EE 1401 ECONOMIC DISPATCH CONTROL

1. What is the purpose of economic dispatch?
The purpose of economic dispatch (or) optimal dispatch is to minimize the fuel costs for the power system.
2. What is meant by unit commitment?
Unit commitment means optimum allocation of generators at each generating station at various station load levels.
3. Name the methods of finding economic dispatch.
The two methods to find economic dispatch are:
(i) Load scheduling
(ii) Unit commitment
4. What is meant by total generator operating cost?
The total generator operating cost includes the cost of fuel, cost of transmission loss, labor and maintenance costs.
5. What are the factors affecting the cost of generation? (or) List the various constraints in the modern power systems.
The cost of generation depends on operating constraints or system constraints. They are:
• Equality constraint.
• Inequality constraint
o Generator constraints
o Voltage constraints
o Running space capacity constraints.
o Transformers tap settings.
o Transmission line constraints.
6. When do discontinuities occur in the fuel cost curve and the incremental cost curve?
The discontinuities occur when the output power has to be extended by using additional boilers, steam condensers, etc. The discontinuities also appear if the cost represents the operation of an entire power station.
7. What are the advantages of using participation factor?
The advantages of using participation factor are:
(a) Computer implementation of economic dispatch is straight forward.
(b) Execution time for economic dispatch is short.
(c) It will always give consistent answers when units reach limits.
(d) It gives linear incremental cost functions or has non-convex cost curves.
8. What is the difference between load frequency controller and economic dispatch controller?
The load frequency controller is a fast acting controller and the economic dispatch controller is a slow acting control. LFC adjusts the speed changer setting every minute in accordance with a command signal generated by the central economic dispatch computer.
9. What is merit order scheduling?
This method ensures that the incremental cost of all the generators is constant over the full range (or) over successive discrete portions within the range. This method of scheduling is known as merit order scheduling.
10. What is Lagrangian multiplier?
The necessary condition for the existence of a minimum cost operating condition is that the incremental cost rates of all the units be equal to some undetermined value(λ) called Lagrangian multiplier.

11. What is all the point to be noted for a economic load dispatch including transmission losses?
1) The incremental cost of production of a plant is always positive; the incremental transmission losses can be both positive and negative.
2) The individual generators will operate at different incremental cost of production.
3) The generation with highest positive incremental transmission loss will operate at lowest incremental cost of production.
12. What are the assumptions for deriving loss coefficients?
1. The ratio X/R for all transmission line is same.
2. The phase angle of all the load currents is the same.
13. How is incremental operating cost related to economic dispatch?
Total system load has to be divided among all units that all the units operate at equal incremental cost.


14. What are the criteria that should be satisfied for economic loading of generating station?
Equal incremental cost criteria.
15. Write the relationship between fuel energy input and input fuel cost.
Input fuel cost Fi (Pi)=K.Fi(Pi) Rs/hr
Where, K = Cost of the fuel in Rs/MKcal.
Fi (Pi) = Fuel energy input in MKcal/hr.
16. Write the quadratic expression of fuel cost.
Fi (Pi)= = αi (Pi) + βi (Pi) + γi Rs/hr
Where αi, βi, γi are constants.
(Pi)=Power generation
17. What is system incremental cost?
The incremental fuel cost of all the generating units must be the same. The common value of incremental fuel cost λ is called the system incremental cost.
18. Define incremental efficiency?
It is defined as the reciprocal of incremental fuel rate or incremental production cost.



26. How can the economic controller be added as the tertiary loop of LFC control?
The economic dispatch control equipment comprises of a function generator to find desired generation from λ, comparator to detect the error by comparing the desired gen eration with actual MW generation, low gain integral controller provided at energy control centre to send the correcting signal for readjustment of speed changer position. This low gain of the integral controller ensures slower action of the secondary control. Thus economic controller can be added to LFC loop
27. Compare with unit commitment and economic load dispatch.

Unit Commitment Economic Load dispatch
Optimum allocation of number of units to be operated (to determine this units of a plant that should operate for a given load is the problem of unit commitment Optimum allocation of generation to each system. (St each generating station at various station load levels)
There are number of subsets of the complete set of ’n’ units that would satisfy the expected demand. The problem assumes that there are ‘n; units already connected to the system.
Purpose of unit commitment is to find the optimal subset among the subsets which provide the minimum operating cost. Purpose of economic dispatch problem is to find the optimum operating policy for these ‘n’ units