Unit Commitment

EE1401 POWER SYSTEM OPERATION AND CONTROL UNIT COMMITMENT ‘Unit’ refers to ‘generating unit’ and to ‘commit’ a generating unit is to ‘turn it on’. That means, start and bring the unit up to speed, synchronize it to the power system to make it deliver power to the network. To commit enough units and leave them online, whatever be the load demand is uneconomical. It is expensive to run more generating units than what is required and decommitting or turning them off, when they are not needed, can save considerable money. For example, assume that there are three units and each unit’s minimum and maximum MW limits and fuel costs are known. Let unit 1 be the most economic and unit 3 the most expensive. Then the optimum commitment will be to first supply from unit 1 and the load can be supplied by operating closer to its best efficiency and then when the load increases the second unit will be committed with both units run at their best efficiency possible so that the cost is minimum. The third unit is committed when the load increases sufficiently so that it is economical to commit unit 3 than to supply the load from two units.

Figure shows a unit commitment schedule in the case of a simple peak - valley pattern of load variation. Unit 1: Min = 150 MW, Max = 600 MW – Most economical Unit 2 Min = 100 MW, Max = 400 MW Unit 3:Min = 50 MW, Max = 200 MW – least economical.

But the unit commitment problem is not just meeting the load and is much more than this as there are other constraints to be considered. Reference: Power Generation, Operation and Control – A J Wood & R F Woolenberg.

MEENAKSHI COLLEGE OF ENGINEERING, CHENNAI 78