slot.

The system now handles networks of up to 4000 nodes and 700 operable switches, and incorporates a network compression module. The scheduler takes about 3 minutes on a SuperSparc20 workstation.

In addition to gantt charts and resource views, PlaNets also offers graphical interfaces to manually operate the switches to reconfigure the network and see resulting load flows, to manipulate problem constraints using known consumer load profiles.

PlaNets - the technology

The PlaNets scheduler is entirely written in CHIP V4. Extensive use has been made of the CHIP++ object system and the XGIP X11 graphics system.

A complete problem of the before mentioned size, is represented by about 22000 domain variables. They are temporal (start dates), electric (current intensities) or topological (switch states). Due to the propagation characteristics of the resulting constraint network, the scheduling and reconfiguration subproblems can be solved efficiently. Optimization is done by a branch-and-bound process. A reconfiguration algorithm in a C library provided by ENHER S.A., for load balancing is incorporated, using CLIC.

PlaNets - the future

Before coming fully operational, PlaNets will be integrated with the other systems at the dispatching centre of the company using DEC alpha technology. Among these are systems for operational planning and the so-called orthogonal graphics system for on-line detailed visualization of the distribution network.

PlaNets results from ongoing technical cooperation between the Spanish power utility ENHER, S.A. (Empresa Nacional Hidroeléctrica del Ribagorzana) and the Institut de Cibernètica (UPC/CSIC).

Authors: Tom Creemers, Jordi Riera and Lluís Ros.