(3) improved decision-support tools. The major technologies within each category are discussed in the following sections, and the annex provides more detail.

Advanced Equipment and Components

Advanced equipment and components include technologies for improving and controlling power flows, enabling greater efficiency in long-distance transmission, storage of electrical energy (to be dispatched into the grid as needed), and grid operation. Advanced electronic equipment is also being used for smart metering and control in the distribution networks. The status of these technologies, likely future technology improvements, and potential for deployment into the T&D system are addressed in the five subsections below.

Power Electronics

A T&D system requires power-flow control and protection against overloads and instability. The electromechanical devices currently used for these purposes are slow and cannot react quickly enough to handle rapid transients, but modern solid-state power electronics can overcome this problem. Power electronics are not new, but their deployment has been limited to particular applications in which their higher cost is offset by their benefits to investors. Power electronics can be used in the transmission system (for both AC and HVDC applications),17 and in the distribution system.

Power electronics on the AC transmission system are referred to as flexible alternating current transmission system (FACTS) devices.18 FACTS devices can control both real and reactive power flows along transmission corridors, thereby maintaining the stability of transmission voltage. FACTS devices can also increase the power transfer capability of transmission lines and improve overall system reliability by reacting virtually instantaneously to disturbances. FACTS can enable wholesale markets, increase security, enable self-healing capacity, and optimize the use of system assets by controlling the flow of power, and they can help to


The transmission system in the United States is almost entirely AC. Transmitting electricity via HVDC involves converting AC to DC, transmitting the electricity in DC, and converting it back to AC at the receiving end.


Annex 9.A describes specific flexible alternating current transmission system (FACTS) devices and their applications in more detail.

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