U.S. patent number 3,683,161 [Application Number 05/070,274] was granted by the patent office on 1972-08-08 for computing economic power distribution in power tools.
This patent grant is currently assigned to Leeds & Northrup Company, Philadelphia, PA. Invention is credited to Bruce F. Wollenberg, Walter O. Stadlin.
United States Patent |
3,683,161 |
|
August 8, 1972 |
COMPUTING ECONOMIC POWER DISTRIBUTION IN POWER TOOLS
Abstract
The economic dispatch of generation in interconnected areas is
computed by determining the generation which will cause the
incremental cost of power at each of the interarea tie points, as
calculated from the interconnected areas, to be equal at the
existing load level. A computer having its own loss matrix is
utilized in each area. It computes the tie point costs on the basis
of cost information sent from the areas interconnected to it, and
it also sends its own costs to those areas. In each area the
desired generation and net tie line interchange are computed to
provide a basis for controlling the area's generation.
Inventors: |
Walter O. Stadlin (Eagleville,
PA), Bruce F. Wollenberg (Chalfont, PA) |
Assignee: |
Leeds & Northrup Company,
Philadelphia, PA (N/A)
|
Family
ID: |
22094279 |
Appl.
No.: |
05/070,274 |
Filed: |
September 8, 1970 |
Current U.S.
Class: |
705/412; 376/215;
307/57 |
Current CPC
Class: |
G06Q
10/04 (20130101); H02J 3/00 (20130101); G06Q
50/06 (20130101); Y04S 10/50 (20130101); Y02E
40/70 (20130101) |
Current International
Class: |
H02J
3/00 (20060101); G06Q 10/00 (20060101); G06f
015/56 (); G06f 015/06 (); H02j 003/06 () |
Field of
Search: |
;235/151.21,150 ;307/57
;444/1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eugene G. Botz
Assistant Examiner: Edward J. Wise
Attorney, Agent or Firm: William G. Miller, Jr. Raymond F.
MacKay
Claims
1. A method of operating a computing system to compute the economic
distribution of the load among the power sources in each of a group
of areas interconnected for transmission of power therebetween when
at least two of the areas are interconnected by a plurality of
transmission lines, comprising the steps of automatically computing
a first incremental cost of power at a boundary point on each tie
line between each area and the areas interconnected thereto, said
computation being based on the incremental costs and the
incremental transmission losses in that area, automatically
computing in each of the areas interconnected thereto, a second
incremental cost of power at the same boundary points based on the
incremental costs and the incremental transmission losses in the
interconnected areas, automatically comparing for each area the
first and second incremental costs calculated for each of the
boundary points, and automatically computing in accordance with the
results of said comparisons the magnitude of power generation
required from each of the power sources and the net interarea tie
line interchange for the respective areas to obtain equality
between the sum of the combination of the total actual generation
and interarea tie line interchange and the combination of the total
desired generation and the desired computed interarea tie line
2. The method of claim 1 in which the computing system includes a
computer for each of the areas to make the automatic computations
for the areas.
3. The method of claim 1 in which the computing system carries out
all of
4. The method of claim 2 in which the computer in each particular
area sends to the computers of each of the areas to which it is
interconnected a signal representative of the incremental cost of
power at the boundary point of each of the individual tie lines to
those interconnected areas for the existing level of incremental
cost of delivered power for the
5. The method of claim 4 in which the computer in each particular
area sends to the computer of each of the areas to which it is
interconnected a signal representative of the level of power flow
associated with the incremental cost sent to the interconnected
areas and signals representative of the rate of change of the
incremental cost at each of
6. A method of operating a computing system to compute the economic
distribution of the load among the power sources in each of a group
of areas interconnected for transmission of power therebetween when
at least two of the areas are interconnected by a plurality of
transmission lines, comprising the steps of automatically computing
a first incremental cost of power at a boundary point on each tie
line between each area and the areas interconnected thereto, said
computation being based on the incremental costs and the
incremental transmission losses in the area involved in
computation, automatically computing in each of the areas
interconnected thereto, a second incremental cost of power at the
same boundary points based on the incremental costs and the
incremental transmission losses in the interconnected areas,
automatically comparing for each area the first and second
incremental costs calculated for each of the boundary points,
automatically computing from the results of said comparison the
magnitude of power interchange required on each of the tie lines to
obtain an economic interchange of power, automatically computing in
each area an incremental cost for generating power from each source
of the area, automatically computing in accordance with said
last-named costs the associated desired generation for each of the
sources, comparing the sum of the combined total generation and
total desired tie line flow for the area with the combination of
the total actual generation and the total actual tie line flow for
the area, and adjusting the incremental cost of delivered power for
the area to bring said sum towards zero.
Description
This invention relates to a method for the computation of the
allocation of generation as between a plurality of generators
interconnected in groups to form separate areas with the areas in
turn being interconnected by tie lines to form a power pool. More
particularly, this invention relates to a method for computing the
generation required of each of the generators making up the
separate areas to establish for the power pool a minimum total cost
for the power generated by the pool for the purpose of obtaining
maximum economy of operation while meeting the load requirements of
the pool and its scheduled interchange with other pools.
In the past, the computation of the allocation of generation among
the separate generators of the interconnected areas of a pool has
involved the use of computers which utilize a loss matrix or
similar method for introducing into the computations the effect of
transmission losses. The parameters of the matrix related to the
overall computation problem involved in the pool as contrasted with
a loss matrix which utilizes parameters which related only to the
particular losses involved in the individual areas of the pool.
Thus, in earlier systems such as the system shown in U.S. Pat. No.
3,400,258, issued to one of the present inventors on Sept. 3, 1968,
means have been described for the calculation of the desired
generation for each of the generating stations of the separate
areas of the pools but by virtue of the incorporation of the
constants relating to the transmission losses of the pool in a
single matrix in addition to the incorporation of the loss constant
dealing with a particular area in still another matrix there has
been a duplication of computational facilities in order to make
possible the separate and individual operation of the areas in the
pool with predetermined tie line flows between them as compared
with the operation of the pool with an economic distribution of the
total generation so that the tie line flows between the areas
carried out the economic distribution desired.
In some systems for computing the generation at the various
generating stations of each area as well as the power interchange
between the areas, it has been necessary to utilize means for
dealing with a plurality of interconnecting tie lines between some
of the individual areas by calculating an average condition for
those tie lines as, for example, in the system described in
"Economic Control of Interconnected Systems" by Leon K. Kirchmayer,
published by John Wiley & Sons, 1959.
Still other systems for calculating the values of generation and
the interchange power between the areas have been disclosed, for
example, in the Kirchmayer U.S. Pat. No. 3,117,221, issued Jan. 7,
1964, wherein there has been incorporated a computation not only of
the transmission losses but also of the cost of wheeling power
through an area.
It is, therefore, an object of this invention to provide a novel
method for determining the desired generation for the generators in
the separate areas interconnected to form a power pool so as to
constantly maintain maximum economy of operation for the pool
consistant with restrictions on generator and tie line loading.
A further object of this invention is the provision of a novel
method for determining the desired generation for the generators of
the stations in the separate interconnected areas of the pool as
required for maximum economy of operation of the pool while taking
into account the transmission losses on the tie lines
interconnecting the areas.
A still further object of this invention is the provision of a
novel method for establishing signals representing the desired
generation for the generators making up the pool as may be required
for satisfying the load of the pool while taking into account the
transmission losses on tie lines between the areas with the use of
a separate computer for each of the areas.
Still another object of this invention is the provision of a means
for computing the economic distribution of total generation in a
power pool for maximum economy without the utilization of a pool
loss matrix or its equivalent.
In carrying out the present invention there is provided a method
for automatically computing the economic distribution of the
generation in a group of areas interconnected for the transmission
of power therebetween when at least two of these areas are
interconnected by a plurality of transmission lines. This method
comprises several steps of which the first is the automatic
computation of the incremental cost of power at a boundary point on
each tie line between that area and the areas interconnected
thereto, based on the incremental generation costs and the
incremental transmission losses in that area. The method also
includes the step of automatically computing in each of the areas
interconnected thereto a second incremental cost of power at the
same boundary points mentioned above based on the incremental cost
of power generation and the incremental cost of transmission losses
in the interconnected areas. In addition, there is included the
step of automatically comparing for each area, the first and second
incremental costs calculated for each of the boundary points as set
forth above and then as a final step there is an automatic
computation from the results of this comparison, of the magnitude
of the power generation required from each of the power sources and
the net interarea tie line interchanges on each of the ties between
the respective areas as needed to maintain equality between the sum
of the total actual generation and the total actual interchange of
the area and the sum of the desired generation and the desired tie
line interchanges as computed.
FIG. 1 is a diagrammatic showing of a power pool showing the tie
lines between the individual areas and the communications channels
needed between the computers of the areas.
FIGS. 2, 3, 4 and 5 are block diagrams of parts of the algorithm to
be followed by a digital computer in making the necessary
calculations.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a power pool 10 which includes 4 interconnected areas,
namely areas 1, 2, 3 and 4. Area 1 is shown as having transmission
lines 12 and 13 connecting it to area 2 for the interchange of
power therebetween. Area 1 is also connected by transmission lines
14 and 15 to areas 4 and 3, respectively. There is also shown a
plurality of tie lines 16 and 17 connecting area 4 and area 3 for
interchange of power between those areas while the tie line 18
connects area 3 to area 2. Areas 1, 2 and 4 have tie lines 20, 21,
22 and 23 which connect to other power pools. For example, tie
lines 20 and 21 connect area 1 to an external power pool and the
tie lines 22 and 23 connect areas 2 and 4 respectively to other
pools.
As shown in FIG. 1, each of the areas may incorporate a number of
generation sources such as the generator 24 whose actual output
P.sub.ga is supplied to a station or generator bus 26 at which
point the incremental cost of generating power C.sub.g may be
determined as will be explained subsequently. The power generated
in each of the areas is absorbed by the loads of the areas (not
shown) and is also transmitted as required over the individual tie
lines interconnecting each individual area to other areas of the
pool.
Each of the areas of the power pool 10 is shown as having a
computer such as the computer 31 of area 1 whose purpose is to
compute the desired generation, P.sub.gd, for each of the
generators in the area as well as the desired net tie line
interchange .SIGMA..sub.P.sub.+A P.sub.td for that area. This
computation in each of the computers 31-34 is preferably made by a
digital computer in accordance with a program following the
algorithm to be described subsequently. This computation requires
that each of the computers have as an input the measured value of
the actual generation for each of the generators in the particular
area P.sub.ga as well as a measured value for the actual tie line
flow over the tie lines into the particular area P.sub.ta. In
addition to these measured values, which are made in the particular
area in which the computer is located (the local area), the
computer of each area must receive from the computer of each area
to which it is interconnected (the foreign areas) the following
information based upon values computed during the last economic
dispatch computation made by the computers of the foreign areas:
P.sub.t ].sub.f = the tie line power flow over each tie line to the
local area from a foreign area. C.sub.t ].sub.f = the incremental
cost at a tie point on each tie line associated with the flow of
the power P.sub.t ].sub.f. .differential.C.sub.t
/.differential.P.sub.t ].sub.f = the rate of change of the
incremental cost of power flow over each tie line with changes in
tie line power flow evaluated in the region of P.sub.t ].sub.f.
In addition it will be necessary to introduce into the computers
31-34 measured values of the sum of the tie line interchanges
between the particular area and the interconnected areas of the
pool .SIGMA.P.sub.ta which for area 1 in pool 10 of FIG. 1 is the
summation of the tie line flows over the ties 12-15. Also, it is
necessary to sum up the tie line flows to other pools as, for
example, by measuring .SIGMA.P.sub.ta which in FIG. 1 is shown as
including the measurement of the tie line power flow over the ties
20-23.
As will be described subsequently, it will be necessary to
calculate the incremental cost C.sub.t of power at the tie points
on the interarea ties such as shown on tie line 12. The points at
which these costs may be calculated can be at a particular bus to
which the tie is connected or at other points, wherever it is
convenient for purposes of power system operation.
It will be noted that each of the computers 31-34 has communication
channels shown by dashed lines interconnecting the computers in the
areas which are interconnected by tie lines. Over the communication
channels 41, 42 and 43 the necessary information is sent from the
respective computers 32, 33 and 34 to computer 31 while over the
channels 41 and 42 similar information is sent back from computer
31 to computers 32 and 33. The communications over channel 43
supplies information between computer 34 and computer 31. Computer
34 is in the reference area, namely area 4. Communications channels
46 and 47, respectively, connect the computers 34 and 33 in one
case and 33 and 32 in the other case with communications over both
channels going in both directions. Thus, as will be evident from
FIG. 1, it will be necessary to have communication channels between
all areas which have interconnecting tie lines between them for the
purpose of interchanging the information described above.
Fundamental economic theory has demonstrated that the incremental
cost (dollars/MWh) at any point on a power system must be the same
when computed from all connecting areas for the system to be in
economic balance. This principle has been applied to generators
within an area as, for example, in U.S. Pat. No. 2,836,730 issued
to E. D. Early on May 27, 1958 and U.S. Pat. No. 2,836,731 issued
to W. G. Miller, Jr. on May 27, 1958. Likewise, the principle can
be applied to the interchange among areas as illustrated in U.S.
Pat. No. 3,117,221 issued to L. K. Kirchmayer on Jan. 7, 1964 and
also illustrated in U.S. Pat. No. 3,400,258 issued to W. O.
Stadlin, one of the present inventors, on Sept. 3, 1968. Further
theoretical background for this proposition may be found in the
previously mentioned publication "Economic Control of
Interconnected Systems," by Leon K. Kirchmayer.
For control of power interchange between areas of an
interconnection to be advantageous to the areas, it is necessary
for the incremental costs of power at the boundary of
interconnected areas to be the same when it is calculated from
either of the interconnected areas. In general an area's
incremental cost at the boundary will increase with increased power
flow out of the area because of the necessity for increasing its
total generation. For the purposes of the computations described
herein, the functional relationship between the incremental cost
and the tie line power flow can be considered monotonic though it
need not necessarily be considered linear. It will be evident that
optimum operation of the power pool 10 of FIG. 1 can be defined as
that operation which causes an interchange over the tie lines
within the pool as well as over the tie lines to external pools as
necessary to produce the greatest monetary benefits for the areas
of the pool, that is, areas 1-4.
For the purpose of computing in each of the areas the desired
generation for each of the generators in the area and the desired
power interchange over the ties to the area, it is advantageous to
make the computations in the area by treating each of the tie lines
to the area in a manner equivalent to the treatment given to a
generator in the area. Thus, if a particular area views an
interconnecting tie line it has with another area as being
equivalent to a generator in its area whose cost function is
determined by the interconnected area, then the desired power flow
on the interconnecting tie line can be computed by comparing the
incremental cost of power at the tie point with the cost of other
sources of energy, that is the generators in its own area as well
as with the cost at other tie points. Thus, for each area, optimum
operation within an area is achieved when all sources are supplying
power at the same values of incremental cost of delivered power,
that is, the cost of power at the hypothetical load center of the
area.
For the purpose of computing in each of the computers 31-34 of FIG.
1 the desired generation of each of the generators of the separate
areas as well as the desired net interchange on the tie lines of
the areas, the computers may advantageously be programmed so as to
carry out the steps of computation set forth in FIGS. 2, 3 and 4,
which will now be explained in detail.
In FIG. 2 the economic dispatch computation program is entered
periodically, for example, every 5 minutes as indicated by block
60. The first block following the block 60 in the flow chart of
FIG. 2 is block 62 which indicates that the iteration carried on by
the program through the outer loop of the flow diagram is to be
carried on "N" times in order to converge to a solution. As
indicated by block 64, the computation of the incremental
transmission losses associated with power from each of the
generators of the area as well as the tie lines of the area
.differential.P.sub.L /.differential.P.sub.n is calculated for each
of the inputs to the loss matrix. This calculation which is carried
out in accordance with the equation shown in block 66 is a well
known calculation and is referred to, for example, on page 49 of
the above-mentioned Kirchmayer book, "Economic Control of
Interconnected Systems" and on page 75 of "Control of Generation
and Power Flow on Interconnected Systems," by Nathan Cohn,
published by John Wiley & Sons, Inc. in 1966. In the equation
in block 66, B.sub.nm is the appropriate transmission loss
coefficient while B.sub.n0.sub. is the transmission loss associated
with zero power from the source being considered while P.sub.m
represents the output of each of the power sources or transmission
lines being considered in computing the transmission losses.
As indicated by the block 68, the computation of block 66 is
continued for each of the loss matrix inputs and after the
transmission loss associated with each of the inputs is calculated,
the program then continues to the step indicated in block 70,
namely the setting of .lambda..sub.A to 8 and the setting of
.DELTA..lambda..sub.A to 4, .lambda..sub.A being the incremental
cost of power delivered to the hypothetical load center of the area
in which the computation is being made. The values 8 and 4 for
.lambda..sub.A and .DELTA..lambda..sub.A respectively are to be
considered as typical and may be altered to extend or contract the
range of .lambda..sub.A in the solution. For example, if we assume
that the particular program being discussed is being carried out by
computer 31 of FIG. 1, then .lambda..sub.A the incremental cost of
delivered power for area 1.
Having set the value of .lambda..sub.A and .DELTA..lambda..sub.A ,
the next step in the program is to enter a series of computations
which are repeated "M" times, as indicated by the block 72. The
value of M is chosen in accordance with the desired accuracy of
solution and may be typically set to 20. These computations include
first a setting equal to zero of the total desired generation of
the generating sources .SIGMA.P.sub.gd as well as a setting to zero
of the value of .SIGMA.P.sub.td, that is the sum of the desired tie
line flow over all of the tie lines between the areas
interconnected to the area for which the computation is being made.
These latter settings are indicated in block 74.
Having made the settings indicated in block 74, the program then
enters a portion which is repeated for each of the sources of
power, where the sources of power include not only the generators
but also the interarea tie lines. That repetition of the
computations for each of the sources is indicated by block 76 which
is followed by a branching point 78 which causes the program to
branch to the series of computations indicated in block 80 if the
source of power is a generator or to the series of computations
indicated in block 82 if the source of power is an interarea tie
line. Normally, the computations relating to the generators as
indicated in block 80 will be carried out first. These computations
include first a computation of the incremental cost at the station
or generator bus for the power provided by the generator. That cost
is indicated as C.sub.g and is calculated as a product of
.lambda..sub.A (the incremental cost of delivered power in the
area) and the quantity (1 minus the incremental transmission
losses) associated with the generation of the particular generator
involved.
The generation desired for the generator being considered is a
function of the incremental cost of generation at the generator
bus, and for a particular incremental cost as calculated by the
previous calculation there will be then an associated generation
value P.sub.g representing the amount of generation required to
provide power at the cost figure C.sub.g. The next step in the
computation is to compare the level of generation associated with
the computed cost figure C.sub.g with the generator's high and low
limits, that is, determine whether P.sub.g is greater than or equal
to the low limit P.sub.Lg or less than or equal to the high limit
P.sub.Hg. If P.sub.g is beyond one of the limits, then the desired
generation P.sub.gd will be set to equal either P.sub.Lg or
P.sub.Hg, depending upon which limit is exceeded; otherwise, the
desired generation P.sub.gd will be set to equal P.sub.g which was
previously computed as the function of the incremental cost C.sub..
The value P.sub.gd for a particular generator is then added to the
total of the values .SIGMA.P.sub.gd which have been accumulated as
a result of the same computations for other generators and the new
total, .SIGMA.P.sub.gd, as a result of this computation will then
be available for summing with the desired generation calculated for
the next generator to be considered. The last step of the
computation as shown in block 80 is to store (save) the value
P.sub.gd for the particular generator being considered as well as
the sum of the desired generations.
Following the computations set forth in block 80, the program then
continues as indicated by block 84 until the computations have been
done for each source of power, as indicated by block 76. If we
assume that all of the generators have been considered and the
calculations of the desired generation for each of them has been
determined, then the block 78 will cause the computations in block
82 to be made for each of the interarea tie lines.
The computations for the interarea tie lines include first a
computation of the incremental cost at the tie point C.sub.t which
is calculated in a similar fashion to the calculation for the costs
at the generator bus except that the incremental loss quantity is
computed with regard to the tie line being considered. After the
cost C.sub.t is computed, it is then necessary to determine the
power flow P.sub.t over the tie line which will provide the level
of power flow associated with the calculated cost C.sub.t. In order
to make this calculation, it is necessary to utilize information
transmitted from the area at the other end of the particular tie
line being considered; thus, as shown by the second equation in
block 82, the value of P.sub.t is determined by adding to the
negative of the tie line flow value P.sub.t ].sub.f transmitted
from the foreign area to which the tie interconnects a quantity
which is computed by dividing the difference between the cost at
the tie point C.sub.t and that which was computed in the foreign
area C.sub.t ].sub.f by the rate of change of the tie line cost in
the foreign area, namely .sqroot.C.sub.t /.sqroot.P.sub.t
].sub.f.
The value of P.sub.t is then compared as is done in block 80 with
the lower and higher limits set for the tie line, namely P.sub.Lt
and P.sub.Ht, and the desired tie line power flow P.sub.td is then
set equal to P.sub.t if P.sub.t is within the limits; otherwise it
is set equal to the particular limit which is exceeded.
Thus, P.sub.td, the desired power transfer over the tie line being
considered, is determined and stored and that value is added to the
previously accumulated total .SIGMA.P.sub.td for the tie lines
previously considered to get a new total .SIGMA.P.sub.td which is
also then stored for purposes of the next computation relating to
the tie line power flow.
Once all of the generators and all of the interarea tie lines have
been considered and the associated desired values for the
generation of the generators and the power flow over the tie lines
has been calculated, the computations carried out by blocks 80 and
82 are not continued and the comparison shown in block 86 is made.
There the absolute value of the sum of all of the calculated values
for the desired generation of the separate generators in the area
and the sum of the desired values of all of the interarea tie lines
to the area are compared with the measured total actual generation
of the generators of the area and the measured total of the tie
line power flows between the areas, and if the comparison gives a
value which is not less than a small number.epsilon. which is
established as a criteria for the accuracy to which the iteration
is to be carried, then the program begins a series of steps which
are intended to alter the value of .lambda. .sub.A either in an
upper or a lower direction depending upon the direction necessary
for convergence of the solution. The next step after that shown in
block 86 would be the step shown in block 88 which is carried on as
long as the program has not iterated more than "M" times, as shown
by block 72, or in other words as long as the block 89 indicates
that the program should continue to iterate the value of
.lambda..sub.A.
The consideration indicated by the steps shown in block 88 is
whether the sum of the total desired generation of the generators
in the area and the total desired power flow over the interarea tie
lines is greater than the actual generation in the area and the
actual power flow over the interarea tie lines. If the desired
values are greater than the actual values, then, as shown in block
90, .lambda..sub.A is decreased by a value .DELTA..lambda..sub.A.
The value of .DELTA..lambda..sub.A, as computed in block 90 for the
next iteration, is computed as one half of the present
.DELTA..lambda..sub.A whose value may initially be 4, as indicated
by block 70. .lambda..sub.A may start at a value of 8 as indicated
by block 70. The program then progresses from block 90 to block 72
and another iteration is carried out. If the total desired values
for the generation and the interarea tie line flows are not greater
than the actual values, then the value for .lambda..sub.A is
increased by a value .DELTA..lambda..sub.A. .DELTA..lambda..sub.A
is altered as in block 90, and the program progresses to block 72
and is continued until the computation converges sufficiently or
has been carried out "M" times as indicated by block 72.
Should the necessary convergence as tested in block 86 fail to
materialize within the "M" times that the iteration is carried out,
then block 89 will cause the program to transfer to block 94 which
will exit from the program and indicate an error. Upon the
occurrence of an error, .lambda..sub.A will have reached either the
upper or the lower value of its range. For the initial value shown
in block 70 the range of .lambda..sub.A would be zero to sixteen
dollars per megawatt hour. An error with a correspondingly high
value of .lambda..sub.A would be an indication of either too little
generating capacity and/or too little tie line capacity.
When the comparison made in block 86 shows that the iteration of
.lambda..sub.A is completed within the desired accuracy, then the
program continues "N" times to convergence utilizing for the
computation in block 66 the new values computed for the various
sources, that is the generators and interarea tie lines as values
for P.sub.m. Actual interarea tie line values P.sub.ta could also
be used for P.sub.m, the choice depending on the convergence
characteristics of a particular power system. Once convergence has
been reached, the next step will be that shown in block 98, namely
a summation of the total desired interchange over the interarea
ties .SIGMA.P.sub.td with the actual measured interchange over the
ties from the area under consideration to external pools, namely
.SIGMA.P.sub.ta so as to thereby obtain the total net interchange
for the area .SIGMA..sub.P.sub.+A P.sub.td.
Having obtained the total net interchange for the area the next
series of steps in the program as shown by FIG. 3 is for the
purpose of determining the information to be sent to the
interconnected areas relating to the cost of power at the tie
points and the rate of change of that cost with changes in tie line
power flow. The first step in that determination is the step shown
in block 100 where a new value .lambda..sub.A is found by summing
the .lambda..sub.A previously stored with the value
.DELTA..lambda..sub.A. After that step, the program goes through
the series of calculations now to be described for each of the tie
lines "t" between the area in which this computer is operating and
the other areas within the pool which are interconnected to it.
A series of computations is carried out as shown in block 102. The
first calculation involves the computation of a tie point cost
which is to be transmitted from the local area to the
interconnected area to which the tie connects C.sub.t ].sub.l. That
cost is obtained by computing the product of .lambda..sub.A, as
obtained in block 100, times the quantity (1 minus the transmission
losses over the tie line).
Knowing the cost just calculated, it is possible to compute a
fictitious value for the power flow over the tie P.sub.t ' by
adding the value -P.sub.t ].sub.f, that is the value received from
the interconnected area to the quantity obtained by dividing the
difference between the local cost C.sub.t ].sub.l and the cost at
the same tie point C.sub.t ].sub.f as sent from the other area by
the change in cost with respect to the change in tie line flow as
determined and sent from the other area, namely
.differential.C.sub.t /.differential.P.sub.t ].sub.f which
represents the slope of the cost curve at the interconnected
area.
The value P.sub.t ' is then compared with the low limit P.sub.Lt
and the high limit P.sub.Ht and if the value P.sub.t ' is not
beyond either of the limits, then the value P.sub.td ' which
represents a fictitious desired tie line power flow will be set
equal to P.sub.t '; whereas if P.sub.t ' exceeds one of the limits,
P.sub.td ' will be set to that limit.
Having a fictitious value P.sub.td ' determined by changing the
value of .lambda..sub.A by .DELTA..lambda..sub.A, it is then
possible to determine a .DELTA.P by subtracting from the fictitious
value P.sub.td ' the computed desired tie line flow P.sub.td and
checking to see if that difference is equal to zero, as shown in
block 104. If it is not, the computation shown in block 106 is
carried out. In block 106 the computation involves a determination
of the change in cost at the tie points with changes in tie power
flow as determined from the area at which the computer is located,
namely .differential.C.sub.t /.differential.P.sub.t ].sub.l by
multiplying the quantity (1 minus the incremental transmission
loss) by the quantity .DELTA..lambda..sub.A and dividing by
.DELTA.P and then subtracting 2 times the constant B.sub.tt (which
represents the self constant of the tie as it relates to tie line
losses) and also multiplying by .lambda..sub.A.
If the quantity .DELTA.P is equal to zero and thus the statement in
block 104 is true, then the incremental cost change calculated for
the local area, as shown in block 108, is set at a maximum value
K.sub.max and the program proceeds to block 110 where the value for
the tie line power flow calculated at the local area P.sub.t
].sub.l is set equal to P.sub.td and as indicated by the block 112,
the computation is then repeated for another tie line.
Once the computation in block 106 is made, the next step after that
computation is to determine whether or not the calculated
incremental cost at the tie point, as computed in block 106, was
equal or less than zero, as shown in block 109. If the value was
less than or equal to zero, then the incremental cost in the local
area would be set to a minimum value of K.sub.min, as shown in
block 111 and the program would progress into block 110; whereas if
the value of the incremental tie point cost was not equal to or
less than zero, the program immediately progresses to block 110 and
the value of the incremental tie costs for the local area as
computed in block 106 is stored as the incremental cost to be sent
to the interconnected area along with the sending of the value
P.sub.t ].sub.l and the value C.sub.t ].sub.l.
From the above description of FIG. 3 it will be evident that by
incrementing the value of .lambda..sub.A the program has computed a
cost figure C.sub.t ].sub.l representing the cost of the power
provided by this area to the tie point when the power flow is at a
value P.sub.t ].sub.l and a comparable incremental change in tie
cost .differential.C.sub.t /.differential.P.sub.t ].sub.l is also
transmitted so that those three items of information can be
utilized in the interconnected area as a basis for determining the
amount of power flow which is desired over the tie line for
economic operation.
The above discussion of FIGS. 2 and 3 relates specifically to the
computations which would be carried on in the computers in areas 1,
2 and 3 of FIG. 1. Similar computations would be carried on in
computer 34 of area 4 with the exception of a few minor changes
which will now be discussed. Since area 4 is acting as the
reference area, it is necessary to take into account in determining
the incremental cost the magnitude of the interchange between the
reference area and the power pool and the external power pool to
which it may be connected as by the tie line 23. This change will
be evident from the modifications of the algorithm shown in FIG. 2
which should be made as indicated in FIG. 4 for the computer 34 of
area 4. In FIG. 4 the blocks 98a, 86a and 88a indicate that those
portions of the algorithm have been changed, the block 98a being
utilized in place of the block 98 of FIG. 2 while the blocks 86a
and 88a, respectively, replace blocks 86 and 88 of FIG. 2. It will
be noted that the block 98a is placed in a different part of the
flow chart as compared with the block 98 of FIG. 2 for it is
advantageous to make the calculation set forth in block 98a prior
to the entry of the iterative portions of the program where the
value calculated in block 98a is utilized as, for example, in block
86a and 88a.
In block 98a the desired net interchange of area 4, that is the net
interchange over the interconnecting tie lines 14, 16, 17 and 23,
.SIGMA..sub.P.sub.+A P.sub.td, is computed as the value
.SIGMA.P.sub.ta representing the actual total net interchange
between pool 10 and the external pools as measured over the
interconnections 20, 21, 22 and 23 minus the quantity
.SIGMA.(.SIGMA..sub.P.sub.+A P.sub.td).sub.f which represents the
sum of the desired interchange values computed in the foreign
areas, that is, the areas interconnected to area 4 and transmitted
to area 4 for this computation over additional communication
channels not shown in FIG. 1.
Between block 98a and block 86a, the steps of the computation
necessary in area 4 are the same as those previously described for
the other areas.
In area 4, the determination as to whether or not the incremental
cost iteration has been completed is based upon the absolute value
of the desired generation of area 4 plus the desired net
interchange of area 4 minus the actual generation of area 4 and
also minus the actual net interchange of area 4. As shown in block
86a, that absolute value is compared with .epsilon. and if it is
not less than .epsilon., the iteration of .lambda..sub.A continues
as stated in block 89 and the test shown in block 88a is made.
In block 88a the test comprises the comparison of a quantity which
is a sum of the desired generation as computed for area 4 and the
desired tie line interchange as computed for area 4 with the
respective actual values for those quantities. The program then
proceeds in a similar fashion as described with regard to FIG.
2.
The above discussion of FIG. 4 relates specifically to the
computations which would be carried on in the computers in area 4
of FIG. 1. Different computations could be carried on in the
computer 34 of area 4 when it is desirable to eliminate the need
for additional communication channels by making the minor changes
which will now be discussed. The calculations shown in blocks 86
and 88 would be varied from those shown in FIG. 2 and instead would
be as shown in FIG. 5, wherein the block 86b and the block 88b
respectively replace the blocks 86 and 88 of FIG. 2. As shown in
block 86b, the test made involves the comparison of the sum of the
desired generations as calculated, namely .SIGMA.P.sub.gd, with the
sum of the measured generations of the area generators
.SIGMA.P.sub.ga. If the absolute value of the difference is less
than .epsilon., then the next step in the computation would be
carried out by block 96 as indicated in FIG. 2; whereas otherwise
the next step would be carried out by block 89 which continues the
computation by carrying out the comparison shown in block 88b where
the value .SIGMA.P.sub.gd is compared to the value .SIGMA.P.sub.ga
to see if it is greater than that value. In accordance with the
results of that comparison the value .lambda..sub.A is either
increased or decreased as previously explained and as set forth in
blocks 90 and 91 of FIG. 2.
Under some conditions it is advantageous to utilize in the pool a
computer, at one area only, which acts as a master computer and
which receives information from each of the areas of the pool
indicative of the actual generation of each of the generators in
those areas as well as the tie line flows. With that information
this master computer can compute the desired generation values for
each of the generators of the separate areas as well as the desired
flow over the individual tie lines and that information can be
transmitted to the respective areas as well as being utilized in
the master computer. Basically, this arrangement allows for the use
of one master computer without the necessity of using a loss matrix
for all of the tie lines in the pool. It will be evident to those
skilled in the art that either of the systems described in detail,
namely for each of the areas with their own computer or with
systems described with a master computer or satellite computer at
the other areas, could be used depending upon which would be most
advantageous for the particular pool involved.
In conjunction with the computations set forth above, each of the
areas desirably incorporates a load frequency control system which
will effectively modify the generation of the generators in the
area in accordance with the computed desired values. The load
frequency control system may be operated as a permissive control
system utilizing the computed (desired) net interchange of the area
as a basis for determining whether the determination should be
increased or decreased. For example, the desired net interchange
for the area can be compared with the actual net interchange for
the area and the difference can be modified by the existing
frequency deviation in the area so as to produce an area control
error sometimes known as area requirement which can be utilized as
an input to a master controller. The master controller can then
produce signals such as pulses of duration corresponding to the
magnitude of the error and those pulses can be selectively allowed
to modify the setting of the governor motor of the generators in
the area in accordance with the relative value of the computed
value of the desired generation P.sub.gd for a particular generator
as compared with the actual generation P.sub.ga of that generator.
Such systems are well known and are illustrated in the above
mentioned book by Nathan Cohn entitled, "Control of Generation and
Power Flow in Interconnected Systems". Particular reference is
directed to that portion of the book following page 103 dealing
with "Control Executions". P.sub.gd, for example, may be used as
the base point setting for a generator. Participation settings,
which are usually used in conjunction with base points, can be
determined on the basis of the units regulating capability and
associated economic factors depending upon the nature of the
control system desired.
The combination of the computations described by FIGS. 2, 3, 4 and
5 and the simultaneous control of the generators in the several
areas is important in that a number of the computations involve the
use of actual measured values and when the above mentioned
computing procedure is combined with an effective control system
operable to carry out the desired economic distribution, the
computation will be effective to provide accurate values for the
desired generation of each of the generators and the desired net
interchange for the areas.
* * * * *