GB2102609A - Turbine control with flameout protection - Google Patents

Abstract

A turbine control system provides improved turbine operation and extended turbine blade life through more reliable control system response to turbine flame-out conditions. The control system includes a plurality of thermocouples, for sensing the temperature of the gases driving the turbine, which are scanned and provide output signals which are analyzed to determine the flame condition of combustor baskets (42) and to elicit control system action to a flame-out or overtemperature condition. <IMAGE>

Description

SPECIFICATION Turbine control with flameout protection This invention relates generally to combustion turbines but is more particularly directed to control systems provided in connection with such turbines.

Still more particularly, the invention relates to a tur bine control system having apparatus for detecting and indicating flame condition ofturbine combus tors.

The proper operation of gas turbines requires controlling fuel flow to meet speed and load control requirements so as to improve component and overall turbine performance. One important parameter affecting component life is turbine rotor blade temperature. Variations in the temperature of driv ing gases in the turbine blade path may cause extreme stress to the turbine blades and may thereby produce unnecessarily rapid weakening of the blades.

Failure of a combustor basket to ignite during start-up of loss of flame in a combustor basket during turbine acceleration may result in damaging thermal stress to the turbine blades. Prior art turbine control schemes typically utilize ultra-violet flame detectors to monitor the status of combustor flame during turbine ignition.

In such a control system the ultra-violet detector generates a signal to a turbine ignition control indicating a flame-out condition. The ignition control may thereafter attempt a reignition. If reignition fails, or of reignition is not attempted, a fuel control then operates to shut off fuel flow and abort the start-up.

Failure to detect the flame-out condition and to take appropriate action results in accumulation of uncombusted fuel on the turbine blades, causing hot spots to develop on the blades.

Structural changes to prior art ignition systems have rendered prior art flame detection schemes ineffective as a means of verifying ignition of all combustors. In the typical prior art control system, an ignition device fires two combustors. Each of the two combustors is serially connected by means of cross-flame tubes to seven other combustors. A flame initiated in the first two combustors is successively propagated through the cross-flame tubes to the seven combustors connected to each of the first two combustors. An ultra-violet flame detector, such as an Edison flame detector Model 424-10433, is located in the last flame detector in each of the two serial strings of combustors. The presence of a flame in the last combustor of a string indicates ignition of all combustors in that string.

Typical prior art ignition systems have been altered by adding a cross-flame tube between the final combustors of the two strings. While this change increases the probability of successful ignition of all combustors, it renders ineffective the two flame detectors as a means of determining successful ignition of all combustors. Hence, there is a need for a turbine control system which will protect turbine blade life by providing a reliable response to flame-out conditions.

One possible resolution to the flame detection problem would involve placing a flame detector in each of the sixteen combustor baskets. Such an approach necessarily implies an eight-fold increase in the cost of detector hardware, which increase should be avoided if unnecessary. In addition, ultraviolet flame detectors occasionally suffer the disadvantage of oil or smoke fouling, rendering the detector inoperative.

It would be advantageous to develop a turbine control system which provides improved turbine operation and extended blade life as a result of more reliable control system response to flame-out conditions, thereby avoiding blade overtemperature conditions. Such a control system should include a flame detection scheme which does not require additional transducers for its operation and which utilizes tranducer hardware already embodied within the turbine control system, which hardware is not susceptible to oil or smoke fouling. It would also be advantageous if the control system would operate in response to a loss of flame during turbine acceleration, as well as during start-up.

It is an object of this invention to provide a gas turbine control system with flameout protection, with a view to overcoming the deficiencies of the prior art.

The invention resides in a gas turbine control system comprising a speed and load control for limiting turbine rotational velocity and turbine electrical loading, an acceleration control for limiting the rate of change of turbine rotational velocity during start-up or sudden loss of load, and a temperature control for limiting the temperature of driving gases and turbine components, said temperature control having an apparatus for determining and indicating variations in temperatures of gases driving a combustion turbine and for determining and indicating to the control system the status of a flame within each of a plurality of combustor baskets in the turbine, said apparatus comprising means for sensing temperature of gases driving the turbine; means for periodically scanning said sensing means to determine a high temperature and a low temperature within the driving gases; means for scanning said sensing means to detect a failed sensing means; means for evaluating the high temperature, the low temperature and the failure status of said sensing means in accordance with a plurality of control inputs to determine and indicate excessive temperature variations in the driving gases and lack of flame in a combustor basket; and means for controlling fuel flow to the combustor baskets in response to said evaluating means to effect an attempt to reignite the turbine or to shut down the turbine.

In accordance with a preferred embodiment of the invention, a combustion turbine control system is provided for improved turbine start-up and load operation through more reliable avoidance of blade overtemperature and safer, more reliable turbine shutdown operation based on improved responses to combustor flameout conditions. More particularly, output signals from a plurality of thermocouples are scanned to detect a highest temperature, a lowest temperature and thermocouple failure. The temperature and thermocouple status signals are processed to obtain output signals which indicate to other portions of the control system variations in temperature spread which exceed certain setpoints, failure of a combustor basket to ignite and loss of flame in a combustor basket during turbine acceleration.

The control system responds to these signals according to the severity of the indicated problem.

Minimal temperature variations are indicated toan operator, while severe temperature variations may result in turbine shutdown. The control system responds to a flame-out condition by attempting reignition or by shutting down the turbine, as appropriate, so as to avoid damage to the turbine blades.

The invention will become readily apparent from the following description of an exemplary embodiment thereof when read in conjunction with the accompanying drawings, in which: Figure 1 depicts a diagram of a typical prior art combustor ignition system.

Figure 2 depicts in block diagram form a combustion turbine control system in accordance with the preferred embodiment of the invention.

Figure 3 depicts a block diagram of a temperature control included in the turbine control system of Fig.

2.

Figure 4 depicts a block diagram of a temperature profile subsystem included in the temperature control of Fig. 3.

Figure 5 depicts in block diagram form logical devices utilized to monitor the blade path temperature and to produce various indication and control signals.

Figure 6 depicts in block diagram form logical devices used to verify combustor basket ignition and to generate an appropriate control and indication signal.

Figure 7 depicts in block diagram form logical devices utilized to detect an outflame condition in a combustor basket and to generate an appropriate control and indication signal.

Figure 1 shows a block diagram of a typical prior art combustor ignition system 10 for igniting each of sixteen combustors 11 in a combustion turbine. In the system 10, an ignition device 12 fires each oftwo combustors 14,16. The flame from each ignited combustor 11 is propagated along a cross-flame tube 18to anothercombustor 11, causing ignition of that combustor 11. The propagation continues until the two combustors 20, 22 farthest from the ignition device 12 have been ignited. Ultraviolet flame detectors 24 detect the presence of a flame in the two final combustors 20, 22, indicating to a turbine control system that all combustors have been successfully ignited.

In order to improve the reliability of the combustor ignition system, the prior art ignition system 10 has been augmented by addition of a cross-flame tube 18 between the two final combustors 20, 22. With this addition, the flame detectors 24 no longer pro vide a reliable indication of successful ignition of all combustors. Hence, it was necessary to device a tur bine control system which does not depend on two strategically placed flame detectors to indicate successful turbine ignition.

To provide improved turbine operation and extended blade life as a result of more reliable control system response to flame-out conditions, a combustion turbine control system is arranged in accordance with the preferred embodiment of the invention. The preferred structure for the control system is depicted in Figures 2-7. Figure 2 discloses a block diagram ofthe turbine control system 30.

The control system 30 comprises three major subsystems: a temperature control 32, a speed and load control 34, and an acceleration control 36.

A simplified representation of a single shaft turbine 38 includes a compressor 40, combustion basket 42 and turbine 44 connected to drive a load 46.

Air entering the compressor inlet 48 supports the combustion of fuel injected by a nozzle 50. The heated exhaust gases exit from a turbine outlet 52 past distributed thermocouples 54. A fuel pump 56 delivers fuel to the nozzle 50 according to a rate set by a fuel control 58. The details of the fuel control 58 are not material to the present invention since it may include any servo-control which produces an output according to an electrical input signal, there being many devices on the market suitable for such use.

The three control subsystems 32, 34,36 receive input signals from transducers positioned appropriately throughout the turbine system. These are depicted generally in Figure 2. Various interconnections between the turbine control subsystems 32, 34, 36 and an operator's panel are not depicted in Figure 2. Each of the subsystems 32, 34, 36 produces an output signal is applied to a signal select gate 58, such as that described in U.S. Patent No. 3,520,133.

In the above-named patent, the signal select gate comprises a low value gate which selects the lowest of the analog signals applied to the gate to provide as an input to a fuel control.

The speed and load control 34 receives signals from plant megawatt meters 62 indicating the electrical loading factor for the turbine and signals from a tachometer 64 indicating rotational velocity of the turbine. The speed and load control 34 functions to limit turbine speed to an acceptable range during a start-up and acceleration process as well as during normal turbine operation. The control 34 also func tionsto limit the rate of turbine loading during a loading and synchronization process.

The acceleration control 36 receives signals from the tachometer 64 indicating turbine rotational velocity and produces an output signal which limits turbine acceleration during the start-up process and during loss of load. The acceleration control thereby limits surges in turbine rotational velocity.

The temperature control 32 receives signals from thermocouples 54 and from pressure sensors 66 and produces a signal which serves to limit turbine temperature so as to prevent damage to turbine components. The temperature control 32, depicted in greater detail in Figure 3, embraces a temperature profile system 68 which operates in cooperation with other portions of the turbine control system 30 to provide improved turbine operation and extended blade life through an analysis of turbine temperatures to ascertain and indicate the flame condition of the several combustor baskets 11.

The temperature control 32 may comprise two distinct sections. One section, such as that described generally in copending U.S. patent application Serial No. 276,508 assigned to the assignee of the present invention, comprises hardware 70 for analysis of signals received from a plurality of thermocouples 72 positioned in the turbine exhaust duct 52. The first section also includes inhibit hardware 74 to inhibit an output signal from this section during the start-up sequence.

A second section of the temperature control 32 comprises hardware 76 for analysis ofsignals received from combustor shell pressure sensors 66 and form blade path thermocouples 78. The analysis hardware 76 may be such as is described generally in the aforementioned application. The output sign alsfrom both sections are applied to a signal select gate 80 which selects the lower of three signals for output from the temperature control 32.

The second section of the temperature control 32 includes a temperature profile system 68 which operates in cooperation with the analysis hardware 76 to provide an indication of excessive temperature variations at the turbine blades. The profile system 68 also provides an independent indication to the turbine control system 30 of the flame condition of the several turbine combustors.

The temperature profile system 68 is depicted in greater detail in Figure 4. The system 68 comprises a plurality of thermocouples 112, a scanner device 114, an open thermocouple detector device 116, and a thermocouple monitor device 118.

Each of from eight to sixteen type K thermocou ples 112, depicted in Figure 4, are arranged in an annular array immediately downstream of gas tur- bine blades so as to detect the temperature of gases driving the gas turbine blades. Each thermocouple 112 is perioically checked by the open thermocouple detector device 116 to verify the validity of signals received from that thermocouple 112. Transducer signals received from any thermocouple 112 which is found to have failed are ignored in subsequent analyses.

The scanner device 114 routinely scans the output of each thermocouple 112 and reports the highest and the lowest temperature readings detected among the plurality of thermocouples 112.

The thermocouple monitor device 118 receives as inputs 115 temperature signals from the scanner device 114 and failure signals from the detector device 116, as well as a plurality of control inputs 120. Control inputs 120 comprise a variety of signals, generally activated or set by an operator or other control personnel, which govern analysis of the temperature signals produced by the scanner device 114. Examples of control inputs 120 include signals for temperature comparison purposes and inhibit signals.

The thermocouple monitor device 118 performs comparisons of the thermocouple status data in relation to the control inputs 120 to determine the presence of an excessive temperature variation or a combustor basket outfiame condition. A plurality of control outputs 122 from the thermocouple monitor device 118 communicate alarm conditions to an operator panel (not shown) and the temperature control 32. The control outputs 122 provide alarm signals when temperature setpoints are violated, when a combustor basket fails to ignite on turbine ignition and when a combustor basket flame is lost during turbine acceleration.

The temperature profile system 68, performing in cooperation with the temperature control 32 and other portions of the turbine control system 30, thus provides a simple and effective means for improving turbine operation and preserving the structural integrity of turbine blades. The system 68, which makes use of preexisting temperature detection devices, combines a detection of an outflame condi tion in a combustor basket with a detection of excessive temperature variations around the turbine blades and initiates action on the part of the turbine control system 30 to respond to the problems.

More particularly, thermocouple failure is ascertained by the open thermocouple detector device 116 by sensing thermocouple loop resistance. A failed thermocouple 112 is defined to be any thermocouple with a loop resistance which exceeds 500 ohms. The open thermocouple detector device 116 comprises an application of known electronic circuitryto detect thermocouple loop resistance.

Thermocouple loop resistance checks are performed at regular intervals and also upon detection by the thermocouple monitor device 118 of a violation of a temperature setpoint. Resistance checks are preferably not performed on each scan cycle of the scannew device 114 because the excessive time required to perform a resistance check unduly impedes the scanning of the thermocouples. It is preferable to minimize the frequency of open thermocouple checks in order to maximize the scanning rate.

The scanner device 114 sequentially detects a millivolt signal from each thermocouple 112, cold junction compensates that signal over a temperature range of 33"F to 120"F and then linearizes the millivolt signal to relate a temperature to the voltage.

The highest and lowest temperature readings from each scan of all thermocouples 112 are communicated to the thermocouple monitor device 118.

The thermocouple monitor device 118 may be subdivided into three sections based upon the nature of the functions performed by each section.

The first section is a blade path temperature spread monitor section (see Fig. 5) and comprises logical devices which function to indicate violations of sertain temperature setpoints. Also contained within this section are logical devices for alarming certain thermocouple failure conditions.

The second section is an ignition flame verification logic (see Fig. 6) which signals failure of a combustor basket to ignite within a predesignated period of time after turbine ignition. The third section is a turbine acceleration flame monitor section (see Fig. 7) which functions is to indicate failure of a flame within a combustor basket after successful turbine ignition. The three sections comprising the thermocouple monitor device 118 may be constructed from known electronic circuit elements such as is depicted in Figs. through 4, or, alternatively, may be implemented by a digital computer or microp rocessor.

The blade path temperature spread monitor sec tion 130 is depicted in circuit block diagram form in Fig. 5. The section 130 compares the difference bet ween the highest and the lowest temperatures ascer tained on each scan to three separate setpoint temp eratures (SP1, SP2, SP3). Each of the three temperature setpoints is a control input 120 to the ther mocouple monitor device 118 (see Fig. 4) and may be adjusted as necessary to suit system specifications.

The comparison logic is essentially the same in the case of each of the three setpoint monitors. A temperature difference signal 132 is first determined as the spread between the highest and lowest temperature signals detected on a given scan. A comparator 131 compares the temperature difference signal 132 to the temperature setpoint signal 134 and provides an output signal which is activated when the temperature difference signal 132 exceeds the temperature setpoint signal 134. An actuated contact input signal 136 designated WCCI-1 enables an AND gate 138 and permits the comparator output signal to set a latch 140. The latch output signal activates a lightemitting diode (LED) 142 on an operator's panel (not shown) and actuates a contact-closure control output 144to a turbine control system (not shown).The latch output may be reset by means of a reset line 46 actuated by a pushbutton on the operator's panel or, alternatively, by a remote reset signal from the turbine control system.

The setpoint three (SP3) monitor has an additional output line 148 which causes a display on the operator's panel to freeze when that stepoint is violated. The frozen display will update if an operator presses a reset button, but will freeze again if the setpoint violation persists.

Other logic devices within the spread monitor section 130 indicate the failure of thermocouples. When the open thermocouple detector device 116 detects a failed thermocouple, a thermocouple failure input line 149 is activated, setting two latches 150, 151.

When the first latch 150 is set, it activates an LED 152 on the operator's panel and a control output 154 to the turbine control system. Twelve hours afterthe second latch 151 is set, a second contact output 156 is set. There is no corresponding LED on the operator's panel for the second control output 156.

Athird control output 158 and LED 160 are activated by a latch 162 when a total ofthree failed thermocouples have been detected and reported by activation of a control input signal line (TF3) 164. The three control outputs 154,156,158 actuated by the three latches 150, 151, 162 may be reset by means of a reset line 166 actuated by a pushbutton or by a remote reset signal from the turbine control system.

The second section within the thermocouple monitor device 130 (see Fig. 4), an ignition flame verification section 170, is depicted in circuit block diagram form in Fig. 6. The flame verification section 170 utilizes temperature detection hardware to verify the presence of a flame in each combustor basket at the completion of the ignition cycle. Operation of the section 170 is initiated by activation of a turbine igni tion signal 172, which is a control input signal desig nated WCCI-2. Activation of the WCCI-2 signal 172 causes a turbine ignition timer 174 to begin counting down from a preset value on the order of sixty sec onds. Activation of the WCCI-2 signal 172 also enables a counter 176.

Each time the lowest temperature signal 178 detected on a given scan ofthethermocouples exceeds an ignition temperature reference signal 180, a comparator 182 causes the enabled counter 176 to be incremented. Each time the lowesttemp- erature signal 178 falls below the reference signal 180, the comparator 182 causes the enabled counter 176 to be reset. A counter value signal 188 is continuously compared by means of a count limit comparator 184 to a count limit signal 186 which is on the order of four counts. Both the ignition temperature reference signal 180 and the count limit signal 186 are control inputs which may be adjusted accordingly to control system specifications.When the countervalue signal 188 does not exceed the count limit signal 186, a comparator output signal is inverted to obtain an alarm signal 185. A blocking gate 183 prevents the alarm signal 185from issuing forth until the turbine ignition timer 174 has expired.

Upon expiration of the turbine ignition timer 174, a latch 181 is momentarily set and then reset. The resultant pulse initiates two events. First, the counter 176 is disabled, stabilizing the output signal ofthe count limit comparator 184. Second, the blocking gate 183 is opened, permitting the count limit comparator 184 to set a latch 190 if an alarm condition exists. The alarm output signal lights an LED 192 on the operator's panel and actuates a contact-closure control output 194. The alarm condition indicates a failure of at least one combustorto ignite within the time period established by the turbine ignition timer 174. Activation of the alarm also freezes the display on the operator's panel (not shown).

The third section within the thermocouple monitor device 130, called the turbine acceleration flame monitor section 200, is depicted in circuit block diagram form in Fig. 7. The flame monitor section 200 utilizes temperature detection hardware to verify the continued presence of a flame in each combustor basket during turbine acceleration. The flame monitor section 200 is enabled by the expiration of the turbine ignition timer 174 (see Fig. 6). Expiration of the turbine ignition timer 174 sets a latch 202, which opens a blocking gate 204, enabling an alarm signal 221. The alarm signal 221 may be disabled by actuation of either a reset line 206 or the WCCI-1 control input signal 136, both of which reset the latch 202.

The alarm indicating loss of flame in a combustor basket occurs when two conditions are concurrently attained. To achieve the first of these conditions, the lowest thermocouple temperature signal 178 detected on a given scan is compared, by means of a comparator 208, to a flame-out temperature reference signal 210. When the lowest detected temperature signal 178 fails to exceed the reference signal 210, a counter 212 is incremented. When the lowest detected temperature signal 178 exceeds the reference signal 110, the counter 212 is reset. After each scan the state of the counter 212 is compared, by means of a second comparator 214to a count limit signal 216, which is on the order of six counts. If the state of the second counter 112 exceeds the count limit 216, the first condition for setting the alarm is met.Both the flame-out temperature reference signal 210 and the count limit signal 216 are control input signals which may be adjusted according to system specifications.

To determine the second condition, the temperature difference signal 132 is compared by means of a comparator 218 to a setpoint temperature signal (SP4) 220. If the temperature difference signal 132 exceeds the fourth temperature setpoint signal 220, the second condition for the alarm signal 221 has been achieved.

The output signal of the comparator 218 for the second condition is "anded" by means of an AND gate 222 with the output signal of the comparator 214 for the first condition. If both conditions are met and the alarm signal is enabled by the blocking gate 204, a latch 224 is set, lighting an LED 226 on the operator's panel and setting a contact-closure control output 228. Activation of the latch 224 also freezes the display on the operator's panel (not shown). As in the case of previous alarms, activation of a reset line 130 resets the alarm condition.

Hence, the turbine control system described herein provides improved turbine operation and extended blade life through a more reliable control system response to flame-out conditions. A turbine control system arranged in accordance with the principles of the invention employs a flame detection scheme utilizing existing transducer hardware to generate reliable control system response to variations in driving gas temperature, to failure of a combustorto ignite, and to loss offlame in a combustor during turbine acceleration.

Claims (8)

1. A gas turbine control system comprising a speed and load control for limiting turbine rotational velocity and turbine electrical loading, an acceleration control for limiting the rate of change ofturbine rotational velocity during start-up or sudden loss of load, and a temperature control for limiting the temperature of driving gases and turbine components, said temperature control having an apparatus for determining and indicating variations in temperatures of gases driving a combustion turbine and for determining and indicating to the control system the status of a flame within each of a plurality of combustor baskets in the turbine, said apparatus comprising means for sensing temperature of gases driving the turbine; means for periodically scanning said sensing means to determine a high temperature and a low temperature within the driving gases; means for scanning said sensing means to detect a failed sensing means; means for evaluating the high temperature, the low temperature and the failure status of said sensing means in accordance with a plurality of control inputs to determine and indicate excessive temperature variations in the driving gases and lack of flame in a combustor basket; and means for controlling fuel flow to the combustor baskets in response to said evaluating means to effect an attempt to reignite the turbine or to shut down the turbine.

2. A gas turbine control system according to claim 1 wherein said evaluating means comprises means for determining and indicating excessive variations in driving gas temperature, means for monitoring driving gas temperatures during a turbine ignition cycle and for determining therefrom and indicating a failure of a combustor basket to ignite during the ignition cycle; and means for monitoring driving gas temperatures during operation of a turbine after ignition and for determining therefrom and indicating a loss of flame in a combustor basket.

3. A gas turbine control system according to claim 1 or 2 wherein said sensing means comprises a plurality of thermocouple devices.

4. A gas turbine control system according to claim 2 wherein said monitor means also comprises means for indicating a failure of said sensing means.

5. A gas turbine control system according to claim 2 wherein said means for determining exces sive temperature variations comprises means for comparing a temperature difference signal to a temperature setpoint signal and signifying when the difference signal exceeds the setpoint signal; and means for latching an output signal from said comparing means so that an output which signifies that the difference signal exceeds the setpoint signal is continuously indicated until reset by a control input signal.

6. A gas turbine control system according to claim 5 wherein the temperature difference signal is a signal proportional to a difference between the low temperature and the high temperature detected by said scanning means on a given scan of said sensing means.

7. A gas turbine control system according to claim 2 wherein said ignition monitoring means comprises a first means for comparing on each scan by said scanning means a low temperature signal detected on that scan to a low-temperature reference signal and for signifying when the reference signal exceeds the low temperature signal; means for counting successive scans where an output of said first comparing means signifies that the reference signal exceeds the low temperature signal; a second means for comparing the state of said counting means with a predetermined count limit and for signifying when the state of said counting means exceeds the count limit; means for latching an output signal from said second comparing means; means positioned between the output for said second comparing means and an input to said latching means for enabling or disabling the input signal to said latching means; means for timing a turbine ignition cycle; and said counting means and said second comparing means continuously reflecting the relationship between the low temperature signal and the reference signal, until said timing means indicates the expiration of the ignition cycle, whereupon said timing means causes said enabling means to be enabled, permitting the final state of said second comparator means to indicate if there is a failure of a combustor basket to ignite.

8. A gas turbine control system according to claim 2 wherein said post-ignition monitoring means comprises means for enabling said post-ignition monitoring means to operate upon conclusion ofthe turbine ignition cycle; a first means for comparing the low temperature signal determined on each scan by said scanning means to a temperature reference signal, and for signifying an alarm condition when the reference signal exceeds the low temperature signal on a predetermined number of consecutive scans; a second means for comparing a temperature difference signal to a temperature setpoint signal and for signifying an alarm condition when the setpoint signal exceeds the difference signal; and means for indicating when outputs from both said first and said second comparing means signify an alarm condition.