V0N_hydro
Member
Hi,
I normally work on small hydro-electric turbines between 600 and 1500 rpm and am able to use PLC based PID control on these. My next project is a small steam turbine which operates at 4800 RPM.
On the end of the STG shaft are 12 magnets providing a 960Hz speed signal to a magnetic pick up unit (MPU). A woodward governor expert tells me that this signal will not provide sufficient resolution for control. Perhaps not even optimal control, but any control.
I was also told that generally 1800Hz speed signal is appropriate for control of diesel generators and 3-4kHz was appropriate for steam turbines.
A PLC with scan time 10ms is only 100Hz. Suppose the PLC gets the speed signal through a speed transducer device which converts the 960Hz-4kHz signal into a 4-20mA. The governor man said already this adds too much delay.
I suppose it is beneficial to be measuring changes in speed faster than PLC scan time of 100Hz (PID loop execution frequency), but I wouldn't have thought changes in the control valve position could be made fast enough for the PLC to be too slow.
The analog input card says sample time is 5ms default, configurable to 1ms for 1 input. resolution 24 bit. so an analog speed input is sampled at 200-1000Hz.
PLC can also have counter modules which sample at 10kHz or 60kHz. Sampling a 3kHz signal at 60kHz doesn't seem fast enough, but using the speed transducer solves this problem.
Interested in everyone's experience with control of steam turbines.
I normally work on small hydro-electric turbines between 600 and 1500 rpm and am able to use PLC based PID control on these. My next project is a small steam turbine which operates at 4800 RPM.
On the end of the STG shaft are 12 magnets providing a 960Hz speed signal to a magnetic pick up unit (MPU). A woodward governor expert tells me that this signal will not provide sufficient resolution for control. Perhaps not even optimal control, but any control.
I was also told that generally 1800Hz speed signal is appropriate for control of diesel generators and 3-4kHz was appropriate for steam turbines.
A PLC with scan time 10ms is only 100Hz. Suppose the PLC gets the speed signal through a speed transducer device which converts the 960Hz-4kHz signal into a 4-20mA. The governor man said already this adds too much delay.
I suppose it is beneficial to be measuring changes in speed faster than PLC scan time of 100Hz (PID loop execution frequency), but I wouldn't have thought changes in the control valve position could be made fast enough for the PLC to be too slow.
The analog input card says sample time is 5ms default, configurable to 1ms for 1 input. resolution 24 bit. so an analog speed input is sampled at 200-1000Hz.
PLC can also have counter modules which sample at 10kHz or 60kHz. Sampling a 3kHz signal at 60kHz doesn't seem fast enough, but using the speed transducer solves this problem.
Interested in everyone's experience with control of steam turbines.