The electricity is one of the most important goods in the modern societies. It is not only essential at domestic leveI, but specially at industrial level.

During the process of economies liberalisation starting from the eighties, the electric sector was no more seen as natural monopoly and it started to be subject to the market rules. Due to that, and a little on the whole side, they are going settling competitive markets of electricity, where producers and buyers dispute the best price, according to established rules.

However the electricity requires technique needs on production side and specially on the transmission side. It is usual, in the electricity markets, the existence of an independent entity that, on transparent and no-discriminatory way, could do the management of alI the process. This entity is named System Operator. There is another entity that manages the market on the economic side and is called Market Operator.

I order to deal with the congestion problems on the electricity transmission we need calculus complex algorithms, naturally processed by computers.

With this report it was intended, in a first phase, to analyse several liberalisation processes of electric sector de-regulation, completed or in course, in several countries or organisations. To do the global framework the report presents the legal framework in Portugal and faced to the European Union Directive 96/92 and finally the theoretical framework with the description how to work the market and how to solve congestion problems, as well some intemational experiences ofthe electric sector reorganisation.

Following, and recognising the importance of the congestion problems and its decisive intluence in the technical administration of the system and in the attempt of assuring high feasibility levels and of quality service, a prototype of computer application was developed that simulates the action of the Market Operator and the System Operator. The producers and buyers auctions are received through the Web, the system marginal price is calculated based on those auctions and, finally, are analysed if the dispatch calculated is practicable faced to technical conditions of the grid exploitation. If happened violations of technical restrictions of system operation, an algorithm is executed that, using adjusting bids, could identify some modifications to do in the injected powers. To conclude is presented an example of the prototype application developed with the purpose of illustrating its operation and contributing to illuminate eventual doubts that can stay in relation to its conception.