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Wednesday, September 19, 2012

1.1 Transmission System


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.
 
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