Planning, design and operation of electrical power systems require engineering studies to evaluate existing status, to size new components and to propose improvements of performance, reliability, safety and economics. Studies are the most efficient way to prevent unexpected failures, to lower costs, and to optimise equipment selection. During the design phase of a new system, the studies foresee the future conditions of a power systems, allowing to avoid potential deficiencies. In existing systems, studies locate the cause of failures and indicate corrective measures.