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Case study - Generator Scheduling in
the Electricity Industry
Peter Kelen - Power optimization
Limited
Power optimization, a consultancy based in
England, has created two types of scheduling applications
for the electricity generating industry using Xpress-MP.
One type of application, called unit commitment, optimizes
the short-term scheduling of the generating units at power
stations, for each half-hour of a day or week. The other type
of application, generator maintenance scheduling, optimizes
the start-weeks of maintenance overhauls for generating units
over a year. These are described in more detail below.
Electricity generating companies and power systems
have the problem of deciding how best to meet the varying
demand for electricity, which has a daily and weekly cycle.
As electricity cannot be stored, it is necessary to start-up
and shut-down a number of generating units at various power
stations each day. The unit commitment problem is to decide
when and which generating units to start-up and shut-down,
in order to minimise the total fuel cost or to maximise the
total profit, over a study period of typically a day, subject
to a large number of difficult constraints that must be satisfied.
The most important constraint is that the total generation
must equal the forecast half-hourly demands for electricity.
Unit commitment is a very challenging optimization problem,
because of the astronomical number of feasible combinations
of the on and off states of all the generating units in the
power system over all the time-points in the study period.
One version of the Power optimization unit commitment
software, developed for Northern Ireland Electricity (NIE),
has been used every day since December 1996 to schedule the
generating units in the Northern Ireland power system. The
users at NIE report that the schedules produced by the software
are consistently of a very high quality. As unit commitment
is such a difficult combinatorial optimization problem, the
software uses a novel multi-stage solution method, which drastically
reduces the computer run-time required to find an excellent
feasible schedule to just a few minutes. A great advantage
of using the Mixed Integer Linear Programming (MILP) method
is that it has proved to be easy and quick to introduce new
constraints and features into the unit commitment software,
in order to model the changes in the plant and operating rules
of the power system that occur from time to time.
Electricity generating companies also have the
longer-term problem of generator maintenance scheduling. This
is to decide in which weeks of the year to schedule the planned
maintenance overhauls for generating units, in order to minimise
annual fuel costs and to maximise the reliability of the power
system, whilst satisfying all the complex logical constraints
on when overhauls can take place. NIE has been using overhaul
optimization software developed by Power optimization since
May 1995 to schedule maintenance overhauls for the Northern
Ireland power system. NIE also uses the software to forecast
the weekly production from each power station over a year,
taking the random breakdowns of plant into account.
Xpress-MP
has been invaluable in the development of these applications.
The Modeller has made it much easier to represent complicated
and difficult constraints, and to experiment with different
mathematical formulations of the optimization problems. The
Optimizer has proved to be a very fast and robust solver,
which is of the required high quality for daily industrial
use for critical applications. By buildingXpress-MP
into the applications, there has been no need to worry about
keeping up-to-date with the latest advances in solution techniques
for MILP problems – these have automatically been incorporated
with each new release of Xpress-MP.
For more information, please visit web site
www.powerop.co.uk
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