Four most common distribution feeder systems applied nowadays are: 1.Radial 2.Parallel feeders 3.Ring main 4.Meshed system...
Four most common distribution feeder systems applied
nowadays are:
1.Radial
2.Parallel feeders
3.Ring main
4.Meshed systems
1. Radial
Many distribution systems operate using a radial feeder
system. A typical radial feeder system is shown schematically in figure below.
Radial feeders are the simplest and least expensive, both to construct and for
their protection system.
This advantage however is offset by the difficulty of
maintaining supply in the event of a fault occurring in the feeder.
A fault would result in the loss of supply to a number of
customers until the fault is located and cleared. The next level of reliability
is given by a ‘parallel feeder’ system.
Radial feeder system
2. Parallel feeders
A greater level of reliability at a higher cost is achieved
with a parallel feeder. A typical parallel feeder system is shown schematically
in figure below.
In the event of a line fault only one of the feeder sets of
cables will be affected, thus allowing the remaining parallel feeder to
continue to supply the load.
To improve the reliability factor it may be possible to have
the separate sets of cables follow different routes. In this case the capital
cost is double that of a radial feeder but there is a greater reliability
factor for the line. This may be justified if the load is higher, more
customers are being supplied, or there are loads such as hospitals which
require high levels of reliability.
Parallel feeders are more common in urban areas or for
feeders to large single customers, where load shedding in an emergency may be
possible.
Parallel feeder
system
3. Ring main
A similar level of system reliability to that of the
parallel arrangement can be achieved by using ring main feeders. This usually
results from the growth of load supplied by a parallel feeder where the cabling
has been installed along different routes. These are most common in urban and
industrial environments.
Whilst the start and finish ends of the ring are at the same
location, power is delivered by both pathways of the ring into substations
located around the ring.
Should a fault occur on a feeder cable at any point around
the ring the faulty section may be isolated by the operation of the protecting
circuit breakers, at the same time maintaining supply to all substations on the
ring.
In typical urban / suburban ring main arrangements, the open
ring is operated manually and loss of supply restored by manual switching.
Ring main feeder system
Current practice is to use distribution automation, where
operation and supply restoration in the feeder rings is done automatically by
centrally controlled supervisory systems.
This gives the advantages of ring main systems as line
voltage drops are reduced at the various load substations there is a ‘firm’
supply (i.e. an alternative path is available if the primary one fails) to each
load substation.
4. Meshed systems
In transmission and sub-transmission systems, usually
parallel, ring or interconnected (mesh) systems are used. This ensures that
alternative supply can be made to customers in the event of failure of a
transmission line or element.
The extra expense can be justified because of the much
greater load and number of customers that are affected by failure of lines at
transmission or sub-transmission levels.
The general rule is that where large loads or numbers of
customers are involved, then some form of standby, in the form of deliberate
redundancy, is built into the network design, through the use of parallel,
meshed or ring type feeders.
Meshed
systems
Only in outer rural areas would one consider using only
radial supply at a sub-transmission level. On the other hand, simple radial
supply is almost universally used for low voltage (400V) feeders, even in urban
areas, because they supply relatively few customers.
Reference: Design guide for overhead distribution systems,
Chisholm Institute of TAFE, Electrical, Electronics Engineering Department.
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