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Topic Title: Zero, +ve & -ve sequence currents
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Created On: 10 September 2008 11:03 AM
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 10 September 2008 11:03 AM
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IainThomson85

Posts: 16
Joined: 02 July 2008

This is one of the things I covered at uni when doing asymmetric fault analysis. I learned all about how to calculate what happens under various faults but one thing I didn't pick up.....what the hell does it actually mean?!?!

I have seen it a fair amount mentioned since and I finally think its time to put my hand up and ask, what actually are zero, positive and negative sequence currents & impedances. Is it to do with which phase is in which portion of its cycle (if L1 was in 0 deg, L2 would be in +ve cycle and L3 would be in - ve portion).

It's something that never really comes up at work since I work for a building services consultancy and we generally steer clear of HV stuff (although I did have to do a per unit calculation last week, that was a first!).

Still, I would be grateful if someone could offer a good explanation.

Iain
 10 September 2008 11:53 AM
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leejohnson33

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Joined: 01 September 2003

Hi there Iain,

In a three phase system, one set of phasors has the same phase sequence as the system under study (ie. A B C). This is called the positive phase sequence and produces a normal rotating field.

The second set has the reverse sequence (ie. B A C). This is called the negative phase sequence and produces a field with opposite rotation.

Finally, a third set exists which is in phase with each other and is called the zero phase sequence and produces a field that oscillates but does not rotate.

In a perfectly balanced system, no negative phase sequence currents would exist. However, it is virtually impossible to acheive this perfectly balanced system in practice and so these negative phase currents need to be considered.

Line voltage imbalances caused by electrical faults or imbalanced loads lead to current imbalances in each conductor. Therefore, the magnetic coupling between windings becomes uneven. A counter rotating field (in respect to the main field) will now exist and the resultant field will cause undesirable eddy currents to flow. The consequenes of this for generators will either be a loss of torque, or depending on the load, will increase the current for the same slip speed and hence raise the temperature of the alternator.

Hope that is of some help to you,

Lee
 10 September 2008 02:52 PM
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IainThomson85

Posts: 16
Joined: 02 July 2008

So the negative sequence is the counter force felt on the generator end caused by the imbalance in phase currents? So when a fault occurs thats why you are considering the positive sequence (reference phasor/normal rotation) and the negative sequence currents (phase imbalance currents).

What part does the zero sequence play? You said it has no rotating force as they are in phase, it only oscillates. Would a neutral point be what you are talking about? No net current flow but a constantly changing but cancelling force?
 11 September 2008 11:15 AM
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jafarpour

Posts: 198
Joined: 14 August 2005

Originally posted by: IainThomson85

So when a fault occurs thats why you are considering the positive sequence (reference phasor/normal rotation) and the negative sequence currents


Are you electrical engineer?
 11 September 2008 12:47 PM
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hpcompaq

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 11 September 2008 01:18 PM
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jafarpour

Posts: 198
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A method of analysis that is applicable to unbalanced faults is Method of Symmetrical Co-ordinates Applied to the Solution of Polyphase Networks. C.L. Fortescue. Trans. A.I.E.E.,Vol. 37, Part II, 1918, pp 1027-40.
By applying the 'Principle of Superposition', any general three-phase system of
vectors may be replaced by three sets of balanced (symmetrical) vectors; two sets are three-phase but having opposite phase rotation and one set is co-phasal.
These vector sets are described as the positive, negative and zero sequence sets respectively.
When a fault occurs in a power system, the phase impedances are no longer identical (except in the case of three-phase faults) and the resulting currents and voltages are unbalanced, the point of greatest unbalance being at the fault point. The fault may be studied by short-circuiting all normal driving voltages in the system and replacing the
fault connection by a source whose driving voltage is equal to the pre-fault voltage at the fault point. Hence, the system impedances remain symmetrical, viewed from
the fault, and the fault point may now be regarded as the point of injection of unbalanced voltages and currents into the system. This is a most important approach in defining the fault conditions since it allows the system to be represented
by sequence networks using the method of symmetrical components.
fault branch changes from 0 to I and the positive sequence voltage across the branch changes from V to V1 ; replacing the fault branch by a source equal to the change in voltage and short circuiting all normal driving voltages in the system results in a current dI flowing into the system, and:

dI = (V-V1)/Z1

Where Z1 is the positive sequence impedance of the system viewed from the fault. As before the fault no current was flowing from the fault into the system, it follows that I1 , the fault current flowing from the system into the fault must equal - dI. Therefore:

V1 = V- I1Z1

is the relationship between positive sequence currents and voltages in the fault branch during a fault.

If only positive sequence quantities appear in a power system under normal conditions, then negative sequence quantities can only exist during an unbalanced fault. If no negative sequence quantities are present in the fault branch prior to the fault, then, when a fault occurs, the change in voltage is V2 , and the resulting current I2 flowing from the network into the fault is:

I2 = -V2/Z2

The impedances in the negative sequence network are generally the same as those in the positive sequence network. In machines Z1 ≠ Z2 , but the difference is generally ignored, particularly in large networks. The negative sequence diagrams are
similar to the positive sequence diagrams, with two important differences; no driving voltages exist before the fault and the negative sequence voltage V2 is greatest at the fault point.

The zero sequence current and voltage relationships during a fault condition are the same as those in the negative sequence network. Hence:

V0 = -I0 Z0

The currents and voltages in the zero sequence network are co-phasal, that is, all the same phase. For zero sequence currents to flow in a system there must be a return connection through either a neutral conductor or the general mass of earth. Note must be taken of this fact when determining zero sequence equivalent circuits.
Further, in genera the value of Z0 varies according to the type of plant, the winding arrangement and the method of earthing.

Edited: 11 September 2008 at 01:26 PM by jafarpour
 11 September 2008 05:40 PM
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IainThomson85

Posts: 16
Joined: 02 July 2008

Originally posted by: jafarpour
Are you electrical engineer?


Yeah I am but I haven't had to deal with HV faults since uni, hence me asking for a bit more information on it. I remember how to do the calculations for various fault types using sync fault analysis, but it was never really explained what we were actually doing. I found uni was full of that, pleanty of info on how to do things, not much on what you were ACTUALLY doing. The problem was the power lecturers were mostly chineese. Lovely guy but the communication barrier made for some very hard to follow lectures. You ended up teaching yourself the method but not knowing why.

Thanks for the replies, will read those links and thanks for detailed reply Jafarpour.
 11 September 2008 05:55 PM
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IainThomson85

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Joined: 02 July 2008

hpcompaq, that first link was an excellent explination, thanks.
 13 October 2008 02:39 PM
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Ayubkseb

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Joined: 13 October 2008

The basics of the zero, positive, negetive sequence may be explained as follows

Any unbalanced three phase currents(voltage) can be split to three phasor components . positive , negetive and zero sequnce for analysing the effect of unbalnce as there is no direct method avilable for analysing the effect of unbalanced currents or voltages

Now the positive and negetive sequence currents are balanced phasors and the only difference is that their sequnce is opposit
As -ve and +ve sequnce currents are balanced , if you add it up at any moment anywhre you will get zero
Hence if you have a star network with neutral earthed , there will not be any -ve or +ve sequence through the neutral ( it add up to zero ! )
The Zero sequnce phasors are equal in magnitude and there is no phase difference b/w the sequnce components of three phases . Hence adding this components( of three phases) at any moment will not give zero but an added value
Hence if you have star nework with neutral earthed , then the zero sequence of each phase will add up in neutral . Hence the currents available in the neutral of a star network will be only zero sequence and +ve and -ve sequnce can not exists in neutral
 19 February 2012 12:03 PM
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abhishekdeepak

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What physically make zero sequence current in same phase? Is zero sequence current hypothetical .There is some Physics of ground to make it in same phase.
 19 February 2012 12:03 PM
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abhishekdeepak

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Joined: 18 July 2009

What physically make zero sequence current in same phase? Is zero sequence current hypothetical .There is some Physics of ground to make it in same phase.
 19 February 2012 12:33 PM
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simongallagher

Posts: 148
Joined: 28 July 2005

That was the defination of my degree, lots of instructions on how to perform these calculation, no explantion what we were actually calculating!

Power was probably the biggest example of this.

Simon
 20 February 2012 02:37 PM
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surajitchattopadhyay

Posts: 6
Joined: 01 December 2009

Resultant of Zero sequence of all three phase can not produce rotational flux.

zero sequence is very common in ground fault like SLG , LLG etc

one can sense it in the ground line
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