The Performance Of The Output Of Combined Cycle

Published Date: 02 Nov 2017

J N Rai1, Naimul Hasan2, B B Arora3, Rajesh Garai4, Rishabh K Gupta5, Rahul Kapoor6

1Department of Electrical Engineering,

Delhi Technological University, India

[email protected]

3Department of Mechanical Engineering,

Delhi Technological University, India

2Department of Electrical Engineering,

Jamia Millia Islamia, New Delhi, India

Abstract— A combined cycle plant generates power by combining both gas turbine and steam turbine. For power generation purposes the Brayton Cycle in open or in combined process is used for the gas turbine in which the air is directly taken from the atmosphere and mixed with the fuel to produce electrical energy, hence the ambient temperature of the air (atmosphere) is an important factor on which the power output of a gas turbine depends. The temperature control of the gas turbine is also important for preventing the excess rise in the temperature of the gas turbine. In this paper the effect of ambient temperature and temperature control on the performance of the output response of the gas turbine has been studied using the practical data from a combined cycle plant when the turbine was being operated at base load. Graphs have been plotted and analyzed on the basis of data obtained.

Keywords- Gas turbine, ambient temperature, exhaust temperature, power.

INTRODUCTION

Power generation is an important factor in the development of any economy. More efficient plants are required for producing more energy by utilizing less amount of fuel and controlled harmful emissions. Gas-fired combined cycle plants are the technologically advanced plants which can achieve efficiency as high as 60% and lower atmospheric emissions. The Gas based power plants allow the plant to be compact using lesser amount of area. Because of these characteristics these plants have become a subject of research [1] and work is required to understand their characteristics under various operating conditions in order to have a better control over their operation. Fig. 1 shows the typical gas turbine configuration.

In a combined cycle plant the load is controlled by the operation of the gas turbine and the output of the steam turbine depends on the exhaust of the gas turbine. Combined cycle plants because of their quick startup time provide high operational flexibility for adjusting the load output fast and load predictability. Changes in demand/load cause deviation in the frequency of the generated power. The frequency can be restored to its nominal value by various governing systems. This can be accomplished by Automatic Generation Control. Because of the flexibility in operation of Combined Cycle Power Plants the units are generally AGC controlled. Fig. 2 shows the start-up characteristics of a gas turbine [2].

Figure 1. Gas Turbine

Figure 2. Starting Characteristics of Gas Turbine

GAS TURBINE THERMODYNAMICS

Assuming the adiabatic compression of air in the compressor, compressor discharge temperature is given as: [4]

(1)

Where Ti (K) is inlet temperature (ambient temperature), nc is the compressor efficiency, x is compressor temperature ratio given as

(2)

Pro is design compressor pressure ratio, and W is airflow in per unit of its rated value, y is ratio of specific heats.

Gas turbine inlet temperature Tf (K) is given by

(4)

Subscript "o" denotes the rated value and Wf is fuel flow in per unit of its rated value.

Gas turbine exhaust temperature Te (K) is given as

(5)

nt is turbine efficiency.

TEMPERATURE CONTROL SIMULATION

Fig. 3 shows the simulation model of the exhaust temperature measurement and control of the gas turbine. It consists of blocks representing various transducer block for indicating electrical signals according to the changes in temperature, thermocouple block for measurement of the temperature, and a reference block for comparison i.e. if the exhaust temperature is above which the turbine can handle then necessary control actions will take place which would reduce the exhaust temperature of the turbine. The temperature of the exhaust of the gas turbine is restricted to a constant value due to which fuel flow and air are varied which affect the output of the gas turbine.

Figure 3. Exhaust Temperature control

Figure 4. Flow Diagram for calculation of variation of Gas Turbine parameters.

CASE STUDY

In the following case study we have considered a multi-shaft combined cycle plant having two gas turbines and a steam turbine. This system is a part of a large interconnected system. The variation in the output of gas turbine with atmospheric temperature keeping exhaust temperature controlled has been plotted. The ratings of Gas turbines are 104 MW and that of steam turbine is 122 MW. [2]

Figure 5. Plot for GT #1

Figure 6 Plot for GT #2

Fig. 5 and Fig. 6 as per Table I [2] shows the plot of power output versus temperature. The exhaust temperature of the turbine being adjusted to 5610 C for steam turbine operation. The graph shows the decrease in power output when the ambient temperature of the compressor inlet rises.

This is because the gas turbine is a constant volume machine, when there is a rise in temperature the density of the air at the inlet decreases so less air flows in the combustion chamber and more fuel remains unburnt due to which the power output of the turbine drops. The graph deviates from the plot given in [4] because the demand of power (load) keeps changing due to which other factors like fuel flow and air flow are varied. The power output for a given ambient temperature can be increased by cooling the inlet air to the compressor.

Figure 7

Fig. 7 as per Table III [2] shows the graph between exhaust air flow and the power output near rated value of the output. Since the fuel flow is negligible compared to the air flow the exhaust flow can be assumed to be equal to the airflow. It can be seen from the equation........ the power output of the gas turbine is proportional to the air flow. Also by varying the air flow in the turbine the temperature of the turbine is also controlled as shown in the next graph.

Figure 8

Fig. 8 as per the data in Table II [2] shows the air flow versus fuel flow characteristics keeping the turbine inlet temperature constant near rated power output. To increase the output of the gas turbine the fuel input has to be increased as can be seen by the equation...... It can be seen that with the rise in fuel flow, air flow also increases so as to maintain the inlet temperature of the turbine at a specified value according to the equation...... which shows that the air flow should be increased in direct proportion to the fuel flow.

TABLE I

Gas Turbine Ambient Temperature Data

Gas Turbine #1

Temperature ( o C)

Load (MW)

32

99.4

26

103.6

28

102.7

30

99.9

35

96.1

34

96.9

19

104.1

16

105.7

12

108.6

13

107.9

11

109.9

9

110.6

Gas Turbine #2

Temperature ( o C)

Load (MW)

33

100.5

27

104.4

28

103

30

101.5

35

98.2

34

98.6

20

107.5

15

106.7

12

109.5

13

108.4

11

110.5

10

112

TABLE II

Air Flow in Gas Turbine

Gas Turbine #1

Air Flow Exhaust (kg/s)

Load (MW)

Exhaust ( o C)

381.7

102.9

555

383

103.2

554

377.1

101.3

557

381.9

103

554

384.7

103.4

553

377.4

100

556

377.5

100.8

556

381.2

102.7

554

381.1

99.7

556

TABLE III

Air flow and fuel flow data in per unit

Air flow

Fuel flow

Power

Turbine Inlet Temperature ( o C)

0.957

0.511

1.038

1133

0.955

0.506

1.019

1132

0.962

0.515

1.038

1132

0.943

0.486

0.981

1133

0.945

0.495

1.000

1132

CONCLUSION

It can be concluded from the above study that

With the rise in the ambient temperature of the atmosphere the output of the gas turbine falls and the output of the gas turbine can be increased by reducing the inlet temperature of the compressor by cooling of the air that is being fed to the compressor.

When the fuel input to the turbine is increased, for increasing the output, the air flow has also to be adjusted accordingly to prevent the turbine temperature to go above a reference temperature. There is a linear rise in air flow with the fuel flow when the turbine is being operated near its rated value.

The exhaust temperature is also controlled by increasing the air flow.

Acknowledgment

The authors are grateful to the staff of Pragati Power Corporation Limited of Indraprastha Power Generation Corporation Limited for their in valuable advices and providing necessary data.