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Rate Of Change in a certain time

+1 vote
i am looking for block to measure the rate of change for sine wave at normal operation the ROC = 0 during short circuit the current is increasing to very high level ... during this increasing i need some function to calculate the ROC and tell me if this increasing is a short circuit case

best regards

asked Mar 26 by Ahmed Raafat (20 points)

1 Answer

+2 votes

I believe this is answered in the Support section of the Plexim site, under "Why is there no “derivative” block in the PLECS component library?".  You can define a value for your short circuit current ROC limit, and use a comparator with that constant value to determine if you're shorted.  If that's not what you're looking for, please clarify.

answered Mar 26 by angrmgmt (43 points)
edited Mar 31 by angrmgmt
thanks for your interesting to answer ...

i asked PLEXIM support and they provide a block for di/dt ... but  i can not use it or in right way i dont know how i can use it ...

i am not looking to compare the current by a certain value , if exceed ,, So that mean i have short circuit ... so time the current it high but still lower than the diecrmental value by few limits ...

So i need another function can monitor the rise of current ,,, comparing to time ,,, if this rising is exceed the normal value it mean that  have short circuit ,, so that i dont have to wait until the current reach to the discremental vlaue

best regards

You have some signal with values of di/dt that fall within a particular range during normal operation.  If the di/dt value is greater (in magnitude) than this range, you have a short circuit, correct?  So using the information I suggested, you can set up a comparison between the value you select for the limit (di/dt = 1000000 A/s, for example) and your measured di/dt, and if the output of your di/dt block is greater than that value, you can do whatever it is you need to do to resolve that problem (open a switch, insert a shunt, etc.).

I don't know what you mean by "time the current it high but still lower than the diecrmental value by few limits" or "until the current reach to the discremental vlaue".  I assume you're talking about exactly what is in the first paragraph of this comment, though.  Please clarify if I have not understood you correctly.
you are completly right ..but if i did that i still detect the short circuit current by changing in magnitude .not the rate of rise

let us supposed  that the short circuit level is (id) = 5000 A , once the current reach to 5000 A , a magnitude comparator works, and i can take action ,....

but some times the current reach to 4500 A . it is very high value but lower than the sitting value. So the comparator will not work , but that this value is not desired ...

So i am looking to add another why for detection the short circuit ... (di/dt) ...  if the nominal currnet = 100 A,  i am lookig to detect the change in magnitude during the rising from the rated value (100A) to the short-circuit value , and if we found that the Rate of change is very high we dont have to wait the current until reach a (id) we take action imeadialty

hope that i did clear explain .. and hope you find something to help me
best regads

I think we may be having a problem understanding one another's mathematical language.  Not to mention I see an easily misunderstood typo in my first answer, which I will fix now.

I use di/dt to represent the rate of change of current with respect to time.  It could either be a 'rate of rise' or 'rate of fall', but either way it is the first derivative of the current with respect to time.  The link I gave you in my first answer tells you how to set up a c-script to obtain this rate of change.

Are you looking for the second derivative (the rate of change of the rate of change of current, or 'acceleration' of current)?  If so, you can chain two of these blocks together.  Taking the derivative of the derivative of the current will give you this value.  If you want a way to implement that directly, you can take a look at higher order finite differences, and change the suggested script accordingly.  I can't imagine a use for this, and certainly not for what you've said you wanted to do, but this might have been our misunderstanding to begin with.

Either way, PLECS cannot possibly know what you consider to be too high a rate of change, so you must define a value yourself (as a constant), and one way for it to determine if the rate of change of current is greater in magnitude than that value is with a comparator.  An alternative would be another c-script with an if/then statement in it.

Please try what I have suggested in my first answer.  If you're not sure how to connect it, simply probe your current, and connect it to the input of the c-script, then connect the signal output of the c-script block to a scope to confirm that this is, in fact, what you're looking for.