Neutral point treatment of a converter transformer
A common question of system integrators or end users is about the neutral point treatment of a converter transformer. Can or shall the neutral point be earthed? What are the consequences for the system design and performance of a drive system? How does the treatment of neutral point affect the fault condition? Despite many concerns, the answer to this question is straight forward.
This article is specific for converter transformers, i.e. input isolation transformers that supply the grid side section (rectifier) of a variable frequency drive (VFD). Transformers as part of VFD applications usually consist of multiple windings with different winding connections.
Neutral point treatment in general
Star (wye) and zig-zag connected windings inherently contain a neutral point. The logical question arises how to treat this neutral point. Three options exist:
- Keep neutral point isolated
- Neutral point solidly earthed
- Neutral point earthed through an impedance
- Resistive earthing of neutral point
- Inductive earthing of neutral point (e.g. Petersen coil)
Neutral point treatment of a converter transformer
Converter transformers have specific requirements on the neutral point treatment that are linked with the earthing (grounding) concept of the system. Users are sometimes unsure and often ask back. In fact, the answer is simple and straight forward.
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