Pruebas A Transformadores De Potencia

Journal I.E.E., January J955

The performance of

S

TAR-STAR transformers are often provided with a
delta-connected tertiary winding for one or more of
the following reasons:

(i) To provide auxiliary terminals at a voltage different
from the primary and secondary voltages of the transformer.
(ii) To allow sufficient earth-fault current to flow for
the operation of protective gear.(iii) To prevent the transformer from introducing
triple-frequency voltages into the network.
(iv) To keep the voltage rise on any phase within
narrow limits in the event of any unbalance of the load
between phases.
The first of these reasons is obvious and does not need
further discussion. When it applies, the winding should
be called auxiliary rather than tertiary.
In practice, whateverthe purpose of the tertiary winding its conductor cross-section is usually determined by
the necessity for it to withstand fault conditions. This
requirement often calls for a much more expensive
winding than would suffice for the purpose for which the
winding is provided.
Flux Distribution during an Earth Fault

Suppose that a star-star transformer has its neutral
points earthed both onthe primary and secondary sides
and that, in addition, the power source connected to the
primary side has an earthed neutral; then in the event of
an earth fault on one phase of the secondary the power
current would be confined to one limb of the transformer
and of the power source and would be able to return
through the earth connections without materially affecting
the other phases. Inthese circumstances the affected
limb of the transformer would behave very much as a
single-phase transformer and the reactance to the earth-

1

STAR-STAR TRANSFORMERS
Professor R. O. KAPP, B.Sc, Member
A. R. PEARSON, B.Sc, Associate Member
To obtain a satisfactory performance with a star-star
transformer it is sometimes, but not always, necessary to
provide the transformer with adelta-connected tertiary
winding. Much misapprehension exists among electrical
engineers about the reasons for such a winding, and most
users of these transformers in Britain demand a delta
winding in all instances, although, in fact, its additional
cost is often not justified and its provision is technically
undesirable. The subject is rarely mentioned in textbooksand then, as a rule, onlybriefly. The authors of this article
explain in plain words when a star-star transformer should
be fitted with the extra winding. This will be familiar to
transformer specialists, but it is believed that the article
will be useful to many other electrical engineers who use
star-star transformers and enable them to decide whether
a tertiary winding is necessary in a particular transformer.
Theauthors show that in most instances a decision can be
reached easily and without an exact numerical assessment.

only and not on the primary, the primary power current
feeding the fault cannot return to the power source via
earth and has to return via the two other phases, as
shown in Fig. 1. As a result, an earth fault on the

Flow of earth-fault current
in star-star transformer

PRIMARYfault current would be of the same order as that to a
normal power current.
In the more usual circumstances where the neutral
point of the transformer is earthed on the secondary side

SECONDARY

secondary connection from one limb of the transformer
produces currents flowing in the windings on the two
other limbs, and the behaviour of the transformer as
regards reactance to the earthfault can be expected to
9

Journal I.E.E., January 1955

differ materially from its behaviour with a normal
3-phase power current.
Let us consider, therefore, a star-star transformer
having the neutral point earthed on the secondary side,
but not on the primary side, and with an earth-fault
current flowing from one secondary connection, with
corresponding currents in the primary as...