interesting the highest end 300 series processor the 319 is faster than the high end 400 series 417.
so far the extra expense of having one PLC per unit isn't justified by the cost of downtime for all units instead of only 1 unit due to a PLC failure, which is a low likelihood event. our average plant is less than 10MW.
we only have the one PLC, although remote IO racks can be the same for each unit. governor control in past designs has been hardwired back to the main rack and not over communications bus, although I'm considering moving to remote IO for the HPU control as well.
Remote IO with quantum is over modbus, with IO scan time of 10ms. IEEE 125 deadtime is 200ms. say 50ms to sense the change in speed, 50ms to calculate the desired change in control variable, 50ms to operate the output and 50ms delay in the action of the hydraulics and the deadtime requirement for IEEE 125 is met.
100ms is allocated to PLC time there. Say the scan time was as high as 20ms, the PLC would still be fast enough to process the input and make the change in output to have the governor respond within 200ms.
In practice scan times on quantum PLC I see are much less than 20ms, so I don't see PLC processing power as being a limiting factor (with Quantum, and assuming S7-319 is just as fast). But please elaborate on any experiences which indicate otherwise, I haven't found many people I can discuss implementations that meet IEEE 125 with.
Meeting the 200ms deadtime in IEEE 125 doesn't guarantee good performance or meeting the speed stability index specification either, so perhaps scan time plays a part in that, although I would have thought it would be mostly determined by resolution of speed sensing and precision of the control of the torque applied to the turbine. Perhaps we should start a new thread for IEEE 125 if you are keen to talk more in detail about the requirements?
thanks for your thoughts
so far the extra expense of having one PLC per unit isn't justified by the cost of downtime for all units instead of only 1 unit due to a PLC failure, which is a low likelihood event. our average plant is less than 10MW.
we only have the one PLC, although remote IO racks can be the same for each unit. governor control in past designs has been hardwired back to the main rack and not over communications bus, although I'm considering moving to remote IO for the HPU control as well.
Remote IO with quantum is over modbus, with IO scan time of 10ms. IEEE 125 deadtime is 200ms. say 50ms to sense the change in speed, 50ms to calculate the desired change in control variable, 50ms to operate the output and 50ms delay in the action of the hydraulics and the deadtime requirement for IEEE 125 is met.
100ms is allocated to PLC time there. Say the scan time was as high as 20ms, the PLC would still be fast enough to process the input and make the change in output to have the governor respond within 200ms.
In practice scan times on quantum PLC I see are much less than 20ms, so I don't see PLC processing power as being a limiting factor (with Quantum, and assuming S7-319 is just as fast). But please elaborate on any experiences which indicate otherwise, I haven't found many people I can discuss implementations that meet IEEE 125 with.
Meeting the 200ms deadtime in IEEE 125 doesn't guarantee good performance or meeting the speed stability index specification either, so perhaps scan time plays a part in that, although I would have thought it would be mostly determined by resolution of speed sensing and precision of the control of the torque applied to the turbine. Perhaps we should start a new thread for IEEE 125 if you are keen to talk more in detail about the requirements?
thanks for your thoughts