information about electric power transmission

1. INDEX
1. INTRODUCTION
2. MODERN DISTRIBUTION SYSTEM
3. INTERNATIONAL DIFFERENCES
4. DISTRIBUTION NETWORK
CONFIGURATIONS
5. CHARACTERISTICS OF THE SUPPLY
GIVEN TO CUSTOMERS

2. INTRODUCTION
An Electric Power Distribution System is the final stage in
the delivery of electric power; it carries electricity from
the transmission system to individual consumers. Distribution
substations connect to the transmission system and lower the
transmission voltage to medium voltage ranging between 2 kV and
35 kV with the use of transformers. Primary distribution lines carry
this medium voltage power to distribution transformers located near
the customer's premises. Distribution transformers again lower the
voltage to the utilization voltage of household appliances and typically
feed several customers through secondary distribution lines at this
voltage. Commercial and residential customers are connected to the
secondary distribution lines through service drops. Customers
demanding a much larger amount of power may be connected directly
to the primary distribution level or the sub transmission level.

3. The modern distribution system begins as the
primary circuit leaves the sub-station and
ends as the secondary service enters the
customer's meter socket by way of a service
drop. Distribution circuits serve many
customers. The voltage used is appropriate
for the shorter distance and varies from 2,300
to about 35,000 volts depending on utility
standard practice, distance, and load to be
served. Distribution circuits are fed from
a transformer located in a substation, where
the voltage is reduced from the high values
used for power transmission.

4. In many areas, "delta" three phase service is common. Delta service has
no distributed neutral wire and is therefore less expensive.
The Americas
Many countries in north, central and South America use 60 Hz AC, the
120/240 volt split phase system is used domestically and three phase is
used for larger installations.
Europe
In Europe, electricity is normally distributed for industry and domestic
use by the three-phase, four wire system. This gives a three-phase
voltage of 400 volts wye service and a single-phase voltage of 230
volts. For industrial customers, 3-phase 690 / 400 volt is also available.
Large industrial customers have their own transformers with an input
from 10 kV to 220 kV.

5. Distribution networks are divided into two types,
radial or network. A radial system is arranged like a
tree where each customer has one source of supply.
A network system has multiple sources of supply
operating in parallel. The secondary network is
commonly found in big cities and is the most
reliable system. Spot networks are used for
concentrated loads. Radial systems are commonly
used in rural or suburban areas.

6. • AC or DC - Virtually all public electricity supplies are AC today. Users of
large amounts of DC power such as some electric railways, telephone
exchanges and industrial processes such as aluminium smelting usually either
operate their own or have adjacent dedicated generating equipment, or use
rectifiers to derive DC from the public AC supply.
• Nominal voltage, and tolerance (for example, +/- 5 per cent).
• Frequency, commonly 50 or 60 Hz, 16.7 Hz and 25 Hz for some railways
and, in a few older industrial and mining locations, 25 Hz.
• Maximum demand (some energy providers measure as the largest mean
power delivered within a 15 or 30 minute period during a billing period)
• Load factor, expressed as a ratio of average load to peak load over a period of
time. Load factor indicates the degree of effective utilization of equipment
(and capital investment) of distribution line or system.