5.1 GENERAL
Speedtronic Mark VI Control contains a number of control, protection and sequencing systems designed for reliable and safe operation of the gas turbine. It is the objective of this chapter to describe how the gas turbine control requirements are met, using simplified block diagrams as shown in Figure (5.1) and one–line diagrams of the SPEEDTRONIC Mark VI control, protection, and sequencing systems. A generator drive gas turbine is used as the reference.
5.2 CONTROL SYSTEM
5.2.1 Basic Design
Control of the gas turbine is done by the startup, acceleration, speed, temperature, shutdown, and manual control functions illustrated in Figure (5.1). Sensors monitor turbine speed, exhaust temperature, compressor discharge pressure, and other parameters to determine the operating conditions of the unit. When it is necessary to alter the turbine operating conditions because of changes in load or ambient conditions, the control modulates the flow of fuel to the gas turbine. For example, if the exhaust temperature tends to exceed its allowable value for a given operating condition, the temperature control system reduces the fuel supplied to the turbine and thereby limits the exhaust temperature.
Operating conditions of the turbine are sensed and utilized as feedback signals to the speedtronic control system. There are three major control loops – startup, speed, and temperature – which may be in control during turbine operation. The output of these control loops is connected to a minimum value gate circuit as shown in Figure (5.2). The secondary control modes of acceleration, manual FSR, and shutdown operate in a similar manner.
Fuel Stroke Reference (FSR) is the command signal for fuel flow. The minimum value select gate connects the output signals of the six control modes to the FSR controller; the lowest FSR output of the six control loops is allowed to pass through the gate to the fuel control system as the controlling FSR. The controlling
FSR will establish the fuel input to the turbine at the rate required by the system which is in control. Only one control loop will be in control at any particular time and the control loop which is controlling FSR will be displayed on the <HMI>. Figure (5.3) shows a more detailed schematic of the control loops. This can be referenced during the explanation of each loop to show the interfacing.
5.2.2 Start–up/Shutdown Sequence and Control
Start–up control brings the gas turbine from zero speed up to operating speed safely by providing proper fuel to establish flame, accelerate the turbine, and to do it in such a manner as to minimize the low cycle fatigue of the hot gas path parts during the sequence. This involves proper sequencing of command signals to the accessories, starting device and fuel control system. Since a safe and successful start–up depends on proper functioning of the gas turbine equipment, it is important to verify the state of selected devices in the sequence.
Much of the control logic circuitry is associated not only with actuating control devices, but enabling protective circuits and obtaining permissive conditions before proceeding. The gas turbine uses a static start system whereby the generator serves as a starting motor. A turning gear is used for rotor breakaway General values for control settings are given in this description to help in the understanding of the operating system. Actual values for control settings are given in the Control Specifications for a particular machine.
5.2.3 Speed Detectors
5.2.3 Speed Detectors
An important part of the start–up/shutdown sequence control of the gas turbine is proper speed sensing. Turbine speed is measured by magnetic pickups and will be discussed under speed control. The following speed detectors and speed relays are typically used:
–L14HR Zero–Speed (approx. 0% speed)
–L14HM Minimum Speed (approx. 16% speeds)
–L14HA Accelerating Speed (approx. 50% speeds)
–L14HS Operating Speed (approx. 95% speed)
The zero–speed detector, L14HR, provides the signal when the turbine shaft starts or stops rotating. When the shaft speed is below 14HR, or at zero–speed, L14HR picks–up (fail safe) and the permissive logic initiates turning gear or slow–roll operation during the automatic start–up sequence of the turbine.
The minimum speed detector L14HM indicates that the turbine has reached the minimum firing speed and initiates the purge cycle prior to the introduction of fuel and ignition. The dropout of the L14HM minimum speed relay provides several permissive functions in the restarting of the gas turbine after shutdown.
The accelerating speed relay L14HA pickup indicates when the turbine has reached approximately 50 percent speeds; this indicates that turbine start–up is progressing and keys certain protective features. The high–speed sensor L14HS pickup indicates when the turbine is at speed and that the accelerating sequence is almost complete. This signal provides the logic for various control sequences such as stopping auxiliary lube oil pumps and starting turbine shell/exhaust frame blowers.
Should the turbine and generator slow during an under-frequency situation; L14HS will drop out at the under–frequency speed setting. After L14HS drops out the generator breaker will trip open and the Turbine Speed Reference (TNR) will be reset to 100.3%. As the turbine accelerates, L14HS will again pick up; the turbine will then require another start signal before the generator will attempt to auto–synchronize to the system again. The actual settings of the speed relays are listed in the Control Specification and are programmed in the <RST> processors as EEPROM control constants.
Speedtronic Mark VI Control contains a number of control, protection and sequencing systems designed for reliable and safe operation of the gas turbine. It is the objective of this chapter to describe how the gas turbine control requirements are met, using simplified block diagrams as shown in Figure (5.1) and one–line diagrams of the SPEEDTRONIC Mark VI control, protection, and sequencing systems. A generator drive gas turbine is used as the reference.
5.2 CONTROL SYSTEM
5.2.1 Basic Design
Control of the gas turbine is done by the startup, acceleration, speed, temperature, shutdown, and manual control functions illustrated in Figure (5.1). Sensors monitor turbine speed, exhaust temperature, compressor discharge pressure, and other parameters to determine the operating conditions of the unit. When it is necessary to alter the turbine operating conditions because of changes in load or ambient conditions, the control modulates the flow of fuel to the gas turbine. For example, if the exhaust temperature tends to exceed its allowable value for a given operating condition, the temperature control system reduces the fuel supplied to the turbine and thereby limits the exhaust temperature.
Operating conditions of the turbine are sensed and utilized as feedback signals to the speedtronic control system. There are three major control loops – startup, speed, and temperature – which may be in control during turbine operation. The output of these control loops is connected to a minimum value gate circuit as shown in Figure (5.2). The secondary control modes of acceleration, manual FSR, and shutdown operate in a similar manner.
Fuel Stroke Reference (FSR) is the command signal for fuel flow. The minimum value select gate connects the output signals of the six control modes to the FSR controller; the lowest FSR output of the six control loops is allowed to pass through the gate to the fuel control system as the controlling FSR. The controlling
FSR will establish the fuel input to the turbine at the rate required by the system which is in control. Only one control loop will be in control at any particular time and the control loop which is controlling FSR will be displayed on the <HMI>. Figure (5.3) shows a more detailed schematic of the control loops. This can be referenced during the explanation of each loop to show the interfacing.
5.2.2 Start–up/Shutdown Sequence and Control
Start–up control brings the gas turbine from zero speed up to operating speed safely by providing proper fuel to establish flame, accelerate the turbine, and to do it in such a manner as to minimize the low cycle fatigue of the hot gas path parts during the sequence. This involves proper sequencing of command signals to the accessories, starting device and fuel control system. Since a safe and successful start–up depends on proper functioning of the gas turbine equipment, it is important to verify the state of selected devices in the sequence.
Much of the control logic circuitry is associated not only with actuating control devices, but enabling protective circuits and obtaining permissive conditions before proceeding. The gas turbine uses a static start system whereby the generator serves as a starting motor. A turning gear is used for rotor breakaway General values for control settings are given in this description to help in the understanding of the operating system. Actual values for control settings are given in the Control Specifications for a particular machine.
5.2.3 Speed Detectors
5.2.3 Speed Detectors
An important part of the start–up/shutdown sequence control of the gas turbine is proper speed sensing. Turbine speed is measured by magnetic pickups and will be discussed under speed control. The following speed detectors and speed relays are typically used:
–L14HR Zero–Speed (approx. 0% speed)
–L14HM Minimum Speed (approx. 16% speeds)
–L14HA Accelerating Speed (approx. 50% speeds)
–L14HS Operating Speed (approx. 95% speed)
The zero–speed detector, L14HR, provides the signal when the turbine shaft starts or stops rotating. When the shaft speed is below 14HR, or at zero–speed, L14HR picks–up (fail safe) and the permissive logic initiates turning gear or slow–roll operation during the automatic start–up sequence of the turbine.
The minimum speed detector L14HM indicates that the turbine has reached the minimum firing speed and initiates the purge cycle prior to the introduction of fuel and ignition. The dropout of the L14HM minimum speed relay provides several permissive functions in the restarting of the gas turbine after shutdown.
The accelerating speed relay L14HA pickup indicates when the turbine has reached approximately 50 percent speeds; this indicates that turbine start–up is progressing and keys certain protective features. The high–speed sensor L14HS pickup indicates when the turbine is at speed and that the accelerating sequence is almost complete. This signal provides the logic for various control sequences such as stopping auxiliary lube oil pumps and starting turbine shell/exhaust frame blowers.
Should the turbine and generator slow during an under-frequency situation; L14HS will drop out at the under–frequency speed setting. After L14HS drops out the generator breaker will trip open and the Turbine Speed Reference (TNR) will be reset to 100.3%. As the turbine accelerates, L14HS will again pick up; the turbine will then require another start signal before the generator will attempt to auto–synchronize to the system again. The actual settings of the speed relays are listed in the Control Specification and are programmed in the <RST> processors as EEPROM control constants.
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