Ritz VES12-02 Voltage Transformer

1.3.1 Choice Limitations of the Rated Burden
1.3.2.2 Rated Continuous Thermal Current
Especially for small rated primary currents, and high rated short time thermal currents (Ith), the rated burden of a
­current transformer is limited due to the maximum p ­ ermissible kAW-value (ampere turns). In this case, information should
be requested from the manufacturer.
This is the value of the current which can be permitted to flow continuously through the primary winding while the
secondary winding is connected to the rated burden, without the rise in temperature exceeding the values specified.
If the rated burden of a current transformer is calculated according to the formula
(AW) 2 . QFe. K [VA]
PN = lFe AW QFe K lFe
primary ampere turns iron cross section (mm2) constant ferromagnetic circuit (cm)
it becomes evident that, if the ampere turns (AW) can be doubled, a burden which is four times higher can be ­achieved.
Physically speaking, however, this is not always feasible, because the ampere turns are limited by the rated dynamic
current (Idyn). The reason for this is the force of the magnetic field intensity which, in case of a short circuit, attempts to
mutually balance the individual primary windings.
Furthermore, the maximum rated burden depends on the size of the resin body.
1.3.2 Definitions
It is common practice that the rated continuous thermal ­current is equal to the rated current but a higher current
can also be defined.
1.3.2.3 Rated Short Time Thermal Current The r.m.s value of the primary current is the value which a
transformer will withstand for one or three seconds without suffering harmful effects, should the secondary wiring be
short circuited.
1.3.2.4 Rated Dynamic Current The peak value of the primary current is the value which a
transformer will withstand, without being damaged electrically or mechanically by the resulting electromagnetic forces,
should the secondary winding be short circuited.
1.3.2.5 Burden Burden is the impedance of the secondary circuit and power
factor in ohms. The burden is usually expressed as the ­apparent power in volt amperes (VA) at a specified power
factor and at the rated secondary current.
1.3.2.6 Rated Burden The value of the burden is based on the accuracy requirements of this specification.
1.3.2.1 Rated Primary and Secondary Current The value of the primary and secondary current indicates
the performance rating of the transformer. A common practice is to use a secondary rated current of 1 or 5 A.
The primary rated current depends on the network and is defined by the end user.
Economically speaking, a secondary rated current of 1A should be chosen in order to keep the rated burden low,
especially for long wiring distances.
PN = I² · R+PB
Medium Voltage Instrument Transformers
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