IEEE C67.92-1987 pdf free.IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems.
IEEE C67.92 The ratios of thesymmetrical component parameters are used tocharacterize the classes of grounding as eithereffectively or noneffectively grounded in accor-dance with their coefficient of grounding,asdefined in 5.5.1 through 5.5.3.
Effectively Grounded. Groundedthrough a sufficiently low impedance ( inherentor intentionally added, or both) so that the coefficient of grounding does not exceed 80% [1],[6],[10]. This value is obtained approximately whenfor all system conditions the ratio of zero-sequence reactance to positive-sequence reac-tance X/X1 is positive and less than three, andthe ratio of zero-sequence resistance to positive-sequence reactance Ro/X is positive and lessthan one [6],[10],[16].
Noneffectively Grounded. The coef-ficient of grounding exceeds 80%[1 ]. This valuecan be exceeded when the ratio of zero-sequencereactance to positive sequence Xo/Xis negativeor is positive and greater than three, or the ratioof zero-sequence resistance to positive-sequencereactance Ro/X is positive and greater than one.Other Grounding Classes. The defini-tions and comments in the following sectionapply for specific grounding classes.
Resistance Grounding. When a system isgrounded through resistance, the zero-sequenceimpedance viewed from the fault may be inductive or capacitive, depending on the size, number,and location of the neutral-grounding resistorsand the capacitance to ground of the remainingsystem. With low-resistance grounding Xowillordinarily be positive,the fundamentalfrequency phase-to-ground voltages will,ingeneral, not exceed normal line-to-line voltage,and the neutral-to-ground voltages will notexceed normal line-to-neutral voltage. With high-resistance grotnding, Xo may be negative.In thatevent,phase-to-ground voltages may be greaterthan normal line-to-line voltages, and neutral-to-ground voltages greater than normal line-to-lineneutral voltages [12],[15].
If low-resistance grounding is used，thenatural-frequency voltages are significantlyreduced.The maximum voltages are essentiallythe fundamental-frequency voltages,which aregenerally higher than the fundamental-frequency voltages obtained with corresponding values ofneutral-ground inductive reactance [12],[15].
Inductance Grounding. When a system isgrounded through an inductance less than thatof a ground-fault neutralizer, the zero-sequenceimpedance viewed from the fault is inductiverather than capacitive and the zero-sequenceresistance is relatively small. Accordingly, thefundamental-frequency phase-to-ground voltages will not exceed normal line-to-line voltage,and the neutral-to-ground voltage will notexceed normal line-to-neutral voltage.
Following the initiation of a fault, systems withneutrals grounded through reactance will havemaximum transient voltages-to-ground on theunfaulted phase not exceeding 2.73 times normal.The voltage-to-ground at the neutral willnot exceed 1.67 times normal line-to-neutralvoltage [12],[15].IEEE C67.92 pdf download.