We have several large AC and DC drive systems in our plant. I recently upgraded the service for a 150HP DC drive that was upsized to a 200HP. In the process I discovered the wye secondary of the drive isolation transformer was not grounded or bonded at the XO. The XO was left unconnected. I bonded the XO on the larger transformer that I installed, in particular because it reduced the arc flash calculation(I'm not sure why). I have since found other installations on other large drive systems installed the very same way. Everything I read online says to ground this secondary for two reasons: 1) eliminate common mode noise in both directions to or from the power system to the drive, or vice-versa. 2) localize all high frequency ground currents generating from the high speed semiconductor switching. So why would someone intentionally leave these isolation transformer secondardies ungrounded?
secondary grounding of delta-wye drive isolation transformer
- Thread starter Tfret
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ptonsparky
Tom
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- EC - retired
Were they bonded anywhere? They do not have to be bonded at the transformer, it could be done at the secondary protection.
Other than that, ignorance?
Other than that, ignorance?
Not really an answer to your question. I have noticed a few times on this forum the mention of drive isolation transformers. It isn't something we very often do here in the UK unless you need a different voltage for the drive. The common LV industrial supply is from a star (wye) 400V 3ph +N with the N grounded at the source.
On larger drives we have quite often gone 12-pulse with a Ddyn0 or similar to reduce harmonics on the supply. Then the primary is sometimes fed from the supply to the facility, typically 11kV.
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