Primary-Side Transformer Protection - Siemens SENTRON 3VL System Manual

Application planning

6.3 Primary-side transformer protection

6.3 Primary-side transformer protection
The circuit breaker as primary-side transformer protection
When switching on low-voltage AC transformers, the extremely high inrush current peaks
place special demands on the trip unit or on the making capacity of the circuit breakers if
these are also used to switch the transformer.
For most applications, an inrush current of 20 to 30 times the rated operating current is
expected in practice and must be taken into account when selecting the circuit breakers.
The maximum short-circuit current I
current). A circuit breaker in the lower setting range must therefore be operated for primary-
side transformer protection.
Example: A transformer with 500 A rated current; 20 times the inrush current
Selected: ETU with I
50% of I
Note
Switching the circuit breaker off
It is imperative to note that the minimum short-circuit current I
VDE 0100 is switched off in every case using a protection facility (e.g. circuit breaker).
With 3VL, the circuit breaker can be shut down using the time-delayed short-circuit release
(S), e.g. a 3VL with an ETU20, where it is possible to set the delay time to up to 500 ms
depending on the duration of the inrush current.
The short delay "bridges" the inrush current peak and the short-circuit protection can then
respond at low current values after a delay.
Circuit breakers with phase failure protection
Circuit breakers with phase failure protection must not be used. Their trip units have
protection against unbalanced network load. This cannot be switched off and could lead to
unintentional trips.
68
= 1000 A; setting range 0.4 - 1 x I
n
= 500 A; I = 11 x I
n i
CAUTION
of the 3VL circuit breakers is 11 x I
k
= 1000 A x 11 = 11000 A = 22 x current setting
n
(rated operating
e
= 400 A to 1000 A
n
in accordance with
kmin
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01