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Participant

Join Date: May 2012
Posts: 2

Zero Sequence Current Flow in Zig Zag Connected Transformer

09/30/2012 6:00 PM

For a Zig Zag connected Transformer, why does only zero sequence current flow in the secondary and not negative and positive sequence? Why is "zero sequence" related to earth faults? For a 3 phase to earth fault, why do all three phases go from being 120 degrees apart to 0 degrees apart? (Probably need a diagram :l)

Appreciate any help

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Guru

Join Date: Oct 2008
Location: 34° 34' 21.60" N, 92° 55' 42.28" W
Posts: 20992
#1

Re: Zero Sequence Current Flow in Zig Zag Connected Transformer

09/30/2012 8:12 PM
__________________
Luck comes and goes. Skill is forever. Intelligence either is, or it ain't. lyn
Participant

Join Date: May 2012
Posts: 2
#2

Re: Zero Sequence Current Flow in Zig Zag Connected Transformer

10/01/2012 3:14 AM

Thanks lyn. What does it mean when the "Fluxes no longer oppose" ? Does that mean the phases are no longer 120 apart but instead 0 deg apart? Zero sequence current exists between the faulted phase and the Tx neutral? Why is there a phase change at all if the Earthing is mostly resistive? What makes the faulted phases become 'in phase' with each other? (Zero sequence).

What doesn't make any sense is that the three phases start off 120 apart, and then magically they become in phase with each other (zero sequence) under a ground fault condition. I understand that under a fault condition the impedance will change the phase angles and the resulting unbalance can be represented by sequence component vectors which are added to give the resultant phasor. I just cant grasp and can't find any resources to determine why the Earthing Transformer only allows zero sequence currents to flow (voltages to each phase are no longer in balance).

Power-User

Join Date: Sep 2008
Posts: 197
#3

Re: Zero Sequence Current Flow in Zig Zag Connected Transformer

10/02/2012 9:52 AM

http://en.wikipedia.org/wiki/Lenz%27s_law

Induced fields in a transformer are a function of magnetic flux.

Disruption of the winding flux by a circuit fault yields disruption of the phase timing relationship between phases.

In an unfaulted (normal) 3 phase circuit the algebraic sum of the currents between phase(s) and ground at any point in time is: zero

The ground conductor in the circuit exhibits/carries the difference in phase-to-phase current during a ground fault.

The circuit ground of the transformer is monitored by either an in-line CT or by the use of a "doughnut" (Circular) inductive pickup CT wherein all 3 phases and the ground conductor are passed through the center of the pickup.

Normally the stray current (noise) flowing through the ground return is minimal and within an acceptable design threshold.

When a ground fault occurs the algebraic sum of the phase currents is no longer at zero and voltage is induced into the inductive pickup or in-line CT thereby activating the ground fault protection circuit. (The ground fault protective trip current value is a design chosen value loaded into the GF protective relay.)

Hope this helps.