I know the difference between PLC and microcontroller. MY question is how Et200s, S7-300, s7-1500

and S7-400 stands compare to some microcontroller. For an example Atmega16 can work on a maximum frequency of 16MHz. Whats the CPU speed of the mentioned CPUs.

Why do I ask this? Well we have need(my company) to do something what normal logic says to use microcontrolell but since we are working mostly with PLCs andd in order to save time/money I'm actually wondering can we do same job with PLC since we are handling with PLC at the moment and certainly not with microcontrollers. 

PLC/microcontroller would need to measure AC current and regulate electrical unit according that measurement. 

Piotr.M

Hi

You need answer some questions - microcontroller has a scan time (i.e. one run of the infinite loop of main program) in microseconds and PLC scantime is in order of 3 to ca. 150ms depending on program complexity and the cpu.

what response time is required?

what analog conversion speed is required? any filtering i.e. filter 50Hz?

etc.

Yes, PLC can control analog outputs based on input and setpoint using standard PID block or any other control method - it is freely programmable.

microcontroller

+ cheaper to manufacture for large series (where one expensive development can be spread in sale priceof  many identical devices)

+ faster

- more time consuming development

- you need create a custom PCB

PLC

+ fast development

+ can be upgraded in the field by service engineers and diagnosed by customer maintenance staff

+ ready off-the-shelf modules for in/out

+ widely available software and software engineers

- more expensive per unit than microcontroller

depends on budget per machine and complexity use S7-1200 or ET200.

if very tight on budget and you like messy programming on markers use unitronics (PLC + HMI in one box)

 Well, Lets say I need to control generator voltage. Generator voltage frequency is 50Hz in stabile grid/ power network. So I would say 20ms would be minimum. 

But since AVR (automatic voltage regulator) needs to answer on some power grid peaks/disturbance it should be faster than 20ms I guess. For how much I dont know.

But lets stay at 20ms. 

Problem with PLC is how to measure governor unit, in this case generator current/voltage.

In a case I'm using Siemens measuring device SENTRON PAC4200 just for measurement and then via PROFINET/PROFIBUS to PLC, I think I'm over 20ms already

I'm also not sure whats sampling time of SENRON PAC4200, but Siemens 7UM62 generator protection device(taking measurements from this device is option too) it's about 30-40ms If I'm not wrong.

Piotr.M

is it a stand-alone (island) genrator or one generating to a grid?

if standalone you need control RPM and excitation. this does not need to be very fast because of inertia of the generator itself. if it works on a grid you will need a synchronization automation (which measures voltages across the opened circuit breaker and then you need equalize rms values on both sides then minimize values across the switch i.e. L1.Mains vs L1.Generator, same for L2 and L3 then close the switch.

If it is a one-off solution then PLC could be cheaper (easier to program)

more analysis is needed on response times, any safety functions, etc.

 First of all I'm glad you know about this section too (I mean on Power plants).

Most of our projects are generators up to 10 MVA working on the grid without island mode of work option. Just on few, client demanded island mode of work.

Regarding frequency controll thats what I'm already doing with PLC, no matter island mode or not.

Regarding which turbine is used and regarding Hydraulic system PLC controls frequency(before synchrnozation), active power, pressure or water level(on the grid - after synchronization). 

In a case of island mode PLC keeps regulating speed even after synchrnozation(synchronization on dead bus) and keeps it to 50Hz in order to satisfy LOAD demands. IN a case of island mode there are usually also a little bit different valves on Hydraulic unit, and usually there are flying wheels on generator-turbine shaft too in order to slow down speed change once LOAD has got changed.

In each case island mode or on the grid, parallel to these regulations I have mentioned that are performed by PLC there are Voltage regulation before synchrnozation by AVR, and reactive power or PF or even voltage regulation on the GRID too. (reactive and PF regulation on the GRID, sometimes even voltage regulation on the grid too). 

These regulation have been done by AVR but reference usually coming from PLC(HMI, SCADA...)

My goal was to avoid AVR device and change it with some PLC in a case microcontroller is not needed. You have said on the GRID I dont need that quick device. Well it depends how good GRID is. Paraller to PLC control there Generator protection devices looking for  U>>,<<U, <<f, f>>, Overexcitation, U/f and so on... They can stop machine in a case that AVR has slow reaction. 

What you have mentioned for syncronization...Usually AVR does not do that, usually there comes devices with just that function, to tell you that both voltages (GRID and Generator) are at some time exactlly the same and they just close contact throught which you energized the closing coil of generator circuit breaker. 

Since I think AVR should be slower device than synchronizer my first Idea was to make AVR inside PLC. Next step would be to make syncronizer too.

SO since you know about this mattery what do you think which speeds are neccessary for AVR and lets say for synchronizer. How PLC can take measurements of the voltage/current without some measuring device like SENTRON and then via communication. It is possible via some Analog input, but in that case measurements also needs to go through some device too before it goes to PLC.

Piotr.M

BTW

You can use the timed interrupts in the PLC for time-critical tasks as well

most probably you can avoid the microcontroller

but you need ensure the analog inputs have required speed (some needs more than 20ms per channel to convert and some even ca 800ms for the whole ai module)

 I've been working with OB35 so far for these regulations I have mentioned but they are not suppose to be quick so I got no problem. Today I have found for a very first time OB61 but I still didnt get it for what it is for exactlly. I can not see the purpose of the OB61 at the moment.

Piotr.M

Hi

OB61 is used in time-driven networks when data exchange occurs at given time interval

it is called AFTER data exchange so you get fresh signals as soon as they are received by the PLC

 You mean data exchange in a case of some communication, so after reading/writting something, or

lets say in my case immediatelly after AI card has refresh(after cycle time has finished - sampling time) measuring values?

I have read this post https://support.industry.siemens.com/cs/document/26170521/how-do-you-calculate-the-cycle-time-for-an-analog-module-?dti=0&lc=en-WW.

So in a case I have 1 channel active that would be after 22ms I would call OB61 and that would be quicker compare to using OB1 instead of