Student Needs Help with X/R ratio's!!

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troyski99

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I am struggling to get my head round the X/R ratio's thing? I understand it's the reactance over the resistance but where do the values come from? Surely they would be different at different locations within the distribution network? If so, isn't that near impossible to then calculate? Which items of plant are included for X/R? I have to investigate how CHP effect the Distribution network's fault levels and X/R keeps cropping up and is standing in my way of understanding how i'm going to be able to answer that question. Could someone please clarify (in lay-mans terms, as i'm not very bright) some points around this area? I would very much appreciate it.
:?
 
From Troy
Hey Bob,

So this forum is for people to discuss and get help with electrical issues they are having, right? Then why would you shoot me down like that in my first thread? Do you not think that is the slightest bit mean? I notice you have posted over 2,000 comments on this forum already, are they always that negative?

I'm a student in my final year of college who has a wife, children and a mortgage. I am struggling to juggle all of these things with full time studies, and from time to time i could use a little assistance to help me save a time, hence my post yesterday. I spent a decent chunk of time looking in to X/R on the internet but couldn't get an understanding of it, so in my desperation put a thread up on here. You should think a little further than the end of your nose before you shoot someone down in the future, or practice what my mother taught me growing up "If you don't have anything good to say, then don't say it at all".

Have a good day

Troy
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Troy,
I am surprised at your attitude. There was no intent to "shoot you down". I don not know you and have no way of judging whether you are google savvy or not. The subject you are having difficulty with is very technical an may require a great amount of time to type a proper response. Many of the members of this forum use google as a quicker method to point the poster toward the answer. Frankly. I do not
care if you are
a student in my final year of college who has a wife, children and a mortgage. I am struggling to juggle all of these things with full time studies, and from time to time i could use a little assistance to help me save a time
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I have been there - done that and succeeded. I made an effort to answer your question the best way I deemed fit at that moment. Use it, if you want, to assist you in finding your answer.
 
Generally, the X/R ratio is measured at the point of common coupling. For the sake of understanding an electrical system's natural resonance frequency, you need to know the X/R ratio at the P.C.C.. With this being said, you can virtually ignore what happens in the facility as it is insignificant to the big picture. So, for instance if you are using the asymmetrical fault current with X/R ratio to determine the natural system resonance frequency of the electrical system, you can determine how, for instance, how a new capacitor bank kVAR will interact, possibly causing a parallel tank circuit.

This will help you determine if the capacitors will stack 5th or 7th , or potentially higher order harmonics, to cause a parallel resonance on the system. If you create parallel resonance at a frequency where sensitive drives operate, you can basically amplify the current on the system, causing drives to fail, motors to overheat and lights to flicker.

Hope that helps.
 
Wow, Troy. Just saw your response. At any rate, there is no one in this forum that has ever been mean or rude to me and I've been known to ask some pretty silly questions at times. Maybe you should re-evaluate your perspective, Sir. Sounds like you still have a lot of time in your life to become grateful. If not for yourself, for your wife and kids!
 
troyski99 said:
I am struggling to get my head round the X/R ratio's thing? I understand it's the reactance over the resistance but where do the values come from? Surely they would be different at different locations within the distribution network? If so, isn't that near impossible to then calculate? Which items of plant are included for X/R? I have to investigate how CHP effect the Distribution network's fault levels and X/R keeps cropping up and is standing in my way of understanding how i'm going to be able to answer that question. Could someone please clarify (in lay-mans terms, as i'm not very bright) some points around this area? I would very much appreciate it.
:?
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What is CHP?
The X/R is not hard to calc if the system is modeled. The X part of the equation relates to the reactance of all the wire (both copper and aluminum) that are able to store energy like an electro-magnet. These components include xfmrs, wire (cabling), capacitors, and anything that may hold or store energy, When there is a short (fault), the magnetic field of the electrical system collapses in the form of DC current. This is typically referred to as the asymmetrical fault current. When the DC field collapses, it does this very quickly.....typically less than a cycle or two. From there, it tapers down to the symmetrical fault current that is also referred to as the static or steady-state current. This steady-state current is the fault produced from generators (or motors) on the system and will typically last until the prime mover quits or the generators are isolated from the fault.

The R is the pure resistance of all the wire mentioned above.

The X/R ratio will affect how much energy is stored and how long the DC bleed off occurs. So, a high X/R ratio, will result in a large asymmetrical current that will take longer to bleed off before turning into the steady-state current.
 
FWIW (< 2 cents), I did not find Bob's response helpful. Telling someone to Google it is akin to saying I don't have time to help you right now. Providing a search phrase to narrow the Google results or links to some helpful articles would be a minimal response.

With that said Troy, I don't have time (actually don't feel like making the time at the moment) to help you with your X/R question. I just felt you were getting beat up a bit too much for a newbie to the site and don't really see where your reaction would be considered unexpected.

Just let it go. Bob's a good guy and can be helpful when he has time and he did not mean anything bad by his response. Typed words don't always convey exactly what we mean or how we feel.
 
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