Bài giảng Chức năng các hệ thống truyền tải và phân phối điện năng - Chương 1: Giới thiệu chung về hệ thống điện - Võ Ngọc Điều

nsmission lines are installed in a tunnel, 
which enables many circuits in a limited area.
Transmission line laid in a trench
Underground Transmission Lines
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Covering the transmission line
Underground transmission line tunnel
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Underground Transmission Lines
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Heat is generated when electricity flows 
through cables, limiting the power transmission 
capacity in tunnels. To increase the capacity, a 
tunnel cooling system can be installed. The 
system above circulates cold water through the 
tunnels.
Cable snaking through underground 
transmission tunnel
Underground Transmission Lines
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Cable installation of underground 
transmission line
Cross section of underground 
transmission line
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Distribution Systems
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- A distribution system originates at a distribution substation and includes the 
lines, poles, transformers and other equipment needed to deliver electric power 
to the customer at the required voltages. Customers are classed as: Industrial 
Customer
 Commercial Customer
 Residential Customer
 Transportation Customer
Distribution Systems
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- A distribution system consists of all the facilities and equipment connecting a 
transmission system to the customer's equipment.
- A typical distribution system can consist of: 
 Substations
 Distribution Feeder Circuits
 Switches
 Protective Equipment
 Primary Circuits
 Distribution Transformers
 Secondaries, and
 Services
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Distribution Feeder Circuits
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- Distribution feeder circuits are the connections between the output terminals 
of a distribution substation and the input terminals of primary circuits. The 
distribution feeder circuit conductors leave the substation from a circuit breaker 
or circuit recloser via underground cables, called substation exit cables. The 
underground cables connect to a nearby overhead primary circuit outside the 
substation. This eliminates multiple circuits on the poles adjacent to the 
substations thereby improving the overall appearance of the substation.
- Several distribution feeder circuits can leave a substation extending in 
different directions to serve customers. The underground cables are connected 
to the primary circuit via a nearby riser pole.
- The distribution feeder bay routes power from the substation to the 
distribution primary feeder circuits.
Distribution Feeder Circuits
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- In the photo of the distribution main feeder the primary circuit is fed 
underground to a nearby distribution system overhead line. The yellow cables 
are the primary feeder lines going underground.
3-phase distribution feeder bay 
Distribution main feeder
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Distribution Feeder Circuits
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Distribution feeder recloser 
Switches
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- Distribution systems have switches installed at strategic locations to redirect 
or cut-off power flows for load balancing or sectionalizing. Also, this permits 
repairing of damaged lines or equipment or upgrading work on the system. 
The many types of switches include: circuit-breaker switches
 single-pole disconnect switches
 three-pole group-operated switches
 pad-mounted switchgear
Air circuit-breaker switches
Air-break isolator switch 
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Switches
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Circuit switchers Single-pole disconnect switch combined
with a fuse is called a fused cutout 
Switches
94Circuit breakers 
Pad mounted switchgear 
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Switches
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Group-operated three-pole air break switch 
Protective Equipment
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- Protective equipment in a distribution system consists of protective relays, 
cutout switches, disconnect switches, lightning arresters, and fuses. These 
work individually or may work in concert to open circuits whenever a short 
circuit, lightning strikes or other disruptive event occurs.
- When a circuit breakers opens, the entire distribution circuit is deenergized. 
Since this can disrupt power to many customers, the distribution system is 
often designed with many layers of redundancy. Through redundancy, power 
can be shut off in portions of the system only, but not the entire system, or can 
be redirected to continue to serve customers. Only in extreme events, or failure 
of redundant systems, does an entire system become deenergized, shutting off 
power to large numbers of customers.
- The redundancy consists of the many fuses and fused cutouts throughout the 
system that can disable parts of the system but not the entire system. Lightning 
arresters also act locally to drain off electrical energy from a lightning strike so 
that the larger circuit breakers are not actuated.
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Protective Equipment
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Substation bus lightning arresters
Fused cut-out
Protective Equipment
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Pole mounted type - lightning arrester
Substation disconnect switch 
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Primary Circuits
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- Primary circuits are the distribution circuits that carry power from substations 
to local load areas. They are also called express feeders or distribution main 
feeders. The distribution feeder bay routes power from the substation to the 
distribution primary feeder circuits.
- In the photo of the distribution main feeder the primary circuit is fed 
underground to a nearby distribution system overhead line. The yellow cables 
are the primary feeder lines going underground. 
3 phase distribution feeder bay Distribution main feeder 
Primary Circuits
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Overhead primary feeder Distribution primary feeder underbuild 
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Distribution Transformers
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- Distribution transformers reduce the voltage of the primary circuit to the 
voltage required by customers. This voltage varies and is usually: 120/240 
volts single phase for residential customers,
- 480Y/277 or 208Y/120 for commercial or light industry customers.
Three-phase pad mounted transformers are used with an underground primary 
circuit and three single-phase pole type transformers for overhead service.
- Network service can be provided for areas with large concentrations of 
businesses. These are usually transformers installed in an underground vault. 
Power is then sent via underground cables to the separate customers. 
Distribution Transformers
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Air Distribution transformer - commercial 
facility Industrial facility distribution transformer 
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Distribution Transformers
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Residential distribution transformer
Pad-mounted residential distribution 
transformer 
Secondaries
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- Secondaries are the conductors originating at the low-voltage secondary 
winding of a distribution transformer. Secondaries for residential service are 
three-wire single-phase circuits. They extend along the rear lot lines, alleys, or 
streets past customer's premises. The secondaries can be overhead lines or 
underground lines.
- Overhead secondary lines are usually strung below the primary lines and 
typically in a vertical plane. When secondaries are strung in a vertical plane, 
they are directly attached to the support pole one above the other. This is in 
contrast to the primary lines which are often strung on a cross bar or other 
attachment in a horizontal or "V" shaped plane
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Secondaries
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Cabled secondaries 
Secondaries in a vertical plane 
Secondaries
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Cabled secondaries, primaries in a "V" plane 
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Services
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- The wires extending from the secondaries or distribution transformer to a 
customer's location are called a service. A service can be above or below 
ground. Underground services have a riser connection at the distribution pole. 
Commercial and residential services are much the same and can be either 120 
or 220 or both
Distribution system lines and associated 
equipment 
Service line to residence 
Services
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Commercial service 
Secondary to underground service 
via a riser 
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Industrial Customer
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- Most industries need 2,400 to 4,160 volts to run heavy machinery and usually 
their own substation or substations to reduce the voltage from the transmission 
line to the desired level for distribution throughout the plant area. They usually 
require 3-phase lines to power 3-phase motors.
Industrial facility distribution transformer 
Commercial Customer
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- Commercial customers are usually served at distribution voltages, ranging 
from 14.4 kV to 7.2 kV through a service drop line which leads from a 
transformer on or near the distribution pole to the customer's end use structure. 
They may require 3-phase lines to power 3-phase motors.
Distribution transformer to 
3-phase service - commercial facility
Commercial service drop 
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Residential Customer
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- The distribution electricity is reduced to the end use voltage (120/240 volts 
single phase) via a pole mounted or pad-mounted transformer. Power is 
delivered to the residential customer through a service drop line which leads 
from the distribution pole transformer to the customer's structure, for overhead 
lines, or underground.
Residential distribution transformer and service 
drop
Pad-mounted residential distribution 
transformer
Transportation Customer
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- Currently the only electric transportation systems are light rail and subway 
systems. A small distribution substation reduces the local distribution voltage 
to the transportation system requirements. The overhead lines supply electric 
power to the transportation system motors and the return current lines are 
connected to the train tracks. 
Public transit train powered by 
overhead electric lines Substation where electricity is conditioned for 
powering commuter trains
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Transportation Customer
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Power runs from the substation underground to 
the poles where power is delivered to the power 
lines. The circuit is completed through the train 
tracks, with lines returning to the substation. 
Electric cables carry electricity to 
power the train's motors