Switching transients - Loads served by converter transformer

[Adam1980]

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.

 

[ScottyUK]

Current chopping is certainly a concern for motors switched by VCBs. Transformers generally are switched less often, unless they are captive unit types for motors, so less of a concern. Motors are often fitted with surge capacitors or arrestors at the motor main terminal box.

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.

 

[Adam1980]

Hello Scotty,

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

 

[ScottyUK]

Is it possible that the converter transformers being considered by Siemens tend to be dry types with a relatively high surge impedance compared with equivalent oil types? I expect Siemens probably base theirs on their GEAFOL range. I expect that you can find some of the equations relating to current chopping, however if you're struggling the IEE's Distribution Switchgear book might be a starting point.

 

[Adam1980]

I believe it is something related to the non-linearity of the variable speed drive. The recommendation states that when the converter transformer serving a variable speed drive experiences an under load switching the behaviour is similar to that of transformer inrush.

If the reason is caused by the transformer type (dry/oil) shouldnt that be also a concern for all transformer application.

Thanks.