Introduction to EOPS

In electric power systems, the first step is to properly assess the load requirement of a given area where electrical power is to be supplied. This power is to be supplied using the available units such as thermal, hydroelectric, nuclear, etc. Many factors are required to be considered while choosing a type of generation such as: kind of fuel available, fuel cost, availability of suitable sites for major station, nature of load to be supplied, etc.
 

The energy requirement on power system is not constant due to the varying demands at the different times of the day. It is expected to supply reliable and quality power to the consumers. Power system operators should ensure the continuity of power supply at all times.
 

The use of a single unit to supply the complete load demand is not practical since, it would not be a reliable one. Alternately, a large number of smaller units can be used to fit the load curve as closely as possible. Again, with a large number of units, the operation and maintenance costs will increase. Further, the capital cost of large number of units of smaller size is more as compared to a small number of units of larger size. Thus, there has to be compromise in the selection of size and number of generating units within a power plant or a station.
 

Electric power generating stations are far away from the load centers. Thus, large and long transmission lines (grid lines) wheel the generated power to the substations at load centers. Many electrical equipment are used for proper transmission and distribution of the generated power.

Optimum economic efficiency is achieved when all the generators which are running in parallel are loaded in such a way that the fuel cost of their power generation is the minimum. The units then share the load to minimize the overall cost of generation. This economical approach of catering to the load requirement is called as ‘economic dispatch’. The main factor in economic operation of power systems is the cost of generating the real power. In any electric power system, the cost has two components as under:

The Fixed Costs: Capital investment, interest charged on the money borrowed, tax paid, labour, salary, etc. which are independent of the load variations.

The Variable Costs: which are dependant on the load on the generating units, the losses, daily load requirements, purchase or sale of power, etc.
 

Economic operation of power systems is concerned about minimizing the variable costs only. Further, the factors affecting the operating cost of the generating units are: generator efficiency, transmission losses, fuel cost, etc. Of these, the fuel cost is the most important factor. Since a given power system is a mix of various types of generating units, such as hydel, thermal, nuclear, hydro-thermal, wind, etc., each type of unit contributes its share for the total operating cost. Since fuel cost is a predominating factor in thermal (coal fired) plants, economic load dispatch (ELD) is considered usually for a given set of thermal plants in the foregoing discussion.

There are two problem areas of operation strategy to obtain the economic operation of power systems. They are: problem of economic scheduling and the problem of optimal power flow.
 

A. The problem of economic scheduling: This is again divided into two categories:

1. The unit commitment problem (UCP): Here, the objective is to determine the various generators to be in operation among the available ones in the system, satisfying the constraints, so that the total operating cost is the minimum. This problem is solved for specified time duration, usually a day in advance, based on the forecasted load for that time duration.

2. The economic load dispatch (ELD): Here, the objective is to determine the generation (MW power output) of each presently operating (committed or put on) units to meet the specified load demand (including the losses), such hat the total fuel cost s minimized.

 

B. The problem of optimal power flow: Here, it deals with delivering the real power to the load points with minimum loss. For this, the power flow in each line is to be optimized to minimize the system losses.