Beckhoff SCT7 Series, Документация

Страница: 11 / 48

Product overview
SCT7xxx 11 Version: 1.2
Technical terms of current transformers
Term Explanation
Primary rated current I
pr
(alternative formula symbol I
N
)
Value of the rated current on the primary side.
Secondary rated current I
sr
Value of the rated current on the secondary side.
Rated power S
r
Value of apparent power (in [VA]) that the transformer can deliver
to the secondary circuit at secondary rated current and rated load.
Rated frequency f
R
Value of the rated frequency.
Accuracy class Indication that the measurement deviations are within specified
limits under prescribed application conditions.
Rated insulation level U
m
Maximum voltage; RMS value of the highest conductor-to-
conductor voltage for which a instrument transformer is rated with
respect to its insulation.
Specified is the value of the rated insulation level in three values:
1. maximum value of the conductor-to-conductor voltage for which
the isolation of the transducers is rated;
2. value of the rated AC voltage (50 Hz, 1 min) with which the
insulation safety of the equipment is tested
3. value of the surge voltage level (this specification is mostly
unoccupied here, since IEC 61869/1 only requires a specification
for transformers with a conductor-to-conductor voltage of >1.2 kV)
Overcurrent limiting factor (FS) Ratio of the rated limiting current to the primary rated current.
Thermal rated continuous current I
cth
Value of the continuous current in the primary winding at which the
overtemperature does not exceed the value specified in the
standard, with the secondary winding loaded with the rated load.
Rated short-time thermal current I
th
Value of short-time current for a limited time in the primary winding
at which the excess temperature does not exceed the value
specified in the standard, with the secondary winding loaded with
the rated load.
Rated impulse current I
dyn
Maximum value of the primary current, the electromagnetic force
effect of which does not cause any electrical and mechanical
damage to the current transformer with short-circuited secondary
winding.
“Open voltage" of
current transformers Current transformers which are not directly connected to a load
must be short-circuited on the secondary side for safety reasons!
A current transformer operated open on the secondary side induces
very high peak voltage values at its secondary terminals. The
magnitudes of these voltages can reach values of up to several
kilovolts, depending on the dimensioning of the current transformer,
and thus represent a danger to persons and the functional safety of
the transformer. For safety reasons, and to avoid magnetization of
the core iron during secondary-side open operation, open operation
should generally be avoided.
Grounding of secondary terminals According to DIN VDE 0141 (01/2000) paragraph 5.3.4, current and
voltage transformers for nominal voltages from Um = 3.6 kV must
be grounded on the secondary side. For low voltages (Um ≤ 1.2
kV), grounding is not required if the transformer housings do not
have any metal surfaces with large contact areas.