Electrical
Energy
Engineering
Department
1
7/4/2016
Distribution Planning
This section describes the general distribution planning
steps that may be taken in order to estimate the magnitude
of the medium and low voltage distribution system loads to
be supplied.
The overall efficiency of the distribution system is as
important in load energy consumption. Therefore load
factor, maximum demand, diversity, losses and growth
characteristics are particularly discussed.
DEFINITIONS:
1- Demand or average demand:
The demand of system is the load at the receiving terminals
averaged over a specified interval of time’.
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The load may be expressed as active power (kW) or reactive power
(kVAr).
The period over which the demand is averaged is known as the
demand interval ,the next figure load curve illustrates average
hourly loads (kW) over a 24-hour period.
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The load curve was based on the following data
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2- Maximum demand (MD):
MD: Is the greatest of
all demands which
have occurred during
the specified period of
time, the maximum
demand
may
be
expressed in kW, kVAr,
etc.
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3- Demand factor (‫)ﻣﻌﺎﻣل اﻟطﻠب‬
DF : Is the ratio of the maximum demand of a system to the total
connected load of the system
4- Utilization factor (UF)
The utilization factor is the ratio of the maximum demand of a
system to the rated capacity of the system, UF indicates the degree
to which the system is being loaded during peak load periods with
respect to its capacity
Note: Both the maximum demand and the total connected load should be expressed in
the same units thus making the demand factor dimensionless.
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5 -Load factor (LDF) (‫)ﻣﻌﺎﻣل اﻟﺣﻣل‬
LDF: is the ratio of the average load over a designated period of
time to the peak load occurring in that period
The load factor indicates the degree to which the peak load is
sustained during the period.
For the figure sample 24-hour period before
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6- Diversity factor (DF)(‫)ﺗﻧوع اﻻﺣﻣﺎل‬
DF: is the ratio of the sum of the individual maximum demands of
the various subdivisions of a system to the maximum demand of the
whole system.
Loads do not normally all peak at the same time. The sum of the
individual peak loads will therefore inevitably be greater than the
peak load of the composite system
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From Figure
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7- Coincident factor (CF) (‫)ﺗﻣﺎﺛل اﻻﺣﻣﺎل‬
The reciprocal of the diversity factor is known as the coincident factor
The coincident factor is dependent upon the type of loads connected
to the system. Typically:
In general, and in the absence of other data:
n-is the number of loads connected to the
system
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8- Load diversity( ‫)ﺗﻧوع اﻻﺣﻣﺎل‬
Load diversity is the difference between the sum of the peaks of two or
more individual loads and the peak of the combined load.
Referring to Figure
9- Loss factor (LSF)(‫)ﻣﻌﺎﻣل اﻟﻔﺎﻗد‬
LSF: is the ratio of the average power loss to the peak load loss,
during a specified period of time.
Since power losses are proportional to the square of the load
current:
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From the simple average hourly load variation and square of the
hourly demand patterns shown in Figures befor
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Although loss factor cannot generally be expressed in terms of load
factor, the limiting values of the relationship may be established and
this is illustrated in Figures.
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In more general terms if:
x peak load of duration, t
yminimum load of duration (Tt)
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Thus if the load remains at its peak value all the time that it is on,
and zero for the remainder of the time period, then the loss factor
(LSF) is equal to the load factor (LDF).
Further, if the following assumptions are considered:
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then rewriting equation above
and applying these assumptions and comparing with equation
above:
The loss factor cannot be determined directly from the load factor
because the loss factor is determined from the losses as a function of
time, which in turn are proportional to the time function of the square
of the load.
However, a relationship has been calculated which gives a
reasonable value of the 30-minute, monthly, kW loss factor in terms
of the corresponding load as shown graphically in Figure.
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In general:
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10- Load duration(‫)ﻓﺗرة اﻻﺣﻣﺎل‬
LD: is
the relationship of demands and the duration of the demands
over a specified time period.
Referring to Figure the hourly demands have been sorted from
grater to lower order and tabulated in Table as shown below to
give:
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11 -Loss equivalent hours
Loss equivalent hours are the number of hours of peak loads which
will produce the same total losses as is produced by the actual
loads over a specified period of time.
Both the actual and peak demand values must be chosen from the
associated load duration:
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With reference to the load duration and loss table
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Load duration and loss table (for the peak day described in Figure
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The loss equivalent hours are also referred to as the ‘Equivalent
peak loss time’ (EPLT). An alternative method of calculating this is:
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12- Peak responsibility factor (PRF)
The peak responsibility factor represents the contribution a
component makes to the system demand losses at the time of
system peak demand.
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It should be noted that no-load losses are continuous and occur both
during system peak demand and at other times. Generation
therefore had to be designed to support these no-load losses.
Load losses vary with the load such that peak losses on a particular
component of the overall distribution system occur at peak load on
that component which may not be at the same as the overall system
peak demand.
Only a fraction of the individual component losses therefore
contribute to the system peak demand.
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Aerial Bundled Conductor (ABC)
A distribution systems the Aerial Bundled Conductor(ABC) is
used ,why?.
-To improving consumer supply reliability.
-The cost of ABC quite naturally.
-The limiting factor is volt drop, this being determined by the
line reactance.
The capital costs per unit length for ABC are approximately
twice that for bare-wire.
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Currents in various distribution
station bus-bar voltages calculation
sections and
Example:
3-phase ring main ABCD fed at A at 11 kV supplies balanced loads
of 50 A at 0.8 p.f. lagging at B, 120 A at unity p.f. at C and 70 A at
0·866 lagging at D, the load currents being referred to the supply
voltage at A. The impedances of the various sections are :
Section AB = (1 + j 0·6) Ω; Section BC = (1·2 + j 0·9) Ω
Section CD = (0·8 + j 0·5) Ω; Section DA = (3 + j 2) Ω
Calculate the currents in various sections and station bus-bar
voltages at B, C and D.
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Solution. shows one phase of the ring main. The problem will be
solved by Kirchhoff’s laws. Let current in section AB be (x+ j y).
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Applying Kirchhoff’s voltage law to mesh ABCDA, we have,
Drop in AB+ Drop in BC + Drop in CD+ Drop in DA= 0
or [(x −0·6y) + j(0·6x+ y)] + [(1·2x −0·9y −75) + j(0·9x+ 1·2y)]+ [(0·8x
−0·5y −143) +j(0·5x+ 0·8y −56)]
+ [(3x −2y −791·8) + j(2x+ 3y −246·2)] = 0
or (6x −4y −1009·8) + j(4x+ 6y −302·2) = 0
As the real (or active) and imaginary (or reactive) parts have to be
separately zero,
∴ 6x −4y −1009·8 = 0 and 4x+ 6y −302·2 = 0
Solving for x and y, we have,
x = 139·7 A ; y= −42·8 A
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Current in section AB =(139·7 − j42·8) A
Current in section BC =(x −40) + j(y+ 30)
= (139·7 −40) + j(−42·8 + 30) =(99·7 − j12·8) A
Current in section CD =(x −160) + j(y+ 30)
= (139·7 −160) + j(−42·8 + 30)
=(−20·3 − j12·8) A
Current in section DA =(x −220·6) + j(y+ 65)
= (139·7 −220·6) + j(−42·8 + 65)
=(−80·9 + j 22·2) A
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**Note : In stead of the current in example ,the power may be given
and we need to calculate the impedance of the conductor in order to
designate the cross-section needed.
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REVIEW QUESTIONS
1. Discuss why:
(a)All ac transmission and distribution systems are 3 phase systems.
(b) The transmission systems are mostly overhead systems.
(c) All overhead lines use ACSR conductors.
(d) Overhead line conductors are invariably stranded.
2. Give reasons:
(a) The transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4 wire
circuits.
(b) It is necessary to use high voltages for transmission systems.
(c) At 400 kV and above the transmission lines have bundled conductors.
(d) The tendency of corona formation is lesser in aluminum conductor lines than in copper conductor
lines.
(e) The voltage drop is a very important consideration in distribution lines but not so important in
transmission lines.
3.Draw a single line diagram showing the essential parts in a modern power system network.
4.Bring out the relative advantages and disadvantages of overhead and underground systems.
5. Write a short note on 'overhead line conductors' bringing out the reasons for using ACSR
conductors.
6.What is a bundled conductor? Why is it used? Give a few configurations of such conductors
commonly employed.
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