economic load dispatch with and without losses co-ordination equation unit commitment-constraints

1. With and without losses
Unit Commitment
By Asha Anu Kurian
Department of EEE
Saintgits College of Engineering

2.  Introduction
 Different constraints
 ELD neglecting losses
 With losses
 Methods of calculating penalty factor and loss coefficient

23. Part II

25. Cost
function

27. Co-ordination equation

32.  The system load changes in cyclic manner.
 It is not advisable to keep all the units available all the time.
 When system load decreases, it is better to shut down one or
more units.
 When the system load increases at a latter time, units are to
be brought in.
◦ Unit commitment problem is finding the shut down and commissioning
rule so that the total cost of generation over a period of time, say one
day, is minimum.

33.  The following are data pertaining to three units in a plant.
Unit 1: Min. = 150 MW; Max. = 600 MW
C1 = 5610 + 79.2 P1 + 0.01562 P1 2 Rs / h
Unit 2: Min. = 100 MW; Max. = 400 MW
C 2 = 3100 + 78.5 P 2 + 0.0194 P 2 2 Rs / h
Unit 3: Min. = 50 MW; Max. = 200 MW
C 3 = 936 + 95.64 P 3 + 0.05784 P 3 2 Rs / h
What unit or combination of units should be used to supply a load of 550 MW most
economically?
To solve this problem, simply try all combination of three units. Some combinations
will be infeasible if the sum of all maximum MW for the units committed is less than
the load or if the sum of all minimum MW for the units committed is greater than the
load. For each feasible combination, units will be dispatched using equal incremental
cost rule studied earlier. The results are presented in the Table below.

37. 1. Spinning Reserve
Total amount of generation available from all synchronized
units on the system – (present load +losses).
Spinning reserve must be carried out such that loss of one or
more units does not cause too far a drop in system frequency.
Should obey certain rules set by reliability council,that specify
how the reserve is to be allocated.
Typical rule
1.Spinning reserves are kept in power system to meet the
unexpected load demand or failure of any generator etc.
2.Spinning reserve are usually allocated as a percentage of given
forecasted peak demand or

38. 3.Reserve should be capable of making up of losses of heavily loaded
line
4.Reserve should be function of Probability of not having sufficient
generation to meet load
5.Reserve must be allocated wisely between fast responding unit and
slow responding unit which allows c the agc to restore the frequency
and interchange quickly in the event of generating unit outage
Beyond spinning reserves,other classes of reserve involve
Scheduled Reserve or off line reserve
This includes quick start diesel or gas turbine units or most
hydro units or pumped storage hydro units-can be brought online
,synchronised and brought to full capacity
These reserves must be spread around the power system
 to avoid transmission system limitations
To run various parts of the system as island if they need to be
electrically disconnected

39. 1.Minimum up time: When a thermal unit is brought in, it cannot be turned off
immediately. Once it is committed, it has to be in the system for a specified
minimum up time.
2.Minimum down time: When a thermal unit is decommitted, it cannot be
turned on immediately. It has to remain decommitted for a specified
minimum down time.
3.Crew constraint: A plant always has two or more generating units. It may
not be possible to turn on more than one generating unit at the same time
due to non-availability of operating personnel.
4.Transition cost: Whenever the status of one unit is changed some transition
cost is involved and this has to be taken into account.

40. Hydro constraints:
Most of the systems have hydroelectric units also. The operation of
hydro units, depend on the availability of water. Moreover, hydro-
projects are multipurpose projects. Irrigation requirements also determine
the operation of hydro plants.
Must run unit:
Sometimes it is a must to run one or two units from the consideration of
voltage support and system stability.
Fuel supply constraint:
Availability of fuel is limited in some situations we need to account such
cases also.
Nuclear constraint:
If a nuclear plant is part of the system, another constraint is added. A
nuclear plant has to be operated as a base load plant only.

41.  A base load power plant is a power station that usually
provides a continuous supply of electricity throughout the
year with some minimum power generation requirement.