Short Answer:

Economic load dispatch is the process of distributing the total load demand among all the generating units of a power system in such a way that the total operating cost of generation is minimum while meeting the required demand and system constraints.

In simple terms, it means deciding how much power each generator should produce at a given time so that the overall fuel and operating costs are reduced. Economic load dispatch ensures the most efficient and economical operation of a power plant or interconnected power system.

Detailed Explanation :

Economic Load Dispatch

Economic load dispatch (ELD) is an important concept in power system operation and control. It deals with the problem of finding the most economical way to allocate the total electrical load among the available generating units of a power station or interconnected grid. The main aim of ELD is to minimize the total fuel cost while ensuring that the total power generated equals the power demand plus system losses.

Every generator in a power system has its own fuel cost characteristics, which depend on the type of fuel, efficiency, and load. Economic load dispatch helps determine the optimum generation level of each unit so that electricity is produced at the lowest possible cost while maintaining system stability and reliability.

1. Meaning and Objective of Economic Load Dispatch:
   When multiple generators operate in a system, each unit has a different efficiency and cost of fuel consumption. Some generators produce electricity more economically than others. Therefore, distributing the total system load equally among all generators would not be cost-effective.

The objective of economic load dispatch is to:

• Generate the required power at minimum cost.
• Maintain system reliability and frequency stability.
• Operate each generating unit within its safe limits.
• Reduce fuel consumption and emissions.

In simple terms, economic load dispatch ensures that the cheapest combination of generators supplies the load at any given time.

2. Principle of Economic Load Dispatch:
   The principle of ELD is based on the incremental costof generation for each unit. Incremental cost means the additional cost of producing one more unit of electricity (kWh).

For achieving minimum total cost, the incremental cost of all generating units operating in the system must be equal. This condition is known as the equal incremental cost criterion, and it is expressed as:

Where:

•  = Incremental fuel cost (Rs/MWh)
•  = Power outputs of generating units
•  = System incremental cost (Lagrange multiplier)

This means that for the total cost to be minimum, all operating generators must have the same incremental cost per unit of output.

3. Mathematical Formulation:
   Let there be ngenerating units in a power system.
   The total fuel cost can be expressed as:

Subject to the constraint:

Where:

•  = Total cost of generation
•  = Power demand
•  = Transmission losses

The objective is to minimize  while satisfying the above equality.
Using calculus and the Lagrange multiplier method, the condition for minimum cost leads to the equal incremental cost criterion mentioned earlier.

4. Steps in Economic Load Dispatch:
   The process of economic load dispatch typically involves the following steps:

1. Obtain Cost Characteristics of Each Unit:
   Each generator has a cost curve showing the relation between power output and fuel cost.
2. Calculate Incremental Cost for Each Unit:
   Derive the incremental fuel cost (Rs/MWh) for all generating units.
3. Equalize Incremental Costs:
   Adjust the power output of each unit such that all operating units have equal incremental cost.
4. Check Power Balance:
   Ensure that the total generated power equals the system demand plus losses.
5. Adjust for System Constraints:
   Ensure that no unit exceeds its maximum or minimum generation limits.
6. Iterate if Necessary:
   Repeat calculations until the most economical combination is found.

5. Constraints in Economic Load Dispatch:
   While determining economic load dispatch, several practical limitations must be considered:

• Generation Limits:
  Each generator has a minimum and maximum generation capacity.
• Transmission Losses:
  Power loss in transmission lines must be accounted for, as it affects cost and efficiency.
• Ramp Rate Limits:
  Generators cannot increase or decrease their output instantaneously; there is a rate limit.
• System Stability and Security:
  The distribution of load must ensure stable voltage and frequency conditions.
• Environmental Constraints:
  Modern ELD models also consider emission limits to minimize environmental impact.

6. Types of Economic Load Dispatch:
   There are two main types of ELD problems based on transmission losses:

• a) Without Transmission Losses:
  Assumes transmission losses are negligible, simplifying calculations. The total generation equals the load demand.
• b) With Transmission Losses:
  Considers the losses in the transmission system, making the problem more accurate but complex. The losses are calculated using loss coefficients (B-coefficients).

7. Importance of Economic Load Dispatch:
   Economic load dispatch is very important for modern power systems due to the following reasons:

• Reduces the fuel and operating cost of generation.
• Improves overall system efficiency.
• Ensures balanced load sharing among generating units.
• Minimizes environmental pollution by reducing fuel wastage.
• Helps in optimal utilization of resources and better planning.

It is also the foundation for unit commitment and economic scheduling, which are advanced methods used in power system operation.

8. Modern Techniques for Economic Load Dispatch:
   With the development of digital technology, modern methods are used to solve complex ELD problems more accurately and efficiently. These include:

• Lambda Iteration Method
• Newton-Raphson Method
• Dynamic Programming
• Genetic Algorithm (GA)
• Particle Swarm Optimization (PSO)

These computational techniques provide faster and more reliable solutions, especially in large interconnected systems.

Conclusion:

Economic load dispatch is the process of allocating the total load demand among available generating units in a power system to minimize the total operating cost while meeting all system constraints. It ensures efficient use of fuel, stable operation, and reduced generation costs. By applying the principle of equal incremental cost, engineers can achieve the most economical and reliable power generation. ELD plays a vital role in optimizing energy production and ensuring cost-effective operation of modern power systems.