Economic dispatch of energy and VARs as an operational problem:

There are two basic problems in economic load dispatch due to frequent changing of loads:

1. Problems when loading units from one load condition to next load condition:

The change in load needs to be assessed for which new economic loading is required. It is not practical to readjust the economic loading for an increment or decrement of 100 KW in a plant with three units supplying 30 MW. But an increment or decrement of 200 KW might be important and therefore new economic loading should be done. In real practice, the minimum increment or decrement value of load demand is fixed in a computer software program which will then automatically control and change the loading.Another factor may be the timing. For example, economic rescheduling for a new 300 KW addition might be delayed up to a few minutes with expectation of this increase to fall down to say 200 KW as compared to a new 1 MW addition for which economic rescheduling must be done within few seconds. For a unit running at maximum capacity, any increment in load should be accompanied immediately by economic reloading so that the unit do not gat overloaded. If any overloading is done, the machine will gat off by over current relay. For a unit running at minimum capacity, it might be further de-loaded when overall decrement in load occurs. So economic reloading should be done as immediately as possible.

2. Operational problems associated with power factor and VAR flow:

VAR flow increases the line losses and causes to decreases the power factor. The machine can be run at full current rating but at low power factor reducing the efficiency of the system. In a circuit with p. f. of 0.5, the current would be twice that of which would flow when the power factor was unity. Due to increased current in a circuit with increased VAR flow (low power factor), voltage drop and line losses due to line resistance will be more than it would be at unity power factor. So for a low power factor in the system, the optimum loading of units will be obtained with increased line loss which is not, in fact, economic and desirable. Therefore it becomes necessary to make VAR flow as low as possible in order to achieve desired optimum scheduling. The series and shunt compensation techniques are used in a transmission lines to minimize the flow of reactive power.