Zig Zag Power Transformer Behaviour During Unbalanced Faults
:One of the most intriguing and least understood topics in power engineering is the power transformer with a Zig Zag connection as one of the windings (Dz or Yz).
The current distribution for unbalanced faults in this case is particularly fascinating. This analysis is detailed in the attached write-up.The Zig Zag connection is a prime example of having one’s cake and eating it too.
It provides a neutral point, allowing for any type of grounding-solid, resistance, or reactance grounding.
Additionally, it achieves complete zero-sequence isolation between the primary winding and the Zig Zag winding, which is essential for ideal ground fault relaying and coordination.
The most surprising finding is the behavior of Yz transformer for ground fault on star side: even when the neutral is solidly grounded physically, it behaves like an ungrounded system!
Wherever a user plans to use a power transformer with a vector group of YNd and an NGT (Neutral Grounding Transformer) for grounding on the delta side, it is recommended instead to use a vector group of YNzn with the Zig Zag neutral grounded solidly or through a resistor or reactor, as required.