Switching transients - Loads served by converter transformer
Switching transients - Loads served by converter transformer
(OP)
Dear all,
in all literature from manufacturers it is always recommended to consider the virtual current chopping phenomena when dealing with switching off loads connected to converter transformers (ex. motors through Variable Speed Drives) using a vacuum circuit breaker.
These recommendations are not considered when dealing with other transformers. For example distribution transformers or motors connected through delta/wye.
I would appreciate any explanation or literature to support this recommendation.
Thank you.
Adam.
in all literature from manufacturers it is always recommended to consider the virtual current chopping phenomena when dealing with switching off loads connected to converter transformers (ex. motors through Variable Speed Drives) using a vacuum circuit breaker.
These recommendations are not considered when dealing with other transformers. For example distribution transformers or motors connected through delta/wye.
I would appreciate any explanation or literature to support this recommendation.
Thank you.
Adam.






RE: Switching transients - Loads served by converter transformer
I imagine that electronic switches are even more susceptible to damage from switching transients than motor windings so I understand why there is a caution on converters transformers but disagree that no cautions exist for motor circuits.
RE: Switching transients - Loads served by converter transformer
thank you for the reply.
What you have noted is true since there is a requirement for protection of motor circuits against surges due to two facts:
1- motor BIL rating is usually not high.
2- the protection usually addresses disconnection of the motor during starting (i.e. when the current is purely inductive).
However my question is regrding disconnection of the motor under normal running conditions (the motor is already running at nominal current and we are switching it off). In this case the recommendation of protection specifically against virtual current chopping is only indicated for motors served by variable speed drives (a combination of line-to-line and line-to-earth (6) arresters since chopping could occur for two poles when the first-pole-to-clear re-ignites).
My reference here is Siemens Switchgear Application Guide.
The guide provides only application recommendation but no background for the recommendation.
I believe this could be just empirical based on experience however what I am looking for is the origin of this phenomena in such circuits.
Thank you.
Adam
RE: Switching transients - Loads served by converter transformer
RE: Switching transients - Loads served by converter transformer
If the reason is caused by the transformer type (dry/oil) shouldnt that be also a concern for all transformer application.
Thanks.