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Changing trip delays on CEM block based on PV

I have a situation where my trip delay on a CEM block needs to change based on the PV. I can change the trip delay no problem but if I am already in a trip delay condition and the trip delay timer is timing, I cannot shorten that time.

Example:

my trip value 0.16 and my trip delay is 30 minutes, if my PV goes to 0.17 I want my trip delay to be 20 minutes. The problem I am having is that the trip delay timer has already started to count down from 30 minutes, changing the trip delay to 20 minutes does not affect the delay timer at all, it is still counting down from 30 minutes. I have tried to write the delay timer and it does not work.

Is there a way to reset the delay timer if the delay time changes? Say the delay time is 30 minutes and the delay timer has counted down to 27 minutes, my PV goes higher and triggers the delay time to be set to 20 minutes, I need the delay timer value to be set to the new delay time.

I have a matrix of 24 PV conditions and time delays that I need to account for. Basically the higher the PV the shorter the time to trip.

Thanks for any responses.

neil redfearn
  • GeloCortez
    not sure if that is possible. if not, you may want to use external function block to delay the trip
  • Bill Thackston
    There are several ways you can do this, but the appropriate answer depends on the behavior you want. In your example, if PV > 0.16 for 3 minutes, and now PV > 0.17, how long do you want to delay? 20 minutes, or 17? You can get a 20 minute time value by dropping the trigger bit for 1 scan, while writing to the delay time. The timer will start counting down with the new delay value. If instead you want to count down from 17 minutes, you can use an integration block and simply write to its upper limit. As an alternative, you could integrate the amount over the limit, which would reduce the time delay proportionally (you'd need to add a reset condition to the integrator of course). A much easier and far clearer method would simply be to use two (or more) timers OR'ed together, with different times and limit conditions. You also need to decide the response to a value momentarily > 0.17 and then > 0.16, etc. etc.
  • neil redfearn

    In reply to Bill Thackston:

    Here is what I have figured out. from a plot of the limits of the time vs PV, I have a linear equation Y=mX+b. I needed that to verify that the SGSR - Signal Characterizer Function Block would work for me, and it does. If my PV (X) increases, my time to trip (Y) will decrease in a linear fashion. Since I can't change the delay timer on the AVTR once it has started, I changed from an AVTR to a DVTR, with 2 votes to trip. First vote is PV above 0.16, second vote is a timer (off delay) expired. The off delay timer was the key. Once the off delay is counting, the elapsed timer of the off delay timer counts and the off delay parameter of the timer can be manipulated based on my linear equation from the SGSR block. This will give me the exact time to trip based on the PV. I will have to show the difference of the elapsed and off delay of the timer on the graphics so the operator knows how much time they have before they do actually trip. Thanks for your response Bill, it helped me get on the right track and eventually come up with a solution.

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