FUNDAMENTALS OF GAS TURBINE
PROTECTION
Objectives
• List the different trips typically used in gas turbine
applications.
• In general terms, describe how each trip signal is
developed and processed.
• Describe what physically happens to shutdown the turbine
when the Mark V processes a trip signal.
Trips
• Most gas turbine have the following protective trips:
– overspeed
– overtemperature
– loss of flame
– vibration
– combustion monitoring (exhaust temperature spread)
Protection System
Overspeed
• Gas turbines will have two means of overspeed that
operate independently of each other, primary and
secondary.
• Primary overspeed is performed in <RST>. It consists of
comparing the speed signal from the magnetic speed
pickups (TNH) against a constant (TNKHOS) and sending
a trip signal when TNH>TNKHOS.
Overspeed
• In modern applications, the secondary overspeed
protection is also electronic, performed in <XYZ>. The
processors also take inputs from magnetic speed pickups
and compares them to a constant to determine when a trip
is needed.
• Either the primary or secondary overspeed protection can
trip the turbine with nothing required from the other
circuit.
Overspeed
• In older applications, secondary overspeed was performed
mechanically with the use of an overspeed bolt mounted
directly on the shaft.
• The bolt was held in by a spring. As the shaft exceeded its
overspeed setpoint, centrifugal force would pull the bolt
out against the spring pressure. The extended bolt would
then hit a lever which would dump control oil.
Overtemperature
• The overtemperature trip is provided as a backup in case of
a failure in the temperature control loop which allows
TTXM to exceed TTRXB.
• If TTXM exceeds TTRXB by the margin established by
the constant TTKOT3 (25º F is typical), an
overtemperature alarm is received.
• If TTXM exceeds TTRXB by the margin established by
TTKOT2 (40º F is typical), the turbine is tripped.
Overtemperature
• The overtemperature protection also provides an
isothermal trip setting, TTKOT1.
• The isothermal trip compares TTXM to TTKOT1and trips
the turbine if TTXM is greater. TTRXB is not used for the
isothermal trip.
Overtemperature Graph
Overtemperature Circuit
Loss of Flame
• Gas turbines are provided with loss of flame trips to
prevent the buildup of a combustible fuel/air mixture in the
hot gas path and exhaust.
• In most gas turbine applications:
– loss of flame by one detector will result in a “Flame
Detector Trouble” alarm
– loss of flame by more than half the flame detectors will
result in a trip
Vibration
• Shaft vibration is detected by magnetic pickups located
along the shaft at bearing housings and other chosen
locations.
• The vibration signals from these detectors are compared to
control constants to determine if an alarm or trip is
necessary.
• Fault detection is also provided in case of a failed detector.
Vibration Protection Circuit
Combustion Monitoring
• Combustion monitoring is performed to detect
deterioration of the combustion system components.
• The combustion monitoring circuits monitor the
temperature of the compressor discharge air and exhaust
gasses and analyzes the pattern of these temperatures to
determine if there is a problem with the combustion
system.
Combustion Monitoring
• Combustion monitoring looks for incomplete mixing of
the combustion gasses to detect deterioration of
combustion components.
• In order for this to be effective, there must be an even flow
pattern in the turbine inlet.
Venn Diagram
Venn Diagram
• S1= the difference between the highest and lowest exhaust
temperatures
• S2= the difference between the highest and 2nd lowest
exhaust temperatures
• S3= the difference between the highest and 3rd lowest
exhaust temperatures
Combustion Monitoring
• The “Exhaust Thermocouple Trouble” alarm will actuate if
any thermocouple causes S1 to exceed a constant (usually
5 time the allowable spread) for 4 seconds.
• The “Combustion Trouble” alarm will actuate if a
thermocouple causes S1 to exceed a constant (usually the
allowable spread) for 3 seconds.
Combustion Monitoring
• A High Exhaust Temperature Spread trip will occur when:
– a Combustion Trouble alarm exists, S2 exceeds a
constant (usually 0.8 times the allowable spread), and
S1 and S2 are from adjacent thermocouples, or
Combustion Monitoring
• A High Exhaust Temperature Spread trip will occur when:
– a Exhaust Thermocouple Trouble alarm exists, S2
exceeds a constant (usually 0.8 times the allowable
spread), and S1 and S2 or S2 and S3 are adjacent,or
Combustion Monitoring
• A High Exhaust Temperature Spread trip will occur when:
– S3 exceeds a constant (usually the allowable spread)
for 5 minutes.
Trip Oil
• The trip oil system system is the interface between the
Mark V control system and the mechanical devices that
control fuel flow to the turbine.
• Besides tripping the turbine in an abnormal situation, the
trip oil system also provides signals to the fuel stop valves
for normal startup and shutdown.
• Finally, in dual fuel applications, the trip oil systems
serves the isolate the unused fuel supply from the turbine.
Trip Oil System
Trip Oil
• Key components:
– the inlet orifice is sized to provide a flow rate that is
sufficient for the trip oil system, but not so much that
the trip oil system will depressurize the lube oil system
when the turbine is tripped
– the orifice in each of the supply lines to the two fuel
dump valves (20FG and 20FL) are sized so that when
the unused system is dumping oil, it doesn’t
depressurize the entire trip oil system
Trip Oil
• Key components:
– the dump valves 20FG, 20FL, and 20TV dump trip oil
to the trip valves for the stop/speed ratio valve and gas
control valves (gas fuel), the liquid fuel stop valve
(liquid fuel), and VIGV actuator (VIGV’s)
– the dump valves are de-energized to trip so that the
turbine will be protected on a loss of DC power
Trip Oil
• Trip signals are 2 of 3 voted on the terminal board before
the solenoid dump valves are de-energized to ensure that a
single failure does not cause a spurious or unnecessary
turbine trip.
• On a trip, FSR is clamped to zero so that the servo valves
will shut the fuel control valves.
Trips
• Trips for the a 61B can be broken down into the following
categories:
– Pre-ignition (failure to ignite)
– Post-ignition (loss of flame, high exh temp spread)
– Protective status (high vibration, turbine overspeed)
– Starting means
– IGV
– Local and remote emergency manual trips
