The problem of economic load dispatch in plant-level supervisory information system (SIS) of thermal power plants has been solved by using the Laglange relaxation method. Taking power generation cost as the target function

the restraint and relaxation of system load have been carried out by using the lagrange multiplier

and the obtained lagrange function being divided into upper and lower levels of an optimization problem

and then being solved separately

ensuring rapidity and effectiveness of the algorithm. A simulation comparison with the chaotic optimization method and with the genetic algorithm has been made

the results show that the given method has fast convergence speed and high precission of solution

being easily to be mastered

is a effective tool for solving the problem of economic load dispatch in the electric system.