UNIT COMMITMENTAND ECONOMICDISPATCH SOFTWARETO OPTIMISE THE
SHORT-TERM
SCHEDULINGOF ELECTRICAL POWER
GENERATIONINTRODUCTION
Electricity generating companies and power systems have the problem ofdeciding how best to meet the varying demand for electricity, which has adaily and weekly cycle. The short-term optimisation problem is how toschedule generation 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
constraints that must be satisfied. If the electricity company has
responsibility for satisfying the demand for electricity, then the mostimportant constraint is that the total generation must equal the half-hourly
forecast demands.
There are two related short-term optimisation problems, 'unit
commitment' and 'economic dispatch'. Unit commitment is the processof deciding when and which generating units at each power station tostart-up and shut-down. Economic dispatch is the process of deciding
what the individual power outputs should be of the scheduled generating
units at each time-point. Unit commitment is a very challenging
optimisation problem, because of the astronomical number of possiblecombinations of the on and off states of all the generating units in the
power system over all the time-points in the study period.
Power Optimisation is an independent consultancy, which develops unit
commitment software called POWEROP for electricity companies. The
software considers both unit commitment and economic dispatch
simultaneously, which improves the quality of the calculatedschedules. The software is customised to the unique features of theelectricity company being modelled and to the precise requirements of
the user, and it can be integrated with the user's own computer systems.
One version of the unit commitment software, developed for Northern
Ireland Electricity (NIE), has been used every day since December 1996 toschedule the generating units in the Northern Ireland powersystem.

The users at NIE consider the unit commitment software to beindispensable and they report that the schedules it produces are
consistently of a very high quality.
Other versions of the unit commitment software are being used for
self-scheduling by a number of generating companies under the British
Electricity Trading and Transmission Arrangements (BETTA) in the British
electricity market. Each version is customised to the individual
requirements of the user.

ADVANTAGESBack to top
The software is based on a proprietary multi-stage version of amathematically rigorous optimisation method, the Mixed Integer Linear
Programming (MILP) Method. This has the following advantages:

·The schedule produced by the software is always feasible, that is
it satisfies all the constraints modelled by the software if the data
are self-consistent. (If the data or constraints are notself-consistent, then a feasible solution does not exist; in suchcases, the less important constraints are relaxed automatically by
the software so that a usable schedule is always produced.)·

The schedules produced are better than those that could be foundmanually or by 'priority-order' methods, leading to large savingsin annual fuel costs and higher annual profits.

·

A wide variety of constraints and plant types can bemodelled, including complicated scheduling constraints, non-linear
cost curves, energy-limited plant, power-purchase agreements andemission constraints.

·

It is relatively easy and quick to introduce new constraints andfeatures and to modify the software as circumstances change.

·

The software is robust to changes in operating conditions andrelative fuel costs, because no prior assumptions are made about
the nature of the solution.

·

As a result of using the proprietary multi-stage version of the MILP
method, the software finds good feasible schedules much fasterthan if it were to use the standard MILP method.

FEATURES
Back to top
The objective of the optimisation is to minimise the total fuel costs overthe study period whilst satisfying the appropriate constraints.
Alternatively, the objective of the optimisation can be to 'maximise'profit', defined as the total revenue from electricity sales minus fuelcosts. The study period, which is divided into half-hourly time-intervals,
can be from a single half-hour up to several days.

The software takes into account any combination of the following
features and constraints of the power system being modelled:

Types of Generating Units
Back to top
The software can model the operation of both thermal andhydro-electric generating units and of electricity contracts with othercompanies. Thermal units can include coal-fired, oil-fired, gas-fired andnuclear-powered steam turbines, and combustion turbines burning
distillate (often called gas turbines). Thermal units can also include
dual-fired generating units, which can burn one of two alternative fueltypes. The software can model and optimise the use of mixtures ofdifferent fuels and/or gas contracts in the same generating unit orgroup of units. The hydro-electric units can include conventional hydrounits and pumped storage units. Gas-fired units and hydro-electric units
are treated as energy-limited plant. Upper or lower energy limits may
also be applied to other types of generating units and may be used, forexample, to represent limited fuel supplies or 'take or pay contracts'.

System Constraints
Back to top·

The total output of all the generating units must be equal to theforecast value of the system demand at each time-point. The
penalty for not doing so is set by the user, so this constraint may bemade 'hard' or 'soft' depending on the user's requirements.

·

The total spinning-reserve from all the generating units must be
greater than or equal to the spinning-reserve requirement of thesystem. This can be either a fixed requirement in MegaWatts (MW)or a specified percentage of the largest on-load output of any
generating unit. Again, the user can choose how strongly to enforcethis constraint. The purpose of the spinning-reserve requirement isto ensure that there is enough spare capacity from the units on-loador 'spinning' at any time to cover the accidental loss of any
individual generating unit, or to satisfy demands that are higher
than their forecast values. The precise definition of how muchspinning reserve a particular unit supplies can be customised to theuser 痴 requirements.
·

The software can also impose a requirement for 30-minutereserve. This is spinning reserve plus the total capacity of off-load
quick-start plant that can be started-up and dispatched within 30
minutes of an emergency. (The latter is sometimes called standing
reserve.)·

The software can also impose a negative-reserve requirement.
This ensures that there are sufficient generating units runningabove their Minimum Stable Generation levels at all times to allow
the total output to be quickly reduced by a specified number of MW.
(There may be a variety of reasons for such a requirement - forexample to cover the possibility of demands being lower than theirforecast values.)·

The software can equalise the outputs of identical generating
units at any one time, but only if this does not cause violations of
any other constraints.

Transmission Constraints
Back to top·

The user can define groups of generating units which are in export
or import-limited transmission-constrained zones. The total
import or export of power from these zones must be less than orequal to the specified transfer limit, after taking into account theforecast local demands in the zones.

Cost Characteristics of Generating Units
Back to top·Each generating unit has a 'no-load' or fixed operating cost and anumber of incremental operating costs, which can define a
non-linear profile of operating costs. These costs can alternatively
be expressed as fixed and incremental heat rates multiplied by afuel cost, in which case the fuel costs can vary over the study
period. The incremental costs or heat rates do not have to be
monotonically increasing.

·

Each generating unit has either a single start-up cost or a numberof warmth-dependent start-up costs corresponding to a numberof warmth conditions of each generating unit (e.g. hot, warm, cold)
as determined by the time that unit has been off-load.

·

Also modelled is works power, i.e. the power taken from theelectricity grid when running a particular generating unit or before agenerating unit is synchronised with the grid. Like start-up costs,
works power can be warmth-dependent, depending on thewarmth condition of the generating unit when it starts up, as
determined by the time that unit has been off-load.

Scheduling Constraints on Individual Generating Units
Back to top·

Each generating unit can have minimum on and off times.

·

There can be upper limits on the numbers of start-up events ofeach generating unit. These are expressed as a limit on the number
in each 'day', where the first 'day' begins at the study start.

·

The user can specify inflexible running of generating units. This
forces a generating unit to run over a specified time-period, with an
output not less than a specified value, provided that the system
demand is greater than a specified value (which can be zero).
Alternatively, the output of a generating unit can be forced to take aspecified value over a specified time-period.

Dispatching Constraints on Individual Generating Units

Back to top·

When a generating unit is on, its power output must be at or above
its Minimum Stable Generation value, except when thegenerating unit is starting up or shutting down.

·

The output of a generating unit must not exceed a specified
maximum value.

·

The user can specify capacity restrictions on generating units.
These reduce the maximum outputs of the units over specified
time-periods. Alternatively, the user can specify different values of
the available capacities of generating units in different time-periods(this is called 'profiling the availabilities').

·

Each generating unit has multiple-segment spinning-reservecharacteristics which depend on the output of that unit.

·

Generating units can be subject to warmth-dependent run-up
rates. When such a unit starts-up, it begins operating at a power
output below its Minimum Stable Generation, and it runs-up to
Minimum Stable Generation by following a warmth-dependent
non-linear run-up profile, depending on the time that the unit waspreviously off-load.

·

Generating units are subject to maximum loading and deloading
rates when running above Minimum Stable Generation. There canbe more than one loading rate for each generating unit, and the user
can specify MW values for breakpoints which define the output
values at which the applicable loading rate changes. In the sameway, there can also be more than one deloading rate for eachgenerating unit.

·

Generating units can be subject to run-down rates. When such agenerating unit shuts-down, it follows a non-linear run-down profile
below its Minimum Stable Generation.

Station Constraints
Back to top·

There can be 'station synchronising intervals' and 'station

desynchronising intervals' at some power stations. These areminimum time gaps between the start-ups and shut-downs ofgenerating units at the same power station.

Dual-Fired Generating Units
Back to top·

Dual-fired generating units can use one of two different fueltypes at a time, for example either coal or oil, but not a mixture of
the two fuels. The software can optimise the scheduling and power
outputs of such dual-fired units, taking into account the costs, heatrates and maximum power outputs for each fuel type. Alternatively,
the user can force a particular fuel type to be used by a dual-fired
unit at any time, for non-economic reasons. When the unit switchesfrom one fuel type to the other, while staying on-load, the software
does not impose any start-up costs or run-up profiles on the second
fuel type.

Mixed-Fuel Generating Units
Back to top·

Mixed-fuel generating units can use a continuously-varying
mixture of different fuel types at a time, for example oil and gas.
The software optimises the scheduling, power outputs and fuelusage of such mixed-fuel units, taking into account the costs, heatrates and maximum power outputs for each fuel type.

Gas Constraints (or other types of fuel constraint)
Back to top·

Gas generating units may be subject to energy-limits, in the formof minimum and maximum daily takes of the total gas used at a
power station or of individual gas contracts. Such daily energy

limits can also be applied to other types of generating unit if desired.
·

The gas contracts can be of the 'take or pay' type, in which there is
one price for the gas below the minimum daily take and anotherprice for gas above the minimum daily take.

·

There can be gas from more than one contract or supplier used
at a power station, with each gas contract having its own prices,characteristics and energy limits.

·

There can be minimum and maximum flow rates of the total gas
used at a power station or of individual gas contracts.

·

The total power output from all the gas generating units can belimited by a 'gas impact factor', which multiplies the system
demand at each time-point.

Pumped Storage
Back to top·

The software can optimise the use of 'pumped storage'hydro-electric units, taking account of the energy lost in thepumping cycle. The software optimises the times and amounts bywhich to pump and / or generate from the pumped storage units,
and when the pumped storage units should be off.

·

The water stored in the upper reservoir of the pumped storage
units can be kept between specified upper and lower limits, toprevent spillage or drainage of the reservoir, allowing for any inflow
of water into the reservoir. The user can also specify minimum,
maximum and target values for the reservoir level at the end of thestudy period. He can also determine how firmly the target level isenforced by choosing appropriate penalty costs.

Electricity Contracts
Back to top·

An electricity contract with another company can be modelled(using the software) as a pseudo-generating unit or as a 'demand'

unit. This option offers all the features described above, for example
ramp rates, energy limits, and minimum on and off times, which canbe useful if the electricity contract contains such features.
For the British electricity market, the software also has an option forthe direct modelling of the electricity contracts which are offered on
power exchanges or by electricity brokers. Electricity contracts (alsoknown as power contracts) may be of type 'buy' or 'sell', The
software models the details of the eelctricity contracts that are
offered on the British contracts market. Electricity contracts may
optionally be grouped, so that if one contract in a group is accepted
then all the other contracts in that group must be accepted. This
direct modelling of electricity contracts allows the software toconsider a much larger number of such contracts than if they weremodelled as pseudo-generating units. This provides guidance to theelectricity company as to which contracts to accept and at whatvolumes, in order to maximise profits, whilst taking into account theknock-on effects of accepting those contracts on the outputs of thephysical generating units, including the effects of ramp rates.
The software also has an option to meet a particular demand
pattern or desired 'Net Contract Position' using electricity contracts
only; this option can be used by an electricity trading company thathas no generating plant of its own. In this case, the software
recommends which electricity contracts are most profitable toaccept among the possibly very large number of available contracts,
and it identifies arbitrage opportunities that may exist but which
may be very difficult to find manually.

Initial Conditions
Back to top·

At the user's option, the software can either use specified valuesof the initial conditions or it can choose its own initialconditions. Using specified values of the initial conditions would beappropriate for short-term operational use. The option of letting thesoftware choose its own initial conditions is suitable for longer-termplanning.

Import and Export across an Interconnector
Back to top·

The software can be used to optimise the import and export ofpower from and to a neighbouring electricity utility via aninterconnector. It can also be used in a 'what-if' mode to evaluate
the costs or benefits of proposed power transfers across theinterconnector.

·

Alternatively, generation and demand may be modelled on bothsides of the interconnector, with user-specified half-hourly
values for the limits on the flow across the interconnector,
which then behaves like a transmission constraint. It is also possible
to treat the demand requirements as 'harder' on one side of theinterconnector than the other.

Minimising Changes from a Previous Schedule
Back to top·

The software has an option to minimise changes from apreviously calculated schedule, whilst taking account of any
changes in demand and plant availability that have occurred since
the previous run of the software. This can be useful if circumstanceschange suddenly, but the system operator does not wish to change
many of the previously issued instructions to the power stations.

Discouraging Changes in Outputs
Back to top·

The 'ramping' of generating units, i.e. changing their power outputs,

causes wear and tear and increases maintenance costs. The
software allows the user to specify a penalty cost on ramping
each generating unit, which is applied per MW change of output.
This discourages unnecessary ramping, and where ramping isneeded, the user can indicate a preference for changing the outputsof one generating unit rather than another, by specifying different
values for the ramping penalty costs of different generating units.

BETTA VERSIONSBack to top
Some versions of the unit commitment software have been speciallydeveloped for self-scheduling by generating companies under the British
Electricity Trading and Transmission Arrangements (BETTA) in the
British electricity market.
The BETTA versions of the software schedule generating units against
half-hourly 'Net Contract Positions' in MegaWatt Hours (MWh). The
software can model general market prices for electricity, and can also be
used to model individual 'buy' and 'sell' contracts for electricity with their
particular characteristics and prices (see the section on electricity
contracts above). This provides guidance to traders as to which contracts
to accept and at what volumes, in order to maximise profits, whilst taking
into account the knock-on effects of accepting those contracts on the
outputs of the physical generating units, including the effects of ramp
rates.
The BETTA versions of the software calculate minute-by-minute output
profiles for each generating unit for the 'physical notifications' that are
required under the BETTA rules. These output profiles satisfy theconstraints on the generating units, whilst minimising imbalances between
the total integrated output profiles and the Net Contract Positions. The

study period of the BETTA versions of the software can be up to several
days long, with half-hourly time-intervals.

VERSION USED BY NORTHERN IRELAND ELECTRICITY
Back to top
The version of the unit commitment software developed for Northern
Ireland Electricity (NIE) has been used every day since December 1996 toschedule the generating units in the Northern Ireland power system. This
has a variety of thermal generating units, consisting of gas, coal andoil-fired steam turbines and combustion turbines, and a number of
combined cycle gas turbine (CCGT) stations. The total gas used by thegas-fired units and the CCGTs are subject to minimum and maximum daily
takes and minimum and maximum flow rates. There are interconnectors
with Scotland and southern Ireland.
The software takes account of the complicated operating rules andonerous spinning-reserve requirements of the Northern Ireland powersystem. It minimises the total fuel cost over the study period of one day
divided into 48 half-hours. The users at NIE consider the unit
commitment software to be an indispensable tool for scheduling theNorthern Ireland power system in the presence of difficult fuel constraints,
and they report that the schedules produced by the software areconsistently of a very high quality.

USER INTERFACE
Back to top
Power Optimisation can provide a sophisticated user-interface to the unit
commitment software, based on Microsoft Excel
workbooks. Alternatively, the unit commitment software can be used as asolution engine, which communicates via simple text files with auser-interface developed by the customer.

CUSTOMISATION OF FEATURES
Back to top
Power Optimisation has a policy of customising its unit commitment
software to meet the individual needs of the user. In fact, each of thecurrent users has a version of the software that has been tailored to its
own requirements. Note that the features described above are the sum
total of the features in all of the different versions of the software, rather
than in any one particular version.

CONTACT INFORMATION
Back to top

To discuss how this software may be of benefit to your company
and for further information,please contact:
Power Optimisation, Woodside Avenue,
Beaconsfield,
Buckinghamshire, HP9 1JJ, England,
United Kingdom.
UK Telephone: 01494 675175.
International Telephone: +44 1494
675175.

E-mail: info@powerop.co.uk