2.1. Introduction
2.1.1. The Evolution of Interconnected Systems
Electricity grid interconnections have played a key role in the history of electric power systems. Most
national and regional power systems that exist today began many decades ago as isolated systems, often as
a single generator in a large city. As power systems expanded out from their urban cores, interconnections
among neighboring systems became increasingly common . Groups of utilities began to form power
pools, allowing them to trade electricity and share capacity reserves. The first power pool in the United
States was formed in the Connecticut Valley in 1925 . As transmission technologies improved, long distance
interconnections developed, sometimes crossing national borders. The first international interconnections
in Europe came in 1906, when Switzerland built transmission links to France and Italy.
One of the great engineering achievements of the last century has been the evolution of large synchronous
alternating current (AC) power grids, in which all the interconnected systems maintain the
same precise electrical frequency. Today, the North American power system is composed of four giant
synchronous systems, namely the Eastern, Western, Texas, and Quebec interconnections. The Eastern
interconnection by itself has been called the largest machine in the world, consisting of thousands of
generators, millions of kilometers of transmission and distribution lines, and more than a billion different
electrical loads. Despite this complexity, the network operates in synchronism as a single system. So
does the Western European interconnection, which reaches from the UK and Scandinavia to Italy and
Greece, embracing along the way much of Eastern Europe (for example, Poland, Hungary, Slovakia,
and the Czech Republic). Synchronous interconnections among countries are expanding in Central and
South America, North and Sub-Saharan Africa, and the Middle East10.
At the same time that synchronous AC networks have reached the continental scale, the use of
high voltage direct current (HVDC) interconnections is also rapidly expanding as a result of technical
progress over the last two decades. HVDC permits the asynchronous interconnection of networks that
operate at different frequencies, or are otherwise incompatible, allowing them to exchange power without
requiring the tight coordination of a synchronous network. HVDC has other advantages as well,
especially for transmitting large amounts of power over very long distances. Fundamentals of both AC
and DC interconnections are discussed below in Sections 2.2, 2.3, and 2.4 of this Chapter.