[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

in the 1960s, it was found that a transformer with a delta-connected primary was more prone to
ferroresonance problems because of higher capacitance between buried primary cables and ground. An
acceptable preventive was to go to grounded-wye-grounded-wye transformers on all but the heaviest
industrial applications.
2.2.5 Operational Concerns
Even with the best engineering practices, abnormal situations can arise that may produce damage to
equipment and compromise the continuity of the delivery of quality power from the utility.
2.2.5.1 Ferroresonance
Ferroresonance is an overvoltage phenomenon that occurs when charging current for a long underground
cable or other capacitive reactance saturates the core of a transformer. Such a resonance can result in
voltages as high as five times the rated system voltage, damaging lightning arresters and other equipment
and possibly even the transformer itself. When ferroresonance is occurring, the transformer is likely to
produce loud squeals and groans, and the noise has been likened to the sound of steel roofing being
dragged across a concrete surface. A typical ferroresonance situation is shown in Figure 2.2.10 and consists
of long underground cables feeding a transformer with a delta-connected primary. The transformer is
unloaded or very lightly loaded and switching or fusing for the circuit operates one phase at a time.
Ferroresonance can occur when energizing the transformer as the first switch is closed, or it can occur
if one or more distant fuses open and the load is very light. Ferroresonance is more likely to occur on
systems with higher primary voltage and has been observed even when there is no cable present. All of
the contributing factors — delta or wye connection, cable length, voltage, load, single-phase switching
— must be considered together. Attempts to set precise limits for prevention of the phenomenon have
been frustrating.
2.2.5.2 Tank Heating
Another phenomenon that can occur to three-phase transformers because of the common core structure
between phases is tank heating. Wye–wye-connected transformers that are built on four- or five-legged
cores are likely to saturate the return legs when zero-sequence voltage exceeds about 33% of the normal
line-to-neutral voltage. This can happen, for example, if two phases of an overhead line wrap together
and are energized by a single electrical phase. When the return legs are saturated, magnetic flux is then
forced out of the core and finds a return path through the tank walls. Eddy currents produced by magnetic
flux in the ferromagnetic tank steel will produce tremendous localized heating, occasionally burning the
tank paint and boiling the oil inside. For most utilities, the probability of this happening is so low that
it is not economically feasible to take steps to prevent it, other than keeping trees trimmed. A few, with
a higher level of concern, purchase only triplex transformers, having three separate core-coil assemblies
in one tank.
2.2.5.3 Polarity and Angular Displacement
The phase relationship of single-phase transformer voltages is described as “polarity.” The term for voltage
phasing on three-phase transformers is “angular displacement.”
2.2.5.3.1 Single-Phase Polarity
The polarity of a transformer can either be additive or subtractive. These terms describe the voltage that
may appear on adjacent terminals if the remaining terminals are jumpered together. The origin of the
polarity concept is obscure, but apparently, early transformers having lower primary voltages and smaller
kVA sizes were first built with additive polarity. When the range of kVAs and voltages was extended, a
decision was made to switch to subtractive polarity so that voltages between adjacent bushings could
never be higher than the primary voltage already present. Thus the transformers built to ANSI standards
today are additive if the voltage is 8660 or below and the kVA is 200 or less; otherwise they are subtractive.
This differentiation is strictly a U.S. phenomenon. Distribution transformers built to Canadian standards
are all additive, and those built to Mexican standards are all subtractive. Although the technical definition
of polarity involves the relative position of primary and secondary bushings, the position of primary
bushings is always the same according to standards. Therefore, when facing the secondary bushings of
an additive transformer, the X1 bushing is located to the right (of X3), while for a subtractive transformer,
X1 is farthest to the left. To complicate this definition, a single-phase pad-mounted transformer built to
ANSI standard Type 2 will always have the X2 mid-tap bushing on the lowest right-hand side of the lowvoltage
slant pattern. Polarity has nothing to do with the internal construction of the transformer
windings but only with the routing of leads to the bushings. Polarity only becomes important when
transformers are being paralleled or banked. Single-phase polarity is illustrated in Figure 2.2.11.
2.2.5.3.2 Three-Phase Angular Displacement
The phase relation of voltage between H1 and X1 bushings on a three-phase distribution transformer is
referred to as angular displacement. ANSI standards require that wye–wye and delta–delta transformers
have 0˚ displacement. Wye–delta and delta–wye transformers will have X1 lagging H1 by 30˚. This
difference in angular displacement means that care must be taken when three-phase transformers are
paralleled to serve large loads. Sometimes the phase difference is used to advantage, such as when
supplying power to 12-pulse rectifiers or other specialized loads. European standards permit a wide
variety of displacements, the most common being Dy11. This IEC designation is interpreted as Delta
primary–wye secondary, with X1 lagging H1 by 11 30˚ = 330˚, or leading by 30˚. The angular displacement
of Dy11 differs from the ANSI angular displacement by 60˚. Three-phase angular displacement is
illustrated in Figure 2.2.12.
FIGURE 2.2.10 Typical ferroresonance situation. (From IEEE C57.105-1978, IEEE Guide for Application of Transformer
Connections in Three-Phase Distribution Systems, copyright 1978 by the Institute of Electrical and Electronics
Engineers, Inc. The IEEE disclaims any responsibility or liability resulting from the placement and use in the described
manner. Information is reprinted with the permission of the IEEE.)
FIGURE 2.2.11 Single-phase polarity. (Adapted from IEEE C57.12.90-1999. The IEEE disclaims any responsibility
or liability resulting from the placement and use in the described manner. Information is reprinted with the
permission of the IEEE.)
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC