Power Flow
Economic Dispatch
PSS/E-30.2
Users Manual
4.8 Economic Dispatch
The Economic Dispatch facility, accessed by selecting Power Flow>Changing>Economic dispatch (ECDI), provides a combined unit commitment and economic dispatch utility. This procedure
places machines in a specified subsystem on or off line to satisfy a given subsystem minimum
capacity. The in-service machines in the subsystem are then dispatched on the basis of equal incremental cost to meet a specified total subsystem generation.
The Economic dispatch process requires minimum and maximum outputs, incremental heat rates,
fuel costs, and start-up priority rankings for all machines to be scheduled. This data should reside
in source file form in an Economic Dispatch Data File (*.ecd), which is read at initiation of the
procedure.
4.8.1 Creating the Economic Dispatch Data File
Each machine to be dispatched must have a dispatch data record specified for it in the Economic
Dispatch Data File. The exceptions are those machines that are the "supplementary" units of a dispatch group. The following standard data record is applicable for all individual machines which will
be dispatched and those machines which are the "principal" machines of a dispatch group. Data
requirements for the supplementary machines in a dispatch group are shown in the following
Section 4.8.1.2.
4.8.1.1 Standard Record Format
Each machine to be dispatched, except for those machines that are the "supplementary" units of a
dispatch group, must have a data record specified for it in the Economic Dispatch Data File in the
following format:
I,ID,PRIOR,FUELCO,PMAX,PMIN,HEMIN,X1,Y1,X2,Y2,X3,Y3,X4,Y4,X5,Y5,X6,Y6
where:
I
Bus number; bus I must be present in the working case and have a nonzero generator
number assigned to it.
ID
One- or two-character machine identifier of the machine at bus I whose data is specified by this record. ID = ’1’ by default.
PRIOR
Priority ranking code (> 0). Machines with a priority ranking of zero do not have their
status changed during the unit commitment portion of the process. Machines with
lowest positive priority ranking codes are switched in-service first even though
machines with higher priority rankings may be more economical to run. Machines with
highest priority ranking codes are switched out-of-service first even though machines
with lower priority rankings may be more expensive to run. PRIOR = 0 by default.
FUELCO
Fuel cost for the machine in dollars per MBtu. No default is allowed.
PMAX
Maximum machine active power output; entered in MW. If this machine is the "principal" unit of a dispatch group, this is the sum of the maximum outputs of all machines
in the dispatch group. If defaulted, PMAX and PMIN for this machine are set to the
power limits currently in the power flow case (see Section 3.2.4).
PMIN
Minimum machine active power output; entered in MW. If this machine is the "principal"
unit of a dispatch group, this is the sum of the minimum outputs of all machines in the
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dispatch group. If PMAX is defaulted, the value specified for PMIN is ignored and the
one currently in the power flow working case is used; otherwise, no default is allowed.
HEMIN
Minimum heat input required by the machine when on-line; entered in MBtu/hr. If X1
(see below) is greater than zero, HEMIN should be specified as the heat input required
at "X1" MW. No default is allowed.
Xi,Yi
Points on the incremental heat rate curve; Xi values are entered in MW and Yi values
are entered in Btu/kWh. At least two points, and up to six points, may be entered. Both
X and Y must be in ascending order, with X1 > PMIN and Xn < PMAX. If this machine
is the "principal" unit of a dispatch group, this curve is the combined curve of all
machines in the dispatch group.
For the user’s reference, the PSS/E Example directory contains an economic dispatch file,
savnw.ecd, which can be used in conjunction with the power flow case savnw.sav. A plot of those
incremental heat rate curves is shown in Figure 4-151.
Figure 4-151. Incremental Heat Rate Curves Provided in the PSS/E savnw.sav Case
The auxiliary program PLINC may be used to plot the incremental heat rate curves of selected
machines. Refer to PSS/E Program Operations Manual, Chapter 11 for details on its use.
4.8.1.2 Supplementary Unit Dispatch Groups
A special record format is recognized on which a "supplementary" unit of a dispatch group is specified. This record has the following format:
I,ID,PRIOR,FRACT,PRNBUS,PRNMAC,CODFRC
where:
I
Bus number; bus I must be present in the working case with a nonzero generator
number assigned to it.
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ID
One- or two-character machine identifier of the machine at bus I whose data is specified by this record. ID = ’1’ by default.
PRIOR
Priority ranking code (< 0). A negative priority ranking code signifies that this machine
is a "supplementary" unit of a dispatch group. No default is allowed.
FRACT
When CODFRC is zero or negative, the fraction of the total power dispatched for the
dispatch group to be assigned to the "principal" unit. When CODFRC is positive, the
fraction of the total power dispatched for the dispatch group to be assigned to this "supplementary" unit. FRACT must be greater than zero and less than one. No default is
allowed.
PRNBUS
Bus number of the "principal" machine associated with the dispatch group of which this
"supplementary" unit is a member. No default is allowed.
PRNMAC
One- or two-character machine identifier of the "principal" machine at bus PRNBUS
associated with the dispatch group of which this "supplementary" unit is a member.
PRNMAC = ’1’ by default.
CODFRC
Defines the interpretation of FRACT on this record by the economic dispatch process.
When CODFRC is zero or negative, FRACT is the fraction of the total power dispatched
for the dispatch group to be assigned to the "principal" unit. When CODFRC is positive,
FRACT is the fraction of the total power dispatched for the dispatch group to be
assigned to this "supplementary" unit. CODFRC = 0 by default.
A Standard economic dispatch data record (see Section 4.8.1.1) must be included in the data input
file for machine PRNMAC at bus PRNBUS. Its power output limits and incremental heat rate curve
must be that corresponding to the combination of all machines in the dispatch group.
4.8.2 Application Notes
A dispatch group provides a means of handling a group of machines which are to be treated as a
single entity in the unit commitment and economic dispatch calculations. A combined cycle plant,
in which multiple steam units and combustion turbines are to be dispatched as one unit, may be
handled as a dispatch group. The high pressure, low pressure pair of a cross compound unit may
also be treated as a dispatch group.
To specify a dispatch group, any one (and only one) of the machines in the dispatch group must be
designated on a standard Economic Dispatch Data File record (see Section 4.8.1.1); this machine
is referred to as the "principal" unit of the dispatch group. The dispatch data specified on this data
record must be that for the combination of all machines which are members of the dispatch group.
Each of the remaining units in the dispatch, must be specified on a supplementary Economic Dispatch Data File record (see Section 4.8.1.2); these machines are referred to as the "supplementary"
units of the dispatch group.
The sum of the FRACT values on all of the "supplementary" data records of a dispatch group must
be less than one. When a dispatch group contains more than two machines, the data record for
each of its "supplementary" machines must have CODFRC specified as a positive number, and
FRACT as the fraction of total group power assigned to the "supplementary" machine. When a dispatch group contains exactly two machines, CODFRC for the "supplementary" machine may be
specified as a positive or negative number or zero, and FRACT must be specified accordingly (see
Figure 4-152).
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Figure 4-152. Assignments for Supplementary Machine Economic Dispatch
The initial service status of each machine in a dispatch group, as determined by the combination of
the type code of the bus to which it is connected and its machine status flag, must be the same:
either all in-service or all out-of-service. If the status of a dispatch group is switched during the unit
commitment calculation, the status of each machine in the dispatch group is changed.
For a dispatch group, at the completion of the dispatch calculation, the dispatch group’s total power
output is split among the machines in the group according to the split fractions FRACT specified on
the data records of the "supplementary" machines in the group (see Section 4.8.1.2).
When PMAX (and hence PMIN) are defaulted in the Economic Dispatch Data File, the machine
limits contained in the working case are used as the dispatch limits (see Section 4.8.1.1). For a dispatch group, the sums of the limits of all machines in the group are used as the dispatch limits. In
this case, it is possible to violate the individual machine limits as contained in the working case if
FRACT is not coordinated with the individual machine limits.
When a subsystem of the power flow working case is being processed, the subsystem assignment
of the "principal" unit of a dispatch group is taken as the subsystem assignment of the group. If the
"principal" unit is in the subsystem being processed, its dispatch group is processed even if one or
more of its "supplementary" units is not in the subsystem. Conversely, if the "principal" unit is not in
the subsystem being processed, the dispatch group is not processed even if some or all of its "supplementary" units are in the subsystem.
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When the subsystem to be processed is specified by owner, the owners to which machines (rather
than buses) are assigned are used in defining the subsystem of machines to be processed. If a
machine has multiple owners (see Section 3.2.4), the owner specified in the owner/fraction pair with
the largest ownership fraction is assumed to have dispatch responsibility. If this largest ownership
fraction is identical for two or more owners of a machine, the first one in the tables, as shown in the
machine data listings is used.
The iterative dispatch calculation in the economic dispatch process is a binary search technique
with an upper limit of 40 iterations. Convergence is assumed when the difference between dispatched power and desired power is less than 0.00001 times the desired power. Convergence
failure, which is usually the result of precision limitations, is alarmed and the economic dispatch process continues as if convergence had been achieved.
4.8.3 Performing an Economic Dispatch
Selecting the Power Flow>Changing>Economic dispatch (ECDI) option from the Main Menu will
display the Economic Dispatch dialog (see Figure 4-153).
The subsystem selector is available for the user to identify which specific subsystems of the power
flow case ("Area", "Zone", "Owner", "base kV") or which specific buses are to be processed. The
dispatch is performed on only those machines in the specified subsystem for which valid dispatch
data had been read. Other machines retain their initial status and power outputs.
The user is provided with the facility to identify the Economic Dispatch Data File (type *.ecd) to be
processed and to decide whether of not to base the subsequent dispatch on an existing unit commitment of a new one. If a new unit commitment is selected, all machines in the specified subsystem
with a nonzero priority ranking for which dispatch data had been read are placed out-of-service prior
to commencing the unit commitment calculation. Otherwise, the current unit commitment status profile is used as the starting point.
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Figure 4-153. Launching the Economic Dispatch Procedure
After defining the subsystem to be processed and identifying the data file, clicking the Go button will
invoke the process and output a summary of the predispatch condition to the Output Bar or output
device of the user’s choice.
This is the result obtained using the savnw.sav power flow case and savnw.ecd file.
The first four quantities in the above summary are totals based upon the present power outputs and
dispatch data specified in the data input file for those machines initially in-service in the subsystem
to be processed. The final total is the sum of the present power outputs of those machines to be
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processed, and those machines in the specified subsystem for which no dispatch data was provided
and whose outputs will not be changed by the economic dispatching process.
Following the summary of predispatch conditions, the process will prompt the user for additional
information including the Desired load and Desired minimum capacity of units being dispatched,
as shown in Figure 4-154. The values specified for these two quantities should be entered as those
applying to the machines to be dispatched. Specifically, they must not include the power outputs
assigned to those machines within the specified subsystem for which dispatch data was not read.
In Figure 4-154, a new load level has been selected for the savnw.sav power case prior to performing the economic dispatch.
Figure 4-154. Redispatching Load and Generation
After clicking the OK button, the process commences the unit commitment portion of its calculation.
Note again that the status of any machine with a priority ranking of zero is not changed during the
unit commitment portion of the process but such machines are included in the economic dispatch
calculation. Neither the status nor power outputs of machines for which either no dispatch data was
provided or whose data was alarmed, and of machines outside of the specified subsystem, is
changed during this process.
If the specified desired minimum capacity is greater than the sum of the maximum power outputs
of the machines currently on-line, the unit commitment process places additional units on-line.
Machines with lowest priority rankings are connected first, and, within a given priority ranking, units
with the lowest full load average cost per MW are placed in-service first.
If the specified desired load is less than the sum of the minimum power outputs of the machines
currently on-line, units are taken off-line. Machines with highest priority rankings are disconnected
first, and, within a given priority ranking, units with the highest full load average cost per MW are
placed out-of-service first.
Any time a machine status is changed, a message is printed at the Output Bar. Any processing
errors encountered are alarmed and the user is asked to respecify the desired load and minimum
capacity.
Once a commitment profile satisfying the desired load and capacity constraints has been successfully determined, the power outputs of the machines to be dispatched are calculated. Power outputs
are set on the basis of equal incremental costs subject to the machine power output limits.
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The dispatch calculation uses an iterative approach and, at the end of each iteration a convergence
monitor is printed which tabulates:
•
The iteration number.
•
The total power mismatch.
•
The incremental cost.
•
The change in incremental cost.
Following the dispatch calculation, the production cost and the incremental cost along with the postdispatch totals of dispatched power, capacity limits of the dispatched machines, and subsystem
generation are tabulated. Plant power outputs and reactive power limits are updated to reflect the
commitment and dispatch calculation results.
Results are summarized in the Output Bar or output device selected by the user. For the example
shown in Figure 4-154, using the savnw.sav power flow case, the results summary and convergence monitor are shown below in Figure 4-155.
Figure 4-155. Summary Results of the Economic Dispatch Process
The dispatch data as read from the Economic Dispatch Data File is not retained with the working
case file following termination of the process. A data input file is required for each new execution.
The dispatch process responds to the following interrupt control codes:
•
AB - Abandon following the completion of the next dispatch iteration.
•
NC - Suppress the dispatch convergence monitor.
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4.8.4 Viewing Economic Dispatch Results
Following completion of the dispatch calculation, the plant power outputs and reactive power limits
outputs are updated to reflect the commitment and dispatch calculation results. One means to
examine the results is to use the Limit Checking report to look at machine terminal conditions. In
Figure 4-156 those conditions can be seen for the savnw.sav power flow case before and after the
economic dispatch calculation example described here.
Figure 4-156. Before and After Machine Terminal Conditions
for Economic Dispatch Example
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