module 5 part 4 unit commitment

1. ELECTRICAL MATERIAL SCIENCE
FARIS K.K [ AP. EEE ] Al-Ameen Engg College , Shoranur
1
Ee 306 power system analysis Chapter – 5 part 2
SYLLABUS
Unit commitment – introduction- constraints on unit commitment-spinning reserve- thermal unit
constraints – hydro constraints
5.1 UNIT COMMITMENT
Unit commitment is the process of deciding when and which generating units at each
power station to start-up and shut-down to meet the load demand.
Defenition: - Determination of Generation mix to achieve estimated output level to meet the
demand of electricity for a specified time interval while satisfying all constraints.
5.2 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. They are
 Spinning reserve
 Thermal units Constraints
 Hydro Constraints
 Must Run Constraints
 Fuel Constraints
5.3 SPINNING RESERVE
 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.
 It is must be allocated among fast responding units and slow responding units based
on far drop in system frequency.
 This allows the automatic generation control system to restore the specified
frequency.
5.4THERMAL UNITS CONSTRAINTS
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

2. ELECTRICAL MATERIAL SCIENCE
FARIS K.K [ AP. EEE ] Al-Ameen Engg College , Shoranur
2
 Minimum up time: once the unit is running, it should not be turned off immediately
 Minimum down time: once the unit is turned off , there is a minimum time before it can be
turned on.
 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 cannot be turned on or off.
5.5 HYDRO CONSTRAINTS
Most of the power systems are hydro electric units. It depends on the availability of water.
5.6 MUST RUN CONSTRAINTS
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.
5.7 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.
5.8 COMPARISON BETWEEN UNIT COMMITMENT & ECONOMIC DISPATCH
UNIT COMMITMENT ECONOMIC DISPATCH
It is the process of when and which
generating units at each power station to
start up& shutdown
It is the process of distributing load among
various generating units based on cost
UC focus on power system reliability ED focus on computational approach
In UC, units are committed based on their
efficiency
In ED, all units are committed for optimal
result