What is causing my equipment to arc over ?

[Scrummer4]

I am trying to find the phenomena (plural) which are causing my equipment to arc over. We are talking about a 400 Vac, 12 pulse, 50 Hz rectifier. We had all 3 phases of the delta rectifier arc to the nearest earthed panel. The equipment has been in service for about a month, and is one of 4 identical equipments. Clearances between the 400 volt bars, fuses and panels are all compliant with the relevant standards. 11th harmonic filters are fitted.
This has only happened once at this 2 MVA installation, but a similar situation has been seen with similar equipments, not only ours, but also competitors, who I believe are also struggling to find the causes.
We are considering transients caused by system switching, resonance triggered by such switching, PFC interactions, and similar causes. But my underlying question is “How is the (more or less) 50 kV generated which causes this arc ? Then, how do we prove, disprove, and test for these apparently random conditions ?
Any ideas, references to papers, websites, etc., will be more than welcome.
Hope I haven't rambled on for too long, but it is a complex situation to explain.
Thanks.

 

[Skogsgurra]

Keith,

Yes, you are correct. It is about rather high energies. Those tubes explode if the prospective sc current is more than a few tens of amperes. We have tens of kA here. Varistors get high impedance after the transient is over, so follow current is only a few milliamps. But they usually need to absorb kilojoules during the transient itself. By selecting them with a high enough energy rating, you avoid the aging that is typical if you select the "just right" varistor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Hi Gunnar.

Most varistors depend on series impeadance to share the hit and limit the actual current. Most fail rapidly if they're the only energy dump.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

No. Not in power applications. Telecom and control circuits are another breed.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...