Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 56

et al . 2000), following the large-scale utilisation of renewable energy sources, energy storage
systems, electrical vehicles, etc. Not only is there a change of scale but also a change of
technology. Large generators are almost exclusively 50/60 Hz synchronous machines. Dis-
tributed power generators include variable speed (variable frequency) sources, high speed
(high frequency) sources and direct energy conversion sources that produce DC. For example,
wind turbines are most effective if free to generate at variable frequency and so they require
conversion from AC (variable frequency) to DC to AC (50/60 Hz) (Chen and Spooner 2001);
small gas turbines with direct drive generators operate at high frequencies and also require AC
to DC to AC conversion (Etezadi-Amoli and Choma 2001), and photovoltaic arrays require
DC-AC conversion (Enslin et al . 1997).
There are several operating regimes possible for distributed generation. One such is for
distributed generators to form microgrids before being connected to the public grid. As a
result, local consumers are largely supplied by the local distributed generation with shortfalls
or surpluses exchanged through a connection to the public electricity supply system (Lasseter
2002; Venkataramanan and Illindala 2002). The use of a microgrid opens up the possibility
of making the distributed generator responsible for local power quality in a way that is not
possible with conventional generators (Green and Prodanovic 2003). Another option is to
connect distributed generation and storage systems directly to the grid.
1.6.2 Introduction to Smart Grids
The change of the operation paradigms of power systems does not stop at distributed generation.
A more advanced concept, the smart grid, has been introduced to power systems to further
improve reliability, quality, operating efficiency, resilience to threats while reducing the impact
of power systems to environment, taking advantage of advanced digital technology. The
main characteristics of smart grids with comparison to today's grids are shown in Table 1.4,
according to (DOE 2009a). The scope of a smart grid is depicted in Figure 1.47, which shows
that smart grids have a layered structure consisting of:
Infrastructure: the traditional generation, transmission and distribution facilities and new
add-ons, such as renewable energy generators, PHEVs, smart appliances, distributed gener-
ation and storage systems, etc.
Control, communication and information systems to facilitate system coordination, opera-
tion, and improvement of energy efficiency, marketing, and security etc.
The areas of the electric system that cover the scope of a smart grid include the following
(DOE 2009b):
Area, regional and national coordination regimes: A series of interrelated, hierarchical
coordination functions exists for the economic and reliable operation of the electric system.
These include balancing areas, independent system operators (ISOs), regional transmis-
sion operators (RTOs), electricity market operations, and government emergency-operation
centres. Smart-grid elements in this area include collecting measurements from across the
system to determine system state and health, and coordinating actions to enhance economic
efficiency, reliability, environmental compliance, or response to disturbances.