IEEE C62.92.5-1992.IEEE Guide for the Application of  Neutral Grounding in Electrical Utility Systems, Part V—Transmission Systems and Subtransmission Systems.
IEEE C62.92.5 Transmission systems are normally connected to generating systems by means of a delta-wye-connected transformer bank with the generator side connected in delta and the transmis-sion system connected grounded wye.This connection provides a ground source for the trans-mission system. It also reduces the magnitude of ground-fault current in the generatingsystem and keeps the third harmonic voltage possibly produced by the generator out of thetransmission system.
Control of Overvoltages Produced by Ground Faults and Degree of Surge-Volt-age Protection With Surge Arresters. There are two components of voltage or overvoltagein electrical systems when a system ground fault occurs or when a circuit breaker or a switchoperates in clearing the ground fault.One of these is the temporary overvoltage or fundamen-tal frequency overvoltage,and the second is the natural frequency voltage, usually of shortduration, that is superimposed upon the temporary overvoltage. Since total voltages are ofgreater interest, the sum of the temporary overvoltage and the natural frequency voltage iscommonly used and termed the transient voltage.
Temporary Overvoltage (TOV) and Arrester Rating.The ultimate surge voltageprotection is obtained through arrester voltage ratings as low as system grounding conditionswill permit during normal and abnormal system conditions. Initially, however, when the surgearrester was adopted as the basic protection device,the equipment design (coordination ofmajor insulating structures) assumed that an “ungrounded neutral” or “100% rated” arresterwould be used, unless otherwise specified [5].
In time, after successful service experience with 100% rated arresters (100% of maximumline-line voltage), it was reasoned that lower rated arresters would be suitable on groundedneutral systems. On these systems, the TOV on the unfaulted phases during a line-to-groundfault would bear the same relationship to arrester rating as “maximum line-line voltage” inan ungrounded system. An“effectively grounded”system was then defined in terms of thesymmetrical-component sequence resistances and reactances [1], for which the Tov on anunfaulted phase does not exceed 80% of the maximum line-to-line voltage.Under this condi-tion, an arrester rated at 80% of maximum line-to-line voltage was deemed applicable, and itwas classified as a “grounded neutral”” arrester.The use of a “grounded neutral” arrester with lower protective levels enabled designs insome electrical equipment, such as transformers, to have reduced insulation levels with ade-quate protection.Reduced insulation allowed reduction in size,weight，and cost. Subse-quently，still lower rated arresters were commonly applied whenever the grounding wassignificantly better than“effective,” particularly at system voltages where these reductionswere significant (above 230 kV).
Usually the TOV produced by a system ground fault is greater than that produced by othercauses (generator overspeed，ferroresonance，harmonics，etc.).An exception to this mightoccur on systems where the coefficient of grounding [1] is less than 80%.The rating of gapped silicon-carbide surge arresters generally exceeded the TOV due to aphase-to-ground fault on the system where it was applied [6].This criterion was based on theassumptions that the maximum TOv is produced by a ground fault and that the arrestermight operate due to a surge while there was a ground fault on another phase. The arresterhad then to seal off against the TOv, which was sustained until the fault was interrupted.There were some arresters that sealed off against voltages higher than their rating.Overvolt-age characteristics for these arresters were published in the late 1960s or early 1970s. Thisfeature has sometimes been utilized to provide lower protective levels.IEEE C62.92.5 pdf download.