Artificial Intelligence in Power System Optimization

CHAPTER

3

UNIT COMMITMENT

*

3.1 INTRODUCTION

Since electrical load varies over a daily period and a weekly period in a cyclic

manner, electric utilities plan in advance an economic schedule to decide which

generating units to switch on/off and connect to/disconnect from the network

system at a certain time. In general, load profi les aren’t uniform. Total loads on

the electric power system are generally higher during day time and early evening

when industries are (still) active while household demand picks up towards the

evening, and lower during late evening and early morning when most people

sleep. Due to less economic activity, the overall load is lower over the weekends.

Suffi cient reliable power generation to meet the peak load demand must therefore

be synchronized prior to the actual load occurrence. Simply committing a suffi cient

number of units to cover the maximum system load and leaving them running

online for the entire duration scheduled may not be economic because the supply

is probably excessive. Note that to ‘commit’ a generating unit is to ‘turn it on’, i.e.,

to bring the unit up to speed, synchronize it to the system and connect it so that

it can deliver power to the network. Since an appropriate schedule can save costs

signifi cantly, unit commitment (UC) is essential to provide an economic on/off

schedule regarding unit characteristics and system restrictions such as reserve and

network constraints etc. It determines start-up, shut down, production levels of all

units in each period considering unit operation constraints and cost.

In general, each generating unit has its own characteristics such as minimum

up and down time, minimum and maximum generation level, and up/down ramping

limit. Furthermore, there are some system requirements such as power balance,

spinning reserve, transmission line limit, fuel supply limit, etc. UC is a scheduling

problem usually covering a discrete time range from 24 hours (1 day) to 168 hours

(1 week) ahead and is handled by the operator in the pre-dispatch stage. An electric

utility system operator, who has the knowledge of system components and operating

*This chapter has been written with assistance from Nit Petcharaks