The
system consisting of high pressure cables and overhead lines of Transmission
Licensee for transmission of electrical power from the Generating Station up to
Connection Point/ Interface Point. The purpose of the electric transmission
system is the interconnection of the electric energy producing power plants or
generating stations with the loads. There are 5 types on how the transmission
system is connected namely:
1.1.1 Ring
System
This
transmission connected between all the step up transformer and only involved
the primary part of the transformer. The
secondary part of transformer is directly connected to the other step down
transformer. For example, the arrangement
composed of an additional 0.9 km length of cable connecting each string at the
end would provide redundancy in the event of a failure at some point in the
string. A faulty cable segment could be
isolated and operation could continue without any loss of generation. However,
a ring arrangement would not lend itself to the graduation of cable sizes and
because the probability of a fault in a buried submarine cable is low, 0.1
faults per year per 100 km, it was decided the slight improvement in
reliability did not warrant the added cable costs.
i. Advantages:
Ø
Transmits the balance current eventhough there
is the change of burden.
Ø
Voltage losses in the transmission system assume
to be non exist.
Ø
Able to patch a lot of electric user eventhough
it is small in size.
Ø
The burden length is easy to be added.
ii. Disadvantages:
Ø
If one of the transformers is damaged, then
there is no power supply in once place or area.
Ø
High cost for the connecting cable.
1.1.2 Radial System
A
radial arrangement leaves the station and passes through the network area with
no connection to any other supply. This is typical of long rural lines with
isolated load areas. An interconnected network is generally found in more urban
areas and will have multiple connections to other points of supply. These points
of connection are normally open but allow various configurations by closing and
opening switches.
Figure
2a and 2b shows the concept of a typical urban distribution system. In this
system a main three-phase feeder goes through the main street. Single-phase subfeeders
supply the crossroads. Secondary mains are supplied through transformers. The
consumer’s service drops supply the individual loads. The voltage of the
distribution system is between 4.6 and 25 kV. Distribution feeders can supply
loads up to 20–30 miles.
i. Advantages:
Ø
Lower investments
Ø
Simple protection
Ø
Successful high-speed and delayed automatic
Ø
Reclosings are likely
Ø
Lower short-circuit currents
Ø
In the event of a fault or required maintenance
a small area of network can be isolated and the remainder kept on supply
ii. Disadvantages:
Ø
If there is a major power outage that causes a
domino effect damaging the power supply systems from the whole network leaving
more customers without power.
Ø
If one of the transformers is damaged, then the
others malfunction.
Ø
The size of cable much more bigger than ring
transmission.
Ø
Limited for area which is needed small power
supply.
1.1.3 Network
System
The
transmission connection is between the primary circumference in radial while
the secondary circumference is in ring connection. Although networks are
classified as either spot or grid, all networks share certain characteristics.
These characteristics are described below.
1.
Each network is served by at least two primary feeders.
2.
A primary feeder may serve a single network unit or
many network units at different sites and may also serve radial distribution
loads.
3.
The primary feeders for a network system are generally
served from a single substation but may be served by different substations. When
supplied from different substations, phase angle difference and voltage
magnitude difference must be minimized if acceptable operation is to be
obtained.
4.
A network unit consists of a high-side disconnect or
grounding switch, a network transformer, and a network protector (with master
relay, phasing relay, and fuses).
5.
The primary network voltage classes range from 5 kV to
35 kV.
6.
Typical network transformer sizes are 300; 500; 750;
1,000; 1,500; 2,000; and 2,500 kVA. Transformers with 208 Y/120 V secondaries
do not exceed 1,000 kVA in rating.
7.
The transformer impedance is specified in ANSI
C57.12.40-2000 and ranges 4%–7%.
8.
The primary feeder can be either a three-wire or
four-wire system.
9.
The transformer connections are commonly delta
primary-wye grounded secondary for three-wire feeders and wye grounded-wye
grounded for four-wire feeders.
i. Advantages:
Ø
The costs of power generation are substantially
larger than the transmission costs
Ø
The transmission capability of the network has
to be such that the power generated at a reasonable cost can be delivered to
the endusers
Ø
Society is highly dependent on electricity
Ø
Network systems are designed based on redundant
facilities. Any single equipment failure will not result in service outage on
the network.
ii. Disadvantages:
Ø
High interruption costs
Ø
Limits for the largest power plants
Ø
Limits for unit sizes of equipment and machinery
1.1.4 Grid System
This
connection is in between the large power generator in country. The change in frequency is inconspicuous for
the secondary transmission. The grid system also consists of an interconnected
grid of circuits operating at utilization voltage and energized from a number
of primary feeder circuits and network units. The number of cables that tie the
secondary buses to one another can be anywhere from one to dozens. These cables
are also referred to as secondary mains. The numerous cables allow for multiple
current paths from every network unit to every load within the grid. Cable
limiters protect some of these cables (by “limiting” thermal damage to the
cables under fault conditions). The Grid systems have the following
characteristics:
1.
The secondary voltages are either 208 Y/120 V or, in
rare cases, 480 vY/277 V.
2.
The integrity of the grid network is based on multiple
paths through individual cables. This integrity is maintained by individual
cables, and, if used, cable limiters burning clear any faulted cable sections.
3.
The conductors from which customer service is tapped
generally follow the geographical pattern of the load area and are located
under streets and alleys.
4.
Load flow within the grid network will significantly
change as a function of:
§
Medium-voltage1 feeder outage conditions
§
Changing customer load conditions
§
Reduced current carrying capacity because of
cleared cable limiters.
i. Advantages:
Ø
The inherent system redundancy generally
prevents any customer from experiencing poor power quality.
Ø
The change in frequency is always stable.
Ø
The small power generator can be reduces.
ii. Disadvantages:
Ø
Primary feeder outages and burned-off cables
because of previous faults within the grid will cause changes in load flow that
are not readily detected.
Ø
High cost.
Ø
Take times to setup
Ø
Need the dapper maintenance.
1.1.5 Bus System
The
bus system is the transmission where the entire primary and secondary
circumference is connected. Any line or other equipment can be supplied from
either bus. Of course this design requires more equipment at additional cost. The
bus configuration at the distribution feeder varies. The configuration can be a
single breaker, double breaker with both breakers closed, or double breaker
with one breaker open and with a transfer switch scheme or sectionalizing bus
tie breaker. Utilities also use ring bus arrangements, double synchronizing bus
designs, and others such that a bus fault in the station will not result in an
outage to more than one primary network feeder.
However, in many cases, such as
the supply to critical loads, major urban centers etc., the additional cost can
be justified. In large stations with
many circuits or transformer banks, the buses can be sectionalized with
sectionalizing circuit breakers and/or bus tie circuit breakers, operable
directly from the operating room or remotely by supervisory control from a regional
operating center. In some situations, bus sectionalizing may be desirable in
order to isolate a portion of a load from the rest of the system, to maintain
special voltage schedules, and so on.
i. Advantages:
Ø
Provides great flexibility in arrangement
ii. Disadvantages:
Ø
Requires the rearrangement of air selector
switches in order to transfer a line from one bus to the other
Ø
The most expensive.
