Power Systems : Types of Tariffs

INTRODUCTION TO ECONOMIC LOAD DISPATCH

In practice, power plants are located far end from the load centres. Therefore, under normal operating conditions, the generation capacity is much more than the total load demand and losses. Thus, there are many options for scheduling generation.

In an interconnected power system, the main objective is to find the real and reactive power scheduling of each power plant in such a way as to minimize the operating cost. This means that the generator’s real and reactive power are allowed to vary within certain limits so as to meet a particular load demand with minimum fuel cost. This is called the optimal power flow (OPF).

The optimal power flow is used to optimize the power flow solution of large-scale power system. This is done by minimizing selected objective functions such as cost functions, present economic costs, system security, etc. Efficient reactive power planning enhances economic operation as well as system security.

 Operating cost of a Thermal Power Plant:

The factors influencing power generation at minimum cost are operating efficiencies of generators, fuel cost, and transmission losses. Therefore, generation of power plants is done such that the total operating cost is minimum.

The fuel cost of generator ‘i’ is represented as a quadratic function of real power generation,

A plot of fuel-cost curve versus the real power, i.e., known as the incremental fuel-cost curve as shown in figure below.

ECONOMIC DISPATCH NEGLECTING LOSSES & NO GENERATOR LIMITS

The simplest economic dispatch problem is the case when transmission line losses are neglected, as shown in figure below,

Since transmission losses are neglected, the total demand P_D is the sum of all generation. A cost function C_i is assumed to be known for each plant. The problem is to find the real power generation for each plant such that the objective function (i.e., total production cost) as defined by the equation

where C_t is the total production cost, C_i is the production cost of i^th plant, P_i is the generation of i^th plant, P_D is the total load demand, and n_g is the total number of dispatchable generating plants.

The economic load dispatch is used to define the production level of each plant, so that the total cost of generation and transmission is minimum for a prescribed schedule of load. The main objective of load dispatch is to minimize the overall cost of the generation.

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