A Earthing ( zigzag) transformer is
a special purpose transformer with a zigzag or
'interconnected star' winding connection, such that each
output is the vector sum of two phases offset by 120�. Its
applications are for the creation of a missing neutral
connection from an un-grounded, 3-phase system to permit the
grounding of that neutral to an earth reference pointand
also harmonic mitigation, as it can suppress triplet (3rd,
9th, 15th, 21st, etc.) harmonic currents, to supply 3-phase
power as an autotransformer (serving as the primary and
secondary with no isolated circuits), and to supply
non-standard phase-shifted 3-phase power.
Nine-winding three-phase transformers, typically have 3
primaries and six identical secondary windings which be
used in zigzag winding connection as pictured. As with
the conventional delta or wye winding configuration
three-phase transformer, a standard stand-alone
transformer containing only six windings on three cores
can also be used in zigzag winding connection, such
transformer sometimes being referred to as a zigzag
bank. In all cases, six or nine winding, the first coil
on each zigzag winding core is connected contrariwise to
the second coil on the next core. The second coils are
then all tied together to form the neutral and the
phases are connected to the primary coils. Each phase,
therefore, couples with each other phase and the
voltages cancel out. As such, there would be negligible
current through the neutral point, which can be tied to
ground.
Each of the three "limbs" are split into two sections.
The two halves of each limb have the equal number of
turns and they're wound in opposite directions. With the
neutral grounded, during a phase to ground short fault,
a third of current returns to the fault current and the
remainder must go through two of the three phases when
used to derive a grounding point from a delta source.
If one phase, or more, faults to earth, the voltage
applied to each phase of the transformer is no longer in
balance; fluxes in the windings no longer oppose. (Using
symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero
sequence (earth fault) current exists between the
transformer’s neutral to the faulting phase.
The purpose of a zigzag transformer in this application
is to provide a return path for earth faults on
delta-connected systems. With negligible current in the
neutral under normal conditions, providing the defective
load will be automatically disconnected in a fault
condition, an undersized transformer may be used only as
short time rating is required (i.e. the transformer can
only carry full rated current for, say, 60 s). Impedance
should not be too low for desired maximum fault current.
Impedance can be added after the secondaries are summed
to limit maximum fault currents(the 3Io path).
An application example: A combination of Y (wye or
star), delta, and zigzag windings may be used to achieve
a vector phase shift. For example, an electrical network
may have a transmission network of 110 kV/33 kV
star/star transformers, with 33 kV/11 kV delta/star for
the high voltage distribution network. If a
transformation is required directly between the 110
kV/11 kV network an option is to use a 110 kV/11 kV
star/delta transformer. The problem is that the 11 kV
delta no longer has an earth reference point. Installing
a zigzag transformer near the secondary side of the 110
kV/11 kV transformer provides the required earth
reference point.
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