Introductory Lecture
Unit Commitment/Economic
Dispatch
Joseph Fisher (PhD)
Senior Lecturer
Unit Commitment (UC) -Definition
• The problem of unit commitment (UC) is to decide which
units (generators) to interconnect over the next T hours,
where T is commonly as time of the day in few hours to
several hours, 24 or 48 hours, or even 1 week to meet the
anticipated load demand.
• The usual practice is that Power Companies keep Daily Load
Curve for 365 days in a year, which provides and anticipated
load demand for each day.
Unit Commitment
Given load profile
(e.g. values of the load for each hour of a day)
Given set of units available
When should each unit be started, stopped and how much
should it generate to meet the load at minimum cost?
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Load Profile
Typical Load Variations
PNG Ramu Grid Load Curve
Which generator to
commit to meet the
load demand at the
least cost?
Realistic and Reliable Power System
The consumers should get the requested power (e.g.
a 60 W bulb), when they push the on-button. This
should work no matter generation outages in power
stations.
The consumers should get an acceptable voltage,
e.g. 240 V, in the outlet.
A power system should be kept with a realistic
reliability
A power system should be kept in an economic and
sustainable way.
UC PROBLEM
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UC is an Optimization Problem
An optimization problem is a mathematical
and computing problem of finding the best
solution (either with least cost or maximum
profit) from all feasible solutions. For
example, Salesman Problem to travel within
provinces at the least cost (transport fare) to
promote his/her products.
Subject to: These Constraints
Real power balance constraint,
Real power operating limits,
Spinning reserve constraint
Minimum uptime & downtime of unit, and
Startup cost of the unit
• Optimization problem: Minimize fuel cost
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Minimize fuel cost
The sum of
generators 1 to N
The sum of fuel cost
for each generator 1
to N at a point in
time
The required power output
to meet the load demand plus
losses in time (at the
least/minimal cost).
Operating fuel cost – The operating fuel cost is the sum of the costs of
the fuels burnt for each generator
Constraint 1
• REAL Power (MW) Balance
Constraint 2
• Real power operating limits
Generator capability curve
Synchronous generators are rated in terms of
the maximum MVA output that they can carry
continuously without overheating, at a
specified voltage and power factor.
Example: PMin= 50 MW, PMax= 320 MW (Operating limits of a Unit)
Constraint 3
• Spinning reserve constraint
• The spinning reserve is the amount of unused capacity in online energy
assets which can compensate for power shortages or frequency drops
within a given period of time.
• Solar with no battery storage has no spinning reserve. Once partial shading
or cloud cover takes place, the power collapses.
• Large Hydro have spinning reserves that the power demand increases, the
gate valve opens up to release more water to meet the increased in the
load demand to maintain stability in frequency.
Constraint 4
• Minimum uptime and downtime of unit
• Minimum uptime is the minimum operation time of the unit to
operate, whereas minimum downtime is the minimum operation
time to not operate during the operation.
Constraint 5
Startup cost of the unit: the fixed cost to start the unit
The start up costs depends on the status of the unit.
If the unit is banked with the boiler in hot standby condition, a hot
start up cost is incurred.
If the unit is in cold standby, a cold start up cost is imposed when a
unit is started.
The cold start up cost depends on the time from shutdown of the
unit.
Other Constraints (?)
• Environmental Constraints
• Climate Changes
• Shift in government policies (energy pricing, ban on coal power,
plants, etc..)
Problem Definition
Given load
Given set of generator units on-line
How much should each unit generate to meet this load at minimum
cost?
UC
Unit Commitment (UC)
Given load profile (e.g. values of the load for each hour of a day)
Given set of units available
When should each unit be started, stopped and how much should it
generate to meet the load at minimum cost?
Tutorial 1
Three generators with the specified cost functions ($/hr) and
specified operation limits (MW) are required to meet a 550MW of
load at a minimum cost.
How much should each unit or combination of units generate at
minimum cost.
Tutorial 1
Unit 1:
• PMin= 250 MW, PMax= 600 MW (Operating limits of Unit 1)
• C1= 510.0 + 7.9 P1 + 0.00172 P12 $/h (Cost function of Unit 1)
Unit 2:
• PMin= 200 MW, PMax= 400 MW (Operating limits of Unit 2)
• C2= 310.0 + 7.85 P2+ 0.00194 P22 $/h (Cost function of Unit 2)
Unit 3:
• PMin= 150 MW, PMax= 500 MW (Operating limits of Unit 3)
• C3= 78.0 + 9.56 P3+ 0.00694 P32 $/h (Cost function of Unit 3)
What combination of units 1, 2 and 3 will produce 550 MW at minimum
cost?
How much should each unit in that combination generate?
Cost for Various Combinations
Observations from this Tutorial
Far too few units committed:
Can’t meet the demand
Not enough units committed:
Some units operate above optimum
Too many units committed:
Some units below optimum
Far too many units committed:
Minimum generation exceeds demand
End OF Lecture