06-05-2013, 04:53 PM
Three Phase Transformer Winding Configurations and Differential Relay Compensation
Three Phase Transformer.pdf (Size: 1.18 MB / Downloads: 305)
INTRODUCTION
either a +300 or - 300 phase shift of positive sequence voltages and currents, and just these two
configurations seem to cause extensive confusion. There are actually many other ways to
configure a wye or delta that give other phase shifts, and to further complicate matters, there is
the occasional zigzag winding application and the additional confusion over what occurs when
CTs are connected in delta. These alternate transformer winding configurations are sometimes
referred to by terms such as Dy# or Yy#, or Yd#, Dz#, and Yz#, and where the # can be,
seemingly, almost any hour of the clock, hence the term “around the clock” phase shifting is
sometimes heard. The paper will review the variety of possible winding configuration and give
examples of the nomenclature that is used with them and how these various phase shifts are
created.
The paper will also show how a transformer differential relay compensates for the effects of the
various transformer winding configurations, as well as account for delta CT configurations. Many
papers and instruction manuals refer to compensation in terms of phase shifting. This leads
engineers to have a vague and misleading understanding that the relay is somehow phase
shifting currents to compensate for the transformer phase shift. While a “sequence component
differential” relay might be able to work this way, most transformer differential relays work
outside of the sequence component domain and do some form of current balance calculation in
the ABC domain.
Basic Transformer Design Concepts
Before proceeding, let us review a few points on transformer design and review some
nomenclature that will be used in the paper.
Since this paper is aimed at describing transformers that use phase shifts and winding designs
that are more commonly found outside the US market, the phase and bushing names of U, V,
W, will be used generally, rather than A, B, and C, or H and X. The means of specifying phase
shift and transformer connection will be the D-Y-Z + clock method. For instance, a transformer
connection will be Dy1 rather than a DAB/Y, though some dual designations will be used for
clarity.
If one browses transformer sales literature, technical papers and books, and industry standards
such as IEEE C57.12.00 and IEC-60076-1, one will find many variations on the nomenclature
and figures used to show how the phases are identified in a three phase system. Figure 1 is a
composite of some of the identification methods that will be found. Besides the terms seen in
Figure 1, the terms R, S, and T are used in some sources to name the phases.
Positive Sequence Phasing and Even More Possible Winding Configurations
The entire paper assumes positive sequence phase rotation and that all the configuration
diagrams were built on positive sequence phase rotation. There are actually more connections
than listed if one wishes to do phase rotation changes within the transformer by swapping two
phases on only one side of the transformer and leaving the third in place in a way that would
convert positive sequence voltage on one side of the transformer to negative sequence voltage
on the other side. This seems a fairly remote possibility.
Phase Shifts Using Clock Notation
Note the use of Y# for naming each of the configurations. The # refers to the phase angle, as
viewed on a 12 hour clock, of winding W1 relative to the voltage applied to the U bushing with a
balanced 3 phase positive sequence voltage (UVW or ABC sequence). Note also the phasor
diagrams in Figure 3 that show the phase angle between the W1 winding and the voltage on the
U bushing. The relationship between the W1 voltage and the U bushing will become important
when one needs a common reference for determining phase shift across the transformer. One
finds the phase shift by lining up the W1 voltages on different sides of a transformer and seeing
how the U bushing voltages on each side compare.
Combining the Various Winding Configurations
An exhaustive listing of all the possible permutations of transformer winding configurations is not
worth presenting. Only a small percentage of all the possible winding configurations is
commonly found in practice, and many are effectively redundant views of the same
configuration. More complete wiring and phasor diagrams of some of the common
configurations are given below in figures 8-19 and the rest can be analyzed as needed, using
the material as a starting point. Except for the two cases of zigzag windings, only transformers
with two basic voltage levels are covered. A somewhat extensive list of the more common
configurations is found in [1] IEC60076-1, Power Transformers. Figures 20 and 21 are drawn
from this standard without the details of Figures 8-19 but annotated with some additional
information.
In the figures to follow, the phasor diagrams found more commonly in US practice are shown on
the left, and phasor diagrams that might be more like those found in international practice are
shown on the right. Polarity marks have been added to the US practice phasor drawings. The
winding diagram in the lower left is intended to show most clearly how the windings would be
interconnected, using US practice bushing names. The figure on the lower right shows a
common international method of showing the winding connections, except the method has been
modified by the addition of polarity marks and winding numbers, and in these figures, when
zigzag windings are shown, one winding set is shown with the polarity mark up, and the other
with the polarity mark down, so that it is easier to see the interconnection of the windings,
compared to IEC drawing practices.