MODULE IV
ECONOMIC OPERATION OF POWER SYSTEM
INTRODUCTION
One of the earliest applications of on-line centralized control was to provide a central facility, to
operate economically, several generating plants supplying the loads of the system. Modern
integrated systems have different types of generating plants, such as coal fired thermal plants,
hydel plants, nuclear plants, oil and natural gas units etc. The capital investment, operation and
maintenance costs are different for different types of plants.
The operation economics can again be subdivided into two parts.
i) Problem of economic dispatch, which deals with determining the power output of each plant to
meet the specified load, such that the overall fuel cost is minimized.
ii) Problem of optimal power flow, which deals with minimum – loss delivery, where in the
power flow, is optimized to minimize losses in the system. In this chapter we consider the
problem of economic dispatch.
During operation of the plant, a generator may be in one of the following states:
i) Base supply without regulation: the output is a constant.
ii) Base supply with regulation: output power is regulated based on system load.
iii) Automatic non-economic regulation: output level changes around a base setting as area
control error changes.
iv) Automatic economic regulation: output level is adjusted, with the area load and area control
error, while tracking an economic setting.
Regardless of the units operating state, it has a contribution to the economic operation, even
though its output is changed for different reasons. The factors influencing the cost of generation
are the generator efficiency, fuel cost and transmission losses. The most efficient generator may
not give minimum cost, since it may be located in a place where fuel cost is high. Further, if the
plant is located far from the load centers, transmission losses may be high and running the plant
may become uneconomical. The economic dispatch problem basically determines the generation
of different plants to minimize total operating cost. Modern generating plants like nuclear plants,
geo-thermal plants etc, may require capital investment of millions of rupees. The economic
dispatch is however determined in terms of fuel cost per unit power generated and does not
include capital investment, maintenance, depreciation, start-up and shut down costs etc.
,PERFORMANCE CURVES
INPUT-OUTPUT CURVE
This is the fundamental curve for a thermal plant and is a plot of the input in British thermal
units (Btu) per hour versus the power output of the plant in MW as shown in Fig.4.1
Fig.4.1: Input output curve
HEAT RATE CURVE
The heat rate is the ratio of fuel input in Btu to energy output in KWh. It is the slope of the input
– output curve at any point. The reciprocal of heat – rate is called fuel – efficiency. The heat rate
curve is a plot of heat rate versus output in MW. A typical plot is shown in Fig .
, Fig.4.2: Heat Rate Curve
INCREMENTAL FUEL RATE CURVE
The incremental fuel rate is equal to a small change in input divided by the corresponding change
in output.
Incremental fuel rate =∆Input/∆Output
The unit is again Btu / KWh. A plot of incremental fuel rate versus the output is shown in
Fig.4.3
Fig 4.3: Incremental Fuel Rate Curve
ECONOMIC OPERATION OF POWER SYSTEM
INTRODUCTION
One of the earliest applications of on-line centralized control was to provide a central facility, to
operate economically, several generating plants supplying the loads of the system. Modern
integrated systems have different types of generating plants, such as coal fired thermal plants,
hydel plants, nuclear plants, oil and natural gas units etc. The capital investment, operation and
maintenance costs are different for different types of plants.
The operation economics can again be subdivided into two parts.
i) Problem of economic dispatch, which deals with determining the power output of each plant to
meet the specified load, such that the overall fuel cost is minimized.
ii) Problem of optimal power flow, which deals with minimum – loss delivery, where in the
power flow, is optimized to minimize losses in the system. In this chapter we consider the
problem of economic dispatch.
During operation of the plant, a generator may be in one of the following states:
i) Base supply without regulation: the output is a constant.
ii) Base supply with regulation: output power is regulated based on system load.
iii) Automatic non-economic regulation: output level changes around a base setting as area
control error changes.
iv) Automatic economic regulation: output level is adjusted, with the area load and area control
error, while tracking an economic setting.
Regardless of the units operating state, it has a contribution to the economic operation, even
though its output is changed for different reasons. The factors influencing the cost of generation
are the generator efficiency, fuel cost and transmission losses. The most efficient generator may
not give minimum cost, since it may be located in a place where fuel cost is high. Further, if the
plant is located far from the load centers, transmission losses may be high and running the plant
may become uneconomical. The economic dispatch problem basically determines the generation
of different plants to minimize total operating cost. Modern generating plants like nuclear plants,
geo-thermal plants etc, may require capital investment of millions of rupees. The economic
dispatch is however determined in terms of fuel cost per unit power generated and does not
include capital investment, maintenance, depreciation, start-up and shut down costs etc.
,PERFORMANCE CURVES
INPUT-OUTPUT CURVE
This is the fundamental curve for a thermal plant and is a plot of the input in British thermal
units (Btu) per hour versus the power output of the plant in MW as shown in Fig.4.1
Fig.4.1: Input output curve
HEAT RATE CURVE
The heat rate is the ratio of fuel input in Btu to energy output in KWh. It is the slope of the input
– output curve at any point. The reciprocal of heat – rate is called fuel – efficiency. The heat rate
curve is a plot of heat rate versus output in MW. A typical plot is shown in Fig .
, Fig.4.2: Heat Rate Curve
INCREMENTAL FUEL RATE CURVE
The incremental fuel rate is equal to a small change in input divided by the corresponding change
in output.
Incremental fuel rate =∆Input/∆Output
The unit is again Btu / KWh. A plot of incremental fuel rate versus the output is shown in
Fig.4.3
Fig 4.3: Incremental Fuel Rate Curve
