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Topic Title: Earth fault Current  Current Transformer Topic Summary: Created On: 06 June 2013 03:10 PM Status: Read Only 
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13 June 2013 10:33 AM


Lets take for 3 phase load,
Phase A, the load is 20+j13, phase B and Phase C are zero load, try to calculate the vector sum of the current. I believe it wont be zero... 



13 June 2013 10:36 AM


In that example you have a earth fault equivalent to the current in Phase A!
So A + B + C + E = 0 (see the examples at the beginning of the thread about the summations) A nonzero sum in the three phases is due to an external circuit (earth fault) or a short circuit (phases shorted). If you have a load that is ONLY connected to one phase, then it must have a neutral conductor otherwise you do not have a circuit! (unless it is a traction load or SWER, which uses the earth as a return path). In a 3phase, 3wire distribution system, the out and in paths are through the 3wires, as in the example. Edited: 13 June 2013 at 10:54 AM by Zuiko 



13 June 2013 10:57 AM


In that example you have a earth fault equivalent to the current in Phase A! So A + B + C + E = 0 (see the examples at the beginning of the thread about the summations) The imbalance in the three phases is due to an external circuit (earth fault) or a short circuit (phases shorted). Im not talking about a fault, im talking about the load, i installed single phase load which is 20+j13 on phase A, while Phase B and Phase C are zero because there are no load on them, try to calculate the vector sum of current....you wont get zero, they will be return current which is neutral current, not fault current as you mentioned... Current in is Phase current, current out is neutral current...Kirchhoff's Laws... 



13 June 2013 11:00 AM


You are confusing yourself!
Think about it practically. You have a 3phase, 3core/wire distribution system So you run your 3core cable down the street and connect 1 core to a load. Where is the return? You have one end of, say L1 plugged into your television, where is the other end of the television plugged into? Without an "out" connection back to the circuit, all you have in, effect, is an open switch. The out wire will have the same current as the in wire. You have to connect a CIRCUIT. Draw it out. So any circuit needs at least 2 "paths"  one into the circuit and one out. Think about the circuit as a number of points  the current goes into the point, and out of the point. Be that 1 phase and a neutral 1 phase and an "earth" return (this could be the sea, or the sky, or yourself or any conductive path!) or 2 phases or 3 phases or a combination of all these! Neutrals are used because there will single phase loads  such as the television above (sometimes they are used in other situations to measure displacement). So if you had an example as you said, the neutral current would equal the phase current. And any protection you had would have to compare the neutral current to the phase current. This would be a simple RCD like you have in your house. If you had a three phase, 3wire installation, with only 1 load switched on, and that load was a single phase load then you would get current flowing in the other 2 phases to balance the load. You cannot get a 3wire situation where you have L1 = xA L2 = 0A L3 = 0A unless you have a fault on L1 what you would get is a ratio of xA in L2 and L3 which would sum to xA, the value on each phase would depend on the impedence of that phase. The fact you cannot get the example you described on a 3wire system without there being a fault that is the basis of powersystem protection! Edited: 13 June 2013 at 11:20 AM by Zuiko 



13 June 2013 11:20 AM


Zuiko,
There are no load on 2nd phase and 3 phase on my example here, the return current is neutral in my case, but there is no CT on neutral, only on phase conductor,how to cancel the 1st phase since the relay cant read or cant detect the current on the neutral because there is no CT on neutral, relay will read it as imbalanced condition.... Im very clear about RCD and low voltage system for consumer unit...for RCD not problem because Phase Conductors and Neutral installed inside the same CT....So in healthy condition, the flux will cancel each other..so there is no residue current 



13 June 2013 11:23 AM


I just check with my commissioning engineer, For 4 wire system, must use for 4 CT...so its clear now...thanks guy who help me out....




13 June 2013 11:24 AM


So you have a 4 wire circuit RYBN
Then you have to compare what is in the neutral to what is in the phases. Simple as that. There is no way you can guess or assume that the current in the phase is returning through the neutral as it should  you have to measure it! Using your example You have 20A in L1 and 0A in L2 and L3 So 20A must be returning though either the neutral (healthy) or the "earth" (fault), or both (fault) You have to look at either the neutral or the earth to see which one it is. 



13 June 2013 11:39 AM


So you have a 4 wire circuit RYBN Then you have to compare what is in the neutral to what is in the phases. Simple as that. There is no way you can guess or assume that the current in the phase is returning through the neutral as it should  you have to measure it! Using your example You have 20A in L1 and 0A in L2 and L3 So 20A must be returning though either the neutral (healthy) or the "earth" (fault), or both (fault) You have to look at either the neutral or the earth to see which one it is. I am clear about that , i was wondering about possibility to use 3CT for 4 wire system... What you were explained totally correct, but u missed out my question about 3CT and 4CT.... I really appreciate you great help 



13 June 2013 12:07 PM


excellent, I am glad you got it sorted.
Do have another think about what you wrote here though: "Current in is Phase current, current out is neutral current" This is not generally true! Think about the simplest circuit where you have a voltage divider for example. Edited: 13 June 2013 at 12:13 PM by Zuiko 



13 June 2013 05:19 PM


I agree. If the residual output of the CT's is say 5A because we only have 5A on L1 this must be a fault on a 3 wire system. If we have 5A on L1 for a single phase load the load would be presented on 2 phases of a 3 wire system so L2 or L3 would have a load too.




14 June 2013 01:52 AM


The return current cannot flow in the neutral wire when there is not one, so think about why we use neutral conductors? Where do you normally see neutral conductors? It find it easier to work the currents out using j notation, and I get (for the unbalanced circuit above) L1A =  375  j130 L2A = 300 +j289 L3A = 75 j159 which is exactly zero. The simulator is using some much more complicated maths routines, meaning that the current is exactly zero when the measured precision is infinite....don't worry about 3 e14, that number is so small in reality it is zero (it would be unmeasurable in any circuit outside of CERN!). The vector sum of currents in ANY circuit is ZERO. This is Kirchoffs Law, and it is fundamental to the understanding of circuits. It is an example of the fundmental law of science  the conservation of momentum and energy. Once you get your head around that, you will understand the way the three CTs and summation TX work. Bro, I thk u are wrong about this, for unbalanced load, there will be return current, the circuit cant be completed without return path, Current goes in, current goes out, that correct, thats why we have the return current over here.. Math is logic, computer calculation is accurate, its not like human calculation where normally we will ignore some of the numbers...from the software, it has been proved that the neutral is not zero for unbalanced load.. U can try with any load on phase A, while phase B and C is no load, or u can try with phase A and phase B load, while the phase C no load.;..im sure they will be the return current in order for the circuit to complete the return path...the load wouldn't working without return path... But it is different from 3 phase balanced load...neutral will zero because the they cancel each other out... Try to use ur software with this load, and post ur result here.. Phase A= 3+j6 Phase B = 45+j87 Phse C=0 Or u can try Phase A=3+j6 Phase B = 0 Phase C=0 Im sure 100% they will be neutral current.. U can see the neutral conductor on LV of the transformer, MSB,SSB or any 3 phase board, neutral as a return path for the unbalanced load current, check the any SLD and u will find the neutral conductor at MSB, SSB... Without neutral conductor for unbalanced load, it will damage some of the equipment if the load unbalanced... Take an example of a three phase motor, Most of the motor is designed without neutral because the load inside is balanced, but if one of the phase fail, the motor will damage if without phase lost relay... In reality, u cant achieve 3 phase load balanced from MSB or transformer site..thats the purpose of neutral wire. Please refer to this note which i googled from internet, a lecture notes from one of the university, u can study it about 3 phase system, u can refer to page 12 of the slide, its about the unbalanced load, Line currents DO NOT add up to zero. In= (Ia+ Ib+ Ic) ≠ 0 



14 June 2013 09:04 AM


Jack,
I'm not sure what you mean by "in reality"; In reality, power distribution in the United Kingdom, and just about every other country I can think of, is 3phase, 3wire. If in reality, the currents are not summing to zero, where do you think they are going? They cannot just "disappear" can they? (let's disregard antennas and such like!!) If you have zero current on one of the phases of a 3phase, 3wire system you have a fault, simple as that. Have a look at a HV power line  how many conductors can you see? (on EHV transmission you will often see an extra earth condutor above the tower in case of lightning strikes) I'll give you an example, when you open a HV switch on a OH line, the first thing you must check is that all 3phases have opened. If one has got stuck, you have created a fault situation which would trip the feeder breaker back at the substation. Same goes when closing it, all the phases must make. Generally, (not always) it is only when supplying single phase LV loads that neutrals are used. If you have an LV 3phase installation such as domestic or light commercial (which is what I think you are talking about) and 2 of the phases are switched off at say the main Distribution board, then the return path for any load connected to the remaining phase is through the LV neutral which is connected to every load in the installation and which is shorted to all three phases back at the substation. But do not confuse this with power distribution where threewires only is sufficient, regardless of load. In fact, power transformers are quite happy with very unbalanced loads, and it would not be uncommon to notice that most of the load is on one of the phases. regarding the software  I think you should try it to convince yourself, rather than me! 



14 June 2013 09:13 AM


Jack, I'm not sure what you mean by "in reality"; In reality, power distribution in the United Kingdom, and just about every other country I can think of, is 3phase, 3wire. If in reality, the currents are not summing to zero, where do you think they are going? They cannot just "disappear" can they? (let's disregard antennas and such like!!) If you have zero current on a 3phase, 3wire system you have a fault, simple as that. Have a look at a HV power line  how many conductors can you see? (on EHV transmission you will often see an extra earth condutor above the tower in case of lightning strikes) Generally, (not always) it is only when supplying single phase LV loads that neutrals are used. If you have an LV 3phase installation such as domestic or light commercial (which is what I think you are talking about) and 2 of the phases are switched off at say the main Distribution board, then the return path for any load connected to the remaining phase is through the LV neutral which is connected to every load in the installation and which is shorted to all three phases back at the substation. But do not confuse this with power distribution where threewires only is sufficient, regardless of load. In fact, power transformers are quite happy with very unbalanced loads, and it would not be uncommon to notice that most of the load is on one of the phases. regarding the software  I think you should try it to convince yourself, rather than me! Bro, i was talking about LV side, and u were talking about HV side...i guess wht u were trying to explained was on power distribution or HV side... yes u are correct, for HV side before consumer load, the distribution is 3phase, 3wires..., and the relay configuration for 3phase/3 wires is different than 3pahse/4wires Im not from england.. 



14 June 2013 09:31 AM


What I was trying to explain was that the sum of currents in ANY healthy circuit regardless of number of conductors or voltage, is zero
Whether that be HV or LV Whether is be 2 wire, 3 wire or 4 wire; or even 1 wire in the case of SWER (in which the earth is considered a circuit conductor) Do not confuse yourself with HV or LV or the number of phases or conductors. The sum is always zero. If your sum is not zero, you have made a mistake! 



14 June 2013 09:47 AM


What I was trying to explain was that the sum of currents in ANY healthy circuit regardless of number of conductors or voltage, is zero Whether that be HV or LV Whether is be 2 wire, 3 wire or 4 wire; or even 1 wire in the case of SWER (in which the earth is considered a circuit conductor) Do not confuse yourself with HV or LV or the number of phases or conductors. The sum is always zero. If your sum is not zero, you have made a mistake! Zuiko, For Unbalanced load, the sum vector is not zero, you can read the detail explanation on the slide i sent to u from previous post, that the function of neutral conductor on LV side, I never confuse with LV/HV or any number of phases, im very clear about that... Have u made u simulation?....i think u are confuse now.... 



14 June 2013 11:25 AM


Jack,
Neutrals are used to supply phase voltage rather than line voltage loads. If you do not have phasevoltage loads you do not need a neutral. Edited: 14 June 2013 at 11:31 AM by Zuiko 



14 June 2013 03:10 PM


"Math is logic, computer calculation is accurate, its not like human calculation where normally we will ignore some of the numbers...from the software, it has been proved that the neutral is not zero for unbalanced load"
oh really? So, what is the exact magnitude of the current 1 + 1j A or you have 400V line voltage, what is the phase voltage? When you work in electrical engineering you are often going to get irrational numbers such as these, which tend to an exact value when the accuracy tends to infinity. Be careful of spurious accuracy  the answer you get out cannot be more accurate than the data you put in. And beware of the trap of thinking that "computers are accurate"; this is not the case. Computers are as accurate as their designers make them. For example, computers that use floating point calculations do not like irrational numbers that you use all the time in electrical engineering! If you see an answer that you think should be zero and the computer throws an answer that is 3e 14 , use your common sense. Edited: 14 June 2013 at 03:59 PM by Zuiko 



15 June 2013 01:44 AM


"Math is logic, computer calculation is accurate, its not like human calculation where normally we will ignore some of the numbers...from the software, it has been proved that the neutral is not zero for unbalanced load" oh really? So, what is the exact magnitude of the current 1 + 1j A or you have 400V line voltage, what is the phase voltage? When you work in electrical engineering you are often going to get irrational numbers such as these, which tend to an exact value when the accuracy tends to infinity. Be careful of spurious accuracy  the answer you get out cannot be more accurate than the data you put in. And beware of the trap of thinking that "computers are accurate"; this is not the case. Computers are as accurate as their designers make them. For example, computers that use floating point calculations do not like irrational numbers that you use all the time in electrical engineering! If you see an answer that you think should be zero and the computer throws an answer that is 3e 14 , use your common sense. To get the magnitude u can use the simple formula where you have learned from school, for 1 + 1j A, the magnitude is A2 = 12+12 = square root of 2 U can find the current by divide the magnitude and voltage, please read the details in the lec note that i sent to u, its simple note about 3 phase circuit. The answer never been zero for unbalanced load, u can use clamp meter on the neutral cable u will get the value of the current... Please dnt blame the math and computer without any basic, there nothing wrong with that formula and it has been proved for long time... i think u are confusing urself about 3 phase, like i said , try to make a simulation with single load, there will be neutral current exactly same on phase side, or 2 phases load..the number is not zero and not almost zero right??, im sure u are aware of this... Please also check with ur lecturer if u are still student,,. Here another link about 3 phase unbalanced load, http://v5.books.elsevier.com/b...478/9780750687478.pdf Turn to page 24 for 3 phase unbalanced topic 'If the load is unbalanced, then the three line (and phase) currents will be unequal. In this case, the neutral has to carry the resulting outofbalance current. I hope u read it to clear your confusion 



15 June 2013 09:41 AM


Dear Jack
you are going around in circles and tying yourself in knots. I will try to explain this one more time If you have phase voltage loads you use a neutral if you do not have phase voltage loads you do not need a neutral This is the entire basis of power distribution, and gets back to your original question which I have answered at least several times now and I will answer once more so you understand: 3wire distribution system, the current sums to zero, if it does not you have a fault. 4wire distribution sytem, the current sums to zero, if it does not you have a fault. This is the basis of protection, as was explained earlier in the workings of a summation transformer. Your quote is only relevent to phase voltage loads; which is obvious because as you know there is no neutral in systems where the loads are all line voltage loads. I think you need to understand where and why neutrals are used. Maybe somevody at your work could show you round a substation and have a look at the protection systems for a threewire system without a neutral, and then a 4wire system with a neutral? It makes a lot more sense once you have installed and tested one in reality. It is all rather abstract on a page, and I think that is why it is bothersome to you? Thanks, for the invite to clear my confusion, but I finished my electrical engineering schooling a long long time ago, and although every day is a school day for an engineer, and we are constantly learning, it was you who came here for help I remember? The advice I have given to you is sound and correct. Do not take everything you read in a book, or the answer from a calculator to be correct, unless you can understand the context! Convince yourself, do your own calculations and experiments. Try to think why a computer will give a nonzero answer when working with irrational numbers. Spurious accuracy is often very dangerous, and has led to many engineering disasters and failures. good luck W Edited: 15 June 2013 at 10:29 AM by Zuiko 



03 December 2013 12:53 PM


I hope it's not to late to say thanks!, yeah I got it



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