I've taken the liberty of submitting this topic to address CSA's, Cephas', and Robbyk's questions... paraphrasing, "Why is the MV system-neutral grounded, especially when a 4th or neutral-conductor is never required!" If it were possible to insulate electrical equipment with glass, this discussion would hardly be necessary.
Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with a one-phase grounded. That is, until such operation leads to over-voltages, both transient and continuous, resulting in a substantial increase in subsequent damage. (The Control List Archive contains numerous cases.) Although, somewhat to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific
situations.
The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, or melted magnetic-iron. A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of ten's of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault detectors or breakers. The phase-over-current protective device is sufficient. Repair or replacement of LV equipment is easily handled, both in material and cost!
Any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of 100's of thousands of kVA.) Also, if damage is kept low, repair is possible. If, however, current magnitude is very large, the magnetic structure damage may preclude repair, but instead will require total replacement. Thus, impedance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.
Cephas... an earthing (or Zig-Zag) transformer is considered an impedance-earthed system
Regards, Phil Corso (cepsicon [at] aol.com)
Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with a one-phase grounded. That is, until such operation leads to over-voltages, both transient and continuous, resulting in a substantial increase in subsequent damage. (The Control List Archive contains numerous cases.) Although, somewhat to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific
situations.
The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, or melted magnetic-iron. A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of ten's of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault detectors or breakers. The phase-over-current protective device is sufficient. Repair or replacement of LV equipment is easily handled, both in material and cost!
Any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of 100's of thousands of kVA.) Also, if damage is kept low, repair is possible. If, however, current magnitude is very large, the magnetic structure damage may preclude repair, but instead will require total replacement. Thus, impedance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.
Cephas... an earthing (or Zig-Zag) transformer is considered an impedance-earthed system
Regards, Phil Corso (cepsicon [at] aol.com)
