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Distribution System Load Characteristics
R. H. SARIKAS
A S S O C I A T E M E M B E R A I E E
H . B . T H A C K E R
M E M B E R A I E E
JAN FEB MAR APR MAY SEPTUNE JULY AUG
MONTH AND DAY
Fig 1 Daily m aximum loads for a typical distribution
transformer
OCT NOV
DEC
NASMUCH AS the sole purpose for the existence of
an electr ical distr ibution system lies in supplying the
requirements of the consumer 's uti l ization devices, i t
is
funda me ntal that a knowledge of these load req uirem ents
is necessary for sound planning of the distr ibution sys
tem an d i ts var ious componen ts . T h e economic impor
tance of the knowledge of load characteristics has become
more and more recognized wi th the growth of load on
util i ty systems. In recent years, much work has been done
in this f ie ld . Many ut i l i t ies have mad e an d are con t
inuin g
to make, in some form, surveys of the load on their sys
tems. Info rm atio n o bta ine d from u til i ty load surveys
can
be used for many purposes other than distr ibution system
lann ing and op t imum equ ipment des ign . Of ten , da ta
from the same load survey is used in studies of rate struc
ture, cost analysis , an d develo pm ent of selective sell
ing
rograms.
One of the most useful survey methods for determining
oad characteristics is the method of group load surveys.
he test groups are selected as being representative of their
In addition to daily load curves, an analysis of this
data furnishes the fol lowing: a correlat ion between the
ki lowat t -hou r (kw-hr) usage of an individ ual consum er
and his peak demand, coincidence factor re lat ionship,
loss an d power-factor info rm atio n, an d load factor d
ata.
Loads on current ly ins tal led dis t r ibut io n t
ransformers
can be es t imated by us ing the kw-hr vs demand relat ion
ship,
and coincidence-factors obtained from the load sur
vey. T h e load of the ind ivi du al custom ers served by
the
t ransformer is es t imated f rom the grap h relat in g
kw-hr
usage dur ing a given per iod to the peak demand within
that same per iod. Appl icat ion of the appropr iate coin
cidence factor provides an estimate of the peak load sup
plied by the transformer.
Safe overloads for distr ibution transformers can be
determined from an analysis of the daily load curves of
the various test groups. The frequency of occurrence of
the peak-load con ditio n is of grea t s ignificance (Fig. 1)
.
A knowledge of load character is t ics i s impor tant not
only to the uti l i ty, but also to the uti l i ty industry
sup
plier . If load characteristic information which is suffi
ciently representative for the entire electr ical uti l i ty
in
dustry could be obtained, i t might serve as the basis for
evaluating the over-all economics of design changes in
equ ipm ent such as dis t r ib ut ion t ransformers . The se
de
s ign changes could, in turn, provide greater operat ing
economy to the uti l i ty.
D iges t o f pape r 57 - 168 , D i s t r i bu t io n S ystem
Load Char ac te r i s t i c s and Th e i r
U s e in P lann ing and D es ign , r ecom mend ed by the A I EE
Com mi t t e e on
Tr ans m is s ion and D is t r ibu t ion an d app r oved by the
A I EE Tec hn ica l O p
e r a t ions D epar tm en t f o r p r es en ta t ion a t t he A
I EE W in te r G ener a l M ee t ing ,
New York, N. Y. , Jan . 21-25, 1957. Schedu led for publ ica t
ion in A IEE
Power Apparatus and Systems.
R. H . Sar ikas is with the I l l inois P ower C om pan y, De
catu r , 111. H. B.
Tha cker i s w i th the Wes t inghous e E lec t r i c Co r p . ,
P i t t s b u r gh , P a .
Sarikas Thacker Load Characteristics
ELECTRICAL ENGINEERING
