VFD Output Step-up Transformer

Electrical & Electronic Engineering gen discussion Electric Motors & Generators
#1

QUESTION
Recently, I have come across 3.3kV VFD output stepped up to 11kV supplying a 7MW motor. The step-up transformer is Y-D with floating neutral on star side. I am used to Dyn connected captive transformers with the star point high resistance earthed. But, that is without VFD.
Is Y-D with floating neutral a requirement from VFD designer? What could be the issues if we use Dyn with high resistance earthed neutral?

REPLIES

electricuwe
The pulse pattern generation in VFDs relies on cancellation of certain harmonics in the current due to having no ground connection on the load side. Usually the load is a motor a no one would consider to ground the neutral of a motor with a star-connected stator.

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=462439
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#2

I agree with electricuwe’s excellent point.
The output of a VFD would not like to find anything grounded.

Think about a full wave bridge rectifier which is the input of all non-isolated VFDs. The rectifier feeds the energy storage capacitor bank. There is no rational ground reference available with a bridge rectifier circuit.

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