VFD's and spiking

[veritas]

Suppose a VFD drive(s) is supplied from an MCC which is in turn supplied from a 480V swbd. The swbd is connected to a 1.5MVA 13.8kV/480V Dyn11 trfr. Trfr neutral has two EF relay CT’s. Trfr neutral has a 5A NER.

CT1 feeds EF1 relay: set to 500mA, 0.2s and trips the LV ACB.

CT2 feeds EF2 relay: set to 500mA, 0.5s and trips the HV breaker.

Thus EF2 acts as a back-up to EF1. Question arose as to whether the EF2 should trip BOTH the LV ACB and the HV breaker. Apparently the VFD’s are subjected to “spiking” (transient voltage spikes) when the HV CB trips and removes HV supply without the LV ACB opening.

I do not know anything about spiking really and so would like to pose the question why does spiking occur only when the HV CB opens?

Thanks.

 

[waross]

Do you have a vacuum breaker on the HV side?
When the current to an induction coil is abruptly interrupted, you get an inductive spike as the magnetic field collapses.
The magnitude of the induced voltage or inductive "kick" is proportional to how fast the current is interrupted and how fast the field collapses.
The very fast current interruption of a vacuum circuit breaker on the primary can cause a voltage spike.
When the secondary breaker is opened, the transformer is still energized so there is no inductive kick from the transformer windings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Google 'current chopping'. It's a problem with VCB's which occurs at relatively low currents, i.e. typical operating currents rather than fault currents.

 

[veritas]

Yes, the HV breaker is a vacuum circuit breaker. Thus it does sound as if it's a good idea to first trip the LV and then the HV. Probably a blessing for the complete LV installation.

Thanks for the tip.

Kind regards.

 

[veritas]

Another thought - the current chopping spikes would be on the 13.8kV side where the VCB is located. Would they still be seen on the 480V side as well? If so would the magnitude not be much attenuated due to the big inductive impedance of the trfr?

 

[Skogsgurra]

Yes, of course. The spike is proportional to the number of turns. So it is lower on the 480 V side. And you shall be very glad that it is.

Still, it is not a voltage being transformed that is the problem. It is the voltage induced by the collapsing magnetic flux in the core / the "kick/back" voltage. And that voltage is induced in the primary as well as in the secondary winding.

It is common practice to include voltage arresters when vacuum breakers are used on transformer primaries. In many cases, it is an absolute necessity.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[veritas]

Ah, I get it! (I think!) - the interruption of the current before current zero results in a dc offset to account for the "instantaneous" change in current. The trfr core, it being an inductance and like all good inductances are born to do, it tries to maintain the collapsing current as the HV breaker poles separate.

The collapse of the trfr core flux results in an induced voltage in the HV and LV windings proportional to N*dphi/dt. The turns ratio in this case is 28.75 and so I expect the HV side to have a higher induced voltage than the LV by a factor of around 28.75 (not multiplied or divided by 1.732 due to the dy windings - as I presume it to be a singlephase phenomenon - depending on the pole trying to interrupt the current at off zero).

 

[Skogsgurra]

Yes.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Side issue: does the VFD mfr know and approve of the source being a resistance grounded Wye (resistance earthed Star?) network? That can have potentially catastrophic consequences to the line side protective devices in the VFD if not addressed properly and in fact some VFD mfrs will tell you that their drives cannot be fed from anything other than a solidly grounded (earthed) source.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Don't overlook Skogs advice in regards to voltage arrestors on the transformer primary bushings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[veritas]

All good points. Will follow all this up.

Thanks.