Short Answer:

In Economic Load Dispatch (ELD), constraints are the limits or conditions that must be satisfied while minimizing the total generation cost. These constraints ensure safe, reliable, and practical operation of the power system during power generation and distribution.

The main constraints include the power balance constraint (total generation must equal total demand), generator operating limits (each generator must work within its minimum and maximum capacity), and sometimes transmission losses, ramp rate limits, and prohibited operating zones. All these must be considered to make ELD results realistic and applicable.

Detailed Explanation:

Constraints considered in economic load dispatch

Economic Load Dispatch (ELD) is the process of allocating total electrical load among different generating units in a power system in a way that minimizes the total fuel cost. While doing this, several technical and operational constraints must be considered to ensure that the generation is not only economical but also feasible, safe, and stable.

These constraints are mathematical conditions applied to the optimization problem to reflect the real-world limits of generators, systems, and networks. Ignoring these constraints can lead to solutions that may be economically ideal but technically impossible or unsafe.

Main Constraints in Economic Load Dispatch:

1. Power Balance Constraint (Equality Constraint):
   This is the most basic and essential requirement of ELD. The total power generated by all units must exactly equal the total load demand plus system transmission losses.
   Σ Pi = PD + PL
   • Pi: Power output of each generator
   • PD: Total demand
   • PL: Total transmission losses
     This ensures that generation meets demand at all times.
2. Generator Operating Limits (Inequality Constraint):
   Each generator has a minimum and maximum power output it can produce safely.
   Pi(min) ≤ Pi ≤ Pi(max)
   • Pi(min): Minimum capacity
   • Pi(max): Maximum capacity
     These limits are set based on equipment design, thermal limits, and safety standards.
3. Ramp Rate Limits:
   Generators cannot instantly increase or decrease their output. Ramp rate constraints define how fast a generator can change its output over time.
   • Ramp-up rate: Maximum increase allowed in power generation
   • Ramp-down rate: Maximum decrease allowed
     These are important for maintaining system stability and protecting equipment.
4. Prohibited Operating Zones:
   Some generators have ranges of power output that are not allowed due to mechanical vibrations, poor efficiency, or other operational risks. These zones must be avoided during load dispatch.
   For example:
   Generator X cannot operate between 150 MW and 180 MW
5. Transmission Losses:
   Not all generated power reaches the load due to losses in transmission lines. Accurate ELD includes losses in the dispatch calculation. Losses are typically a function of generator output and line resistance.
6. Spinning Reserve Constraint:
   A certain amount of backup capacity (spinning reserve) must be kept ready to meet sudden increases in demand or generator outages. This reserve must be factored into the ELD.
7. Environmental and Emission Constraints (in advanced ELD models):
   Some modern ELD models include constraints for CO₂ or NOx emissions, where generation must remain within emission limits to meet environmental standards.
8. Network Constraints (in Security-Constrained ELD):
   In complex power systems, line capacity limits and bus voltage limits are also considered to prevent network congestion or voltage instability.

Conclusion:

Constraints in Economic Load Dispatch are essential to make the generation schedule practical, safe, and reliable. These include power balance, generator capacity limits, ramp rates, transmission losses, and prohibited zones. Considering these constraints ensures that the ELD solution is not only cost-effective but also technically sound and operationally feasible in real-world power systems.