In order to protect against transient overvoltages it is important to implement resistance grounding. These transients are caused by arcing ground faults, and they supply suitable current for selective ground-fault detection. An important aspect to consider is that delta-connected supplies and wye-connected supplies with inaccessible neutrals cannot have neutral grounding resistors (NGR) connected to them. The only way to achieve resistance grounding in this circumstance is on an ungrounded system after instituting an artificial neutral with a zigzag transformer.
The zigzag transformer, also known as the star connection, consists of six winding coils on three-phase transformer which are wye-connected with a pair of series-connected, phase-displaced windings which are symmetrically attached on each side. Additionally, the zigzag transformer is able to connect its neutral point to ground through a NGR and create an artificial neutral for a three-phase system. As a result, only a small magnetizing current flows through the zigzag transformer. This provides a singular path for the ground-fault current to flow after one phase is grounded.
It is possible to have an operation with only one ground-fault on a resistance-grounded system if the NGR is appropriately sized. Littelfuse recommends the use of a Startco SE-330 or SE-325 to monitor the NGR.
The entire report is located here, courtesy of Littelfuse.
By Maximilian Teodorescu
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