Unit Commitment Constraints

EE1401 POWER SYSTEM OPERATION AND CONTROL CONSTRAINTS IN UNIT COMMITMENT: Unit commitment problem can have many constraints depending on different rules imposed on scheduling by individual power system, power pool, reliability council etc. Spinning reserve is the total amount of generation available from all units synchronized (i.e., spinning) on the system, minus the present load and losses. When a generator is lost, there must be ample reserve on other units to compensate up for the loss in a specified time period. Reserve is calculated as a percentage of forecasted peak demand, or it must be capable of making up the loss of the most heavily loaded unit in a given period of time. It is also sometimes calculated as a function of the probability of not having sufficient generation to meet the load. Reserve capacity must be allocated based on unit being fast-responding or slow-responding. Unit commitment problem include “scheduled reserves” or “off-line” reserves that can be brought on-line, synchronized, and brought up to full capacity quickly. Reserve must be spread around the power system so that they can be made use of even in the case of islanding when system gets disintegrated due to some problem. Thermal units require a team of personnel to operate them, especially when turned on and turned off. Since temperature changes can only be gradual some hours are required to bring the unit on-line. Due to these restrictions in the operation of a thermal plant, various constraints arise, such as • Minimum up time: once the unit is running, it should not be turned off immediately • Minimum down time: once the unit is decommitted, there is a minimum time before it can be recommitted. • Crew constraints: In a plant with more than one unit there may not be enough personnel to attend both the units if both are turned on or off at the same time and hence at the same time both can not be turned on or off. • A certain amount of energy is expended to bring the unit on-line. This is not generated and is included in the unit commitment problem as a start-up cost. Hydro-Constraints Unit commitment if separated from the scheduling of hydro-units as a separate hydrothermal scheduling or coordination problem may not result in an optimal solution. Must Run Some units must-run during certain times of the year for voltage support on the transmission network or for supply of steam for uses outside the steam plant itself. Fuel Constraints Some units may have limited fuel, or else have constraints that require them to burn a specified amount of fuel in a given time, presenting a challenging unit commitment problem. Ref: Allen.J.Wood and Bruce F.Wollenberg, ‘Power Generation, Operation and Control’, John Wiley & Sons, Inc., 2003.

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