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An electric power consumer is an enterprise utilizing electric power. Its
operating characteristics vary during the hours of day, days and nights, days of
week and seasons.
All electric power consumers are divided into groups with common load
characteristics. To the first group belong municipal consumers with a predominant
lighting load: dwelling houses, hospitals, theatres, street lighting systems, mines,
etc.
To the second group belong industrial consumers with a predominant power
load (electric motors): industrial plants, mines, etc.
To the third group belongs transport, for example, electrified railways. The
fourth consists of agricultural consumers, for example, electrotractors.
The operating load conditions of each group are determined by the load
graph. The load graph shows the consumption of power during different periods of
day, month, and year. On the load graph the time of the maximum loads and
minimum loads is given.
Large industrial areas with cities are supplied from electric networks fed by
electric power plants. These plants are interconnected for operation in parallel and
located in different parts of the given area. They may include some large thermal
and hydroelectric power plants.
The sum total of the electric power plants, the networks that interconnect
them and the power utilizing devices of the consumers, is called a power system.
All the components of a power system are interrelated by the common processes of
protection, distribution, and consumption of both electric and heat power.
In a power system, all the parallelly operating plants carry the total load of all the
consumers supplied by the given system.
The building up of a power system is of great importance for the national
economy. An economical utilization of the power plant installations and of the
sources of power is achieved by interconnected operation of a series of power
plants in a common power distribution system.

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