8/2/2019 Pri Sec Distribution System
1/22
After reading this Module , you should be able to:
Provide classification of distribution systems
Discuss design considerations
Provide an analysis of AC and DC distribution systems
8/2/2019 Pri Sec Distribution System
2/22
The part of the electrical-supply system existing between
the distribution substations and the distributiontransformers is called the primary system.
It is made of circuits, known as primary feeders or
primary distribution feeders. The most commonly used
nominal primary voltage is 11 kV.
8/2/2019 Pri Sec Distribution System
3/22
The secondary distribution system receives power from the
secondary side of distribution transformers at low voltage
and supplies power to various connected loads via service
lines.The secondary distribution System is the final sub-system of
the power system.
The secondary distribution systems are generally of the
radial type except for some specific service areas such as
hospitals, business centers and military installations, which
require highly reliable service, and therefore, may be of a
grid or mesh type.
8/2/2019 Pri Sec Distribution System
4/22
Figure : Radial secondary distribution system configuration
8/2/2019 Pri Sec Distribution System
5/22
Good voltage regulation is the most important factor in a
distribution system for delivering good service to the
consumer. For this purpose, careful consideration is
required for the design of feeders and distributor networks.
Feeders are the conductors that connect substations to
consumer ports and have large current carrying capacity.
The current loading of a feeder is uniform along the whole
of its length since no tappings are taken from it. The design
of a feeder is based mainly on the current that is to be
carried.
8/2/2019 Pri Sec Distribution System
6/22
Distributors are the conductors, which run along a street
or an area to supply power to consumers.
These can be easily recognized by the number of
tappings, which are taken from them for the supply to
various consumer terminals.
The current loading of a distributor is not uniform and itvaries along the length while its design is largely
influenced by the voltage drop along it.
8/2/2019 Pri Sec Distribution System
7/22
Service Main and Sub Main The service mains are the
conductors forming connecting links between distributors
and metering points of the consumer's terminal. Figure
14.1 shows the layout of a distribution system. The term
sub main refers to the several connections given to
consumers from one service main.
8/2/2019 Pri Sec Distribution System
8/22
8/2/2019 Pri Sec Distribution System
9/22
It has been established that 70% of the total losses occur
in the primary and secondary distribution system while
transmission and sub-transmission lines account for only
30% of the total losses.
Distribution losses amount to 15.5% of the generation
capacity and the target level is to bring it down to 7.5%.
Therefore the primary and secondary distribution systems
must be planned with care to ensure losses within
acceptability limits
8/2/2019 Pri Sec Distribution System
10/22
Inadequate Size of Conductor :
The rural load is usually scattered and fed byradial feeders.
Feeder Length In practice, 11 kV and 415 V lines in rural
areas are hurriedly extended radially over long distances to
feed loads scattered over large areas. This results in high
line resistance, low voltage, and high current, and
therefore, leads to high I 2R losses in the line.
8/2/2019 Pri Sec Distribution System
11/22
Location of Distribution Transformers Often the distribution
transformers are not located centrally in relation to the
intended customer. Consequently, the farthest customers
obtain an extremely low voltage even though a reasonablygood voltage level is maintained at the secondary
transformer.
This again leads to higher line losses.
Therefore, in order to reduce the voltage drop in the line
transmitting power to consumers located farthest from the
line, the DT should be located at the load center to keep the
voltage drop within permissible limits.
8/2/2019 Pri Sec Distribution System
12/22
Low Voltage Whenever the voltage applied to an induction
motor deviates from rated voltage, its performance is
adversely affected. A reduced voltage, in case of an induction
motor, results in higher currents drawn for the same output
leading to higher losses. This can be overcome by adjusting
the tap changer at power transformer and at distributiontransformer, if available.
8/2/2019 Pri Sec Distribution System
13/22
Use of Over-rated Distribution Transformers
Studies on 11 kV feeders have revealed that often the
rating of distribution transformers (DTs) is much higher
than the maximum kVA demand on the LT feeder. An
overrated transformer produces an unnecessarily high iron
loss.From the above, it is clear that the rating should be
judiciously selected to keep the losses within the permissible
limits.
8/2/2019 Pri Sec Distribution System
14/22
Low PowerFactor In most of the LT distribution
systems, it is found that the power factor varies from as
worse as 0.65 to 0.75. A low power factor contributes
towards high distribution losses.
For a given load, if the power factor is low, the current
drawn is high, consequently the losses proportional to a
square of the current, will be more.
Thus, line losses owing to the poor power factor can be
reduced by improving the power factor by using shunt
capacitors for the purpose.
8/2/2019 Pri Sec Distribution System
15/22
The following methods are adopted for the reduction of
distribution-system losses:
1. HV distribution system
2. Feeder reconfiguration
3. Grading of conductor
4. Construction of new substation5. . Reactive-power compensation
8/2/2019 Pri Sec Distribution System
16/22
1 . HV Distribution System
The low-voltage distribution system contributes a majority of the
total distribution losses because of poor voltage regulation .
The present situation is that LV lines are extended, irrespective of
voltage drops, up to the full capacity of the DT and sometimes
over and above the transformer capacity. Hence, no purpose will
be served by prescribing low kVA-km loading limits for LV lines.
The only practice and feasible solution is to eliminate or
minimize LV lines by switching over to single- phase high voltage
distribution. By adopting HV distribution, the losses in the LV
distribution can be minimized.
8/2/2019 Pri Sec Distribution System
17/22
2 . Feeder Reconfiguration
It is defined as the process of altering the topological
structure of distribution feeders by changing the
open/closed status of the sectionalising and tie switches.
Feeder reconfiguration allows the transfer of loads from
heavy loaded feeders to moderately heavy loaded feeders.
Such transfers are effective in improving the voltage
profile along the feeders and effecting reduction in the
overall system power losses.
8/2/2019 Pri Sec Distribution System
18/22
3. Grading of Conductor
In normal practice, the conductor used for radial distribution
feeders is of uniform cross-sectional area. However, the load
magnitude at the substation is high and it reduces as we
proceed to the tail-end of the feeder. This indicates that the
use of a higher-sized conductor, which is capable of supplying
load from the source point, is not necessary at the tail-end
point. Similarly, the use of different conductor cross-sectionsfor intermediate sections will lead to a minimum capital
investment cost and line loss.
8/2/2019 Pri Sec Distribution System
19/22
4. Construction of New Substation
If a new substation is to be constructed and connected
to an existing network, several possible solutions are to
be studied. These solutions may include various
connection schemes of the substation and several
feasible locations .
The optimum site for a substation is defined as that
location which will result in minimum cost for construction
and minimum losses. Therefore, by constructing a new
substation at load centres, the line osses will be reduced
due to an improvement in the voltage profile and a
reduction in the length of the lines.
5 R i P C i
8/2/2019 Pri Sec Distribution System
20/22
5. Reactive Power Compensation
Voltage-reactive power control function has a pivotal role
to play in the distribution automation.
The problem of reactive power compensation can be
attempted by providing static capacitors.
The method presently used to compensate the reactive
power component is to increase the reactive power by
increasing the terminal voltage of the generator (or), by
increasing the field current of the synchronous machine in
condenser mode at the generating stations. This procedure isnot effective because the power-system losses will be further
increased due to the increase of reactive power in the
transmission system.
8/2/2019 Pri Sec Distribution System
21/22
An alternative method for compensating the reactive power isthe use ofcapacitors in distribution systems at customer
points. Shunt capacitors supply the amount of reactive power
to the system at the point where they are connected.
There are two methods of capacitor compensation viz.
1. Series compensation (capacitors are placed in series with line)
2. Shunt compensation (capacitors are placed in parallel with
load)
8/2/2019 Pri Sec Distribution System
22/22
The fundamental function of capacitors, whether they are
series or shunt in a power system is to generate reactive
power to improve power factor and voltage, thereby
enhancing the system capacity and reducing losses.
In series capacitors, the reactive power is proportional to
the square of the load current, whereas in shunt capacitors
it is proportional to the square of the voltage.