336602256-zero-sequence-impedance-calculation.pdf

Zero Sequence Impedance calculation Ruhollah BaziarGraduated student with the Electrical Engineering at University of Tehran Dear All, Could anyone please expalin me about Zero Sequence Impedance? And what is the difference between Positive, Negative and zero sequence impedance? I have searched through internet for an hour. But I'm really confused about the chapter. I want to understand clearly the concept and how to do testing and calculate the same in power cables after laid. I really appreciate if anyone please give me the clear explanation. Thanks in advance     

Beğenin (18) Yorum Yapın(58) Paylaşın Takip Edin 7 Ocak 2013 Yorumlar Santosh Kumar Bavisetti, Pelqim Spahiu BSc MSc CEng MIET ve 16 kişi daha bunu beğendi 58 yorum En yeni yoruma atlayın

 Geoff Garber Geoff Garber Engineer at First Solar, experienced in protection, integration, automation, I&C, and telecommunications. To start with, the topic of symmetrical components is difficult to grasp for most people. I find it best to start out with a description that does not include math. One of the first jobs for a protection engineer when trying to determine an overcurrent or distance relay setting is to find the magnitude of a possible fault on the 3-phase electric system. For some faults, particularly line to ground faults (they are also the most common), this can be very difficult. Because of the impedance of the lines from the source to the fault, and the impedance of the source itself, the uneven voltage drops and currents are very difficult to calulate using trigonometry. Back in 1918, Charles Legeyt Fortescue, developed a system called symmetrical components that made the math much easier. Fortescue said that any unbalanced set of 3-phase phasors (rotating vectors) could be represented by the sum of three sets of balanced phasors. (Balanced phasors are much easier to work with mathematically.) He called these the positive, negative and zero sequence phasors. The positive sequence represents the part of the phasors that are balanced. With no fault and balanced voltages and loads, the positive sequence equals the total. (Both negative and zero sequence are zero.) The negative sequence loosely represents the imbalance between the three voltages or currents. A line to line fault will have high negative sequence currents. The zero sequence loosely represents the ground current of a line to ground fault. A fault that includes ground will have some zero sequence current. The important thing to remember here is that these sequences do not exist separately. Each is only a mathematical construct. Only their sum can actually be observed.

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For further explanation, brush up on your algebra and visit the web site of any relay manufacturer. Each has an explantaion of symmetrical components that is heavy in math. I find the SEL guide is one of the best. Beğenin (32) Uygunsuz olarak işaretleyin 7 Ocak 2013 Rob M., SEKHAR B. ve 30 kişi daha bunu beğendi

 SEKHAR BHATTACHARYA SEKHAR BHATTACHARYA TECHNICAL ADVISOR LARSEN N TOUBRO SARGENT N LUNDY LTD VADODARA o o o

Well said in a NUTshell by Geoff Beğenin Uygunsuz olarak işaretleyin 8 Ocak 2013

 K A Radhakrishnan K A Radhakrishnan GM ENERGY o o o

Briefly well explained for a beginner by Geoff. Beğenin Uygunsuz olarak işaretleyin 8 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Well Explained by Prof Geoff ! It would have been also better to add the associated conversion chart of various harmonic components and their pathways of breaking down in to various vector sequence components! Beğenin Uygunsuz olarak işaretleyin 9 Ocak 2013

 DANIEL DIMITROV DANIEL DIMITROV Lead Electrical Design Engineer at FLUOR K.S.C. All this said by geoff is correct only for the directly grounded systems. The zero sequence impedance is different for the compensated (through Petersen coil or active resistance) menworks as well for the isolated networks,

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where the zero sequence impedance is equal to zero, when the three phase compex conductance to ground are equal. There would be some value of the zero sequence impedance in the isolated networks only at earth faulth. The value would depend on the capacitive conductance to ground. Beğenin Uygunsuz olarak işaretleyin 11 Ocak 2013

 Geoff Garber Geoff Garber Engineer at First Solar, experienced in protection, integration, automation, I&C, and telecommunications. o o o

Like I said, this is a difficult topic. Brush up your algebra (particularly matrix operations) and dive in. Beğenin Uygunsuz olarak işaretleyin 11 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Typically all Zero Sequence Impedence calculations are done at say 50/60 Hertz, it must be multiplied by 3 to get the real value of calculated Zo impedence. The reasons are simple all Io the Zero Sequence Currents are primarily third harmonics and above. The Zero Sequence Current Component at 50/60 Herz is only 0.01 % of the total Zero Sequence Currents being observed and hence it encounters more impedence in its pathway under any fault conditions. The study of Zero Sequence Currents is at the heart of all the electrical faults, system losses, corona losses, switching transients and power quality related issues electrical system based operational matters. In the real world of electrical engineering the fun lies often in trying to solve the challenge of the unknown ! Interestingly, by simply putting in laymans words, any current which is going directly in to the grounding point and not coming out is a Zero Sequence Current and it then travels a long way to the NeutralGround Base of the main generating source along the earth path conductor. Beğenin (2) Uygunsuz olarak işaretleyin 11 Ocak 2013 santhakumar V., SOCKALINGAM S. bunu beğendi

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation As explained by the fellow members , the sequence components used to analysis a 3 phase unbalanced system by splitting the unbalance current ( or Voltage ) to some balanced components . Suppose the unabalanced currents are IA, IB and IC then IA can be made up of 3 componenents ie positive , negative and zero sequence ( say IA1, IA2, IA0 respectively similarly IB ( IB1 , IB2, IB0) and IC ( IC1,IC2,IC0)

Now the positive sequence components in each phases are ( IA1 , IB1, IC1 ) and they are balanced and have the same phase sequence as that of original unbalanced IA , IB. IC (120deg apart ) Similary Negative sequence componenets in each phases are ( IA2, IB2, IC2) and they are balanced and have the opposit phase sequence as that of the original unblaned IA, IB and IC ( 120 deg apart ) and the zero sequence components are IA0, IB0 , IC0 they are in phase .( zero phase displacement ) Now one must try to find out the path at which the sequence components flow . As the positive sequence components ( IA1 , IB1 and IC1) are balanced - meaning they add up to zero at any instant , they can not flow in the neutral to earth ! Similarly negative sequence components ( IA2, IB2, IC2 ) are balanced , they add up to zero at any moment and hence it can not flow from neutral to earth ! But the zero sequence component ( IA0, IB0, IC0 ) are in phase - meaning they do not add to zero at any instant , they add up at the neutral and can flow to the earth !!! Similary in a delta winding , the positive and negative sequence current can not circulate as they add up to zero hence they come out in lines and the zero sequence componenets will circulate in delta winding as its magnitud are same !! The conclusion is positive and negetive sequence current can not flow to earth but the zero sequence can . It is very clear that the sequence components take different paths in the network because of its nature . In transmission network the positive and negative sequence take the same path which does not include the earth where as the zero sequence current takes different path including earth . Now the impedance offered by the network against positive sequence current is called postive sequence impedance and that to negative is called negative sequence impedance and the impedence offered by the network to zero sequence current is called the zero sequence impedance . Now to determine the positive sequence impedance of transmission line Apply balanced 3 phase voltage to 3 phases of the transmission line at one end keeping the other end of the line short circuited with out any earth at the shorted end and the ratio of phase voltage to current gives the positive sequence and negative sequence impedance Zero sequence impedance Remember the zero sequence currents are in phase in all phases and naturally apply the in phase voltage to the three phases !! procedure Short all the phases of the transmission line at one end apply single phase voltage keeping the other end shorted and earthed !!! measure the ratio of voltage to current and multiply by 3 will give the zero sequence impedance

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Remember . The sequence component can not exists in one phase only . if it exists it exists in all the phases ! even in case of a single phase to earth fault Beğenin (11) Uygunsuz olarak işaretleyin 11 Ocak 2013 K A R., Rakesh K. ve 9 kişi daha bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT o o o

Where is Zero Sequence Current in a Symmetrical 3L Bolted Ungrounded Electrical Fault ? Beğenin Uygunsuz olarak işaretleyin 11 Ocak 2013

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation o o o

@Rakesh . If the fault is ungrounded the zero sequence current can not flow Beğenin Uygunsuz olarak işaretleyin 12 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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It can surely exist but it shall circulate betwen the line conductors and the neutral conductors and shall try to find a nearest available path to the ground in the system ! Technically Zo=Z1=Z2 in balanced 50/60 Hz electrical systems. Please advise why this multiplication by three is required to calculate Zo ? Beğenin Uygunsuz olarak işaretleyin 12 Ocak 2013

 Ganesh Hajare Ganesh Hajare Lead Commissioning Engineer at Energoprojekt Entel Co Ltd

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Rakesh, multiplication of three while calculating Zo is necessary, because it is contribution by all the three phases and as we know they all are in phase. Suppose we get the Zero sequence value as X, then it must be equal to 3Io. So Io=X/3. Beğenin (2) Uygunsuz olarak işaretleyin 12 Ocak 2013 Krishna V., Feroz H. bunu beğendi

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation @Rakesh . 3 phase bolted fault does not include neutral conductor and hence no zero sequence component. In fact bolted faulted is a symmetrical fault which does not require sequence component to analyse . . If there is a neutral wire included in the fault there will be zero sequence current .

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Regarding the multiplication by 3- Whenever there is zero sequence current , it exists in all the phases and adds in the neutral and flow as 3 times I0 in the earth path . But the zero sequence impedance shall be defined for per phase value . Hence if you take the neutral current it shall be divided by 3 or the V/I term shall be multiplied by 3 to get the zero sequence impedance. . Beğenin (2) Uygunsuz olarak işaretleyin 12 Ocak 2013 Feroz H., Varun V. bunu beğendi

 Yanming Fu Yanming Fu Engineer at Huawei Enterprise

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Great explained by Prof Geoff. I also have a question for the vector sequence,this confused me for 4 years.Actually the passive,negative and zero sequence are defined by human just because of the convenience for calculation.For example, in a balance system the 3 phases A,B, C can be composed of 2 small balanced system(certainly I can use Autocad to draw it),so my question is why I can't define the 2 small systems as one passive sequence and the other negative system which means in balanced system the negative sequence is not Zero? Another question: Is any rotating vector composed of the only one group of three sets of sequence? Thanks all. Beğenin (1) Uygunsuz olarak işaretleyin 14 Ocak 2013 K A R. bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer Comments to Rakesh Kapila • The calculated Zo should not be multiplied by 3. It is not true that all Io the Zero Sequence Currents are primarily third harmonics and above in Hv and EHV transmission systems. I do not know why the Zero Sequence Current Component at 50/60 Herz is only 0.01 % of the total Zero Sequence Currents. In fact, for each

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line to ground fault, a high value of zero sequence current (50/60Hz) will appear in the fault current. It is why I0 is used as main indication of a ground fault. Beğenin (1) Uygunsuz olarak işaretleyin 16 Ocak 2013 Alexandre N. bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Please check the frequency and current profile of any typical Line Ground Fault profile, which consists over 98% all electrical faults. It is the basics of any Power Quality related facts. Even the DC component exists which often is high. Check the chart called the Holy Grail of Harmonics. Even under common electrical faults the behaviours of sequence currents are standard behaviours Positive Sequence Currents travel towards the Line Loads, Negative Sequence Currents travel towards the Generating Sources and Zero Sequence currents go towards the Ground. You will find the answers there ! Beğenin (1) Uygunsuz olarak işaretleyin 16 Ocak 2013 Muzaffer B. bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Please take a Power harmonics Meter or a Drentz 5000 and take a few correct measurements and find the frequency profile of any fault current at the base of the Neutral-Ground Connection. Interestingly all system Z0, Z1, Z2 are typically calculated at 50/60 Hertz, the basic fundamental system frequency. Given the fact that the Io current is mostly current in the neutral-ground connection is part of the triplins (3rd, 9th, 15th harmonics) a minimum multiplication by 3 is essential for all realistic Zo impedence calculations under any fault conditions ! Essential to know the Holy Grail of Harmonics Chart which establishes the sequence breakdown of all fault currents consisting of various frequencies first. Beğenin Uygunsuz olarak işaretleyin 16 Ocak 2013

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

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In a neutral-grounded system, for all line-to-ground faults I have seen, the fundamental component normally is main part in fault current. Harmonics never are so high as you said. Beğenin

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Uygunsuz olarak işaretleyin 16 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Have you measured the neutral loading for various switching transients for diffent types of loads ? Taste of pudding is in keeping a record of the measurement made on the site. Different faults behave differently and have varying profiles. As evidence of the fact that all sizing of the system-neutrals have this multiplication with 3 as a criteria for grounding resistances and calculating the values of the system ground grid impedences due to the fact Io(zero sequence current) in the return path mostly consists of third harmonic current components. Believing otherwise is some thing, but seeing and measuring it is another. Please advise what is the other reason for this multiplication by 3 in all the main system neutral sizing calculations in the power electrical engineering ? All books and references on fault current analysis shall confirm this observation. All fault currents are really a very big Current Surge with all other frequency components riding on top of a basic DC current componet, so are the load switching transients related loadings on the neutrals. There is the theoritical basis for this behaviour and can also be observed in practical measurements any where in the world.The presence of the 50/60 Hertz components at the fault point are again related to the value of the fault impedence and the observed connected line load impedence. This comes in to play when the ground fault impedence is low in the initial fault current cycles.

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IEEE has tons of literature and specs and reference materials on this issue. If you do not like IEEE then try out the Shangai Transformer Power Research Institute based materials they can also be a great asset and are also one of the best in the world. Unfortunately, most of the Electrical Protective Devices are rated and set for the fundamental frequency operations and hence all Low Level Faults do tend to bypass the main protective devices in any electrical power system. Beğenin Uygunsuz olarak işaretleyin 16 Ocak 2013

 Dr. Subhash Joshi Dr. Subhash Joshi Power System Consultant and Educationist In a three phase unbalanced system has a set of three voltages which may have imbalance of magnitude and phase difference. Fortescue has beautifully evolved a mathematical way of analyzing currents established by a set of such voltages in a multiphase system. He has assumed Three sets of +ve, -ve and Zero sequence currents in three phase system. Magnitude of current in each phase is equal, but not necessarily equal among the three sequences. But, +ve and -ve sequence currents in three phases are equally displaced by 120 degree between the phases. Sequence of phase difference in +ve sequence is opposite to that of -ve sequence, hence the names so derived. While, Zero sequence components in the three phases are in-phase. Algebraically, the six variables magnitude and phase angles of three voltages are simplified into three sequence voltages and phase angles. As regards Third harmonic or more general triplen harmonics (harmonic order as a multiple of three) +ve and ve components resolve into in-phase quantities and hence qualitatively akin to Zero Sequence component, but quantitatively demonstrate effect of triplen frequencies in impedance and in-turn current/voltage drops.

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This aspect is dealt in more detail in my Ph.D thesis (April'12) at IIT Roorkee. Beğenin (3) Uygunsuz olarak işaretleyin 16 Ocak 2013 Varun V., K A R. ve 1 kişi daha bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Yes! Mr. Joshi your very right qualitatively the Zero Sequence Components do have a fundamental frequency in mathematical analysis, but do tend to disappear quantitavely in all fault current measurements and now the triplins now appear as Zero Sequence currents. Often the presence of fundamental frequency in the neutral ground currents also includes the negative sequence currents which were not able to find a path through lines but found a path through the ground/earth as part of a return path.Color Books from IEEE give a beautiful chart indicating the breakdown of various harmonic components in to different sequence currents.

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The value of impedence Zo does tend to get effected by the value of these frequencies too, that is where this multiplication by 3 is done for all common impedence calculations. The mechanics of fault are very simple and with any small finite value of existing fault-resistance/impedence the high fault current profile gets clipped (due to high ground condition) and which does create other frequency current spectrums (typical to each fault) including the big DC component on which the overall fault current rides and then decays and finally stablizes shortly in few cycles. If the fault resistance was of absolutely ZERO value then there will be no high ground, no triplin components will be present in any such fault conditions. Even a minor near zero fault impedence creates a finite ground voltage when a very high Kilo Ampere currents flow through it. This is the main reason of conversion of a regular Zero Sequence current at the fundamental frequency getting converting in to triplin components. Understanding of this behaviour is a first step to understanding any Power Quality Applications and issues and in the mitigation of Harmonics and Transients in any electrical power system. Beğenin (1) Uygunsuz olarak işaretleyin 16 Ocak 2013 Varun V. bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

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I agree with Dr. Subhash Joshi. However, what Rakesh is saying is different. according to your idea, Z0 is multiplied by 3 due to 3rd harmonic. I have a simple question for you. For an ideal linear balanced 3-phase circuit, There are multual coupling effect among three phases. When we apply a positive seqeunce voltage V1, a positive sequence current I1 appears. The measured or calculated positive sequence impedance is Z1=V1/I1. When applying a zero sequence voltage Vo, a zero sequence current Io appears in each phase. My question is what the zero sequence impedance Zo is for this circuit. During the test, no any harmonics exists. Beğenin Uygunsuz olarak işaretleyin 17 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT We are talking about Neural-Ground Currents and Zo Sequence Neutral Ground Resistance of a System Earth. Interestingly, 3rd harmonic and triplins are major part of Io Zero Sequence Neutral to Ground Current (which is often high) due to finite Neutral Ground Impedence and hence the transient voltage at the ground GPR is generated which clips the regular waveform of the "Io" starting at fundamental frequencies. These transient conditions do not last more than several cycles before the regular protective devices on the power system come in to play. These harmonics are due to the Fourier conversion of regular Zero Sequence Currents travelling at fundamental frequencies and their distrortion due to high ground conditions occuring due to any electrical faults. The presence of Non Linear Loads in the system is some what similiar and causes the sinusoidal voltage and current wave forms to be distorted and thus Harmonics are generated and are represented due to equivalent Fourier conversion which takes place automatically. This is the basic mechanics of any fault current profile formation any where.

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If this was not a reality then there will be no transients and Harmonics existing in any power systems and life shall be very easy for all electrical engineers especially Protection Engineers as all the faults shall be at fundamental frequency and the protective devices shall trip with great ease and comfort. Further there shall be no 3rd Harmonic Restraining Relays to prevent trippings under any Inrush Current conditions for transormers, large Reactors and Motors etc. Remember all Harmonics are generated at the Point of Couplings near the major electrical faults, or near the non-linear loads and due to other transient conditions coming on the system due to high GPRs etc. Harmonics are not generated up stream under balanced conditions which Mr. Sui is trying to measure and declaring its fully absence in his sytem! Beğenin Uygunsuz olarak işaretleyin 17 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Please do the check the Electrical Engineering Classic of TD reference book by Westinghouse in the chapters for the line harmonics, if the line impedance towards the source is higher for the negative sequence currents then they do enter through the fault in the ground and reappear back on the power lines. Technically the power lines are a recognized and a designated return path towards the power generating source for all the negative sequence currents but nothing stops them travelling via earth and appearing back on the lines. All currents travel or take a path not we like them to take but due to the presence of resistances and impedances across their paths. Under many fault conditions it does happen it all depends upon what equipment is on the line side. If there was an inverter used as source negative sequence currents will always find a high impedance path on the line side and will travel through the ground at the fault point. Unless, you decide to filter them out from the line side and have a resistive dissipation for them. Some Hydro-Utilities put it as compulsory requirements for many power plants which have non linear loads and have great amount of negative sequence currents in their electrical systems. In Power Quality Applications the rule is that you always protect the system from the line side from all Harmonics and Transients in the AC systems. Hydro Utilities like to protect their lines and their grounds at all time ! Beğenin

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Uygunsuz olarak işaretleyin 17 Ocak 2013

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation @ Rakesh If so it is an intresting information . As you have mentioned , the sequence current will not take the path we like but based on the electrical rules . Hope you are refering to the Electrical transmission and distibution Reference book by Westinghouse . Can you tell us which chapter and sub clause is talking about the negative sequence current flowing to the earth so that , it can be explored more . To my understanding , the positive and negative sequence current can not flow to the earth from the neutral as it adds to zero at neutral due to its basic charecteristics . In fact the same book mentioned it in number of places .

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Todays quote Our understanding is true until it is proved otherwise ! Beğenin (2) Uygunsuz olarak işaretleyin 17 Ocak 2013 Kamal B., Fenghai S. bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

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For zero sequence impedance Zo, there are two 3s related to it according to my experience and knowledge. First, for a overhead line, Zo is around 3Z1 (not accurately). This relation between Z1 and Z0 can be used for simple and quick assessment. However, this number 3 has nothing to do with harmonics, instead, It comes from mutual coupling between conductors of 3-phase circuit. Secondly, for an impedance in the returning path (neutral or earth), the impedance should be multiplied by 3 because the zero sequence currents in each conductor are in phase with same magnitude. It also has nothing to do with harmonics. Rakesh said in his last comment that negative sequence current will flow into earth or neutral. It is totally wrong. The earth only is the returning path for zero sequence current. For positive and negative sequence currents, each conductor is the returning path for other two conductors. As one public discussion place, I have a suggestion as below: 1. all points should fit the question topic; 2. answer should be simple and direct; 3. only answer question the question starter asks. Thanks Beğenin (3) Uygunsuz olarak işaretleyin 18 Ocak 2013 Feroz H., Xuna L. ve 1 kişi daha bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT The rules of electrical engineering are very simple and straight forward. All Negative Sequence currents flow through the lines towards the Generating source and Zero Sequence currents travel towards earth/ground and Postive Sequence currents flow through the lines towards the loads. * no easy paths can be established say for any Negative Sequence currents due to equipments like Inverters/VFDs or any such high impedences devices on the Line Side coming from the generators the ground carrying conductors tend to become a pathway and the Negative Sequence currents can then travel through them and can climb back to the incoming lines and can continue their journey of travelling towards the generating sources. Often all the earthing points are connected with each other and/or are supoosed to have low impedence paths between them. -higher frequency components existing on the power system also breakdown in the Sequence Networks(Z1,Z2 & Zo) and they also have associated Sequence currents(I1,I2 &Io) flowing through the same system. However the most interesting part is that each frequency current component typically is associated with only one type of sequence current, and it is stated clearly in both the TD Reference book and IEEE color books in dealing with the chapters on the harmonics and transients on AC systems. For example 3rd, 9th and 15th harmonic, 21st Harmonic is associated with the Zero Sequence Currents in any balanced symmetrical fault conditions.If these low level faults become unsymmetrical then the even harmonics also come in to play and also create a DC component and they also have similiar Sequence Current Components residing in any power system.( there is simple chart for it in the IEEE color Book for it. * common difficulty we fail to recognize is that all these Sequence Impedences associated with these Harmonic Components are directly dependent on the frequencies of these harmonic voltages and have to be multiplied by the integers representing these harmonic components. * under any ground fault conditions which may have large currents and even a very small Fault Resistance or Fault Impedence can cause a a high residual voltage to be generated at the point of the fault, and will cause the Fault Currents (which are mostly Zero Sequence in nature and were initially set at fundamental frequency) to clip due to high ground condition and thus this fault current or zero sequence current can breakdown in to various harmonic components. The fault point acts similiar to a point of coupling (but is not the same) and can be theoritically modelled as multiple current sources at different frequencies connected in parallel with their respective zero sequence impedences which are mostly triplins in nature along with the DC current component. * nature and the shape and the size of equivalent sequence circuits remain the same except in their numeric values due to associated harmonic frequencies, all these are independent circuits in parallel to the ones obtained at the fundamental frequency. The most interesting part is that these impedences are always higher and hence harmonic frequencies based voltages do not tend to travel 60/70 feet or a max of 100 feet from the point of coupling or point of fault. However they always leave a foot print of high or finite GPR on the ground grid at the time of any ground fault condition. * protective devices in any power system are designed around the main fundamental frequencies and often tend to ignore sub-harmonic frequency based components, which do also cause enough damage to the electrical appratus and other power protective and disconnect devices & fuses etc., leave hot spots at many contact points, in CBs and tend to bypass the conventional power system Protection and even not picked up by various relays etc.

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Please do remember the rules of Electrical Engineering are fairly simple and they follow all the applicable mathematics and the results can be observed in lab or on any site!. Beğenin (1) Uygunsuz olarak işaretleyin 18 Ocak 2013 K A R. bunu beğendi

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation The fundamental negative sequence current can not flow to the earth even if the transmission line ( or equipment ) impedance is high . The important reason is as stated in my first post that the component of negative sequence from each phases are balanced and they add to zero ( same as positive sequence ) The point here to emphasis is that none of the balanced component currents ( meaning the three phase currents when added instantaneously results zero )in phase A,B,C irrespective of their frequency can flow to earth as the return path is the star point of the equipment .It is easy to visualize the sequence component once this fundamental concept is understood. Now coming back to the tipple harmonic and multiples , They are just like zero sequence currents as there is no phase difference between each phase ( IA is in phase with IB and so also IC ) . May be you are talking about the sequence components of third harmonic and multiples and as they are in phase their negative sequence must also have the same frequency and phase as that of the original frequency . If so it can flow through the neutral ! The point here is that the third harmonic and multiples will not add to zero at any moment . It can be added strightway with out any phase difefernce . They will come in to picture only during transients ( just like DC ) As far as the Protection is concerned the sequence current means that of the fundamental frequency and the negative sequence will not flow to earth as far as setting is concerned and there are methods to measure it in the field .

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The relay will filter the harmonics and respond for the fundamental component . There are also various settings to block the relay operation during transients like 2nd and 5th harmonic blocking etc. . Beğenin Uygunsuz olarak işaretleyin 18 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT o o o

Sequence network of transmission line is connected in series with fault resistance ! Beğenin Uygunsuz olarak işaretleyin 18 Ocak 2013

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation

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It is interesting to note that ,In case of a single phase to earth fault the line currents are IA1 IA2 and IAo and they are equal . Even though there is no current in the B and C phase there assumed to be sequence components in B and C phase . Here the line current in phase A is positive negative and Zero but the same current when flows in the earth and return to the neutral is treated as IA0, IB0,IC0 ! No positive or Negative sequence current in the earth and neutral point! Beğenin (3) Uygunsuz olarak işaretleyin 18 Ocak 2013 Pranesh P., Fenghai S. ve 1 kişi daha bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer o o o

I agree with Ayub's comment: no positive or negative sequence current in the earth and neutral point! Beğenin (1) Uygunsuz olarak işaretleyin 19 Ocak 2013 Kamal B. bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer In each his comment, Rakesh Kapila always tries to lead readers to a new place such below. However, i do not think they are necessary to explain his points. He said:

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* nature and the shape and the size of equivalent sequence circuits remain the same except in their numeric values due to associated harmonic frequencies, all these are independent circuits in parallel to the ones obtained at the fundamental frequency. The most interesting part is that these impedences are always higher and hence harmonic frequencies based voltages do not tend to travel 60/70 feet or a max of 100 feet from the point of coupling or point of fault. However they always leave a foot print of high or finite GPR on the ground grid at the time of any ground fault condition. Beğenin Uygunsuz olarak işaretleyin 19 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Unfortunately, the rules of electrical engineering are very simple and straight forward. Interestingly, the mechanics of any current flows during any electrical faults and the subsequent current formations have been well documented, well defined, field tested and have been tried out in the past. The theory of Power Quality Applications also has clear mathematical and conceptual basis and is closely related to the electrical faults, sequential circuits and is also related to both fundamental frequencies and any subharmonic components and the various effects they generate on the electrical systems. You can not isolate any sub-system and ignore the whole picture. Typical electrical current behaviours at the point of couplings, at fault points, leakage current losses, coronal losses, non-linear load conditions, lightening strikes and many such behaviours of finite GPRs all have effect on the main electrical systems and are related to each other in varying ways across the spectrum. One is free to respect ones opinion and his/her feelings in the matter which may or may not have any rational basis at times. There has been no intention of mine to deflect or to side track the main topic here at any time. I also believe some times it is very difficult to convert the folks already converted and committed to their ideologies. May be the human ignorance at times similiar to the devine bliss which many persons experience in their life times and in their professional careers and at work places !

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I do come across this type of experience on a daily basis as I teach this PQ theory for living to many university graduates of the electrical engineering school on a daily basis. These are varied but difficult concepts and also have many abstract mathematical-constructs to grasp and understand at times. Unfortunately Zo sequence impedences, currents and voltages are critical to many of the above concepts and electrical behaviours ! Beğenin Uygunsuz olarak işaretleyin 19 Ocak 2013

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

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According to so many comments, I do believe that you have very wide knowledge about power system and power quality. However, the topic for this discussion is about sequence impedances. I do not think the sequence impedances have anything to do with leakage current losses, coronal losses, non-linear load conditions, lightning strikes etc. I also believe it is not intention of the topic starter to expend his topic to so wide discussion. The positive, negative and zero sequence impedances are basic concepts for each power system engineers. It is not necessary to check any color books. If you have excellent experience on these things than than this topic, you can start a new discussion topic. By the way, I firmly believe that no positive or negative sequence current in the earth and neutral point! Beğenin Uygunsuz olarak işaretleyin 19 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Please let me know what happens when all sequence impedence circuits are in series to the ground fault resistance ? All the above losses indicated and not recognized by you are closely related to zero Sequence Current losses, and are part low the Low Level Fault conditions typical on any system. Even a small lightening strike can create a ground diffrential conditions or a rise in GPR, it shall easily cause Zero Sequence currents to be generated, and potentially a Surge & Sag formation on the electrical systems. What happens if no path is available along the lines for negative sequence currents ( on the line side) where do they go ? Altimately they have to reach the generating source bus !

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Please advise ! Beğenin Uygunsuz olarak işaretleyin 19 Ocak 2013

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer Let me answer your question: what happens if no path available along the lines for negative sequence currents (on the line side)? Where do they (I2) go? Assumption for simplification: 1. No non-linear element; 2. No capacitive charging; Answer: 1. for a single phase to ground fault, positive, negative and zero sequence networks are connected in series at point of the fault. You have said this point. 2. If the negative sequence path is not available (your language) or Z2 is infinite (my language), the total impedance Z1+Z2+Z0 is infinite. It will result in i1=I2=i0=0. The condition you gave is that the point of fault does not have direct connection with a power source. Therefore, no fault current appears.

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I have shown my answer to public. What is your answer for your own question? What are I1, I2 and I0? Beğenin (1) Uygunsuz olarak işaretleyin 19 Ocak 2013 John H. bunu beğendi

 Ayub Kunnanolly Ayub Kunnanolly Protection Design and System analysis Engineer at Qatar General Electricity and Water Corporation

A clarification on the multiplication factor of 3 used in the neutral impedance or resistor;

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The zero sequence current in each phase adds up in the neutral and the neutral current become the 3 I0 . If the neutral resistance is R , then the actual power that would be developed when there is neutral current flow will be 9 (I0 *I0) * R . But the sequence network is represented as/phase equivalent circuit . Obviously the per phase power shall be 9 *(I0*I0)R/3 ( or 3I0*I0*R). As the per phase zero sequence current is I0 . The actual resistance is multiplied by 3 so that single phase power is maintained at 3*I0*I0*R in the per phase sequence circuit. Beğenin (1) Uygunsuz olarak işaretleyin 19 Ocak 2013 John H. bunu beğendi

 Sanjeev Kumar Sanjeev Kumar GM Projects at Skipper Electrical Nigeria Ltd. Dear all We really enjoy the discussions we appreciate Mr. Rakesh for in depth coverage and crispy Fenghai. Overall it was good to read all comments i agree with many as the main question be still not answered fully as only first part is answered. Second part is left unattended The description is boring to describer as well to describer. Subject should not be diverted. Many times we need information as we feel it is required now to make a decision thus if it is short and simple it paves way for a quick learning of average person. We need to access the level with script of the question accordingly we need to provide the details. Practically speaking all protection and power systems are standardized in most of the countries. now mostly confirmation calculation in circulation, which are seldom reviewed.

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I thank all participants. Beğenin Uygunsuz olarak işaretleyin 19 Ocak 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Ayub, you are right here regards the resistive component of the fault, think of a situation if there was an inductive component also exists with the fault resistance making it a Zf instead of merely a Rf. At any higher sub harmonic frequencies say that of a third harmonics in nature the value of Zf impedence shall become atleast three times. This will further lead to a finite but momentary GPR which in turn shall create even harmonic components which shall include a short DC component also. No wonder immediately after an electrical fault we observe a finite Voltage Surge followed by Sag a common observation here. This is the basic mechanics of an electrical fault generation. This multiplication factor due to the higher frequency components has nothing to do with the topology of a sequence network of say a common TL based LG fault where the factor of 3xRf is automatically placed. I have never questioned the sequential circuit

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topology which is clearly defined by curcuit theory and the vector algebra of the power system components above and is clearly indicated in the text books and reference books on sequential circuits for both symmetrical and unsymmetrical faults for Transmission Lines, Transformers, Cables and Generators and Motors etc. Beğenin (1) Uygunsuz olarak işaretleyin 20 Ocak 2013 K A R. bunu beğendi

 Casper Labuschagne Casper Labuschagne Development Manager at SEL

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Very interesting discussion! I disagree that zero-sequence current is the same as 3rd harmonics. Zero-sequence current are present at power system frequencies, but are similar to 3rd harmonics (triplen). The simple proof lies in micro-processor based relays, particularly the earlier ones. In these relays, the currents on which the protection elements operate are filtered to only allow quantities at power system frequency. All second, third and higher frequencies are filtered out. If zero-sequence currents were at the 3rd harmonic frequency, then these relay elements will never operate for earth faults. Beğenin (2) Uygunsuz olarak işaretleyin 23 Nisan 2013 Pranesh P., Fenghai S. bunu beğendi

 SEKHAR BHATTACHARYA SEKHAR BHATTACHARYA TECHNICAL ADVISOR LARSEN N TOUBRO SARGENT N LUNDY LTD VADODARA o o o

Good discussion Beğenin Uygunsuz olarak işaretleyin 23 Nisan 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Line Impedences for Harmonics are always higher than the Impedences for a Fundamental Frequency. Most of the Power associated on the lines with Harmonics is inductive except on the NG connection, as Earth acts as a very large Capacitor and the fault currents assume real power for Zo Currents travelling through NG connections. I do suggest that you create an artificial high impedence LG Fault on an Electrical System and them with the help of an Harmonics Power Meter and try to measure the frequency profiles of the power currents flowing

through the main NG connection. Just check out and you will reconfirm what I have told you. Invariably electrical faults do lead to a temporary GPR and hence the associated clipping of the voltage & current waveforms, leading to higher order harmonic frequencies. Often common system analysis for Fault currents is done based upon fundamental frequencies for convenience, but the real world is much more complex and interesting.

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If you deny that 3rd harmonics is not a Zo Sequence Current then you deny the existance of the whole science and the art of the Power Quality! Beğenin Uygunsuz olarak işaretleyin 23 Nisan 2013

 Casper Labuschagne Casper Labuschagne Development Manager at SEL Seems you touch on a more than one point here. 1. "...except on the NG connection, as Earth acts as a very large Capacitor ..." Earth cannot be a capacitor on its own; it can be the one "plate" of a capacitor, and the other "plate" is, for instance, the line. This is precisely what the Ferranti effect is: the capacitance between the line and earth. However, if the neutral is solidly grounded, there cannot be a capacitive connection between neutral and earth. 2."...and the fault currents assume real power for Zo Currents travelling through NG connections." Since the power system is mainly inductive, very little real power flows during faults when the neutral is solidly grounded. This is why the maximum torque angle of relays installed on transmission networks is set to 80+ degrees; i.e. mainly reactive power. 3. "...the associated clipping of the voltage & current waveforms, leading to higher order harmonic frequencies." Clipping how? Clipping is normally associated with some sort of saturation. In this case the saturation of what? I do agree if you saturate a non-linear device, you will generate harmonics. Transformer inrush detection is based on detecting the second and fourth harmonics generated when the iron core of a transformer saturates during inrush. However, all sorts of frequencies are thus generated, not only the third harmonic. Of course I deny that 3rd harmonics are zero-sequence currents. Zero-sequence currents are generated at system frequency, while 3rd harmonic currents are generated at a frequency three times higher than the system frequency. However, I don't deny that zero-sequence currents can behave like 3rd harmonic currents, or the same as the other triplens, i.e. 6th, 9th, etc.

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Perhaps you are thinking of the harmonics generated if a resonance circuit exists during fault conditions? But even then, these frequencies are normally much higher than the 3rd harmonic (of course, it could be at the 3rd). Beğenin (2) Uygunsuz olarak işaretleyin 24 Nisan 2013 Pranesh P., Ayub K. bunu beğendi

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

Dear Friends, please go back to original question. Some answers have gone to other way. Below is the initial question: =================================================== Zero Sequence Impedance calculation Dear All, Could anyone please expalin me about Zero Sequence Impedance? And what is the difference between Positive, Negative and zero sequence impedance? I have searched through internet for an hour. But I'm really confused about the chapter. I want to understand clearly the concept and how to do testing and calculate the same in power cables after laid. =================================================== Please do not mention third harmonics again in this topic, which has nothing to do with the harmonics. If you want to sell your knowledge, please open a new topic.

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Thanks Beğenin (2) Uygunsuz olarak işaretleyin 24 Nisan 2013 Kamal B., Ruhollah B. bunu beğendi

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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You can not take a course in power systems and symmetrical components and in the power quality all at the same time through this discussion ! Beğenin Uygunsuz olarak işaretleyin 24 Nisan 2013

 Fenghai Sui Fenghai Sui Senior Power System Protection Engineer-Sr. Network Management Engineer

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This question is only about the zero, negative and positive sequence impedances. In order to describe these impedance, we may have to discuss symmetrical components. However, I believe that it is not necessary to discuss power quanlity such as harmonics. we should focus on the question only. Beğenin Uygunsuz olarak işaretleyin 25 Nisan 2013

 Jim McIntosh-BEEF Jim McIntosh-BEEF

Senior System Dispatcher at Regional utility TO and BA

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exactly Fenghai and Rakesh, Remember guys that if you are using old school relaying (real coils) that they don't care about harmonics, current is current, any calculations and subsequent settings need to take all current into account. Sometimes the effort to get to the 5th significant decimal gets in the way of actually creating useful and practical data. Just about all of you have far superior math skills to me, but it can be taken to a point to where it actually impedes decision making. That is why engineers make poor system operators. It takes all of us to properly run a power system. We don't have to agree always. Beğenin Uygunsuz olarak işaretleyin 25 Nisan 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Criticism of others is another way of self praise! I have no choice but to agree with you. Beğenin (1) Uygunsuz olarak işaretleyin 25 Nisan 2013 Jim M. bunu beğendi

 Casper Labuschagne Casper Labuschagne Development Manager at SEL

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Gentlemen, We had an interesting discussion. However, let us now, as Fengai suggests, focus on the original question. Did we answer this question? Beğenin (1) Uygunsuz olarak işaretleyin 26 Nisan 2013 Peter M. bunu beğendi

 Peter Macalua Peter Macalua Reliability I/E PLanner at Flint Hills Resources Yes Casper, very interesting discussion on a very interesting "positive,negative and zero sequence" electrical topic. Rakesh,Fenghai and Ayub were excellent discussion agents. Electrical Engineering has become richer because of them.Perhaps, there is a need for another theory to unbalanced AC systems and AC fault calculation methods based on theory of chaos. Any comments to start a new topic from Rakesh or Fenhai?

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Peter Macalua Port Arthur,Texas Energy Country Beğenin Uygunsuz olarak işaretleyin 30 Nisan 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Under nery High GPR Zo impedence has practically no meaning, Io current shall not flow through to the ground ! Beğenin Uygunsuz olarak işaretleyin 30 Nisan 2013

 Jim McIntosh-BEEF Jim McIntosh-BEEF Senior System Dispatcher at Regional utility TO and BA o o o

Imbalance has to where else does it go? Beğenin Uygunsuz olarak işaretleyin 30 Nisan 2013

 Casper Labuschagne Casper Labuschagne Development Manager at SEL Hi Peter,

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I am so not qualified to comment on your suggestion! Beğenin Uygunsuz olarak işaretleyin 1 Mayıs 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT Interestingly, all Imbalance currents, which are mostly Io currents travel through lines to other grounding points in the system and dissipate. A high GPR at one point, could again be due to several other reasons however, all Io

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currents do have real energy and are not friendly from energy conservation point of view also. Hence avoid their formations for sure! Beğenin Uygunsuz olarak işaretleyin 2 Temmuz 2013

 Geoff Garber Geoff Garber Engineer at First Solar, experienced in protection, integration, automation, I&C, and telecommunications. Symmetrical Components (positive, negative and zero sequence) are difficult for most people to understand (even engineers). The are simply a math trick to allow the calculation of power system fault currents in asymmmetrical faults (faults where the three phase currents and angles are not equal. These are usually ground faults.) These currents are very difficult to calulate using trigonometry so in 1918 Charles Legeyt Fortescue, developed a system called symmetrical components that made the math much easier. Fortescue said that any unbalanced set of 3-phase phasors (rotating vectors) could be represented by the sum of three sets of balanced phasors. (Balanced phasors are much easier to work with mathematically.) He called these the positive, negative and zero sequence phasors. I'll try to give an explantion without math.

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The positive sequence represents the part of the phasors that are balanced. With no fault and balanced voltages and loads, the positive sequence equals the total. (Both negative and zero sequence are zero.) The negative sequence loosely represents the imbalance between the three voltages or currents. A line to line fault will have high negative sequence currents. The zero sequence loosely represents the ground current of a line to ground fault. A fault that includes ground will have some zero sequence current. The important thing to remember here is that these sequences do not exist separately. Each is only a mathematical construct. Only their sum can actually be observed. Beğenin (10) Uygunsuz olarak işaretleyin 3 Temmuz 2013 SEKHAR B., Peter M. ve 8 kişi daha bunu beğendi

 Zulfiqar Ahmed Zulfiqar Ahmed Protection Design Engineer o o o

Very intersting discussion, it may leads us to some new dimentions. Beğenin Uygunsuz olarak işaretleyin 6 Temmuz 2013

 Rakesh Kapila Rakesh Kapila Professor Electrical Engineering / Professional Engineer at GNIT

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Geoff: I liked your summary above, however under the double electrical fault conditions life is much more complicated. Especially when one fault feeds in to another local fault. It is very difficult to do any meaningful analysis mathematically speaking. There are other un-natural behaviors and responses you encounter commonly on the lines and on the equipment. As a typical example when there is already an imbalance on the system and you could easily expect Zero-Sequence currents flowing on two lines which are again displaced by 120 degrees and basically do consist of 3rd harmonic currents. If there were a sudden line to line electrical fault condition under this previous imbalanced fault, the subsequent fault currents shall consist of the main fundamental frequency (50/60Hz) or will it merely consist of the sub harmonic frequencies of the 3rd harmonic based sequence components? Not very sure! Beğenin Uygunsuz olarak işaretleyin 6 ay önce

 Geoff Garber Geoff Garber Engineer at First Solar, experienced in protection, integration, automation, I&C, and telecommunications. Rakesh, That's why we have software programs like DigSilent, Aspen, or ETAP. In previous employment we used CAPE from Electrocon. I once modeled a fault on a monotube tower carryng two circuits. Lightning hit the top phase on one side of the tower, and made a short circuit to the two lower phases on the other side of the tower. If I thought I would have to compute faults like this by hand, I wouldn't be able to sleep at night.