Economic Load Dispatch

1. Economic Load Dispatch
By
Mangesh S Kulkarni
Sharad Institute of Technology College of Engineering,
Yadrav-Ichalkaranji

2. • Process of allocating generation levels to the
generating units in the mix, so that the system
load is supplied entirely and most economically
• The objective is to calculate, for a single period of
time, the output power of every generating unit so
that all the demands are satisfied at the minimum
cost, while satisfying different technical
constraints of the network and the generators

3. Overview
• Long term system planning: Production cost
– Decide what to build
• Hourly to monthly decisions: Unit commitment
– Decide which plants to have warmed up and ready to go
– Different technologies have different requirements
• Minutes to Hour: Economic dispatch
– Decide which plants to use to meet the expected load now
– 5 minutes to 1 hour
• Cycles to Minutes: Short term system operations and
Load Flow Model
– Maintain supply and demand balance moment to moment
– ~17msec per cycle up to 5 minute control functions

4. System Constraints
• Constraints that affect more than one unit
– Load/generation balance
– Reserve generation capacity
– Emission constraints
– Network constraints

5. Reserve Capacity Constraint
• Unanticipated loss of a generating unit or an
interconnection causes unacceptable frequency
drop if not corrected rapidly
• Need to increase production from other units
to keep frequency drop within acceptable
limits
• Rapid increase in production only possible if
committed units are not all operating at their
maximum capacity

6. Environmental constraints
• Scheduling of generating units may be affected by
environmental constraints
• Constraints on pollutants such SO2, NOx
– Various forms:
• Limit on each plant at each hour
• Limit on plant over a year
• Limit on a group of plants over a year
• Constraints on hydro generation
– Protection of wildlife
– Navigation, recreation

7. Network Constraints
• Transmission network may have an effect on
the commitment of units
– Some units must run to provide voltage support
– The output of some units may be limited because
their output would exceed the transmission
capacity of the network

8. Start-up Costs
• Need to “balance” start-up costs and running
costs
• Example:
– Diesel generator: low start-up cost, high running cost
– Coal plant: high start-up cost, low running cost
• Issues:
– How long should a unit run to “recover” its start-up
cost?
– Start-up one more large unit or a diesel generator to
cover the peak?
– Shutdown one more unit at night or run several units
part-loaded?

9. Summary
• Some constraints link periods together
• Minimizing the total cost (start-up + running)
must be done over the whole period of study
• Generation scheduling or unit commitment is a
more general problem than economic dispatch
• Economic dispatch is a sub-problem of
generation scheduling

10. Algorithms Proposed for Economic Dispatch
Merit Order Loading, Range Elimination,
Binary Section, Secant Section, Graphical/Table
Look-Up, Convex Simplex, Dantzig-Wolf
Decomposition, Separable Convex Linear
Programming, Reduced Gradient with Linear
Constraints, Steepest , Descent Gradient, First
Order Gradient, Merit Order Reduced Gradient,
etc.

11. Economic Dispatch: Formulation
• The goal of economic dispatch is to determine
the generation dispatch that minimizes the
instantaneous operating cost, subject to the
constraint that
• total generation = total load + losses
T
1
1
Minimize C ( )
Such that
m
i Gi
i
m
Gi D Losses
i
C P
P P P


 
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